What Happened on Impact Night?
Posted on Dec 16, 2009 12:10:20 PM | Paul.D Tompkins
The LCROSS mission was ultimately focused on the final four minutes of flight, starting at the time of the Centaur impact, and ending with the impact of the Shepherding Spacecraft. During that time, the Payload Engineer and the Science Team took operational center stage. Once the science payload was powered on, the team’s job was to confirm the full functionality of the instruments, and then to adjust instrument settings to make sure the data we received was the best it could possibly be. For Impact, there were no second chances – the Shepherding Spacecraft was to be destroyed as a forgone outcome of its observation of the Centaur lunar collision.
In this post, I’ve invited Payload Engineer Mark Shirley to provide his perspective of impact night. Mark was the Payload Software Lead, in charge of the design and implementation of onboard instrument command sequences and other onboard software components, as well as a large fraction of the science-related data processing software used on the ground. During flight, Mark sat in the Mission Operations Control Room (MOCR) with the rest of the Flight Operations team. On the final day, his job was to assess and maintain the engineering functionality of the science payload in those minutes before impact.
In the impact video sequence, from the public’s perspective, there were a lot of things that happened operationally that were probably difficult to understand. Mark will explain the plan for gathering data from the impact and also describe what actually happened in flight. On a lighter note, Mark was one half of our now famous (or notorious) “high-five malfunction” that created such a buzz on the social media circuit after impact. He’ll explain that as well. I’ll let Mark take it from here. Enjoy!
I'd like to describe our plan for collecting data about the Centaur impact from the shepherding spacecraft (S-S/C), how actual events differed from the plan, and what that says about the process of developing and flying spacecraft. In particular, I'll cover why some of the pictures were fuzzy and some were white and why we were sending commands during the last minutes. I won't touch on the scientific interpretation of the data, only the process of gathering it. This story contains some hard work, a few mistakes, a little nail biting tension, and finally, success.
Central to the story is the type of mission LCROSS was: a cost-capped, fixed-schedule mission. That meant if LCROSS had been late, LRO would have flown with a dead or inactive LCROSS. If the project had run out of money, whatever hadn't been done wouldn't have gotten done. Within the LCROSS project, the instruments were in a similar position. The original idea was to observe the Centaur impact from Earth only. Onboard instruments were soon added to the design but with a total instrument budget of approximately $2 million. That's much less than single instruments on many other missions.
Two things made success possible. First, project managers kept a tight focus on using the barest minimum hardware and testing required to perform the science, and went beyond that, only as the budget allowed, to increase the likelihood of success. Second, everyone stayed on budget. The payload team had no choice, but if any other part of the project had overrun by a lot, the payload might have been eliminated or flown only partially ready.
The LCROSS Instruments
LCROSS carried nine instruments. Five were cameras to take pictures over a large range of wavelengths, that is, colors. One was for visible light that our eyes can see. Two near-infrared cameras captured mineralogy and water signatures, and two mid-infrared cameras captured thermal signatures from -60C to +500C). LCROSS carried three spectrometers to measure color very precisely. One covered ultra-violet and visible wavelengths and two covered near-infrared wavelengths. The latter spectrometers were the best at searching for water, and one looked down toward the Centaur impact the vapor cloud it kicked up and the other looked to the side as LCROSS passed through that cloud. Finally, LCROSS carried a high-speed photometer to measure the brightness of the impact flash.
The instruments are described in detail here. Data from all nine instruments had to share LCROSS' one Mbps (one megabit or million bits per second) radio link to the ground. At that rate, it takes 2 seconds to transmit a typical cell phone picture. This was the maximum data rate available for the LCROSS mission and used only twice: lunar swingby on June 22nd and impact on October 9th. All other instrument activities that took place during the 112 day mission used speeds less than 256 kbps (kilobits or thousand bits per second), which was sufficient for collecting data to calibrate the instruments.
The Observation Plan
The two components of the LCROSS mission, the Centaur and the Shepherding Spacecraft (S-S/C), separated about 10 hours before they reached the moon. At the moment the Centaur impacted, the S-S/C was still 600 kilometers above the surface. Falling at 2.5 kilometers per second, the S-S/C reached the surface 4 minutes later. Observations of the Centaur impact event made during those 4 minutes were the purpose of the mission. Unlike orbital missions that can usually try multiple times to collect data, we had just one shot.
The diagram below shows the plan for observing from the S-S/C, starting one minute before Centaur impact, at the beginning of what we called “Sequence 2” in the NASA TV video. The diagram plots our intended schedule of instrument observations against time: each row represents one of the instruments (instrument abbreviations appear below each row of data), and each tick mark along a row represents one observation, either an image or a spectrum. Over some intervals, the observations are spaced so closely that the plot looks like a solid bar.
Figure 1 The LCROSS impact observation plan: These timelines indicate when each image and spectra was planned to occur during the final four minutes of the mission. The horizontal axis represents time. Each row represents an instrument, and each tick mark represents the timing of a sample (an image or spectrum) from that instrument.
The last four minutes were divided into three periods, called FLASH, CURTAIN and CRATER. Each period focused on a different aspect of the expected impact event and emphasized data collection from different instruments.
FLASH started one minute before the Centaur impact and focused on the very short burst of light generated by the Centaur impact itself. Starting from the top of the diagram, the plan was to stop both the Visible Light Camera (VIS) and the Near-Infrared Camera #2 (NIR2). This would allow us to focus on NIR1 images which we felt had the best chance of catching the location of the impact flash which we expected to be visible for less than one second. These three cameras shared a common input to our payload computer, called the Data Handling Unit (DHU), and could not be used simultaneously. By stopping VIS and NIR2, we could run NIR1 at a faster rate (see the segment labeled 'A'), increasing the odds it would image the flash. The planned sequence also increased the NIR1 exposure time to capture the flash signature even if it was very faint. We knew this would produce a badly overexposed image of the illuminated lunar surface, but our goal was to locate the impact. We'd have plenty of other pictures of the surface. This sort of shifting attention between cameras accounts for the periods where one camera image would stop updating for a while.
We also designed the FLASH strategy for the spectrometers around our expectation of a dim, short duration flash event. Near Infrared Spectrometer #1 (NSP1), the main water-detection instrument, was put into a high-speed, low resolution mode (represented by the yellow bar). The Visible and Ultraviolet light Spectrometer (VSP) was commanded to take long exposures, and Total Luminescence Photometer (TLP) was powered early enough to reach equilibrium and be at its most sensitive for the flash event.
The second phase, CURTAIN, started just after the Centaur impact and ran for three minutes. Its purpose was to take spectra and images of the expanding vapor and dust clouds thrown up by the impact. CURTAIN was the most important period and also the simplest. All instruments ran in their default modes, as follows. The DHU shifted between the three analog cameras in a stuttering pattern - VIS, VIS, NIR1, NIR2 - repeating. Both thermal cameras monitored the plume shape and temperature. The two downward-facing spectrometers (NSP1 and VSP) looked
for water and other chemicals. The side-looking spectrometer (NSP2) also looked for water and other compounds, but from sunlight scattered or absorbed by the dust and vapor cloud. The TLP continued to take data during this period, but it’s primarily function was during FLASH.
The goal of CRATER, the final period, was to image the crater made by the Centaur impact to get its precise location and, more importantly, its size. From its size and their detailed models of crater formation, the LCROSS Science Team can potentially tell us how the crater evolved over the few seconds of its formation and how much material was excavated. The primary instruments in this period were the two thermal cameras, MIR1 and MIR2. Their sample rates were increased relative to those for CURTAIN. To image the crater in a second frequency band, NIR2, the more sensitive near infrared camera, was commanded to its most sensitive setting. NIR1 and VIS would not be used during this period because neither was sensitive enough to see anything in the permanently shadowed area. All spectrometers would continue running to look for light reflected off of any plume or vapor cloud. At the end of this phase, the S-S/C would fall below the rim of Cabeus Crater, cutting off radio transmission to Earth, and then impact the surface a couple of seconds later.
There were three keys to making this plan work:
- Downlink Bandwidth: the data collected had to fit within the 1 megabit radio downlink. We did a lot of testing before launch to work out a data collection plan that was further confirmed and refined based on on-orbit performance. We gave priority to data from the most important instrument, the near-infrared spectrometers, to provide robustness to the design. The best scheme was pre-programmed and ready to go in case we were unable to command the spacecraft in the final hour.
- Camera Exposure: We had to change camera exposure settings during the descent to reflect the changing brightness of the impact event and the surrounding scene. Defaults were pre-programmed based on the latest lighting models for impact morning from NASA Goddard Space Flight Center.
- Command Timing: In the instrument command sequences governing FLASH, CURTAIN and CRATER periods, we had to orchestrate changes in instrument configuration as they were needed to focus on different aspects of the impact event. Sometimes these changes had to be interleaved with instrument data collection in a way that was vulnerable to small timing changes.
So What Actually Happened?
Well, as reflected in the recent LCROSS press briefing, we collected a very rich and interesting data set that met the needs of our science objectives. However, we had challenges in all three areas – bandwidth, exposure settings and timing - although all ultimately proved minor. However, in some ways, it was a close call. This diagram shows what data was actually collected during the final four minutes of the mission.
Figure 2 Actual performance of the LCROSS payload: These timelines indicate when the images and spectra were taken on the morning of the impact. The pattern gives clues about the performance of the hardware and software systems that collected the data.
First, the rows representing the spectrometers, NSP1, NSP2 and VSP, look almost exactly as they should. Except for one problem with the Visible and Ultraviolet light spectrometer (VSP), which I describe later, our plan for collecting spectra worked perfectly. This is very good, because the spectra carried most of the information we were trying to collect.
As for the cameras, several differences from the plan jump out. The most obvious is that the timing of observations along some timelines is irregular with many observations missing, e.g., the visible camera pointed to by note E. This occurred with all five cameras (the first five timelines) but not with the spectrometers (the next three timelines).
Scene Complexity and Bandwidth Limitations
Image compression is the process of finding and reducing redundancy in an image in order to transmit it more efficiently. On LCROSS, to fit within the 1 Mbps data rate limitation for our downlink, we used a lossy compression algorithm that typically reduced each image to 1/20th its original size. Lossy methods achieve greater compression than lossless methods by actually removing parts of each image. The algorithm tries to find subtle details whose removal the human eye won't notice. Being able to combine many different kinds of data into a single digital data stream is so useful that this approach has been standard practice for many years.
In flight, the irregularity of observations occurred because we underestimated the complexity of the lunar scene during ground testing. We had done much of our testing with a large reproduction of the moon's pole in front of the cameras, but it turned out this didn't mimic the high contrast and detail of the real scene. Scene complexity mattered because the images were highly compressed and changes in the moon scene changed the sizes of the compressed images by a factor of 4. We first observed this behavior during the lunar swingby LCROSS performed during the first week of its mission. Turning on the instruments during the swingby was intended as a learning experience, and it proved critically important. It provided the best operational practice we got for the impact as well as data to calibrate the instruments.
After the lunar swingby in June, I changed the thermal camera sampling rates in the instrument command sequences for the final hour. Unfortunately, the compression problem turned out to be about 20% worse during the final hour of the mission than during the lunar swingby. This forced us to change the thermal camera rates again in real-time, but we had practiced changing them during rehearsals, just in case. In the NASA TV impact video sequence, you can hear the Science Team requesting a change of MIR1 rates to 1 Hz, and MIR2 to 0.1 Hz. See note F in the figure. The rate for thermal camera #1 (MIR1) changes just before this note and changes for MIR2 just after it. Even though we'd practiced, this was still a very tense time as we were losing some data while the changes were being made. Changing the MIR rates felt like it took forever.
