# How does a GPS reference satellite "know" it's own position so precisely? **URL:** \ **Category:** Factual Questions\ **Created:** [December 16, 2004, 4:26am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094 "2004-12-16T04:26:46Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 16, 2004, 4:26am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/1 "2004-12-16T04:26:46Z")
Re the GPS satellites that tell GPS users where they are. Re the positional information they braodcast, what allows to GPS satellite to know what _it’s_ location is with such accuracy relative to the earth? What GPS’s the GPS satellite? ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [December 16, 2004, 4:35am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/2 "2004-12-16T04:35:34Z")
Ground stations: [http://en.wikipedia.org/wiki/Gps](http://en.wikipedia.org/wiki/Gps) [http://en.wikipedia.org/wiki/Orbital\_element](http://en.wikipedia.org/wiki/Orbital_element) ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [December 16, 2004, 5:01am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/3 "2004-12-16T05:01:58Z")
They ask other GPS satellites where \*they \*are. ---
**Author:** ![jayjay](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jayjay/32/6765_2.png) [@jayjay](https://boards.straightdope.com/u/jayjay)\ **Post date:** [December 16, 2004, 5:19am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/4 "2004-12-16T05:19:29Z")
> [@CynicalGabe](#): > > They ask other GPS satellites where \*they \*are. Thus proving that GPS satellites are female. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [December 16, 2004, 5:40am UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/5 "2004-12-16T05:40:36Z")
Turn off the GPS, Luke. Trust the force! ---
**Author:** ![Kymodoce](https://avatars.discourse-cdn.com/v4/letter/k/8797f3/32.png) [@Kymodoce](https://boards.straightdope.com/u/Kymodoce)\ **Post date:** [December 16, 2004, 12:46pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/6 "2004-12-16T12:46:32Z")
You may be interested to know that the satellites broadcast their locational data (ephemeris) continually, but that this is updated after the fact to a precise ephemeris, with a latency of about 12 days. A better-than-broadcast Rapid Ephemeris is available after around 17 hours and Ultra Rapid data is issued 4 hourly. ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [December 16, 2004, 3:15pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/7 "2004-12-16T15:15:34Z")
> [@](#): > > Thus proving that GPS satellites are female. Or maybe they _know_ where they are, and don’t need to ask directions, if only the wife would stop badgering it long enough to get its bearings. And _no_ we are not low on gas and _am not_ tailgating the other satellites! ---
**Author:** ![pullin](https://avatars.discourse-cdn.com/v4/letter/p/c57346/32.png) [@pullin](https://boards.straightdope.com/u/pullin)\ **Post date:** [December 16, 2004, 6:13pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/8 "2004-12-16T18:13:03Z")
Don’t they broadcast a time-tag in the signal too? I assume my little GPS receiver is interested not only in where the satellite is; but _how far_ away it is. Right? Does it (my rcvr) use some sort of time-since-signal-was-sent calculation to determine the distance? Or am I all wet? Now another question (sorry). Since these elapsed times are very very small, how do the satellite and and receiver sync up? I have never figured this one out. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [December 16, 2004, 6:18pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/9 "2004-12-16T18:18:32Z")
I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about. ---
**Author:** ![Kymodoce](https://avatars.discourse-cdn.com/v4/letter/k/8797f3/32.png) [@Kymodoce](https://boards.straightdope.com/u/Kymodoce)\ **Post date:** [December 16, 2004, 6:23pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/10 "2004-12-16T18:23:45Z")
That is a question requiring a complex answer, **pullin**. Yes, the satellites include time data in their signals… in fact, timing is the very essence of GPS position fixing. Once your receiver has locked on to three or more satellites, it calculates an extremely accurate time (based on atomic clocks in the satellites) and this is used to determine the distances from the SV’s. ---
**Author:** ![Kymodoce](https://avatars.discourse-cdn.com/v4/letter/k/8797f3/32.png) [@Kymodoce](https://boards.straightdope.com/u/Kymodoce)\ **Post date:** [December 16, 2004, 6:26pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/11 "2004-12-16T18:26:07Z")
> [@Smeghead](#): > > I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about. Sorry, but this is wrong. GPS satellites are not in geosynchronous orbits, they move relative to the earth… quite rapidly actually. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [December 16, 2004, 6:27pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/12 "2004-12-16T18:27:25Z")
> [@Smeghead](#): > > I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about. GPS satilites are not in geosynchronous orbits. ---
