[aprssig] Does GPS work in space ?

[Robert Kirk]

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.

Bob Kirk
N3OZB