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jumpjack Occasional Visitor
Joined: Nov 03, 2005 Posts: 7
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Posted: Thu Nov 03, 2005 11:18 am Post subject: GPS: how does it work? |
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Ok, very newbie question...
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. |
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MarkHewitt Frequent Visitor
Joined: Nov 16, 2004 Posts: 1077 Location: Chester-le-Street & York
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jumpjack Occasional Visitor
Joined: Nov 03, 2005 Posts: 7
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Posted: Thu Nov 03, 2005 12:06 pm Post subject: |
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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... [/b] |
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peterc10 Frequent Visitor
Joined: Aug 21, 2005 Posts: 1761 Location: Kent, England
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lbendlin Pocket GPS Staff
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
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Posted: Thu Nov 03, 2005 3:43 pm Post subject: Re: GPS: how does it work? |
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jumpjack wrote: | I know a GPS receiver receives signals from several satellites 36000 km above our heads |
Nope, that's not how it works. GPS satellites are not geostationary, they circle the earth every 12 hours (plus/minus), and are on an orbit of roughly 12000 km.
If you get differing time signals from different sats when the time should be the same from all then you can use this information to calculate your position. _________________ Lutz
Report Map Errors here:
TomTom/TeleAtlas NAVTEQ |
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jumpjack Occasional Visitor
Joined: Nov 03, 2005 Posts: 7
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Posted: Thu Nov 03, 2005 7:59 pm Post subject: |
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Great site, thanks! |
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jumpjack Occasional Visitor
Joined: Nov 03, 2005 Posts: 7
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Posted: Fri Nov 04, 2005 8:25 pm Post subject: |
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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? |
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MarkHewitt Frequent Visitor
Joined: Nov 16, 2004 Posts: 1077 Location: Chester-le-Street & York
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Posted: Mon Nov 07, 2005 12:04 pm Post subject: |
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jumpjack wrote: | 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? |
There is a lot of other jiggerypokery that goes on but that's bascially it, yes. |
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