|Posted on Sep 26, 2009 09:34:26 PM | System User | 4 Comments ||
With our first orbit around the Earth behind us, the Flight Team team looked optimistically ahead to a number of upcoming events, including two science instrument calibrations, and two Trajectory Correction Maneuvers, TCM 5b and 5c. Thankfully for us, the schedule of planned events for the next 34 days to be a little more relaxed than for the previous 42 days. The key distinction being the planned events.
August 1 (DOY 213): A Look at Planet Earth, and a “Discovery” of Water
Before the mission began, our team had identified key geometric opportunities for performing our Earth and moon-looking science instrument calibrations. One of the important factors in determining the best time is spacecraft-to-target distance. The closer the Earth and moon are to LCROSS, the better resolution we can attain, and the stronger the signals will be (hence why Lunar Swingby was so important). The below plot shows the distance between LCROSS and the Earth (light blue) and LCROSS and the moon (dark blue), over the entire mission. The labels indicate when specific LCROSS events happened (or will happen), including launch, Lunar Swingby, Earth Look Cal 1, Earth Look Cal 2, Moon Look Cal, and Impact. As you can see, the Earth Look events coincide with low points on the curve, while Moon Look timing was influenced by other factors.
Another factor is “phase” with respect to sun illumination on the target. Both the Earth and the moon go through cycles depending on the relative positions of the sun, the Earth or moon, and the observer, in this case LCROSS. So at various times, the Earth will look like a crescent, at other times will be “full” (like a full moon) and still other times will be “new”, with its face completely un-illuminated. For science measurements, our Science Team tended to prefer “fuller” views of the Earth and moon.
As planned prior to launch, Earth Look started by rotating the payload boresight (pointed along the LCROSS –X axis) to point at the center of the Earth, then repeatedly “swept” the boresight in alternating East-West and North-South passes across the Earth’s disk. The objective was to collect spectra of the known Earth signal (for spectral calibration) and also re-affirm the alignment of the instruments. Similar to the limb crossings in Lunar Swingby (see “Lunar Swingby: Development of a Procedure”) by sweeping the cameras across the disk, the Science Team could compare the timing of the rising and falling signal with the well-known motion of the spacecraft relative to the position of Earth. Consistent mismatches in timing would indicate a misalignment.
We collected some cool photos of the event, included here. Note that our instruments are optimized for lunar impact and finding water, so you can’t make out the landforms in the visible images, and the IR images detect thermal differences in the upper atmosphere. It was really exciting to be looking back at our home planet from so far away. In contrast to images taken from the Space Shuttle, International Space Station, or even Apollo, our images really show how insignificant the Earth is in comparison to the immensity of space.
LCROSS, via its spectrometers, also “discovered” water on the Earth – an event that didn’t exactly make headlines, but made the Science Team quite happy. Detailed analysis of the spectra and imagery indicated that all of our instruments were all performing very well.
In an unrelated piece of good news, the Mission & Maneuver Design team determined that TCM 5a had been so accurate that TCM 5b would be unnecessary. Originally, TCM 5b would have incorporated another Omni Pitch maneuver, but now with TCM 5b unnecessary, we’d perform a standalone Omni Pitch instead.
August 6 – 7 (DOY 218, 219): Re-Pointing our Antenna – Omni Pitch 3
Nearing the “top” of our orbit (see the flight path diagram), it was time again to perform an Omni Pitch maneuver to re-point the primary omni antenna towards the Earth. These times are tricky because at the time of the maneuver, neither the initial nor the final orientation provide very good angles to the omni antenna. You can plan to perform the maneuver early (favoring the initial attitude, but making the final attitude margins really bad), or perform the maneuver late (favoring the final attitude). Omni Pitch 3 favored the final attitude. We lost telemetry data sporadically before it began, indicating very poor link margins (downlink rate at our lowest rate of 2 kbits per second or kbps), but then had sufficient margin to boost our downlink data rate to 32 kbps for post-maneuver virtual recorder playbacks. Throughout the mission, the Flight Team constantly has to evaluate LCROSS link margins to make sure we can command the spacecraft and monitor maneuvers via telemetry.
August 8 – 13 (DOY 220 – 225): Good and Boring
The mission operations philosophy states that “a boring spacecraft is a good spacecraft.” This is certainly true because “exciting” usually means risky, dangerous and proximity to failure. As a Flight Team member, you really don’t want to be responsible for a mission disaster! Despite the boredom that sets in on those uneventful days, slow and steady is a good thing. Thankfully for us, LCROSS was behaving very well, and occasionally we found ourselves in long stretches of time between planned events. During those times, the Flight Team simply monitored spacecraft health, and hoped for boredom.
Cruise Phase health monitoring passes typically include the following steps:1. LCROSS activates its own transmitter just before the scheduled start of a DSN contact. LCROSS normally only transmits when scheduled for DSN contacts to minimize the potential for interference with other spacecraft.
2. Acquisition of Signal (AOS). The Flight Team (specifically, the Flight Controller) coordinates with operators at one of the DSN antennas to establish communications with LCROSS. Telemetry (downlink) comes up first, then commanding (uplink), then “ranging” from which we derive our orbit knowledge. We typically transmit at 2 kbps at AOS then, depending on the link margin available, switch to a higher data rate to allow faster telemetry updates. 64 kbps is our standard Cruise downlink rate. The Flight Team also re-configures some elements of LCROSS onboard fault management for in-view conditions. For some problems, the Flight Team is better suited to respond than simpler, automated software responses. Hence we disable those when in-contact.
