"What Navigators Need to Know" Pieter Kallemeyn - June 19, 1997 Navigation Team Leader Jet Propulsion Laboratory, Pasadena, California |
Right now we are preparing for our final maneuver, Trajectory Correction Maneuver
#4, which will occur June 25. We must to be able to get the best current orbit model
possible using the tracking data up to the last minute. What I'm doing right now is
preparing an input file for the orbit determination program. This file describes the
variation in the spin rate of the spacecraft since February. One of the things we have to
be able to know and model in the program is is how the spacecraft is spinning because that
affects how the tracking data looks.
The spacecraft controls how fast it spins. It receives ongoing instructions that say
it is supposed to be within so many rpms. On June 8 at 23.46.46 UTC, Pathfinder was
spinning at 1.957 rotations per minute. That's the value that the spacecraft thinks it is
spinning at. It travels twice as fast as a second hand on a watch. This is on the list of
things that navigators need to know!
Pathfinder is now within 100 km of its landing site. We did an orbit determination
solution last week for the purpose of doing a preliminary design of TCM #4. The center of
that solution, when mapped to the surface, is within our requirement ellipse, or
footprint. The footprint measures 200 km from one edge to the other and 100 km from edge
to edge in the other direction. What we want to do is put Pathfinder in the center of the
ellipse as best as we know it. The chances of doing this are pretty good. Imagine there
are two football-shaped ellipses: one is our requirement of what we want to be inside and
the other is our current orbit knowledge. These two footballs are offset from each other
by about 60 km off target. We want to be within 100 km in the worst case. We want to move
those footballs so they're one on top of the other. This will be done with a course
correction.
TCM #4 be the smallest maneuver we've done on Pathfinder--.028 meters per second (the
velocity change). Once Pathfinder receives the command it takes only a few seconds for
this change to take affect. We'll develop a sequence that will be clocked off at a given
time and sent to the spacecraft hours before the event. Then as Pathfinder is racing
toward Mars it will activate the command at the appropriate time.
Maneuvers are performed in one of two modes: turn and burn, or vector. In the turn and
burn mode the spacecraft is turned in the direction we want the velocity change to occur.
A pair of thrusters is fired for a predetermined period, which results in a velocity
change. The vector mode is a maneuver that is performed in two parts: axial, which is
along the direction of the spin axis, and lateral, which is roughly 90 degrees from that.
The interesting thing about a lateral maneuver is since the spacecraft is spinning and
the thrusters are mounted on the spacecraft, the thrusters have to be pulsed at given
times. An axial maneuver is a continuous burn. If the lateral thrusters were continuously
fired, they'd cancel each other out after one revolution. We wait until the thrusters are
at just the right orientation in the spin and then we fire them for a few seconds and
turn them off. The spacecraft spins another half a rev until the other thruster cluster
comes into the same orientation and then the thrusters are fired. Basically, thrusters
are fired every half rev.
The Deep Space Network (DSN) is measuring Pathfinder's doppler and range on a
continuous basis. We receive our tracking data from the DSN in batches--one a day. The
Radiometer Data Conditioning group looks at the data first, applying any adjustments that
they think ought to be made. They do some editing, some error checking and data
accountability. When they believe the tracking data are good enough, they package it into
a particular binary format and send us an email message that the data are ready to be
picked up. We capture it via an ftp transfer and then add it to the master tracking data
file, which contains data we've received since launch.
The output arrives around 10 a.m. every day, afterwhich it takes us about 30 minutes
to do one orbit determination run. This is the first step in doing navigation -- using
the tracking data with a model on the ground of what the trajectory is like. We have a
program that uses that trajectory model and the timing of the data points on the tracking
file. From this we can come up with a predicted value for each one of those points. This
is what we call the predicted tracking measurement.
Point for point, we compare the predicted measurement with the actual measurement
taken at the DSN. If everything was done correctly, the error ought to be zero. But
there's always a little bit of error due to a number of reasons: our trajectory isn't
perfect and there are errors outside of navigation's control that we have to take into
account. For example, the atmosphere distorts the signal so we have to calibrate for that.
We also have to calibrate for Earth's rotation. We're also modeling the spin rate of the
spacecraft. So we're getting data from a bunch of places, incorporating it into the model,
and looking at the residuals (the observed value minus the computed value). It takes the
difference of those two values and determines what the true updated trajectory ought to
look like. The end of this is a trajectory file. From the trajectory file we know where
we're going. First you have to figure out where you are and where you want to go and the
TCM takes you from one to the other. .
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