The bandwidth problem could have been avoided if in addition to changing camera sampling rates in the command sequences, we had also changed the stuttering pattern for the analog cameras mentioned above to eliminate one VIS image during each repetition. However, our instrument simulator didn’t have the full set of instruments like the spacecraft, which made it impossible to adequately test this change on the ground. At one point, we discussed testing this change onboard before the impact, but lost the opportunity due to the fuel loss Paul described in his blog on October 4th (see “A Test of the Flight Team”).
One other problem caused by the complex lunar scene was damaged images. After compression, some of the visible camera images were still too large to fit within a single data packet for transmission to Earth. Here's an example of the kind of damaged image that resulted. The shadowed area should be completely dark, but instead contains wispy bright areas. These compression-artifacts are intimately linked to the scene and need to be taken out with image post-processing.
Figure 3 Example of damage to downlinked images due to clipping in the telemetry packet formatting software.
What caused these compression artifacts? I didn't know it at the time, but the software for compressing these images had been written some years before to clip the compressed form of images to ensure they always fit within a single data packet (maximum size 65536 bytes). We used a wavelet-based compression algorithm, and clipping the compressed images removed some information needed to recreate the image accurately. The alternative would have been to split the images across multiple packets and reassemble them on the ground. This certainly could have been done in principle, but doing so would have introduced significant changes right at the heart of software that we had planned to reuse without change after its successful use on previous projects. With what we know now, changing this would have been justified, but it would also have been risky given how central image compression was to the overall design. With what we knew at the time, I believe we would have left it alone because of our short development schedule and all the other things that had to work.
Most of the commanding we did from the ground was to adjust the exposure times of the near infrared cameras as the scene changed. The other cameras either controlled themselves (VIS) or had only one appropriate setting (the thermal cameras, MIR1 and MIR2). We controlled the exposure setting for the near infrared cameras explicitly because we were trying to image a relatively dim flash and ejecta curtain close to bright mountain peaks.
Near the beginning of the FLASH period, we discovered we didn't get this balance right. To image the dim centaur impact flash, we deliberately overexposed the sunlit peaks. This setting combined with the Cabeus scene overdid it. The sunlit areas electronically bled into nearby parts of the image. That occurs when electrons in overexposed pixels move across the image detector to other pixels. In this case, the shadowed area of Cabeus crater was completely covered, obscuring our view of the impact. That was why the only image that was updating just before the Centaur impact was white. We hadn't seen this level of bleeding earlier in the mission, or in almost any of our testing. However, after searching through our data archive, I realize now we did see it occur once, two years ago, in one flashlight test in a darkened room but did not fully comprehend the implications.
The FLASH period was designed to start 1 minute before the Centaur impact, so we had a little time to recover once we saw the problem. During this minute, our first priority was to confirm that the spectrometers (NSP1 and VSP) and photometer (TLP) were working properly. Once that was done, we focused on the NIR1. Since we still had commanding during this period, we tried to change the exposure setting (Payload Scientist Kim Ennico called out “Flight, this is Science, please change NIR1 to OPR 9, over.”). We had less than 30 seconds to get this command sent up to the spacecraft. The command was actually sent but arrived a few seconds too late to capture the impact. In hindsight, this was a challenging stretch for the camera’s range, due to the scene and the potential for bleeding. Our strategy - to aim for the most sensitive exposure setting followed by one attempt to back-off depending on the data-might have worked had we been looking at another region of the moon, that is, had, we launched (and impacted) on a different date, where the terrain and lighting would have been different. While all this was going on, the impact flash was captured by NSP1, so the key science measurement was made.
We intentionally caused the same issue later, during the CRATER period, but we had better success (see above figure at the segment labeled 'B’). Initially, the NIR2 camera images were badly overexposed for the same reason as during FLASH (hence the white images that appear in the NASA TV video just after entry to DV Mode). Kim Ennico, the Payload Scientist, made the call to reduce the exposure time slightly, from what we called OPR 15 to OPR 10. (Again, you can hear this request over the voice loop in the NASA TV video.) She was using live-information from the NSP1 spectrometer and checking those values in real time against a spreadsheet near her seat. You can see her checking and rechecking on the video before making the choice. We only had only one chance to choose the right one. The command was sent and received 30 seconds before the S-S/C’s impact. Kim's call initially left the images overexposed, but as the lit peaks slid out of the field of view, her choice produced excellent images of the very dark crater floor, including the image that gave us our best estimate of the Centaur crater size. These images go all the way down to 2 seconds before S-S/C impact where the craft was 5 kilometers above the surface. The crater floor of Cabeus was indeed brighter than any of the predictions, at least in the infrared. That’s another reminder of science and exploration. Sometimes you are surprised as you collect new data, especially data from areas never looked at before.
Figure 4 This image sequence was captured just before the end of the mission and shows the NIR2 camera going from badly overexposed to acceptably exposed as the lit peaks surrounding Cabeus leave the field of view.
How do we know that's the Centaur crater? Because it was also seen by the two thermal cameras (MIR1 and MIR2) while it was still warm, and we can overlay the images. The left figure below shows aligned images from NIR2 and MIR1, taken before the Centaur impact. The figure on the right shows aligned images from these cameras taken just before the S-S/C impacted and showing the Centaur impact crater (see inset). These images don't align perfectly because they were taken about a second and 2.5 kilometers apart. To obtain images of the Centaur crater using three different cameras was our goal, and we succeeded.
Figure 5 The right image shows the Centaur impact crater in both near-infrared and mid-infrared images. The left images overlays images taken before the impact by the same two cameras.
Kim's call of "NIR2 to OPR 10" yielded a great image of the Centaur crater, but it also caused some confusion. The name for Near-Infrared Camera #2 (NIR2) was too similar to the name of Mid-Infrared Camera #2 (MIR2). We had practiced this interaction over our voice communication loops, but we hadn't practiced it enough to do it quickly and perfectly under time pressure. Kim had to repeat her call using the phonetic term 'November' for the 'N' at the beginning of NIR2 (during CURTAIN phase in the NASA TV video, you can hear the Flight Controller, Jim Strong, ask “is that ‘November’ or ‘Mike’?”, referring to NIR2 or MIR2, ). We didn't realize when we picked the obvious names for these cameras three years ago that the names could cause confusion when spoken over the voice loops connecting our mission control rooms. Back then, our plan avoided real-time commanding completely, but as we learned through our practice sessions before launch and actual experience after launch, we realized we needed the flexibility. We retrofitted a process for proposing and confirming real-time commands into our mission operations architecture as best we could given the facility and time constraints. Note that we didn’t consider doing this for maneuvering the spacecraft or for the most critical science instruments, only the secondary instruments.
The commanding side of the automatic sequence ran almost perfectly. We did have one problem with the Visible and Ultraviolet Spectrometer during the CURTAIN period, though. Because the instrument data handling unit (DHU) was at its maximum data throughput capacity during the first part of CURTAIN, one command to change exposure time was delayed and sent during a period when the instrument wasn't listening. That command was ignored. This resulted in capturing fewer spectra with longer-than-planned exposure times. Luckily, the longer exposure times turned out to be a blessing, since the ejecta curtain was much fainter than some models predicted. The loss of more frequent sampling due to the longer exposures did not affect the science measurement.
High Five Fail
Yes, I should have. After the end of the mission, I missed a high five that was captured on camera. I was teased about it by my colleagues that morning and by my kids that night. The other operator involved, our Telemetry Data Manager, known as "Data" over the voice loop, is both a good colleague and a friend. I didn't intend to embarrass him and have since apologized. We had been told to avoid high fives to prevent exactly the sort of mistake I made, but once the hand went up, I should have responded. So, here it is, for the record:
Why didn't I respond? I honestly don't remember the moment clearly, but I did have two things on my mind. First, my job at that point was to move to the Science Operations Center (SOC) to prepare for the post-impact press conference. We had two hours to make sure we'd gotten the data we expected, to prepare presentation charts and to look for anything obvious. I was concerned about that because I had missed seeing the plume we had hoped for like most everyone else.
More importantly, I was really stressed out. The DHU, the computer that processed all instrument data, was struggling with the very large packet sizes of visible camera images, and the DHU almost crashed a number of times during that final hour. We had developed and practiced a procedure to recover from such a crash to prevent a substantial loss of science data. Flying a spacecraft is a group effort with lots of cross-checking, and as the Payload Software Lead, I felt especially responsible.
During payload development testing, we found and fixed several problems that would have been problematic for the payload. This problem which led to potential crashes of the DHU was known and was the most difficult software problem we saw. The root of the problem was a small chip that controlled the data bus connecting the video capture and compression chips to each other and to the main processor within the DHU. Under certain circumstances this bus controller chip would stop responding, and the DHU software would crash. Since we didn't have access to the chip's design to understand why it would stop, and we didn't have time to replace it, our approach was to create a method for quickly recovering on orbit. This method had two parts. The first part was a software patch we developed that reset the bus controller when the DHU’s main processor noticed it had stopped responding. The second part was a procedure for quickly rebooting the whole DHU from the ground if the software patch didn’t catch the problem.
We developed and tested the software patch just a few weeks before the payload was shipped to Southern California for integration with the rest of the spacecraft. From that point on, through the rest of our testing on the ground and in orbit, we didn’t see this problem again. That is, we didn’t see it until the morning of the impact. That morning, the patch needed to reset the bus controller two dozen times. The vertical green lines in this figure show when.
Figure 6 During the final hour of the mission, Data Handling Unit (DHU) software detected and corrected an anomalous condition on a bus controller chip multiple times. The green lines show when these events occurred. The right end of this figure, starting at the label 'I' (the time of Centaur impact) corresponds to the time spans of the planned (Figure 1) and actual (Figure 2) performance plots. Earlier events happened while monitoring payload performance in the 56 minutes prior to Centaur impact.
Once these events started, I was prepared, on a hair trigger, to start the process of rebooting the DHU if the patch didn’t work. I was constantly checking and rechecking the fault response procedure I had developed for our payload. The details of this procedure varied over time. As the on-board sequence progressed and we got closer and closer to the Centaur impact, we had different decisions to make to recover if something went wrong. This strategizing was being done over another voice loop with Kim and Tony Colaprete, the LCROSS Principal Investigator, in the Science Operations Center (SOC) which was not audible to the audience watching on NASA TV. We had to keep track of a lot of independent data threads and contingencies simultaneously. Our actual trigger for starting this recovery, a gap in numeric sequence of the data coming from the instruments, even occurred once, but it was unrelated and didn’t need a response. In the end, our defenses worked. The software patch performed exactly as intended and no crash occurred. After the S-S/C impact, I breathed a sigh of relief and moved to the next room to start preparing for the press conference.
As I said above, we had challenges in all three areas critical to making our plan work: downlink bandwidth, camera exposure, and command timing. Ultimately, all of the problems we had proved minor, and we collected the data we needed to draw conclusions about the presence or absence of water and other substances in Cabeus crater.
We at NASA all too often strive to give the impression that complex, difficult missions are routine. They're not. They're complex and difficult. What makes them possible is long planning, teamwork, and careful review by people both inside and outside the project. One name for this process is "Systems Management", which recognizes that
people need backup just like the parts of a complex machine. I personally made some mistakes and caught some mistakes. Together, we caught enough of them that we were successful. For me, it was a huge privilege and a wonderful experience. I'm very grateful to have been a part of this mission.
