OFJ Field Journal from Dave Atkinson - 10/22/95

Only 46 days to Jupiter!

It has been a long, exciting, and successful summer. We are now well into the fall semester at the University of Idaho and I am teaching one four-credit course. I am also teaching our department's research colloquium, a course that meets once a week and has a wide variety of speakers from our department, other departments at the University, other Universities, and industry. Last week Marcie Smith, the Galileo Probe Project Manager from NASA Ames was here to talk about the Galileo mission.

In my "free" time (that I don't seem to have nearly enough of) I am hard at work preparing, checking, and testing software for the Doppler Wind Experiment (DWE); this is the probe experiment for which I have primary responsibility. The DWE is designed to measure the winds at the location of the probe descent by looking at the Doppler Shift of the probe to orbiter signal frequency. To conduct this analysis requires that I know three things:

1) the location and velocity of the orbiter during the probe mission

2) the location and velocity of the probe, and

3) the frequency measurements of the probe signal.

Even without winds, there will be a Doppler Shift of the probe signal due to the movement of the orbiter, the probe being carried eastward by the planet's rapid rotation, and the probe descending on its parachute. If it were possible to somehow know the exact locations and velocities of the probe and orbiter, and to know the precise frequency of the oscillator on the probe, in principle we could model the probe signal frequency exactly. But when we make this model we expect it will not be in perfect agreement with the measured frequency. This is due in small part (we hope!) to the fact that we do not know the precise orbiter and probe trajectories. If this error is small, the leftover error is due primarily to the winds. And it is from this small frequency error (called the frequency residuals) that the winds can be determined. So to measure the winds I will need to obtain the orbiter trajectory data (location and velocity) from the project navigation team.

Additionally, the navigation team will supply their best guess of the probe trajectory. However, we can't get the precise probe trajectory until after analysis of the probe data. Most importantly, we'll need the information we get from the Atmospheric Structure Instrument (ASI). The ASI will measure pressures, densities, and temperatures. Then, based on what we know about how the physics of atmospheres (things like "The Law of Hydrostatic Equilibrium" and "The Gas Law"), we can then calculate the distance the probe travels from the entry point to the vertical descent location (where it is on parachute and making measurements) and the speed of the probe as it is falling on its parachute. But this precise data will have to wait until the ASI scientists have a chance to analyze the data - probably sometime in 1996. It is a little disconcerting to know that my preliminary wind analyses will be based on a model atmosphere that might not be what the probe really finds. And if the atmosphere model is not very good, then I know that the probe descent velocity on the parachute will probably be incorrect. In addition, a probe descending through the real atmosphere (unlike a model atmosphere) will be bounced and buffeted around; also it may feel updrafts and downdrafts (like in a thunderstorm). So my early measurements may not be very accurate.

Finally, the last data set I will need is the probe signal frequency as measured by the orbiter. Interestingly enough, out of these three data sets (the orbiter trajectory, the probe trajectory, and the frequencies), I should have the first two well before the probe arrives at Jupiter. Although the Galileo Navigation team promised to get the orbiter trajectory to me sometime in September, things always seem to be a little bit late. Especially when I am anxious to get them. I have also been promised the probe trajectory sometime in September. And, as of late September, neither had arrived. Of course, nothing can take the place of the probe signal frequency data - for that I will have to be REALLY patient and wait until December!