3. Virtual Recorder Playback: Since we are often not in contact with LCROSS, but want to keep track of how it’s behaving at all times, LCROSS constantly stores telemetry in RAM storage, even when out-of-contact. For most DSN passes, we perform a “VR” playback to retrieve the data from the recorders for analysis. The Navigation team uses this data to reconstruct when thrusters fired, and to predict how they might affect the orbit. The remainder of the data is useful to the Engineering Analysis team, who looks for strange behavior in each of the spacecraft subsystems. If the spacecraft comes up in a bad state, this data becomes critically important in determining how it got there (note: foreshadowing). Once the data is linked back to Earth, we free up the memory so that we can store another several days’ worth.
4. Loss of Signal (LOS): Again, the Flight Controller coordinates with the DSN to bring down communications with the spacecraft. Before that happens, the Flight Team ensures LCROSS is behaving properly and then configures fault management for out-of-view conditions.
5. LCROSS automatically de-activates its own transmitter just after the scheduled end of the DSN contact period.
August 14 (DOY 227): Opportunistic Science and Design on the Fly
During the mission, the Science team and Engineering team occasionally requested activities that were not on the original schedule. The Science team wished to find an opportunity to image a point source with a strong infra-red (IR) signal in order to work on a solution to an Mid-IR 2 camera focusing issue. The considered Venus and Jupiter, and though both were very bright in the visible spectrum, neither provided a very good IR signature. Working with the Mission & Maneuver Design team, the Science team discovered a golden observation opportunity – on August 16, just as LCROSS would be passing through orbital plane of the moon, the Earth and moon would appear only a few degrees apart from each other (see the plot of the angle between the Earth and the moon, as viewed by LCROSS). The moon has known IR reflective properties, and because the moon would be on the opposite side of the Earth from LCROSS, it would approximate a point source (look very small). About the only problem with the proposed timing is that LCROSS would be very far from Earth (see the “Special Earth/Moon Cal” in the above plot of Earth and moon distance. However, our team considered the opportunity, and decided to go for it.
With a simple mission like LCROSS, a Flight Team is wise to pre-plan as much as possible before launch. As the saying goes, “the devil is in the details”, and that certainly holds true when piecing together command sequences and operational procedures. We worked very hard before launch, without the pressure of a flying spacecraft to care for, to pre-plan, construct and test commands and procedures for each event in the mission plan.
This new activity, dubbed “Special Earth/Moon Look” didn’t match any of our pre-planned maneuvers, so we had to design it on the fly. We designed the overall approach in a meeting between the Science team, Mission & Maneuver Design Lead, Sequencing Lead, Systems Engineers, Flight Directors and Flight Controllers. To the credit of the team, especially the Activity Planning & Sequencing Lead (John Bresina), we were able to create command a sequence, adapted from our Star Field Calibration sequence, that could accomplish the goals of the activity. One major difference was that we’d be looking at the Earth and moon (two targets) instead of a star field (one target), so it was like merging two Star Field Calibrations together. There were a lot of other changes at the detailed level to make this work. The Engineering Analyst had to extend his automated checking tools to work on this new command sequence. The Simulation Engineer also simulated the commands on the LCROSS simulator to make sure things were going to work. Within a few days we had a set of command products ready to go.
A day before the event, to save time on the scheduled observation DSN pass, we loaded the new command sequences to LCROSS and hoped we’d gotten everything right.
August 16 (DOY 229): Family Portrait: Observing the Earth and Moon
Our Special Earth/Moon Look Cal went off pretty much just as it should have. We collected images of both the moon and the Earth on all of the cameras, and met the full objectives set out by the Science team. There were some minor issues with payload heaters, and a small command sequence timing error, but we were satisfied with the results.
The photo here shows a family portrait taken by the LCROSS visible camera. At the time of the calibration, the LCROSS was approximately 520,000 km from Earth and 881,000 km from the moon.
August 19 (DOY 231): Re-Scaling our Faulty Coarse Sun Sensor
Recall from my previous post (“Our First Orbit Around the Earth”), one of our Coarse Sun Sensors (CSS 1) had been producing low signal outputs since Day 1 of the mission. Now “South” of the moon’s orbit plane, on the second half of our second orbit around the Earth, we took the opportunity of an open DSN contact period to finally load parameters to the spacecraft that would effectively re-scale the output of CSS 1 to have it match the output levels of the other CSS’s. This closed out another of our original spacecraft anomalies, and left LCROSS even safer.
August 20 - 21 (DOY 232 - 233): Planning for Cold Side Bakeout 3, Omni Pitch 4 and TCM 5c
We had a challenging few days ahead of us. The Navigation team had identified another good opportunity to perform a third Cold Side Bakeout (unfortunately on a Saturday). Then on Monday and Tuesday, we’d have a back-to-back execution of TCM 5c and another Omni Pitch maneuver. We generated, tested and reviewed commanding products for all three events on Thursday and Friday. Excited to observe our next Cold Side Bakeout, I elected to take the weekend DSN pass so that I could oversee the maneuver.
August 22 (DOY 234): Innocence Lost
I came onto shift on Saturday morning, very excited for a difficult, busy shift in overseeing Cold Side Bakeout. This third repetition was planned to look a lot like the first one, which ended up being pretty exciting and somewhat complicated. After the standard coordination with our Canberra antenna, we prepared for Acquisition of Signal or AOS.
We acquired telemetry, and ran through our standard set of health checks, but it was immediately clear that LCROSS was in trouble. Inertial Reference Unit (IRU) faulted, Kalman Filter faulted. Attitude control system using the Star Tracker for body rate information. Propellant tank low pressure fault. Thrusters firing nearly continuously.But this is a story in itself…you’ll have to read my next post to know what happened next!
Tags : General