Recap of the Final Day,Part 1: Separation and Braking Burn
Posted on Nov 25, 2009 12:19:14 PM | Paul.D Tompkins
LCROSS, the flight mission, is over, but we’re still analyzing the data that was collected on impact night. Our team is very excited about the recent announcement – that LCROSS confirmed the presence of water at the lunar south pole. But, there’s a lot more data to analyze, both for science and engineering purposes, and I’m guessing there will be more interesting announcements in the months ahead.
With the completion of mission operations, before concluding my account of the LCROSS journey, I wanted to add a few more posts relating our team’s experiences in the last day of the mission. In this post, I’ll describe my last shift in the Mission Operations Control Room (MOCR), covering the Separation event and the final delivery of the Centaur impactor. In a near-future post (I promise!), I’ve invited our Payload Engineer, Mark Shirley, to post with a detailed look at payload operations during the Impact event. For all of you who watched the impact video and are wondering what happened that evening, Mark’s post should be very enlightening.
Now, a note about terminology – the two major pieces of hardware on LCROSS are the Centaur (the impactor) and the Shepherding Spacecraft (abbreviated as “S-S/C”), which carries the science instruments and that has acted as the “living” part of LCROSS since the Centaur’s batteries ran out on Day 1 of the mission. Up until now, I’ve referred to the combined Centaur and S-S/C as “LCROSS”. But in describing what happened after Separation, when the two pieces decoupled, I’m forced to refer to each vehicle independently. Finally, as a reference for timeline details, check out my post entitled “Brace for Impact: A Schedule of Events for the Final Day”.
Shift A: Release of the Centaur and Slowing Down the Shepherding Spacecraft
On October 8, I was the Flight Director in charge of the second-to-last shift (Shift A) of the LCROSS mission. Shift A’s main task was to perform final targeting of the Centaur, via the separation of the Centaur from the S-S/C. The Separation event was the final influence our team would have on the Centaur impact location and time. Soon after Separation, we were to command the S-S/C to image the Centaur as it receded from view. Shift A was also in charge of the “braking burn”, a delta-v maneuver performed after Separation that would decelerate the S-S/C relative to the Centaur to delay its own impact on the moon, thereby improving our view of the impact event during the collection of science data.
Figure 1: Diagram of Separation, Centaur Observation and Braking Burn
In all, we could not have had a better shift. Everything went incredibly smoothly. Here are some highlights:
Once “on console”, Shift A’s first responsibility was to load all of the command sequences to the S-S/C that would govern the initial attitude change to Separation attitude, the Separation itself, the Centaur Observation and the Braking Burn events.
With so much at stake, the Flight Team was very focused, and we completed those command loads very early, with no problems. The onboard commands were designed to execute the whole sequence through Braking Burn (minus the Centaur Observation) without human intervention. Barring unforeseen problems, the Flight Team could have walked out of the control room at that point. But in reality, there’s no way we’d leave the success of the mission to chance. As I’ll describe later, Separation had a lot of potential risk. If anything went wrong, our team needed to be there to take corrective actions and to set up for Impact. Furthermore, the Flight Team had an integral role in initiating the Centaur Observation.
Reorienting to Separation Attitude
To precisely target the Centaur into Cabeus crater, our plan was to use a fraction of the velocity change caused by Separation to push the Centaur into the right impact trajectory. The interface ring between the Centaur and S-S/C contains springs that push the two vehicles apart as soon as they’re released. They produce up to an estimated 500 lb of force, and were predicted to induce approximately 0.7 meters/sec of relative velocity between the two vehicles. However, since the Centaur far outweighed the S-S/C, we expected only 0.15 m/s would be imparted to the Centaur, the rest to the S-S/C.
But we didn’t need all of that extra velocity for a precise impact. To use only a fraction of this added speed, we’d have to perform Separation in a specific direction, so that only the needed component of the velocity change would affect the impact position, while the other components would have no effect on impact position and little effect on timing. Our Maneuver Design team determined, coincidentally, that this direction was only 7 degrees away from our starting Cruise attitude.
So, what happened? A portion of the commands we had just loaded automatically performed the change in orientation. We needed to take every precaution not to disturb our orbit prior to Separation. Rather than commanding the entire 7 degree change in one step, the command sequence performed the maneuver in a long series of small attitude updates. This minimized the number of thruster firings required, though it took over 20 minutes to complete. The whole sequence of commands executed perfectly and with one task completed, I prepared mentally for the next. With the rotation completed, we were less than one hour from Separation.
Other than Impact, Separation was perhaps the most critical event of the mission. Since the Centaur was not independently controllable after Day 1 of the mission, the de-coupling of the S-S/C from the Centaur was LCROSS’s last influence on the Centaur impact trajectory. As Separation approached, my adrenaline started to flow. Here are the reasons Separation made me a little nervous:
- Centaurs typically separate from their payloads within just hours of launch. LCROSS’s Separation was 112 days after launch, far longer than ever done with a Centaur before. The space thermal environment often wreaks havoc on mechanical elements, with extreme temperatures and many swings between hot and cold. Analysis showed that the mechanism could operate over a very wide range of temperatures and after many warming/cooling cycles. Still, we didn’t know if something had been overlooked. The good news was that temperatures from the separation mechanism had remained well within allowable limits for the entire mission, increasing our probability of success. However, if Separation were to fail, LCROSS would impact in one piece, the science payload would be unable to watch impact, and our mission success would be entirely reliant on ground-based and other orbiting observatories to collect impact data.
- The Centaur separation mechanism was designed to very reliably push the S-S/C away smoothly, without causing the spacecraft to tumble. To prevent attitude control from interfering with Separation, the command sequence would disable ACS thruster firings briefly through the transition. But what if something snagged or re-contacted? Disabling the ACS would allow rotations induced by separation to go unnoticed (and uncorrected) for a few seconds. Again, analysis predicted success, but there were a lot of unknowns here.
- In the instant of Separation, LCROSS would transform from a 2894 kg spacecraft to a 617 kg spacecraft (about 1/5 of the mass). The moments of inertia (the spacecraft’s tendency to resist changes in rates of rotation) would also decrease dramatically. To remain in control after this enormous change, the ACS would use new sets of control gains optimized for post-separation conditions. But none of these controllers had been tested on the real spacecraft (only in good simulations – they wouldn’t have worked well with the Centaur attached). Dynamically, it would be like getting a whole new spacecraft, but with only 9 hours to figure out how it really behaved before Impact. To make matters more exciting, our command sequence utilized 5 out of 6 attitude control modes in the first 40 minutes after Separation. Each mode employed an independent set of control gains. One mode might work, and another might be flawed and cause an instability.
- Though the Centaur and S-S/C would recede from each other at very slow speed (0.7 m/s, or about 1.6 mph) after Separation, it wouldn’t take very long for the Centaur to be quite far from the spacecraft (42 meters or half of a football field farther each minute). The Science Team really wanted to film the departing Centaur to determine whether the separation had induced any tumble into the Centaur, and to use that knowledge to better understand the Impact behavior later. To “see” any tumble, which might be very slow, we would need to be able to distinguish the Centaur long axis from its short axes. To do that, the Centaur would have to be close enough to span at least a few pixels in the LCROSS cameras for several minutes. But at Separation, the LCROSS cameras would be facing away from the Centaur. The mission plan involved flipping the S-S/C 180 degrees to point them toward the Centaur as soon as possible after Separation. But this ran counter to the desire of our engineers, who would have preferred to watch the ACS control behavior for a while. As a compromise, we planned on rotating just one minute after Separation. Good for Science, but a little uncomfortable for the Flight Team, which would have to confirm a good separation, then oversee a 180 degree flip on a new set of control gains!
- Neither the S-S/C nor the Centaur could automatically confirm that separation had actually succeeded, so the Flight Team needed to confirm success from the ground via telemetry. A set of three small wires would break at Separation, and enable us to determine a physical separation. Then indirectly, we expected to observe a dynamic response from the S-S/C from the push and any resulting torques. When designing this event, we had a choice: either have the Flight Team initiate the 180 degree flip from the ground on successful Separation (but risk being late on the Centaur observation if we lost communications), or execute the flip using onboard commands and have the Flight Team terminate the sequence if Separation failed (but, if we lost communications and the ability to command, risk performing the flip with the Centaur still attached). Mission-wide, we had adopted a policy that assumed success, so we opted for executing these commands onboard. This meant that within 60 seconds, the Flight Team had to identify whether Separation had been successful, and terminate the running command sequence if it had failed. Sixty seconds may sound like plenty of time, but with multiple operators on console evaluating multiple telemetry indicators, allowing time for a possible termination of the command sequence, sixty seconds was barely enough.
So, what actually happened? At 10 minutes until the designated Separation time, the onboard command sequence started to run. Game time. Months of design, weeks of training would all come together now. I counted down the last 5 seconds until the commands to fire the Clamp Band Ordnance Device (CBOD) would have issued from the onboard sequence. Then we focused on telemetry. Silence on the voice loop.
Ten seconds after the separation time, the Flight Team broke into chatter over the voice loop. I polled the System Engineer. He returned with “three wires, Flight”, meaning that all three wires that indicate a clean break between Centaur and S-S/C all showed positive. I polled ACS (Attitude Control System Engineer). He announced “GO Flight”, indicating he had observed a significant attitude disturbance that could only have happened with a successful separation. I polled the Flight Controller, and he confirmed “three wires”. In the meantime, the control system had re-enabled, and quickly brought the attitude back under control. Separation was a “GO”! Relief. No need to terminate the nominal sequence. I instructed the Flight Controller to waive that option. Then, at the expected time, the S-S/C began to perform its 180 degree flip.
With no time to celebrate, the Flight Team re-focused on the next task – Centaur Observation.
Figure 2: Depiction of of Shepherd and Centaur, Post-Flip: The Centaur appears unrealistically close. At 4 minutes after separation, the Centaur would have been about 170 meters away. Also, unfortunately, with the change in separation attitude from the original plan, the moon did not appear in the background of the Centaur images.
To ready for Centaur Observation, the Flight Team’s next job was to reconfigure the communications subsystem and payload to enable the spacecraft to collect imagery of the Centaur as it receded from view. To enable the downlink of high-rate imagery, we first had to switch from our nominal downlink data rate of 64 kilobits per second (kbps) to 256 kbps. Once the data rate had been increased, we’d then increase the allowable data output rate of the science payload, and switch from the low-rate to the high-rate imagery sequence designed for this event.
Data rate changes and antenna changes were two operations that we typically didn’t perform via onboard commands. Each requires the DSN antenna providing the link to re-acquire the spacecraft signal. The time needed to do that varied significantly, but fixed-time commanding doesn’t allow for that flexibility. For this event, we had three Madrid DSN antennas watching simultaneously (two 34-meter dishes and a 70-meter dish), and each needed to re-acquire the LCROSS signal after the downlink rate change. To complicate things, we knew we couldn’t achieve an adequate link margin at 256 kbps until the spacecraft had completed its 180 degree flip (to point the center of the secondary omni-directional antenna pattern towards Earth), the duration of which wasn’t known with high certainty given our lack of in-flight testing with the new gains. By design, we planned to utilize ground-based commanding.
