OFJ Field Journal from Lou D'Amario - 10/9/95
PRACTICING FOR MANEUVER DESIGNS AND TWEAKSThe Navigation (NAV) Team has been very busy preparing for events occurring during Galileo's final approach to Jupiter and the Jupiter encounter. (The Galileo atmospheric Probe and Orbiter both arrive at Jupiter on December 7.) There are three small trajectory correction maneuvers (TCMs) scheduled to adjust the flight path of Galileo before Jupiter arrival: TCM-27 on November 17, TCM-28 on November 27, and TCM-28A on December 2. The large Jupiter Orbit Insertion (JOI) burn, which slows down Galileo so it will be captured into orbit about the giant planet, occurs on December 7 (Pacific time), about one hour after Galileo is finished receiving data from the atmospheric Probe. Then the first orbit trim maneuver (OTM-1) will be performed on December 9 to correct any errors in the first Jupiter orbit. The NAV Team must determine the necessary velocity changes for each of these maneuvers.
Usually, the Galileo Flight Team has about 7 days to do the work of generating the spacecraft commands for a maneuver (called a "sequence"); this time includes the NAV Team work to calculate the desired velocity change. However, the Flight Team has only 24 hours (working around the clock) to generate the last maneuver before Jupiter arrival (TCM-28A) and the first maneuver after arrival (OTM-1). Similarly, we will be computing a late update (called a "tweak") for both TCM-28 and JOI on the last day before each of these maneuvers, using the same 24-hour schedule. There will be many tired people on the Flight Team after the Jupiter encounter period! The Flight Team must practice these 24-hour maneuver designs and tweaks before they happen to be sure so that there will be no problems. When the Flight Team practices a maneuver design or maneuver tweak, it's called "test and training."
For the past several weeks, I have been heavily involved in test and training. I am the NAV Team coordinator for three test and training exercises. I have to supply a schedule to the Project test and training coordinator showing how much time each team (such as the NAV Team or the Orbiter Engineering Team) has to do their part of the work for a maneuver design. I also have to supply the names of people from the NAV Team who will be doing the test. During the test, I monitor the work of the NAV Team. (And, if I have the time, I do some trajectory or maneuver design work myself!) Then, after the test is finished, I have to give a presentation to the Project managers on how the NAV Team did, what problems we had, and suggestions for improving the process. The first test and training exercise was done during the week of September 18 to test the generation of the TCM-28 tweak. The second test and training exercise was done during the week of October 2 to test the 24-hour maneuver design used for TCM-28A and OTM-1. Overall, both of these tests were successful, although there were some minor problems (not unexpected) that need to be worked out. The third and final test and training exercise will be done near the end of October to test the JOI tweak process.
Normally, I have quite a few meetings to attend each week. Some of them have had to do with preparing for and conducting the test and training exercises I mentioned earlier. I also attend regular meetings that are held each week. There is a weekly NAV Team meeting where we discuss the activities that people on the NAV Team are currently working on and what work is coming in the future. Each week I also attend a TCM Design Team Meeting and a Relay/JOI Working Group Meeting. At the TCM Design Team Meeting members of the Flight Team discuss TCM design issues. The Relay/JOI Working Group is made up of the people who are responsible for producing the critical sequence that controls Galileo during the time it is receiving data from the Probe and performing the JOI maneuver. (This is the most important sequence the spacecraft will ever execute. Receiving data from the Probe and getting Galileo into orbit about Jupiter are prime mission objectives that must be completed successfully This sequence is designed to work in the presence of numerous spacecraft failures and to continue to issue commands regardless of what is happening to Galileo during this critical time.) There are usually several informal meetings each week of NAV Team personnel to discuss navigation strategies for the upcoming Jupiter encounter. Other meetings I have to attend are approval meetings for the sequences that control the spacecraft during Jupiter approach and the orbital tour. I must make sure that these sequences have been checked by NAV Team people to see that the expected navigation tracking coverage, optical navigation pictures, and maneuvers have been put in the sequence correctly.
Last month I also assisted the NAV Team Chief in preparing a memo containing the NAV Team budget for the remainder of the baseline Galileo mission, which ends in December 1997. This involves figuring out the number of people needed and the cost for the work the NAV Team has to do.