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Mon Apr 16 15:45:47 UTC 2012

from you can back track and work out where you are.

- Andrew -

----- Original Message ----- 
From: "Stephen H. Smith" <wa8lmf2 at aol.com>
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Sunday, May 06, 2012 2:28 PM
Subject: Re: [aprssig] How gps works

> On 5/5/2012 11:21 PM, Andrew Rich wrote:
>> Hello
>> I am learning how gps works
>> I understand that gps satellites contain accurate clocks
>> How does the receiver know the distance to the bird ?
> Basically, the very simplified concept:
> 1) All the super-accurate atomic clocks in all the satellites "tick" in 
> unison.    These "ticks" are "broadcast on spread-spectrum signals at 1575 
> MHz (for the civilian GPS service).  The signals for all satellites are 
> "stacked" on top of each other on the same center frequency.  Each 
> satellite uses a different spreading code, which allows it's signal to be 
> separated from others by the receiver on the ground.
> 2) The "ticks" for all the satellites travel outward at the speed of light 
> (300,000,000 meters/sec --or-- about 300 meters/uSecond --or-- about 0.3 
> meters/nanosecond.   I.e. about 1 foot / nanosecond.
> 3) Each satellite is also constantly broadcasting it it's OWN location. 
> (The US Air Force measures the precise location of each satellite several 
> times a day with ground-based radar, and uploads updated orbital data into 
> each satellite several times a day, which the satellites broadcast 
> continuously until the next update.)
> 4) The receiver compares the DIFFERENCE in time-of-arrival of "ticks" from 
> several satellites.   The receiver, based on knowing where each satellite 
> was at the time of the tick, computes the one location in 3D space where 
> this particular combination of delays, due to 
> differing-distances-traveled,    must be.     Differences of 5 or 10 nano 
> seconds in time of arrival  (corresponding to delta distance of  1.5-3 
> meters) are quite easily measured with basic logic circuits.
> 5)  Comparing the DIFFERENCE is easy; having a reference clock in a cheap 
> device, not endowed with a rubidium time standard, that can provide the 
> ABSOLUTE time to compare to, is not.   Instead GPS receivers "cheat" by 
> using the ticks from one satellite as the triggering absolute time 
> reference, and then compare three or more others against the first one.
> Compared to a reference time, the delay for a SINGLE satellite says you 
> must be somewhere on the surface of a sphere of approx 10,000 miles radius 
> from that satellite.    (GPS satellites orbit about 10,000 miles above the 
> earth's surface.)
> Measuring the delay for TWO satellites will define two intersecting 
> spheres. You must now be somewhere on the circle where the two spheres 
> intersect.
> Measuring the delay for THREE satellites will define three intersecting 
> spheres where only two unique points will match the time delays observed 
> for all three. One of these will be an obviously impossible solution 
> because it is located deep inside the earth, which leaves a single point 
> that is your location.
>> Does it look at the pattern coming from the gps sat ?
>> Can it work out when comparing the code from others sats to know the 
>> difference in time between the different birds, much like trying to line 
>> up a set of rulers ?
>> What does sending empheris data do to help ? Does that help the receiver 
>> picture the constellation ?
> More or less.  This is how the receiver "knows" where each satellite is at 
> a given instant.
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