In actual flight, as expected, the science payload powered-on just as the S-S/C began to flip. Payload Engineering reported good status. Soon, the cameras were transmitting low-rate imagery, but we hadn’t rotated far enough to see the Centaur yet. But without the Centaur on its aft end, LCROSS behaved less like a school bus and more like a sports car. It was making short work of the 180 degree flip. At three minutes, with the flip complete, we commanded the downlink rate change. At about the same time, Science reported the receiving the first image of the Centaur, under the low sampling rate. The DSN stations re-acquired the signal, and as soon as possible, we commanded the payload to begin the high-rate image sequence. We has successfully configured the S-S/C for the Centaur Observation, and we now had a little time to breathe, and to watch the Centaur images flowing into the control room.
This link points to a movie made by the Science Team using the images collected during the Centaur Observation (with time sped up significantly).
In the movie, you’ll notice it looks like the Centaur is moving all over the place, but really it’s the S-S/C rotating back and forth. The attitude control system keeps the spacecraft pointed to within a small “deadband”, a small imaginary box around a target pointing direction. The ACS only fires thrusters if the S-S/C is getting near the edge of the deadband. What you’re seeing is the ACS “bouncing” on the edge of the deadband. If you look carefully, you’ll notice that the ACS seems to change to a smaller deadband later in the movie. This not an illusion. The spacecraft switched from a wider deadband (1.0 deg) to a narrower deadband (0.5 deg) partway through the observation sequence, as part of a test of the new control gains. You’ll also observe that the Centaur is in a very slow tumble.
At the time of the Centaur impact, to meet the observation requirements, the S-S/C needed to be roughly 600 km above the lunar surface. This was a trade-off between being too close (and possibly being hit by debris from the Impact), and being too far away (the spatial resolution of our imagery would decrease with distance from the surface). The velocity induced by Separation between the S-S/C and Centaur (0.7 m/s) would be too little to achieve that distance in only 9 hours 40 minutes, so the Braking Burn was inserted into the mission design to slow down the S-S/C down by an additional 9.0 m/s (about 13 times the velocity change of Separation). The Braking Burn was also designed to independently target the S-S/C impact point, also selected by the Science Team.
In flight, nineteen minutes after Separation, the onboard sequence terminated the Centaur Observation and began configuring the spacecraft for the burn. The Flight Team quickly commanded the return to 64 kbps downlink rate (necessary before changing our attitude again), and the DSN re-acquired the signal. The onboard sequence performed another attitude change, this time to the optimal burn attitude. Ten minutes later, after passing through two more new attitude control modes, the burn started on time. Four minutes of firing the 22 N thrusters was predicted to be sufficient to slow ourselves down to meet the impact distance requirement. With the burn over, we had crossed Shift A’s last major hurdle.
Shift A Cleanup and Farewell to LCROSS
We had a few final things to do before our handoff to Shift B. We biased our attitude to keep our thrusters warm, re-enabled payload heaters, and loaded a preliminary Impact command sequence to the spacecraft. Planned before Separation using predictions of the post-Separation and Braking Burn orbit, those commands were designed to perform the full Impact sequence without the Flight Team. They were an insurance policy to protect the team in the off-chance that we lost communications with LCROSS until the last few minutes. However, without accurate knowledge of the S-S/C post-burn orbit, they wouldn’t point the cameras as accurately as the re-planned versions. That planning was up to the next shift, using the actual, measured orbit after Braking Burn.
With that, Shift A was done. In a rush all day, we sat on console for a few more minutes, soaked up our last experiences as members of the LCROSS Flight Team. It was a bittersweet moment – so many experiences in those chairs in such a short time, and the culmination of three years of preparation. Our final shift had gone perfectly – the result of months of thought and practice, and a great spacecraft design. The moment we’d all been waiting for was only 8 hours away, and then the LCROSS flight would be over. Officially, our time was over now. Shift B folks began milling into the control room, with a combination of big smiles and game faces on, ready to get started on their last shift. With just a little reluctance, we stood up from our chairs, and got on with shift handover.
Figure 3: MOCR Shift A: The Ames-local part of Shift A that ran Separation through Braking Burn. Pictured from R to L: Matt D'Ortenzio (Flight Controller, NASA Ames), Matt Reed (Attitude Control Engineer, Northrop Grumman), Tony Lindsey (Data Management Engineer, NASA Ames), Tony Colaprete (LCROSS Principal Investigator, NASA Ames), Darin Foreman (Systems Engineer, NASA Ames), and myself, Paul Tompkins (Flight Director, Stinger Ghaffarian Technologies at NASA Ames). A lot of other people also worked this shift and contributed to its success!
Remember, the next post will feature Mark Shirley, our Payload Engineer. Stay tuned, and thanks for reading!
Brace for Impact! A Schedule of Events for the Final Day
Posted on Oct 08, 2009 02:51:54 AM | Paul.D Tompkins
Our last day in flight promises to be the most challenging and the most rewarding for the project. Our 112 days in orbit are focused entirely on the last four minutes, after the Centaur impacts our target crater and raises a plume of lunar material for the LCROSS Shepherding Spacecraft to observe for signs of water, but before the Shepherd also impacts the moon.
From a Flight Team perspective, the LCROSS impact sequence is a dream occasion, and yet provides some cause for trepidation. Many things can go wrong, and with so little time, there is only so much that can be done.
During you day tomorrow, I thought it might be fun for you to know what the Flight Team will be doing in lead-up to the event. To put it plainly, we won’t be idle! Enjoy!
A Recent Development: TCM 9 put LCROSS On-Target
The latest data from our Navigation team indicates that TCM 9 has already put LCROSS on target to hit the designated impact area, without the need for executing TCM 10 on Thursday evening. Our predicted impact point is already within our target 3.5 km diameter circle, and our team will only make very small adjustments to improve our impact accuracy. This alters our original plan for Thursday.
Instead of performing TCM 10, the team will plan and execute a very slow rotation by feeding the spacecraft new target attitudes each minute (see “Once More Around the Earth” for a description of our “quaternion creep” attitude change) to minimally disturb the current orbit while turning to an orientation that is optimal for Separation. At Separation, LCROSS will use the velocity imparted by the springs between the LCROSS Shepherding Spacecraft and the Centaur (adding an estimated 15 cm/s to the Centaur) as a final means of nudging the Centaur toward the center of our target. Analysis of the Centaur separation springs, along with actual tests of the system conducted to simulate very harsh conditions of space (far harsher than LCROSS has actually experienced) indicate the separation will impart a fairly precise change in velocity to the Centaur.
This plan represents less risk (the slow attitude change will be simpler to plan, test and execute than a TCM), and introduces less uncertainty into the prediction of our impact point (firing thrusters for very short durations adds a lot of uncertainty, while the separation springs in the LCROSS-Centaur interface mechanism are very repeatable). We’re fortunate to find ourselves in this situation, and we’ll take full advantage of it to ensure we impact on-target.
DOY 281 (October 8): TCM 10 (or not), Separation, Centaur Observation and Braking Burn
The last 24 hours of the mission, bridging DOY 281 and 281 (October 8 and 9), will be a flurry of activity. Here is the sequence of events. I’ve provided both UTC and Pacific Daylight Time references:
- 08:00 UTC/01:00 PDT: Final orbit determination delivery for Separation. The Navigation team delivers its final orbit determination to the Maneuver Design Team. This final trajectory estimate will be the basis for planning our slow rotation, Separation and Braking Burn.
- Maneuver planning and communications link analysis for slow rotation through Braking Burn. For eight hours, the Maneuver Design team will determine the optimal attitude for Separation (when we let go of the Centaur), then plan and double-check plans for the slow rotation, Separation and Braking Burn. Braking Burn happens after Separation, so will have no influence on the path of the Centaur. It accomplishes two things: first, it slows our Shepherding Spacecraft with respect to the Centaur, such that at the time of Centaur impact, the spacecraft will be 4 minutes behind it. This allows LCROSS to observe the Centaur impact while not being too close (risking damage from debris) and not being too far away. Second, the Braking Burn independently targets the Shepherding Spacecraft impact point, which will be a few kilometers away from the Centaur impact point. During the same time period, the Communications Link Analyst will refine his estimate of our communications link margin through all phases of the slow rotation, Separation and Braking Burn.
- 13:00 UTC/06:00 PDT: Separation Activity Selection Review (ASR): Our team knows every last detail of what activities we’ll be running, but this meeting is our last chance to change any part of the command sequence, based on late-breaking data (e.g. changes on the spacecraft, etc). The Maneuver Design Team and Communications Analysis Teams will present their results here, and will form the basis of command generation.
- 14:00 UTC/07:00 PDT: Command generation and checking for Separation through Braking Burn. Our Activity Planning and Sequencing Engineer will generate all of the command sequences for TCM 10, Separation, Braking Burn, a preliminary version of Impact, as well as for several contingency cases. He will hand his products over to both Engineering Analysis, and to the Simulation Engineer, who provide different aspects of quality assurance checks. Engineering Analysis performs a number of computer-based checking against LCROSS flight rules to make everything in the sequence is legal. Both the Simulation Engineer and the Engineering Analyst will run the commands on our spacecraft simulator to confirm that they do what we want. During this time, Shift B (Flight Directory Rusty Hunt’s shift) will hand off to Shift A (my shift). Shift B will get some sleep, and return before Impact. Shift A will oversee all of the events through Braking Burn.
- 19:00 UTC/12:00 PDT: Separation Command Approval Meeting (CAM): This is our final, team-level review of all plans, command products and quality assurance data before executing the slow rotation through Braking Burn. We’ll make sure everything is correct, over a 90 minute review. Then we’ll move to our seats in the Mission Operations Control Room (MOCR) to begin execution.
- 20:30 UTC/13:30 PDT: Command loads for Separation, Centaur Observation and Braking Burn, and slow rotation to Separation attitude. Once “on console”, Shift A’s first priority will be to load the commands for Separation, Centaur Observation and Braking Burn. We then turn our attention to the slow rotation to the Separation attitude, by loading the burn commands to the spacecraft in an alternate memory bank. The slow rotation command sequence will re-orient the spacecraft from our Cruise attitude to the Separation attitude. We’ll confirm the loaded parameters, and then wait for the reorientation to start.
- ~00:00 UTC/17:00 PDT: Slow rotation to Separation attitude starts. The maneuver is small, only 6 degrees or so, but will happen in chunks of less than 0.5 degrees each minute. The onboard command sequence automatically switches over to our Separation, Centaur Observation and Braking Burn command sequence, just in case we lose communications with LCROSS. In that off-nominal scenario, Separation would still happen without ground-based commanding by our team.
- 01:40 UTC/18:40 PDT: Separation onboard command sequence starts. The pre-Separation command sequence starts running. Ten minutes to Separation.
- 01:50 UTC/18:50 PDT: Separation. Commands temporarily disable our ACS, then fire the relays that unlock the Centaur from our spacecraft. Heavy springs push the Centaur and spacecraft apart at roughly 0.7 m/s, a firm but gentle shove. The Centaur will accelerate approximately 15 cm/s, but with our optimal orientation, only 3.5 cm/s will be used for Centaur targeting. After Separation, the ACS is re-activated with an entirely new set of parameters to handle the vastly different mass properties. With the Centaur separated, LCROSS will just have lost 2000 kg of mass. The spacecraft motion (dynamics) will now behave very differently. The Flight Team has only 10-15 seconds to confirm that Separation has occurred, and if not, only 50 seconds more to terminate the command sequence to progress any further. We have practiced this critical timing many, many times.