**Author:** ![Tapioca\_Dextrin](https://avatars.discourse-cdn.com/v4/letter/t/3d9bf3/32.png) [@Tapioca\_Dextrin](https://boards.straightdope.com/u/Tapioca_Dextrin)\ **Post date:** [December 16, 2004, 6:33pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/13 "2004-12-16T18:33:13Z")
From [Garmin](http://www.garmin.com/aboutGPS/) (GPS receiver manufacturer) > [@](#): > > GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map…The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. ---
**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\ **Post date:** [December 16, 2004, 6:36pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/14 "2004-12-16T18:36:37Z")
> [@Smeghead](#): > > I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about. They’re not geo-sync. Each of the 24 satellites circles the earth twice a day. When you’re navigating with GPS, your receiver is constantly tracking “inbound” and “outbound” satellites - satellites that are coming into view and leaving view. [Link](http://www.gpscentral.ca/GARMIN%20What%20is%20GPS.htm) ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [December 16, 2004, 6:37pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/15 "2004-12-16T18:37:09Z")
> [@Smeghead](#): > > I would assume that since GPS satellites are in geosynchronous orbits, they don’t really need to worry about time lag. They’re always in exactly the same place relative to the earth. I’m probably completely wrong, though, for some obscure technical reason I don’t know about. They aren’t - read the Wikipedia link provided by **AndrewT** above. They orbit at about 12,000 miles, circling the Earth twice a day. However, their orbit is predictable, and they can calculate their position quite accurately given periodic corrections to their orbital parameters broadcast by the ground stations that have been mentioned. It’s the ground stations that continually monitor them and keep them accurate. ---
**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\ **Post date:** [December 16, 2004, 6:38pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/16 "2004-12-16T18:38:27Z")
Which is, apparently what like 15 other people just said. Man - you guys are quick. ---
**Author:** ![Roches](https://avatars.discourse-cdn.com/v4/letter/r/bc8723/32.png) [@Roches](https://boards.straightdope.com/u/Roches)\ **Post date:** [December 16, 2004, 7:31pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/17 "2004-12-16T19:31:44Z")
What about a GPS receiver? When you turn mine on (and I presume all others are like this), it shows where the satellites are in the sky. Does it know where they should be by taking into consideration the last position it knew it had, the time, and the orbits of the satellites? (i.e. calculating how much the satellites have moved since the receiver was last turned on) ---
**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\ **Post date:** [December 16, 2004, 8:12pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/18 "2004-12-16T20:12:19Z")
> [@Roches](#): > > What about a GPS receiver? When you turn mine on (and I presume all others are like this), it shows where the satellites are in the sky. Does it know where they should be by taking into consideration the last position it knew it had, the time, and the orbits of the satellites? (i.e. calculating how much the satellites have moved since the receiver was last turned on) Garmin has some online [manuals](http://www.garmin.com/aboutGPS/manual.html) in .pdf. From _The GPS Guide For Beginners_: > [@](#): > > The GPS receiver picks up two kinds of coded information from the satellites. One type of information, called “almanac” data, contains the approximate positions (locations) of the satellites. This data is continuously transmitted and stored in the memory of the GPS receiver so it knows the orbits of the satellites and where each satellite is supposed to be. The almanac data is periodically updated with new information as the satellites move around. > ~snip~ > So, having received the almanac and ephemeris data, the GPS receiver knows the position (location) of the satellites at all times. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [December 16, 2004, 8:34pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/19 "2004-12-16T20:34:17Z")
> [@Garmin](#): > > GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user’s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user’s position and display it This is a good explanation except for the use of the word “triangulation”. The accepted meaning of this is to use bearing (i.e. angle) measurements. But (as the quote says later) a GPS receiver is in fact using time-difference measurements - not bearings or their differences. Some reasons why GPS satellites are not in geosynchronous orbits: There is lots of demand for such orbits - space is rather crowded there Lower orbits are less expensive to reach A transmitter closer to the earth needs less transmit power Since the satellites move, a single earth-based control station can talk to all of them ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [December 16, 2004, 11:14pm UTC](https://boards.straightdope.com/t/how-does-a-gps-reference-satellite-know-its-own-position-so-precisely/280094/20 "2004-12-16T23:14:28Z")
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