"A Day in the Life of a Gravity Man" Bill Sjogren - March 17, 1997 Principal Investigator, Gravity Experiment, Mars Global Surveyor Jet Propulsion Laboratory, Pasadena, California |
Today was a very busy day. If you could have spent the day with me, here is a list
of some of the things we would have done together:
1. Evaluated Doppler and ranging data coming from the Surveyor spacecraft (now in
cruise phase), so the data during the orbiting phase will be high quality for gravity
parameter extraction.
Radio tracking of the MGS is done at a frequency of 8,400,000,000 cycles per second.
A shift in this frequency is a measure of the spacecraft speed. It's like a train whistle:
When the train is coming fast toward you, its whistle will have a high pitch; when it is
going away from you, it will have a low pitch. We can measure the speed of the MGS
spacecraft to better than one inch per hour (that's slower than a snail crawl!) when its
moving at a speed of 5400 miles/hour on its way to Mars.
Presently the spacecraft is rolling around the line toward Earth so as to keep its
attitude correct using its star sensors. However, it is not precisely rolling about this
line for we can see in the radio tracking data that it is off by 8 millimeters. This must
be corrected by the time we go into orbit about Mars for this effect will greatly degrade
our gravity field determination. I'm in constant communication with the spacecraft team
in Colorado to reduce this effect and send them plots of the data as evidence that they
have a problem. There is also a problem at the Deep Space Tracking station in Australia.
The ranging data are not being calibrated correctly, which causes the trajectory
computation to be in error and again this will have its effect on our gravity reduction.
2. Interviewed a new candidate to help program our H.P. supercomputer, where we will
be estimating, simultaneously, approximately 6000 unknowns with Doppler radio tracking
data.
Interviewing candidates for a job takes a lot of time. This is the procedure I must
follow: Following this process takes months to bring someone new onboard!
3. Made viewgraphs for a presentation to MGS Project on a low orbit that will produce
high-resolution gravity observations and new scientific results.
The best gravity-field observations are obtained when the spacecraft is at low
altitudes. This is because gravity is a direct measure of acceleration on a particle at a
particular location from it. So the closer the spacecraft is to Mars, the better the
variations in gravity are sensed. Gravity variations tell us something about the
internal (and also surface) mass distribution. Denser materials beneath the surface
will show more gravity, while craters on the surface will show less gravity.
Immediately after aerobraking the nominal plan is to circularize the spacecraft's
orbit to 400 kilometer altitude. My suggestion to the MGS Project is to not go to 400
kilometers, but go down to 200 kilometers for two weeks and then go up to 400 kilometers
for the mapping mission. I must convince the project that we will have excellent science
results and that the risks are worth the payoff and would not cause the mission to fail.
|
Back to Field Journals Menu | Bill Sjogren's BIOgraphy | A Day in the Life of a Gravity Man 1 |