- 01:51 UTC/18:51 PDT: Flip to point LCROSS instruments at Centaur. Just 1 minute 6 seconds after Separation, the onboard command sequence initiates a 180 degree pitch flip to point spacecraft cameras at the departing Centaur. This takes less than 3 minutes to perform. The command sequence also powers up the Data Handling Unit (DHU), which powers the science instruments, in preparation for Centaur Observation. Following the pitch flip, commands roll the spacecraft to optimize the pointing of our omni-directional antenna toward Earth for best downlink rate. At the end of the pitch maneuver, the Flight Team will re-configure the LCROSS downlink data rate for 256 kbps, and will command the DHU to go to a high-rate camera sampling sequence. Imagery of the departing Centaur, with the moon in the background, will begin flowing to Earth.
- 02:01 UTC/19:01 PDT: End of Centaur Observation. Nineteen minutes after Separation, with the Centaur nearly 800 meters away, the Centaur Observation will terminate. The Flight Team will reconfigure the communications downlink rate for Braking Burn (64 kbps). The onboard command sequence automatically re-orients the spacecraft to the final burn attitude, and then squeezes down our attitude control deadband from 3.0 degrees to 0.1 degrees, in preparation for Braking Burn.
- 02:30 UTC/19:30 PDT: Braking Burn starts. This burn is longer our last few TCM’s, just over four minutes. This is because there’s not much time remaining in the mission to build up a 4-minute delay between the Centaur and the Sheperding Spacecraft. At the end of the burn, the onboard command sequence will re-orient the spacecraft to our Cruise attitude.
- ~03:00 UTC/20:00 PDT: Preliminary Impact command load. As a precaution, the Flight Team will load a preliminary command sequence for Impact to the spacecraft. If we lost communications with LCROSS sometime after this point, up until the final few minutes, this command sequence should be sufficient to point the LCROSS cameras at the Centaur impact point, run the instruments, and meet all mission objectives. However, before Impact, the team will re-estimate the orbit of the Centaur and Shepherding Spacecraft, and re-plan Impact with the best possible information.
- 03:30 UTC/20:30 PDT: Shift Handover. Shift A (my shift) hands control over to Shift B. Shift B will oversee the Impact event. We’ll review the status of the spacecraft, in particular the dynamic behavior following Separation, and any last-minute items.
- 04:30 UTC/21:30 PDT: Final Orbit Determination Delivery. The Navigation team delivers its final estimate of the spacecraft and Centaur orbit. The spacecraft’s orbit can be measured directly, while, without a communications transponder aboard the Centaur, we have no direct measure of the Centaur’s orbit after Separation. This final orbit determination will become the basis for Impact command sequences, in particular the spacecraft attitude sequence to maintain pointing on the Impact site, and the Impact timing.
- Final Impact Planning and Command Generation: The Mission Design team will re-plan Impact using the latest orbit data from the Navigation team. The changes between preliminary and final Impact plan will be very subtle. The plan involves literally hundreds of Shepherding Spacecraft orientation changes to keep the onboard science instruments pointing at the expected Centaur impact point as we approach the moon. The new orbit estimate will change all of these orientations very slightly. The Sequencing Engineer will re-implement the command sequences, then pass his results to the Engineering Analyst and Simulation Engineer for final checking.
- 6:30 UTC/23:30 PDT: Disabling LCROSS Fault Management. Shift B will begin configuring LCROSS for the Impact. One of the first steps is to nearly completely disable the LCROSS onboard fault management system. Fault Management responds automatically to correct problems it detects onboard. Sometimes these are benign responses, like switching from a primary sensor to a backup sensor. Other times, the responses can be all-encompassing. It might seem strange to disable this function right before our most important phase of the mission. However, the last thing the Flight Team wants is for a problem onboard the spacecraft to interrupt our Impact observations. Some fault management responses are designed to throw LCROSS into a Survival State, turning off all power to the science payload, and disabling any onboard command sequences. This could mean disaster for the Science Team, since there would not necessarily be sufficient time to recover and return to the pre-Impact configuration. So, only minor fault management is enabled, but the more severe responses are disabled. In preparation for Impact, aside from disabling fault management, Shift B will also coordinate with the Deep Space Network to transfer our downlink path from a 34 meter diameter antenna (DSS-24) to the Goldstone complex’s 70 meter dish (DSS-14). The 70 meter antenna enables LCROSS to return science data at 1 megabit per second (1 Mbps).
- 8:30 UTC/01:30 PDT: Impact Command Approval Meeting (CAM). Shift B will review the final Impact plan and the associated onboard command sequences and ground commanding products. This is our last chance to get things right. Since the team is focused on a very specific set of checks, and for lack of time, this CAM lasts only 30 minutes. Then Shift B goes back to the MOCR to perform Impact.
- 9:00 UTC/02:00 PDT: Loading Impact command sequence to LCROSS. Shift B loads the final command products to the Shepherding Spacecraft, including a set of contingency command sequences to cover off-nominal scenarios. In the event of a building fire or an earthquake, our team even has a command sequence that would allow Shift B to leave the building and have the entire Impact sequence and observation be automated. The Deep Space Network has dedicated four antennas to this period of time, three from the Goldstone complex in California, and a fourth located at Madrid in Spain. Shift B, with the help of DSN operators at JPL, will coordinate those antennas as LCROSS changes its communications configuration. Hours earlie
- 10:00 UTC/03:00 PDT: Start of Impact onboard command sequence. Its first commands will perform a reorientation of the Shepherding Spacecraft to point the science instruments towards the expected Centaur impact point on the moon. The cameras and other instruments will not yet be on. This reorientation will also point the –Z Medium Gain Antenna (MGA) towards the Earth, enabling the team to switch the LCROSS downlink path from the omni-directional antenna to this MGA, in preparation for high-rate science data transmission.
- 10:10 UTC/03:10 PDT: Switch to –Z MGA. Shift B will command the switch from omnidirectional to the –Z MGA antenna. This is a potentially critical step in achieving full-rate science data transmission after the Centaur impact. However, since we did our combined Cold-Side Bakeout #3/MGA Test on September 24, we’re pretty confident this will work again.
- 10:15 UTC/3:15 PDT: Transitioning to Science Rate. The Flight Team will now command a transition from a standard downlink data rate of 64 kbps to our full science rate, 1 Mbps. This is another very important step to achieving full science return. However, we do have backup procedures that would allow us to transmit science data at a lower rate, 256 kbps, if the DSN 70-meter dish were to fail, or if the MGA was non-functional.
- 10:36 UTC/3:36 PDT: Payload powers on. The onboard Impact command sequence powers on and enables the DHU and science instruments. At 10:41 UTC, the command sequence also starts DHU NVM sequence 1, a sequence of instrument commands that tests each instrument in the LCROSS payload, save the Total Luminescence Photometer (TLP). The MOCR at NASA Ames begins to receive data from the science instruments, and the Payload Team and Science Team begins analyzing the preliminary data to make sure everything is working. This is still nearly one hour from Impact, but it’s the team’s last chance to find a problem in our suite of payload instruments that might otherwise foil our Impact observation. The team continues checking the instruments, and via the Flight Controller and Flight Director, commanding small adjustments to exposure settings, for 35-40 minutes.
- 11:10 UTC/4:10 PDT: TLP Instrument powers on. The Total Luminescence Photometer (TLP) instrument powers on for the first time since before launch. This instrument is very sensitive, and can only be powered on a limited number of times. The Science Team has been very careful not to overuse the instrument in tests. However, if the instrument powers on as expected, this is a major success on the road to the Impact event. The TLP, which gathers light measurements at 1000 times per second, will “catch” the Impact flash as the Centaur hits the moon, and is hence a very important instrument for water detection.
- 11:30:20 UTC/4:30:20 PDT: Flash Mode begins. One minute prior to Centaur impact, the DHU will command NVM command sequence 2, which begins Flash Mode. For the next 1 minute 3 seconds, Flash Mode will run the TLP and other instruments to capture the flash of light coming from the impact event.
- 11:31:20 UTC/4:31:20 PDT: Centaur Impact. Centaur impacts the moon at Cabeus. The energy of impact emits a brief, intense flash of light. A plume of lunar debris will rise in a pattern similar in shape to an inverted conical lampshade.
- 11:31:23 UTC/4:31:23 PDT: Curtain Mode begins. The DHU will switch from Flash Mode to Curtain Mode, which is a sampling sequence optimized to observe the evolution of the debris plume as it rises from the lunar surface. With this debris rising above the altitude of the Shepherding Spacecraft, our side-looking spectrometer will look towards the sun to measure light as it is transmitted through the debris. The remainder of the payload will be pointed down towards the impact point. This mode lasts for 3 minutes.
- 11:34:23 UTC/4:34:23 PDT: Crater Mode begins. At this late stage, the DHU will now switch from Curtain Mode to Crater Mode, which is designed to capture data about the properties of the new crater generated by the Centaur impact. The Shepherding Spacecraft now has less than one minute of time to capture and transmit data before it also hits the moon. With the Centaur impact point now off to the side, LCROSS will continue to try and track that point until its own contact with the moon.
- 11:35:39 UTC/4:35:39 PDT: Shepherding Spacecraft impact. The Shepherding Spacecraft will also hit the moon at roughly this time. The Flight Team will abruptly stop receiving telemetry a few seconds later, as the photons from LCROSS’s last transmission travel back to Earth to be received by the DSN 70 meter antenna. The LCROSS flight mission will be over.
This will be my last post until after Impact. I hope you enjoy the show tomorrow – it should be very exciting. Though we won’t have immediate feedback for water detection, I hope to report good news to you on Friday regarding the accuracy of our impact, and the collection of the science data. Then, over the coming weeks after Impact, the Science Team will review their data and interpret the observations. I’m sure you’ll be hearing news one way or the other.
Thanks for reading!
Posted on Oct 07, 2009 05:02:45 PM | Paul.D Tompkins
Saying Goodbye to a Really Amazing Spacecraft (and Team)
Well, we all knew it was going to happen. It was inevitable. It was the whole design of the mission. LCROSS was destined to end its wonderfully fantastic journey by intentionally crashing into a permanently shadowed crater at the south pole of the Moon. We are the ones who devised this fate for LCROSS. So why should we be surprised (and just a little bit sad) now that the time has finally come?
As a proud member of the LCROSS Science Team and as the Observation Campaign Coordinator, I would have to say that working on this mission has been one of the highlights of my career thus far. The mission itself is truly amazing (We’re impacting the Moon! We’re looking to see if there’s water ice at the poles! We’re going to this utterly unexplored place in our Solar System, so close to home, and are so excited about what we’re yet to learn!). LCROSS is so important to both science and exploration. This mission is blazing a new path in how to build small, robust spacecraft both on schedule and on budget. LCROSS uses eight (yes eight!) commercial off-the-shelf instruments for its payload – also a very novel way for NASA to get more bang for the buck as well as good science to boot. The technical aspects of the LCROSS mission are astounding, but none of this would be possible without the dedication of the *people* working on this project.
The LCROSS Team is made up of an amazing cadre of individuals. LCROSS has a relatively lean and nimble team. There’s still a lot of work to be done to send a spacecraft to the Moon, and so that means that everyone has to pull together to make things happen. If somebody is extra busy and needs help, you help them. If there’s something that needs to be done and you’ve never done it before, you figure out how to do it. If you are stuck and need some assistance, just ask your teammates and without hesitation people are willing to help. We all naturally come together to get the job done. There is a high level of trust and commitment on this team, starting with the top Project management and all the way through the people working the nitty-gritty technical aspects. It is truly a glorious experience to work with a team such as this. The best part is that everyone is working towards a common goal, and everyone is willing and able to contribute in whatever way is needed in order to achieve the objective. It is amazing what a group of people can do when presented with a fascinating project and an exciting challenge.
