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From: Brooke Clarke (no email)
Date: Sat Oct 04 2003 - 13:50:19 EDT
Hi Jared:
They use an elliptical model for the orbit.
GPS is a decedent of an idea that came directly from Sputnik. I can
still remember the sound of the signal and can still see in my mind's
eye the loud speaker at my high school from which the beep beep sound
came.
Some guys at Princeton measured the doppler shift on the signal and were
able to determine the satellites orbital parameters. They quickly
figured out that if a satellite was transmitting it's orbital parameters
then someone could figure out where they were. The Transit satellites
were put up based on this for updating the inertial nav systems in
missile subs. There were some problems: The receiver needed an atomic
clock (read $$$$) and it took maybe 15 to 30 minutes of reception to get
the doppler curve.
GPS was designed so that you don't need an atomic clock, but do need to
track 4 sats, 3 for position and 1 for time. And you don't need to wait
for a doppler solution, but instead you use direct ranging to each sat.
I've been paying with some 1900 Golf War vintage Trimble Trimpack GPS
receivers, and within minutes of a warm start it knows within less than
10 feet it's Lon and Lat but it takes maybe 15 minutes for it to figure
out it's elevation within 10 Feet. But there is a disagreement between
Trimble and Motorola about the elevation when they are connected to the
same antenna. Still have not figured why.
Have Fun,
Brooke Clarke, N6GCE
Jared Sherman wrote:
>Trevor-
> <I don't know the details of this case but I imagine that it is because
>the satellites provide more accurate measurements >
>
> I think the new accuracy of satellites comes from the fixed orbital physics involved. The path of the satellite, the spherical nature of it (or not<G>), can all be defined, predicted, and conformed by observations from one orbit to the next. From various ground stations with telescopes and synchronized atomic clocks. Or in the case of GPS, onboard atomic clocks.
>
> Then the satellite, and precisely timed observations of it, provide a much more precise "rod and chain" to measure the underlying distances with. An invisible rod and chain, but a surprisingly rigid one.
>
>
>
>
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