FIELD JOURNAL FIELD JOURNAL FIELD JOURNAL FIELD JOURNAL
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
1997-127T01:00:00.000 LATERAL_TCM 37.9436 -0.348121 -0.828953 -0.437778
0.956677 -0.265867 -0.118676 AB 450
1997-127T01:26:54.831 J2000_TRACK 0.956677 -0.265867 -0.118676 800 1997-127T02:02:45.831
AXIAL_TCM 20.6263 0.956677 -0.265867 -0.118676 MINUSZ 885
1997-127T02:35:22.457 J2000_TRACK 0.956677 -0.265867 -0.118676 800 1997-127T03:11:13.457
LATERAL_TCM 46.8061 0.231928 0.85967 0.455167 0.956677 -0.265867 -0.118676
AB 495
1997-127T03:39:19.875 J2000_TRACK 0.956677 -0.265867 -0.118676 800
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.