And it’s not just the Project folks who have helped make this happen, but it’s all of the students and members of the general public who have so substantially contributed to the successes of LCROSS. Student interns at NASA Ames have had the opportunity to work with real honest-to-goodness flight hardware. Not everyone has the opportunity in college to hold an instrument that will be on the Moon within the next year! Such opportunities are tremendously powerful for encouraging the students of today to continue the pursuit of careers in math, science, and engineering. Amateur astronomers from around the world have been imaging LCROSS in the night sky during its trip to the Moon and are planning to collect observations of the impacts as well. This is a great way to actively participate in a NASA mission. We’ve also been having a great time keeping folks updated regarding LCROSS activities through our NASA website as well as the LCROSS Facebook and Twitter accounts. Thousands of people are following LCROSS on these sites and we’re thrilled to be able to have a two-way dialog to discuss all things lunar!
So, although this mission was destined to end in a spectacular grande finale culminating with two lunar impacts, it is a bit sad to see this phase of the project come to a close. However, next up is the exciting analysis of the data to try and learn all we can about these enigmatic regions on our very own Moon. And here’s to hoping there are lots more missions coming up in the future, because we are fired up and ready to go!
Our Centaur: Launch Vehicle Upper Stage Turned Lunar Impactor
Posted on Oct 07, 2009 03:46:15 PM | Paul.D Tompkins
The LCROSS Mission Design Team included the Centaur as a critical component of their mission from the very beginning. It’s essentially a bonus impactor that stays with LCROSS after performing it’s normal function of delivering LRO and LCROSS on their way to the moon. For the LRO-LCROSS launch and particularly for its contribution to the LCROSS mission, the Centaur was the focus of extra attention from both the LCROSS team and from the KSC/ULA-Atlas launch team.
Think of it as energy recycling. The kinetic energy [1/2 MV2] provided by the launch vehicle to get LCROSS and Centaur to lunar swing-by are later used to create the ejecta plume at the moon’s south pole.
Impacting a 2-ton Centaur upper stage enables the LCROSS mission to significantly increase the amount of lunar ejecta expelled from the target crater over what could have been achieved using only the spacecraft.
Centaur is a cryogenic (liquid hydrogen and liquid oxygen) powered rocket upper stage. Its engine can be shut-down and restarted after one or more guided coast phases. Centaur has a long history of successful missions. For LRO-LCROSS, instead of being parked in a safe orbit after sending LRO and LCROSS toward the moon, this is Centaur’s first role as a guided impactor for the science gathering portion of a mission.
The Centaur upper stage used for the LCROSS mission is the same as Centaurs used to propel the Mars Reconnaissance Orbiter and Pluto-New Horizons on their way to Mars and Pluto, respectively. The only significant modification to this Centaur is the addition of white thermal paint to help balance the temperature of the empty stage.
Upper Photo: The LCROSS logo, painted on the Centaur. Lower Photo: The Centaur upper stage is lifted for mating to the Atlas-V first stage (Tail Number 020) at the Vertical Integration Facility in Florida (April 30, 2009). The Centaur was painted white to help manage the thermal environment during the 100-plus day LCROSS mission. Yellow lifting hardware is ground handling equipment and is removed after stacking on the first (booster) stage. Courtesy of NASA Kennedy Space Center.
What is particularly unique about this Centaur was the way that the United Launch Alliance (ULA) Atlas Team designed the flight profile and maneuvers after LRO separation to minimize the amount of residual hydrogen, oxygen, hydrazine, and helium left on Centaur. This special care to ensure that the Centaur is as empty is possible, makes the Centaur very clean. It reduces the hydrogen (H2), oxygen (O2), and hydroxyl (OH) species that could be confused with in-situ water ice ejecta.
Since Centaur would be attached to LCROSS until just before impact, all of the Centaur hardware and functional system were analyzed and scrutinized and if necessary tested to ensure that they would not adversely affect LCROSS after LCROSS took over control from Centaur.
The final function to liberate Centaur as a guided impactor will be the release of the clamp band holding the LCROSS/Centaur stack together. This separation command will be sent by LCROSS approximately 9 ½ hours before impact. The clamp-band and the separation system that will push LCROSS and Centaur apart, underwent long-duration testing to give the Mission team confidence that it will function smoothly after 100-plus days in orbit.
Additional Centaur info and history, courtesy of NASA’s Glenn Research Center, can be found at: http://www.nasa.gov/centers/glenn/about/history/centaur.html
Once More Around the Earth: September 4 - October 5
Posted on Oct 05, 2009 03:14:59 PM | Paul.D Tompkins
The anomaly robbed the LCROSS Flight Team of precious time to prepare for Impact. But with a healthy spacecraft, and enough propellant to do the job, our team was all too happy to prepare for the future. Ahead of us were five more Trajectory Correction Maneuvers (TCM 6 – 10) to precisely refine our crater targeting, then Separation, Centaur Observation, Braking Burn, and finally, Impact. In the midst of our TCM series, the Science Team continued refining their selection our target – the specific crater, and the point within the crater - based on the latest data from other missions. As a final confirmation of the payload instruments, the Science Team also wanted to look at Earth one last time before Impact. On top of all that, we needed to practice those final two critical days as much as we could.
DOY 247-251 (September 4 – 8): Housekeeping
Out of Emergency Status, we resumed operations gradually, monitoring spacecraft health and performing typical housekeeping duties. Of special note, as a result of the anomaly and our very small propellant margin, we decided to substitute Earth Look Cal 2, originally scheduled for DOY 250 (September 7), with a new, more propellant-efficient calibration maneuver that we termed “Earth Gaze”, on DOY 261 (September 18; see below).
DOY 252 (September 9): TCM 6 Waived
We received more good news about our trajectory. Despite the long train of thruster pulses that resulted from our anomaly, LCROSS’s orbit was still right on target. The Mission Design Team and Navigation Team agreed that TCM 6 would be too small to be worthwhile. This meant one less event to attend to in our busy schedule, and one less reason to take risks at this late, sensitive stage in the mission.
This decision worked in our favor for saving propellant too. Each TCM typically requires the spacecraft to reorient to a specific direction so that LCROSS can add velocity in a particular direction via the 22 N Delta-V thrusters, and then back again to the Cruise attitude. All of our remaining “burns” were expected to be small; so much so that the propellant devoted to the reorientation maneuvers was expected to be several times more than for the burn itself. Waiving TCM 6 meant we could save more propellant, and reduce risk.
DOY 253 (September 10): Omni Pitch “Creep”
Throughout the mission, Omni Pitch maneuvers have been necessary to keep our primary antenna pointed towards Earth. In all previous Omni Pitch renditions, the “slew” was performed in SIM DB2 (Stellar Inertial Mode, deadband 2), an Attitude Control System mode that enables relatively efficient attitude changes. However, SIM DB2 significantly accelerates the spacecraft rotation to induce the change in orientation, and this costs propellant.
In our bid to save propellant, we re-examined all of our previous maneuver approaches for opportunities to be more efficient, including for Omni Pitch maneuvers. Someone on the Flight Team came up with the idea of providing LCROSS a long sequence of small attitude changes rather than one big one, with the goal of avoiding the accelerations of SIM DB2 (small attitude changes don’t justify large rotational accelerations, and therefore are cheaper propellant-wise). Simulations of this new approach, what we informally called “Quaternion Creep”, indicated this would lead to a significant propellant savings, so we baselined this approach for our Omni Pitch maneuver on DOY 253.
Normal Omni Pitch maneuvers used to take 40+ minutes to execute. This new approach took 4 hours 20 minutes, but saved a lot of propellant. Well worth the bargain.
DOY 257 (September 14): Guarding Against Single-Event Upsets
Cosmic rays are high-energy photons that are known to interfere with spacecraft computers by in effect reprogramming single bits of memory as they pass through. When a bit in computer memory is reprogrammed by radiation, but not permanently damaged, it is called a Single-Event Upset (SEU). Most spacecraft computers and onboard memory chips are “radiation-hardened” to prevent or otherwise sidestep the effects of cosmic rays.
The spacecraft Data Handling Unit (DHU) is the independent computer that controls the operation of the science instrument payload. Command sequences onboard the DHU control all of the sampling sequences and instrument settings to make sure LCROSS collects the best possible set of data at impact, and for other calibration events. They are clearly very important to the success of LCROSS.
The DHU, and those instrument command sequences specifically, are somewhat vulnerable to SEU, and the Payload Team had not checked the contents of the DHU in a long time. On this day, our team dumped (downloaded) the full contents of DHU non-volatile memory and confirmed that the sequences were still intact.
DOY 257 – 259 (September 14 – 16): Full Rehearsal of TCM 10 through Impact
It was hard to find time in our busy operational schedule to practice all of our pre-Impact procedures. In the months before launch, we spent all of our time readying for Transfer Phase and the first parts of Cruise Phase. Impact seemed so far away. Then Cruise Phase, with the anomaly, was far busier than expected. With only weeks remaining, our perspective was entirely different.
We took full advantage of two free days in our schedule to hold the “Last Two Days Rehearsal”, a full-team, high-fidelity rehearsal of the last two days of the mission, similar in style to our First Week Rehearsal (see the post on “First Week Rehearsal”). For 42 hours, following the exact schedule we planned to follow in flight, and synchronized with the actual times of day when events would occur, the Flight Team practiced nearly every aspect of those operations. We ran 24 hours a day, with three overlapping execution shifts (Shift B, then A, then B again), and two planning sessions. In the rehearsal, the team successfully hit the target crater and collected all of the science data. However, the there were some procedural and process shortcomings that made things just a bit rocky at times. We learned a lot, and began working improvements in the days after the test.
DOY 260 – 261 (September 17 – 18): Our Last Gaze at Earth
In substitution for Earth Look Cal 2, we conceived of a more propellant-efficient calibration event we coined, for lack of something more official, “Earth Gaze Cal”. We loaded the commands for this event on DOY 260 (September 17), and executed the calibration the following day. Rather than sweep back and forth over the Earth (see the full description of our first Earth Look in “During Our Second Trip Around Earth”), Earth Gaze looked straight at the Earth for an extended period, during which time we collected camera and spectrometer data. This was the Science Team’s last chance to evaluate instrument performance and settings before Impact.
We captured some more images. Here are a few, along with a simulation to show the orientation of Earth during the event.
DOY 267 (September 24): Cold Side Bakeout #3 and a Test of the –Z MGA
Despite previous efforts to rid the Centaur outer skin of water on Cold Side Bakeout #1 and #2, our Navigation team continued to observe the accelerating effects of escaping water at the end of the second of those events. With the amounts of water remaining, the Science Team was no longer concerned that this water could interfere with water measurements at Impact – there was just too little left. However, Navigation was still concerned that remaining water might push our Centaur off course in the hours before Impact, after Separation when we no longer had any control over its orbit.
Recall that we had planned to execute Cold Side Bakeout #3 on DOY 234 (August 22), but our plans were thwarted by the discovery of the anomaly. Cold Side Bakeout #3 was unfinished business that had to be completed.
In an unrelated thread, we also wanted to test the antenna we’d be using for Impact. LCROSS has two Medium Gain Antennas (MGA’s), one on the +Z axis, the other on the –Z axis, used to downlink high-speed science data to Earth. We had used the +Z MGA during Lunar Swingby, but had never tested the –Z MGA in flight. We didn’t want to discover a problem with this antenna in the hours before impact, so we devised a test that would expose any issues immediately, and that would couple very nicely with Cold Side Bakeout #3.
