GPS: how does it work?

jumpjack · Occasional Visitor Joined: Nov 03, 2005 Posts: 7

Ok, very newbie question... Laughing

I know a GPS receiver receives signals from several satellites 36000 km above our heads, but I'd like to know a bit more...

I heard that a GPS receiver requires less time to "get the fix" if it already have ephemerys (how the hell should I write it???) of satellites: why?.
What kind of data does a receiver use to calculate its position? The distance from the satellites?

How does it calculate these distances? From time the signals spent to reach it?

How can it determine this time? Basing on its internal clock?!?

Is there a GPS guide somewhere, ore something?

Thanks in advance.

MarkHewitt · Posted: Thu Nov 03, 2005 11:45 am Post subject:

http://www.pocketgpsworld.com/modules.php?name=FAQ&myfaq=yes&id_cat=1&categories=Basic+GPS+Theory

jumpjack · Occasional Visitor Joined: Nov 03, 2005 Posts: 7

Thanks.

1. All satellites have clocks set to exactly the same time
ok

2. All satellites know their exact position from data sent to them from the systems controllers
Quite strange... Looks like GPS sats receive THEIR position from ground controllers! Anyway, I don't care this...

3. Each satellite transmits its position and a time signal
ok

4. The signals travel to the receiver delayed by distance traveled
yes, but...

5. The differences in distance traveled mark each satellite appear to have a different time
Ok: how can my receiver know this difference? It does NOT have an atomic clock in it, so if its clock is not correct, it can't calculate its position??
I don't understand.

6. The receiver calculates its own position.
Yes, if it knew its distance from sats... Laughing

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peterc10 · Posted: Thu Nov 03, 2005 3:32 pm Post subject:

Suggest you look here http://electronics.howstuffworks.com/gps2.htm

lbendlin · Posted: Thu Nov 03, 2005 3:43 pm Post subject: Re: GPS: how does it work?

jumpjack · Occasional Visitor Joined: Nov 03, 2005 Posts: 7

jumpjack · Occasional Visitor Joined: Nov 03, 2005 Posts: 7

I think I understood (eventually!!! 8O )

1) Four satellites send each one a "data package"; this datapackage contains the time of its sending;
2) Receiver adjust its clock;
3) Satellites send another datapackage;
4) Now receiver can compare its clock to received times, and determine its distance from satellites.

Right?

MarkHewitt · Posted: Mon Nov 07, 2005 12:04 pm Post subject:

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