On 11/11/08 2:55 AM, "Brian Kirby" <[EMAIL PROTECTED]> wrote:
> Somewhere out there is the specs that GPS was designed to. It list some
> of what they had to do, to make the rubidiums and cesiums work in the
> environment they put them in. Believe they are called ICD-GPS-200 or
> something like that
>
ICD-GPS-200 is the spec that defines the interface among the various GPS
segments, specifically, the RF waveform and nav message is defined there.
I don't know that it gives the environmental requirements for the s/v
hardware.
In any event, space qualified GPS receivers are an "off-the-shelf" item (as
much as anything space qualified is) and will set you back a good chunk of a
million dollars, by the time you get it and the accompanying paperwork.
For that matter, flight qualified Xos and OCXOs are readily available, and
much cheaper, but 1E-10 is a pretty stringent tolerance. Check into the
UltraStableOscillator (USO)s made by, e.g., Applied Physics Lab, which are
used for deep space missions.
Such things are used for doing precise measurements, not only of of s/c
position, but also radio science (occultations, gravity measurements),
although two way coherent ranging is also done. For the latter, you
basically send a signal locked to a hydrogen maser signal to the spacecraft,
where it is recovered and used to synthesize a return signal. A typical spec
might be ADEV<4E-16 in 1000 seconds. (and, why yes, verifying that the box
sitting on the bench can do that performance is a non-trivial matter)
Jim Lux
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