The combined Cold Side Bakeout #3 and –Z MGA Test took advantage of the fact that LCROSS was passing right through the ecliptic plane, the plane of the Earth’s orbit around the sun. The LCROSS +X axis was perpendicular to the ecliptic plane at the time, and so by rotating about the +X (roll) axis, we could simultaneously face the “cold side” of the Centaur towards the sun, and the –Z MGA towards the Earth (required to test communications via this antenna). It was a perfectly-timed opportunity.
On DOY 267 (September 24), we performed the maneuver. Unlike previous versions of Cold Side Bakeout, we stopped the spacecraft twice, once at 135 degrees rotation, and again at 225 degrees. The first position pointed the –Z MGA (which is canted by 45 degrees) straight at the Earth, and warmed one side of the cold skin of the Centaur. After 20 minutes, LCROSS rotated another 90 degrees, moving the MGA off the Earth, and moving another part of the cold face of the Centaur into full sunlight. We characterized one “slice” of the –Z MGA antenna gain pattern, confirmed that it was operational and mounted according to specification, and removed more water from our impactor.
One of the risks of Cold Side Bakeouts is that we might induce a thermal instability in our thrusters, as we had in Cold Side Bakeout #1, prompting our improvised fault management to fire the thrusters to keep them warm (see the post entitled “Our First Orbit Around the Earth” for details). Happily, our thrusters remained thermally stable, and we avoided any additional propellant cost.
DOY 268 (September 25): TCM 7
LCROSS had not performed a Trajectory Correction Maneuver since TCM 5a, way back on July 21. We’d been literally coasting along our orbit since then. By this time, the Science Team had selected a satellite of the Cabeus crater, Cabeus A1, as our impact target. Trajectory Correction Maneuver 7 (TCM 7) was performed on September 25 to target that location on Impact Day. According to predictions on burn pointing errors, TCM 7 would guarantee a hit within 38 km of our intended target. Future burns would reduce the error a lot further. Still 14 days from Impact, TCM 7 nudged LCROSS by 32 cm/s (just over 1 foot/sec), but enough to make a big difference in impact position.
DOY 273 (September 30): TCM 8
Following TCM 7, the Science Team continued to receive data from other lunar missions. Lunar Reconnaissance Orbiter (LRO), our partner at liftoff, devoted much of its operational time to scouring our top target regions, and data from Chandrayaan-1 and other missions continued to improve our knowledge of the mapping of possible water concentrations at the lunar south pole. New information gathered after TCM 7 prompted the Science Team to redirect LCROSS to the main Cabeus crater. Relatively close to the previous target, changing represented a small amount of extra propellant, but a potentially significant improvement in science data.
TCM 8 was performed on September 30, and changed LCROSS’s velocity by 35 cm/s. Not only did TCM 8 target the new impact position, but it also refined our impact timing. Our narrow target time window will allow the Hubble Space Telescope to take images of our impact plume.
DOY 274 (October 1): Impact Contingency Rehearsal
Our Last Two Days Rehearsal on DOY 258-259 did not emphasize off-nominal events, but rather the full integration of the team in a single push. On October 1, Shift B, who will be overseeing Impact, practiced two complete run-throughs of the final two hours of the mission. I helped our Test Conductor in setting up the tests, and planning a series of anomalies that would really challenge Shift B. During the simulations, we failed banks of temperature sensors and heater circuits, caused various instruments to malfunction and the DHU to crash. We faulted the IRU, forcing a recovery in the midst of early pre-Impact science operations. We failed the –Z MGA switch (even though this is far less likely having conducted our MGA test), and forced the team to perform the Impact sequence at a contingency low data rate. Their team did very well, and succeeded in meeting mission objectives on both runs. The members of Shift B were very happy they had that final opportunity to practice.
DOY 275 (October 2): Planning Rehearsal
The flight planning team has one of the biggest challenges in the final 24 hours before Impact. Based on a final assessment of the LCROSS orbit, they have to plan the final series of maneuvers that culminate in the Impact event: TCM 10, Separation, Centaur Observation, Braking Burn, and Impact itself. From the maneuver plan, the team must generate command sequences for each of these events, and then run them on our simulator to prove that they are flight-worthy. Furthermore, they also need to generate and test command products to handle specific contingency events, like a failed Separation, or a late Braking Burn. They even generate a preliminary Impact command sequence, just in case we lose contact with LCROSS for hours before the event. Some of these products can be produced well in advance, but a great number of them depend on the final orbit assessment, and the specific orientations that are required for TCM 10, Separation and Braking Burn to ensure we accurately hit our target.
The Last Two Days Rehearsal proved that the planning team could generate all of its products, but could not quite perform its full list of quality assurance tasks in time. Following the rehearsal, the team took some significant steps to streamline the planning, command generation and quality assurance process. We found opportunities for working in parallel, for skipping unnecessary steps, for nailing down some variables in advance to reduce the amount of variance on Impact Day.
To test these changes, we conducted a Planning Rehearsal for DOY 275 (October 2) to span the full 12 hours of the process, exactly as it will happen on the Impact Day. I was really happy with the results. The team actually finished most of its tasks early, including simulations of all command products, and their delivered products contained no errors. The Command Approval Meeting, our last visual evaluation of commands, finished early. I was very encouraged by these results, and the planning team is now fully confident that they could repeat this level of performance on Impact Day.
DOY 278: Final DHU NVM Sequence Loads and TCM 9
The remainder of this post has to do with current and future events. I’m finally caught up!
This morning, as I write this post, Shift B is loading its final set of science payload command sequences to the spacecraft. The Science and Payload Teams have pored over the data we collected from all previous science calibrations to derive a set of camera exposure/gain settings for Impact. Over-exposure means washed-out images, which under-exposure means less contrast and detail. Six of nine sequences will be changed to reflect the Science Team’s best knowledge of camera performance, and expected conditions on our final descent to the lunar surface.
Later this afternoon, Shift A will be performing TCM 9, our second-to-last, and possibly our last orbit adjustment. After TCM 8, based on the latest lunar mapping data, the Science Team selected a different spot within the Cabeus crater. This is our final target. With only 7.6 seconds of firing, TCM 9 moves our impact point from our old point in Cabeus to this new location roughly 9 km away. TCM 9 should put us within 1.75 km of our target. If we can “nail” this maneuver by staying within that range, we’ll be able to skip TCM 10.
TCM 9 is our last spacecraft activity, other than monitoring and housekeeping, until our final rapid-paced series of maneuvers in the final 24 hours.
As you can imagine, things are starting to get very busy! I will submit one more post with a detailed description of the final 24 hours. I also have a few guest authors who would like to share some things with all of you before Impact. Stay tuned...this is going to be an incredible week.
A Test of the Flight Team: The Near-Loss and Full Recovery of LCROSS
Posted on Oct 04, 2009 08:09:46 PM | Paul.D Tompkins
Early in the second half of our second Earth orbit, while out of DSN contact, LCROSS experienced an anomaly. An error detected with our Inertial Reference Unit (IRU) resulted in an automatic response that consumed a large amount of propellant in a short amount of time. We spent the remainder of our second orbit recovering from that anomaly, and protecting against any future excessive propellant usage. We emerged safely, but with so little propellant remaining that, since that event, we’ve had to step very carefully to avoid wasting any more of this precious resource. This posting describes our discovery of the problem, and our recovery steps to protect LCROSS against a reoccurrence.
My intent with this blog entry is to relate the facts as we know them, to demonstrate what an anomaly is, through a real example, and to show how our Flight Team responded. My hope is that you come away with a better understanding of the challenges of space flight.
August 21/22 (DOY 234): Anomaly Detection, Spacecraft Safing, and Entry to Emergency Status
Shift A came into the Mission Operations Control Room (MOCR) on Saturday with a big agenda on LCROSS – to execute a third “Cold Side Bakeout” to rid the Centaur of more ice trapped in its skin, and to perform an Omni Pitch maneuver to flip the spacecraft to re-point the primary omni-directional antenna at the Earth, all under a tight schedule with little margin for error. The shift started at 2:25 AM Pacific time, but despite that early hour, we were excited to have such a challenging pass ahead of us, and eager to get started.
We acquired spacecraft telemetry at 3:25 AM. In the MOCR, each Flight Team operator sits behind a set of computer monitors that display fields of telemetry data – numbers, status indicators, etc. that indicate the health of LCROSS. Many of the telemetry data fields show up in stoplight colors – green, yellow, or red, to indicate whether the value is “nominal” (green), or approaching an emergency state (yellow), or is in an emergency state (red). Typically, telemetry comes up entirely green.
That day, LCROSS came up with lots of yellow and red alarms. We had never seen anything like this in flight. The summary of our observations:
- The Inertial Reference Unit (IRU), our onboard gyro, and primary means of measuring rotation rates around each axis for attitude control, had faulted;
- The Star Tracker (STA) had replaced the IRU in providing rotation rate estimates to the attitude controller;
- Spacecraft rotation rates were periodically exceeding yellow high alarm limits (rotating too fast);
- LCROSS was firing thrusters almost continuously,
- The propellant tank pressure sensor (our primary means of determining how much fuel we have left), indicated we had consumed a LOT of propellant.
In short, we were in serious trouble, and needed to make corrections immediately.
First, we needed to save our remaining propellant by reducing our thruster firings. LCROSS has an Attitude Control System (ACS) to control its orientation automatically, by firing thrusters (LCROSS has no reaction wheels). The ACS controls each of the axes (roll, pitch and yaw; see the diagram in “First Orbit Around the Earth”) to within an acceptable error bound, defined by a control “deadband” from a fixed orientation in space. The ACS has several modes, all with different characteristics. The mode we were in, “Stellar Inertial Mode, deadband 2” (or “SIM DB2” for short), controlled each axis to a +/- 0.5 degree deadband. Through the anomaly, LCROSS was keeping to within its assigned deadband. Under attitude control with a deadband, thrusters typically fire most often when the spacecraft rotates to the very edge of the deadband, causing the spacecraft to stop its rotation and preventing it from exceeding the deadband limit. So, in an effort to slow down the firings (and our propellant consumption), we switched to a wider deadband, SIM DB3, which controls to a 10 degree range. Unfortunately, this didn’t reduce our thruster firing rate very much. We needed to do something else.
The anomaly had caused LCROSS to automatically switch from using the IRU to the Star Tracker (STA) for measuring spacecraft rotation rates. However, when we studied the output of the IRU, it was producing good rotation rate data (or just “rate data” for short). So, we ran through the IRU recovery procedure to re-engage the IRU with attitude control, and sure enough, that finally returned the thruster firing profile to normal and bought ourselves time to think.
One thing was immediately clear – we had suffered a significant “anomaly”, or bad problem and our plans for a Cold Side Bakeout and Omni Pitch maneuver were dashed. We’d be spending the rest of the shift, at least, and possibly a lot longer, figuring out what had happened and how to prevent it from ever happening again.
With the spacecraft stable, but still in jeopardy, the next order of business was to extend our DSN coverage. The Mission Operations Manager, with full agreement from the rest of the Flight Team, declared a “Spacecraft Emergency” with the Deep Space Network. Under DSN guidelines, we could only call this if we thought the spacecraft was in immediate jeopardy of partial or total mission failure. Under an emergency declaration, all missions using the Deep Space Network volunteer their normally-scheduled antenna time in a community effort to help the ailing mission. Their help provided LCROSS with enough antenna time to work out its problems.
