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A Day in the Life of a Gravity Manby Bill Sjogren
March 17, 1997
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.