How it works
Please bear in mind that the GPS is a very complex system. It cost the US government US$16 billion (at 1995 rates) to install the whole system and get it running, and US$6 billion (at 1995 rates) more to keep the entire system up and running till the year 2016.
The 24-GPS satellites are in geosynchronous orbit over the Earth, giving coverage to all areas at all times. Assuming you are not in an area which the US government blocks from GPS coverage, all you need to receive the signals is a GPS receiver and knowledge on how to interpret the readings. GPS receivers are available commercially. Companies such as Garmin and Trimble .
Given that you are in the open and there are clear skies, the GPS receiver will begin broadcasting a signal to the GPS satellites, and the satellites will respond by sending back the same signal, but with a time lapse (due to the radio waves travelling back and forth). Once 3 satellites have responded in this manner, the receiver calculates your position by triangulation. This is possible because the positions of the satellites are fixed in the sky (due to their geosynchronous orbit) and the receiver has the positions of all 24 satellites stored in its memory. That is why when you move from region to another, you have to set the GPS receiver to that country's settings, as this will greatly increase the speed at which the receiver acquires the satellite information ie the satellites above any given region is fixed at all times, therefore the receiver need only search for those satellites above it when in any given region.
Confused? Don't be. Here's a simplified version. Note: Radio waves travel at
Location of S2
Position of S1
Position of S3
Receiver (R) sends a signal to Satellite 1 (S1)
And S1 sends the signal back, but with a time lapse, say 0.04s.
Therefore, distance between R and S1 would be 0.04s/2 x 3.0x10 ms = 6.0x10 m.
R also sends a (different) signal to Satellite 2 (S2).
And S2 sends the signal back, but with a time lapse, say 0.06s
And S3 sends the signal back with a time lapse, say 0.02s..
Therefore, distance between R and S3 would be 0.02s/2 x 3.0x10 ms = 3.0x10 m.
Now, R has 3 sets of data to calculate its location.
Distance of R from S1 = 6.0x10 m
Distance of R from S2 = 9.0x10 m
Distance of R from S3 = 3.0x10 m
Note: 1. all views are from above the terrestrial position of R.
2. The possible locations of R from each of the satellites forms a sphere, not a circle.
3. The intersections of 2 spheres forms a circle.
The intersection of the possible positions of R using the possible distances of R from S1, S2 and S3 lead to two possible locations of R : R1 and R2. Which one to choose? By simple calculation, the receiver R will eliminate one of these two values on the basis of their altitude ie how high they are from sea level. This is easily done as one of these readings puts you more than 100km from the surface of the Earth.
We thus see that although the GPS is a complex system that costs a whole lot of money, it relies on some basic principles of geometry (intersection of objects, triangulation). The big problem is getting the accuracy required.
Therefore, distance between R and S2 would be 0.06s/2 x 3.0x10 ms = 9.0x10 m.
R also sends a (different) signal to Satellite 3 (S3).
the speed of light ie 299 792 458 ms
» 3 x 10
(for ease of calculation)
GPS, a godsend for hopeless navigators