As a step to protecting LCROSS, the Flight Team had to figure out what caused the anomaly. Since the spacecraft was healthy on our last contact, the anomaly had occurred while LCROSS was in a normally-scheduled 66 hour “out-of-view” period with the Deep Space Network (DSN). The Flight Team could not collect telemetry during that time. However, just for this purpose, LCROSS constantly records a part of its telemetry onboard, to enable our team to diagnose problems that happen while out of contact. We set immediately to “downlinking” and analyzing our virtual recorder telemetry data to gather clues.
Using this data, and over an hour or two of analysis, our team gradually pieced together the story. As with most anomalies, this one stemmed from multiple vulnerabilities whose combined harmful effects had not been anticipated.
In summary, a spurious, short-lived error on the IRU was interpreted as a more serious fault by the spacecraft fault management system, resulting in a switch to a backup rate sensor (STA). That rate signal was noisy (including random variations over the “true” rate signal), but was misinterpreted as real, “clean” rate data, causing over-control by the attitude control system and resulting in a great deal of propellant consumption. LCROSS detected the associated tank pressure drop, but with no fault management option available for disabling thruster control (thrusters are required to keep the LCROSS solar array pointed to the sun), and no ability to determine the specific nature of the pressure loss, the spacecraft fault management system performed steps to stop a leaking thruster (another potential reason for a pressure drop), and to power-up its transmitter to “phone home” to warn the operations team that there was a problem. However, this call could not be detected over the Southern hemisphere, since there were no DSN assets that could “see” LCROSS in that location, so the call was missed.
The LCROSS team knew that it had a serious problem on its hands. Did we have enough propellant to complete our mission? Was the IRU truly healthy, or would it fault again and trigger another loss of fuel? Thanks to the DSN Emergency Status agreement, LCROSS was able to get continuous ground antenna coverage, whenever the spacecraft was in view of one of the three DSN antennas. However, given our position in our Cruise Phase orbit, only the Canberra antenna complex could actually “see” LCROSS at that time, and only for 15 hours per day.
Given our limited DSN visibility, our third (now primary) order of business was to protect our spacecraft from events that might trigger another propellant loss, and hence a loss of the mission. We didn’t have much time before we’d go below the horizon with Canberra, so we had to resort to simple fixes first – ones that would protect us, but not add even greater risk. Our actions were:
- To increase the “persistency” of the IRU fault check by flight software. By requiring an IRU fault to persist for 5 consecutive seconds rather than 1 second before tripping to STA, we hoped to avoid switching to the STA for an inconsequential IRU fault, yet remain protected against a serious IRU fault.
- To augment the IRU fault response by automatically recovering the IRU (as we had done manually) first, before switching to STA for rate information. Even if an IRU fault lasted for 5 seconds, this second-tier change would try to re-instate the IRU first, and fail over to the STA only if this had failed.
We designed, implemented, tested and loaded these changes to the spacecraft in our remaining time. By 1:50 PM Pacific Daylight Time, with these temporary fixes in place, we began configuring LCROSS for our forced DSN outage. Just 10 ½ hours after discovering our problem, we had to release our hold on LCROSS again and anxiously look to the next time when we would reacquire communications with the spacecraft.
August 23 – September 3 (DOY 235 - 246): Our Recovery
The extended LCROSS team mobilized to get the mission back on track. Our goals evolved over the recovery period, but here’s what we had to do:
- Improve our fault protection against an IRU failure for the long term. We had short-term fixes in place from the first day, but we ultimately needed to implement a more complete solution. These would take longer to design and test.
- Improve fault protection against further excessive propellant loss. This was a core issue. The loss of the IRU was one potential vulnerability, but there might be others we didn’t yet know about. LCROSS could not tolerate another similar loss of propellant.
- Determine whether the IRU was really showing signs of failure, or whether it was actually fine.
- Assess our propellant margin. Could we complete all of our mission objectives? Would we have to give up some of our planned activities to save propellant?
- Develop a plan for LCROSS health monitoring for the remainder of the mission. With so much at stake, we felt we needed a way to regularly monitor LCROSS after we emerged from Emergency Mode. However, with a team as small as ours, watching the spacecraft 24 hours a day for the rest of the mission would exhaust us. Besides, we needed to devote a lot of time in the coming weeks to prepare for impact.
- Continue performing the nominal events that could not be put off until our anomaly resolution was complete.
We implemented a schedule of 16 hour days, covered by two overlapping execution shifts in the Mission Operations Control Room, and a back-room design, implementation and test team split between the Mission Support Room at Ames and the Remote Operations Centers at Northrop Grumman.
Propellant Usage Monitor and Free-Drift Mode
Our first priority was to develop fault protection against excessive propellant consumption. The sure way to save propellant is to not fire thrusters. But firing thrusters is the only way to maintain attitude control, and that is critical for generating solar energy and to keep the spacecraft thermally stable. It seemed we were in a bind. But then we recalled a strategy the team had originally designed to fight large Centaur gas leak torques (see the post entitled “Real-Life Operations: Day 3” for a description of the Centaur leak issue).
The attitude control system has a special safe mode called “Sun Point Mode” (SPM) that points the solar array at the sun using special Coarse Sun Sensors, and without the use of the Star Tracker. SPM spins LCROSS very slowly, end over end, but keeps the solar array pointed at the sun at all times. SPM still requires thruster control to maintain stability. However, through simulations, our engineering team discovered that by modifying SPM to spin faster, LCROSS would have enough angular momentum to keep it stable for a very long time.
We still needed a means to detect excessive propellant usage. We deemed propellant tank pressure measurements a little too coarse for such an important job, and yet there was no other single, direct indicator of propellant usage in the system. However, one of our team members came up with a great idea. LCROSS generates and stores special telemetry “packets” each time a thruster is fired. These packets accumulate over time in spacecraft memory, and are downloaded to Earth for analysis. His idea was to monitor the accumulation of thruster telemetry packets as an indirect indicator of propellant usage. It was unconventional, but the idea worked.
So, we designed an excessive propellant usage monitor that watched telemetry accumulation, and if too many packets accumulated over a period of time, would cause LCROSS to transition to our modified SPM, “free drift” mode to stop thruster firings, yet keep the spacecraft safe. This was onboard and operating by DOY 241 (August 29). There were other “free drift” ideas, but the beauty of using a modified SPM is that SPM was already fully integrated into our fault management approach for serious spacecraft anomalies. So, much of the software design and testing was already in place. This was far more appealing that developing a new, complicated control mode that might have introduced more risk than it retired.
Efficient Control under Star Tracker Rates
With a general propellant savings approach, we also set to improving the ACS performance under Star Tracker rate information. Northrop Grumman developed an entirely new controller with filters to remove the STA noise. In simulations, it promised to drastically reduce propellant consumption under STA rate information if our IRU failed. We loaded the final version to the spacecraft on DOY 246 (September 3), replacing our old SIM DB3 mode, and it performed even better than expected, reducing propellant consumption by a factor of approximately 50 as compared to our anomaly. An IRU failure would now be far less severe in propellant cost. Even better, it was more efficient than the original SIM DB3, so now we’d be saving propellant, relative to the initial plan, for the rest of the mission.
We never determined the root cause of the momentary IRU glitch, but it never happened again. Between discussions with the IRU vendor and analysis of many days of telemetry, we ruled that there was no reason to think the IRU was in jeopardy.
Enough Propellant for Full Mission Success
There is no way to directly measure the amount of propellant left in the LCROSS tank. One can estimate the remaining propellant load by measuring pressure, temperature and tank volume (and compute the result using gas law equations), and alternatively one can estimate the propellant consumed through the mission by measuring the accumulated on-times for all the thrusters, along with predictions of how much propellant each firing consumes. Neither method is perfect.
Analysis shows that LCROSS expended about 150 kg out of the 200 kg it had remaining in its tank before the anomaly, roughly half of its launch propellant load. However, even under worst-case assumptions, our engineering team determined that LCROSS would have enough propellant to meet the criteria for full mission success. However, we wanted to be very protective of our remaining propellant. In agreement with the Science Team, following great successes with Lunar Swingby, Earth Look Cal 1 and the Special Earth/Moon Look Cal, we canceled our final Earth Look Cal and our Moon Look Cal, but added an Earth Gaze Cal that would efficiently allow the scientists to check the instruments following the anomaly. We’d also continue to look for other opportunities to save what little propellant remained.
The Mission Goes On
During our anomaly recovery, we had to support a number of activities that couldn’t wait until later. The day after our discovery of the anomaly (DOY 235, August 23), orbit geometry dictated we needed to rotate our spacecraft to re-point our omnidirectional antenna toward the Earth again, or suffer very poor communications for many days to come. We performed another of these 10 days later on DOY 246 (September 3).
In a bit of good news, TCM 5c, originally scheduled for DOY 236 (August 24), was determined to be unnecessary. As with so many TCM’s in our original plan, the Mission & Maneuver Design and Navigation teams planned TCM 5a so accurately that neither of the follow-up burns (TCM 5b and TCM 5c) needed to be performed.
However, not all of our news that day was good. Also on DOY 236, we thought we had also experienced signs of a failing Star Tracker. Out of concern for its survival, we shut down the STA for two days, and spent time away from our main anomaly recovery developing protections for the STA. As it turns out, the unusual STA signature was related to an onboard clock calibration we had just performed. The STA was just fine, but you can imagine, in the midst of the tension of the anomaly recovery, this extra scare didn’t help matters! As in so many cases with space flight, even seemingly benign changes can lead to unexpected results.
Finally, the anomaly recovery partially interfered with two planned DSN rehearsals for the separation and impact events. Both events require a lot of coordination, since we’re juggling three antennas, each with multiple receiver configurations. We were able to follow through with a rehearsal of TCM 10, Separation and Braking Burn, but our very busy schedule forced us to abandon our plans for the Impact DSN test. Another victim of unexpected events.
Negotiations with Deep Space Network and other Missions
To address the need to monitor LCROSS more often after our exit from Emergency Status, the LCROSS project negotiated a schedule with the DSN mission community that shortened all out-of-contact periods to 9 hours duration or less. This came at great impact to other missions, but was invaluable to LCROSS in achieving its Emergency Status exit criteria.
To address the concern of Flight Team fatigue that would result from having to staff all of these extra hours, we designed a way for the DSN to perform simple, regular health checks on the spacecraft. Our team standardized our communications downlink rate to 16 kbps to indicate a healthy spacecraft. In the event of an emergency, LCROSS automatically switches to a lower downlink rate (2 kbps). The DSN can easily distinguish between these two rates when it acquires the LCROSS signal, and so our team worked out a protocol for monitoring the signal during each view period, and a call-tree of LCROSS operations personnel should they discover LCROSS is transmitting at 2 kbps. Under the modified Concept of Operations, the Flight Team must now check spacecraft telemetry daily to ensure LCROSS remains healthy.
DOY 247 (September 4): Emergency Over
Twelve days after the anomaly discovery, having augmented LCROSS fault protection and our contact plan with the DSN and other DSN-supported missions, we requested that DSN remove LCROSS from Emergency Status.
One of the gratifying aspects of this effort was how the LCROSS team responded to make sure LCROSS stayed safe. Members of the Flight Team, LCROSS project and NASA Ames worked tirelessly. Northrop Grumman mobilized to provide a number of valuable improvements to LCROSS. The DSN mission community generously volunteered their antenna time to support our bid to restore LCROSS to nominal status. NASA management, from Headquarters to NASA Ames, provided constant support and valuable assistance.
Having survived this test and emerged with the moon and full mission success still in our sights was quite an accomplishment. I want to thank the dedicated members that make up this team, and to everyone else that helped with our recovery!