[aprssig] Does GPS work in space ?
isobar at bcpl.net
Sun Apr 23 14:56:14 CDT 2006
At 07:44 PM 4/23/06 +1000, Andrew Rich wrote:
Yes, GPS works beautifully in space at sun sync orbit. I ran a project at
NASA to fly a relatively cheap and cheaply budgeted ocean viewing
satellite. You can keep track of where a satellite is most conventionally
with active transponders or in a pinch with passive "skin tracking" with
radar - the Air Force likes to know where non-transponding objects are. Not
wanting to pay the expense of tracking, we became the first (maybe still
only) satellite in NASA to do self tracking only with a couple of GPS
receivers, using what is now, 15 year old technology receivers. However,
they were able to ignore the height/speed security limits as well as the
accuracy limits then in effect. At 705Km altitude you lose very little of
the geometric accuracy with shallower receive angles, but in return get
great signal strength and no refraction or multipath problems. The
position accuracy turned out to be better than active transponder tracking.
I have not worked on, but have heard, that new satellites can use several
highly accurate receivers on board, not just to determine position, but
actually determine 3 axis attitude by the difference in arrival times. That
would save problems with pesky horizon sensors and expensive star sensors.
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