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[aprssig] How gps works

Andrew Rich vk4tec at tech-software.net
Sun May 6 05:44:17 UTC 2012


And some other thoughts

Radio waves travel at different speeds in a vaccum to air as we know it.

I am sure what they did was get a known point and work out what the system 
is telling them and adjust the satellites to send the correct signals to 
give the correct answer.


Does that mean each GPS has a satellite tracking program running inside ?

If you loose your battery or move alot, you will need to spend 12 minutes 
downloading your "elements" again ?

Cold start ?

- 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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