How does GPS work?

Question

Can anyone explain it in simple but comprehensive terms?

The main question is:

What pieces data do GPS satellites send to the receiver?

And then an explanation of what else goes on if the answer isn't obvious.

Answer 1

Bouncing signals off Satellites,.

Answer 2

he location of the satellites is known to great precision. Each satellite continuously transmits it's time and position, which receivers pick up, and use to calculate it's precise position and time. At a minimum, four satellites are needed to establish the position of the receiver.

Answer 3

GPS works by having 24 satellites in what is called a GEOSYCHRONOUS ORBIT above the earth. These satellites orbit the earth such that they stay over the same location on the earth at all times.



The GPS Receiver on the ground listens to at least three and preferably four satellites. Each satellite sends out a signal telling the time it sent the signal. The GPS receiver on earth measures the time it took the signal to reach it from teh satellite, and using this information can figure out how far away the satellite is.



Using three satellites, the GPS receiver on the ground can figure out where it is using a set of formulas known as TRILATERATION (which is slightly different from triangulation). Using FOUR satellites, it can also tell your altitude (how high you are off the ground), and so GPS can ALSO be used for airplanes, Low Earth Orbit (LEO) satellites, and missiles. I understand that there are even some artillery shells with GPS in them!



Since the GPS receiver on the ground only LISTENS TO the satellites (it does not actually "talk to" them), GPS is known as a PASSIVE SYSTEM. SInce GPS uses Very High Frequency radio waves, and the receiver doe not have to transmit, the ground receiver can be very small and compact, and take little power -- like what would be needed inside a Cell Phone.

Answer 4

If none know how they work and you have looked at the links, then why do you keep asking? The receiver calculates to the earth geode in its software once it partially computes the location - don't have to know altitude - obviously each satellite identifies itself as well as sending current atomic clock time - objecting to this as major omission is just being a troll.

Answer 5

"Obviously this is impossible."



nonsense. gps works and it works very well. i use it when i fly my plane. i use it when i drive my car.



i'm not going to type in what is already well-documented. the wikipedia article (link) is good. each satellite signal places you on the surface of a sphere centred on the satellite. with three signals you have three intersecting spheres, a point. in practice you need four, to know *when* the spheres intersected. if you have more signals (as you usually do) you can do a least-squares fit.



later: if you know the time you can solve three pseudoranges for (x, y, z). if you don't you solvefour pseudoranges for (x, y, z, t). hence the requirement for four satellites to get a 3d fix, and you get accurate time for free. i work with communication systems that use gps for microsecond-accurate time.



still later: the gps navigation signal really is just a sequence of "now...now...now..." signals. the satellite adds information to say who is saying "now" and exactly when "now" is. the receiver calculates where the satellite was when it sent its signals (the satellites send the necessary data in a separate data stream), compares when it received the signals with when they were sent, and calculates its own position.



part of the calibration work the u.s. naval observatory does is determining the positions of the satellites so they can broadcast the necessary data to gps receivers. they use specialized orbital models that are a lot more complicated than sgp-4 and related orbital models folks like nasa use.



some of us really do know how this stuff works.

Answer 6

There are many specialized satellites in orbit around the Earth, and each satellite has an atomic clock synced with all other satellites and ground clocks. The location of the satellites is known to great precision. Each satellite continuously transmits it's time and position, which receivers pick up, and use to calculate it's precise position and time. At a minimum, four satellites are needed to establish the position of the receiver.





*EDIT* - The satellites send their position in orbit (which is a precise orbit that it does know), and the time based on the atomic clock on board the satellite (synced to all other GPS satellites, and ground atomic clocks). Your receiver just has a basic quartz clock, which gets reset based on the time received from the satellite, and calculated to the nanosecond based on how far away from the satellite the receiver is.



The receiver only knows how far away from the satellite it is, so it needs other satellites to determine how far away from those it is. Where the radius of the circles around which satellite intersect, that's where the receiver is. The more satellites it can connect to, the more accurate the position of the receiver can be determined.

Answer 7

So, basically, there is a bunch of satellites in very precise orbits around the Earth. They're all basically atomic clocks, broadcasting the exact time, everywhere.



Your GPS unit listens to the broadcasts. Because the time from one satellite will be slightly different than another - because the distance it's signal has to travel differs, your GPS unit listens to the differing times, and calculates where you are.



Edit: Impossible...?



So, your unit knows the orbit of the 24 satellites in the constellation, based on the time of day. It receives a signal from, say, 4 of them:

Sat 1: 12:49:52.000093

Sat 2: 12:49:52.000087

Sat 3: 12:49:52.000091

Sat 4: 12:49:52.000097



Then, your unit uses something like the Pythagorean theorem to decipher where it is. Since they're all broadcasting the same time - and, we're *receiving* it with differences - we know that we're closest to Sat 3, and furthest from Sat 2 - and, we can calculate that difference easily. 2 satellites will give our position on a plane, in a circle; 3 will determine where our location on that circle is, and additional satellites help further pinpointing our location. You don't need 7, but it would be more accurate if you did.



Here.... all your questions answered: http://www.physics.org/article-questions.asp?id=55