In this tutorial we’ve been assuming that we know where the GPS satellites are so we can use them as reference points.
But how do we know exactly where they are? After all they’re floating around 11,000 miles up in space.
A high satellite gathers no moss
That 11,000 mile altitude is actually a benefit in this case, because something that high is well clear of the atmosphere. And that means it will orbit according to very simple mathematics.
The Air Force has injected each GPS satellite into a very precise orbit, according to the GPS master plan.
GPS Master Plan
The launch of the 24th block II satellite in March of 1994 completed the GPS constellation.
Four additional satellites are in reserve to be launched “on need.”
The spacings of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe.
On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment.
The basic orbits are quite exact but just to make things perfect the GPS satellites are constantly monitored by the Department of Defense.
They use very precise radar to check each satellite’s exact altitude, position and speed.
The errors they’re checking for are called “ephemeris errors” because they affect the satellite’s orbit or “ephemeris.” These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites.
The errors are usually very slight but if you want great accuracy they must be taken into account.
Getting the message out
Once the DoD has measured a satellite’s exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals it’s broadcasting.
So a GPS signal is more than just pseudo-random code for timing purposes. It also contains a navigation message with ephemeris information as well.
With perfect timing and the satellite’s exact position you’d think we’d be ready to make perfect position calculations. But there’s trouble afoot. Check out the next section to see what’s up.
- To use the satellites as references for range measurementswe need to know exactly where they are.
- GPS satellites are so high up their orbits are very predictable.
- Minor variations in their orbits are measured by the Department of Defense.
- The error information is sent to the satellites, to be transmitted along with the timing signals.