[time-nuts] GPS at 60,000 feet

[Jim Lux]

On 1/31/13 1:09 PM, Chris Albertson wrote:
>> I know  for sure my handheld Garmin works at 27000 feet, at
>> 530mph...   ...I was actually surprised it worked up there.
>> It made me  wonder what the actual limits are.
>
> What are the limits of your hand held unit or what are the limits of
> GPS in general.   I think GPS works as long as you are under the orbit
> of the satellites.  The company I used to work for placed GPS on some
> low orbit spacecraft, so say roughly 200 miles up and 18,000 mph but
> I'd guess most hand held units would not work in those conditions
>

GPS will even (maybe) work at the Moon: with a gain antenna pointed back 
at earth.. you're looking at the satellites on the opposite side of the 
earth radiating around the limb.  I don't know that anyone has actually 
tried it but it's certainly been analyzed to death.

The potential problem with a handheld GPS in space (depending on where 
you are) would be whether you can keep track of the constellation and 
acquire new s/v's fast enough with lots o'Doppler.

You already have to deal with the Doppler from the S/Vs buzzing around 
at 3-4 km/sec.  Whether your receiver can handle the extra 7 km/sec 
Doppler in LEO is a good question.  7 km/sec is about 20 ppm, and I 
suspect that the receiver can already deal with that much change in the 
oscillator frequency.  It might be doppler rate that it would have a 
hard time with (because the designer cranked down on the loop bandwidth 
for noise reasons)

What are those folks flying GPS on CubeSats using?