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Re: [gpsd-dev] PPS over USB

From: Jim Thompson
Subject: Re: [gpsd-dev] PPS over USB
Date: Wed, 2 May 2012 07:11:07 -0500

On May 2, 2012, at 2:58 AM, Hal Murray <address@hidden> wrote:

> address@hidden said:
>> Clock time should be OK even with a single sat - multiple birds are needed
>> for the spherical trig to compute position, but atomic time you get directly
>> from each sat sample. 
> I think it's more complicated than that.
> You can get time with only 1 sat if know where you are located.  It takes 
> special software, usually called "timing".
> The usual approach for finding the location is to do a "survey".  That 
> requires more special software which is usually included in the timing 
> package.  It collects a 1000 (or whatever) good samples and averages the 
> position.  The "good" qualifier means they don't use low-quality samples 
> which might be way off.
> It's worth asking Navisys if they have the timing mode software.

A GPS receiver functions by precisely measuring the transit time of signals 
received from several satellites (the pseudoranges). These distances combined 
geometrically with precise orbital information identify the location of the 
receiver. Precise timing is fundamental to an accurate GPS location. The time 
from an atomic clock on board each satellite is encoded into the radio signal; 
the receiver determines how much later it received the signal than it was sent.

Thus, even a very small clock error multiplied by C (the speed at which 
satellite signals propagate) results in a large positional error.  An error of 
one microsecond corresponds to an error of 300 meters. Therefore, GPS receivers 
solve for the time using the pseudoranges.

tl;dr: One bird won't do.


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