A Question about GPS (Global Positioning System)?

Question

By now almost everyone knows that if you have access to a minimum of 4 GPS satellites (in 3 dimensions you need 4 satellites) then one can determine their latitude, longitude and altitude anywhere on the globe.

So the question is, how does a GPS satellite know its position?

Note: No wikipedia, google, yahoo etc., answer in your own words.

Answer 1

GPS satellites "know" their position because they have an ephemeris in their computers. In effect, this is a set of math equations where the computer starts with the current time, and the equations spit out the current position of the satellite.

The ephemeris is uploaded from the GPS ground station. The ground station figures out the equations by tracking the satellites, with radar and GPS's own signals, and projecting the orbit into the future. The ephemeris are also available on-line, see the link for example.

---- P.S. the GPS satellites are NOT in geo-sync orbit, and do not use any other satellites. The orbit period is about 11 hours and 58 minutes. And you do need 4, not 3 satellites, in order to get decent position accuracy and a good time estimate.

Answer 2

One would technically need 4 satellites, but only 3 are used. This would give 2 places a person could possibly be, 1 on Earth, and the other somewhere up in the sky or space; the system just omits the answer up in space. This means that the satellite has some way of monitoring distance, and can therefore tell how far away from Earth it is. Then, by using signals from the other satellites in the GPS system, it can tell where it is over Earth.

Answer 3

Great question! But (so far) a few wrong answers.

GPS satellites are NOT in geosynchronous orbit. They are in 12-hour orbits. Combined with the earth's rotation, this makes them appear to track around the earth's surface once every 24 hours. The satellite that is above your head right now will be below your feet 12 hours from now.

The satellites know their posistions by means of "almanac" and "ephemeris" messages which are sent from special ground stations. The almanac message is a "rough" description of the satellites' position, and the ephemeris message fine-tunes it.

These messages are then transmitted from the satellites to your GPS unit. Your unit reads & stores the ephemeris message each time you power up your unit (that's why it takes a minute before you're ready to navigate). The almanac message is "good" for a longer period of time, and your unit doesn' t need to read it unless it's been powered down for a long time.

Once your unit has this information inside it, it uses an internal mathematical model of the orbit (based on Newton's laws of motion and so on) to predict the satellite's exact location as it changes from second to second.

Answer 4

It doesn't know or care. On board each is an atomic clock, and it just broadcasts what time it has. The GPS unit in your car (or palm) receives the signals, determine which satellite is broadcasting those times, and it figures out (based on the slightly different times it hears from the 3 or 4 satellites), where it is on (or above) the Earth.

Answer 5

The GPS satellite doesn't need to know where it is. People on Earth need to know where it is.

I believe GPS works by signals from geo-stationary satellites. As long as the GPS unit knows which satellite emits which signal, then the whole global GPS system will work :-)

Answer 6

I'm not sure that "it" knows its position. GPS's do know the positions of satellite though. Exactly how...I'm not sure. But, just like I know the Hubble Space Telescope position over Tulsa, Oklahoma tonight will be over my head from 23:13:05 until 23:17:06, the ground base GPS units know the positions of the satellites that concerns their functioning. Not exactly an answer, but no satellite knows where it is, we do and our "gadgets" do. If your interested in any specific satellites and their locations, you can go to: www. heavens-above.com and enter your location to see what will be passing over.

Answer 7

GPS satellites are in a geosynchronous orbit, they stay where they are relative to a point on the surface of the Earth (mostly with very good accuracy). I don't know where this location is stored, on the satellite or the devices trying to figure out where they are, but they use these reference locations to determine where they are. I believe you only need 3 satellites to truly know your location on the planet. Each one will give you a radius, where the 3 intersect is your location.

Answer 8

The PS sats are in polar orbits, not in geosynchronous orbits. Each sat gets accurate timeignals from the ground and its own database then tells it where it is.