[vsnet-chat 3735] Re: why is Galileo looking at delta Vel?

[Bish Ishibashi <bish@howdy.gsfc.nasa.gov>]

Fraser, 

For the latest interplanetary missions, NASA (or its PI institution
elects to) use at least two functionary attitude and articulation 
control systems: Sun sensor is one to lock on the Sun's coordinate, 
and star scanner is the other. The latter namely tracks not only 
Canopus but a number of other stars at the same time. I don't recall
the number (interplanetary missions are not exactly my expertise), 
but the scanner needs quite a few bright stars to "learn" its attitude
and maneuver. [Even for the HST (not cruising interplanetary space), 
fine-locking guidance requires a handful of acquisition stars in the field
of view of Fine Guidance Sensor.] So capturing the Sun and Canopus is
not nearly enough for reliable interplanetary navigation. 

On the issue of telemetry, a certain bandwidth is usually reserved 
for engineering/housekeeping telemetry. That's aside from science
telemetry (i.e., primary mission data). These housekeeping telemetry 
contain the information on temperature, roll and pitch angle, 
radiation monitor, etc. etc., and most probably *count rates* of 
acquisition stars captured by star scanner in order to navigate. 
My educated guess is that the information on acquisition stars is 
stripped down to a minimum; i.e., most likely it only contains the 
position(*) (x,y) and integrated count rate of an acquisition star detected. 
If true, then that doesn't take up much bits. Like John asks back, 
if we do not know the relative brightness of an acquisition star, 
we generally do not know what a star scanner is locking at. And in 
a case when a satellite lose its lock and "safed" incorrectly, 
it helps tremendously to *guesstimate* to which direction its antenna 
is pointed if we have an extensive set of data from navigation/attitude-control 
department (of course that's really a worst case scenario...). 


(*) It shouldn't be that simple in reality. This isn't a star tracker. 


I speak based on educated guesses gained from other space missions, 
not as a fact. Please take the comments with a grain of salt. 
But if you need me to find out more explicitly, then you will have 
to tell me your address where I can send a bill for my consultation fee ;-). 

Cheers, 

Bish Ishibashi

PS. Besides, there aren't much to look at between the Earth and Jupiter. 
    I bet some plasma monitor and some other instruments had been turned
    on for calibration purposes and such. How unfortunate they couldn't open
    that high gain antenna...that'll lower the bit rate quite a bit 
    at a distance of Jupiter.

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