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[aprssig] How gps works

Andrew Rich vk4tec at tech-software.net
Sun May 6 05:49:27 UTC 2012

Does seeing "part" of the sky make a difference ?

If I sit beside a buidling were I can only see 1/2 the sky does my position 
skew ?

- Andrew -
----- Original Message ----- 
From: "Andrew Rich" <vk4tec at tech-software.net>
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Sunday, May 06, 2012 3:47 PM
Subject: Re: [aprssig] How gps works

> So its a bit like multi latereation but in reverse
> You are being hit with lots of signals that were sent at the same time.
> From the diffreence between when you get "hit" and knowing where they came 
> 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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