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Just How Big is This Maneuver, Anyway?by Pieter Kallemeyn
April 29, 01:00-05:00 PM
[This is part two of a four-part series on the design and execution of Mars Pathfinder's third trajectory correction maneuver (TCM-3). In my last journal I spoke at length about what is involved in determining the orbit of Mars Pathfinder by examining tracking data, the first step in planning a TCM. The next step is determining the characteristics of the maneuver itself, specifically the size and direction of the maneuver. - PHK]
A few hours ago we determined where the spacecraft is through orbit determination. It is now after lunch and we start the next stage. We can take the orbit solution and project it forward into the future to predict where the spacecraft will be when it gets closest to Mars. This gives us our best guess of where Mars Pathfinder will be two months from now. This prediction shows that on its current course, the spacecraft will come closest to Mars on July 4, 1997 at 16:52:52 GMT, and at that time it'll be 169.3 km from the surface. Since the atmosphere of Mars is less than 150 km, this means we're on what we call a flyby trajectory. If we did nothing else for the rest of cruise, we would fly right past Mars, never entering the atmosphere and never landing. Clearly we have to do *something* at TCM-3 in order to land successfully.
But what exactly do we do? First, the navigation team must determine what the target is for this maneuver. The target is simply the coordinates required to enter the atmosphere at the right time and at the right angle so that the entry vehicle will slow down in the atmosphere, the parachute deploys on time, and we land where the mission needs us to land. To find this target, we run a program called AEP, short for the Atmosphere Entry Program. Our local expert on atmosphere entry is David Spencer, a member of the NAV team, so he performs this analysis. David takes the desired landing site along with a computer model of the Martian atmosphere and the entry vehicle, and computes the target at the top of the atmosphere. After about an hour of computer time and some consultation with another atmospheric flight expert, (Bobby Braun in Langley, Virginia, who is a part-time member of the NAV team) we have our answer. The spacecraft must enter the atmosphere at an altitude of 125 km and flight path angle of 14.2 degrees below the horizon at exactly 16:51:49 GMT.
Now we have the target, so we combine it with our current orbit solution in a program that determines the size and direction of the maneuver. We have to input the time of the maneuver, which we know will be May 7 at 01:00 GMT, based on the tracking schedule. The answer is (...drum roll please...) a maneuver size of 10.5 centimeters per second. This is a very small maneuver, approximately 1/4 of a mile per hour. Despite its small size, this maneuver will alter our trajectory by over 200 kilometers over the next two months.
The results of the maneuver calculations are passed along to Sam Sirlin, who will determine the parameters needed for the commands we'll send to the spacecraft. It's his job to take this maneuver size and direction and convert it into a language the spacecraft will understand. This is what the language looks like:
1997-127T00:15:49.000 J2000_TRACK 0.956677 -0.265867 -0.118676 1200
These are command parameters and they're part of a group of commands called a sequence. Don't worry about understanding this... I'll explain it later.
April 30, 1997 - TCM-3 Design Status Meeting
At 10:00 today, members of the Flight Team involved with the design of TCM-3 gathered in a conference room to go over the game plan for the maneuver, now less than eight days away. The person in charge of the meeting is Guy Beutelschies, the flight engineer responsible for building and testing the maneuver sequence. The rest of the people in the room represent Navigation, the various spacecraft subsystems, and the mission manager Richard Cook. This meeting is our first chance to discuss the TCM-3 plan in detail, ask each other questions, and obtain a consensus on the sequence of events for this activity.
The meeting starts with an explanation of the orbit determination, which Robin Vaughan presents. Then Guy describes the sequence of events. Normally, we would execute this maneuver with a combination of two maneuvers. The first would be along the direction of Earth (called the axial mode) and the second would be along a direction at nearly a right angle to the Earth direction (called the lateral mode). However, we want to do something new with this maneuver. We want to test the capability to perform a 0.4-meter-per-second maneuver in the lateral mode. Why? Because on July 4 there is a slight possibility we may need to do such a maneuver to correct our flight path, and we haven't yet tested that large a maneuver in the lateral mode. In order to assure ourselves that we can rely on such a maneuver in the last few hours before approach, we want to test it out while we perform TCM-3, 60 days before we land. This will give us plenty of time to understand how the spacecraft behaves in such a test, and to debug any problems we may encounter. A good deal of spacecraft operations involves testing the capabilities we later need to rely on for doing the real mission.
Therefore, the team decides to do the maneuver as follows: We will first update the attitude of the spacecraft so we are pointed 5 degrees from Earth, a routine activity we've done many times in the past. About an hour later we will perform a lateral mode maneuver of 0.4 meters per second. An hour and a half later, we do a .1-meter-per-second axial maneuver (toward Earth), and finally another lateral maneuver of 0.5 meters per second, an hour and a half after that. The last maneuver is in the opposite direction from the first one, so this is what I like to call a zig-zig-zag maneuver. We zig a bit to the left, zig a little bit forward, and finally zag a bit more to the right, so to speak.
The rest of the status meeting is devoted to other concerns: What data rate shall we be at? Who will notify the ground stations? What emergency actions should we be ready to perform if something goes wrong? Etc., etc. Since we've done two maneuvers before (on Jan 10 and Feb. 4) we know a lot more about this type of activity, so the meeting goes pretty fast. The meeting ends after an hour. The next step is for Guy to build the sequence of commands that will tell the spacecraft what to do on May 7 and to test it on the ground before sending it to the spacecraft.
Next journal... Building and Testing a Maneuver Sequence.