Basics of GPS

The US Government essentially needed a way to figure out where its troops, planes and missiles were an where they were headed. So, in 1979 the US government started launching satellites into orbit to act as man made stars that orbit the earth. The first eleven satellites were the proof of concept, and the system was finally completed in 1994 for full worldwide coverage. These satellites send back very low power signals to earth. A Global Positioning Satellite receiver, commonly known as a GPS, will pick up those signals, and when it has a strong enough signal from enough satellites, it can triangulate to see where it is. Right now there are 30 GPS satellites in orbit, but only 24 of them are in active use, the other 6 are back-ups. The launch of the 24th block II satellite in March of 1994 completed the GPS constellation. The spacing of the satellites in orbit are arranged so that a minimum of five satellites are in view from every point on the globe. Not bad, so as long as you can see the sky, you can use a GPS.

So what happens if you don’t have a clear view of the sky like when you are in urban canyons of tall skyscrapers? Well some GPS units can deal well with signals bouncing around off of those buildings. When that happens, it’s called multi-path error and there are some pretty complex techniques to sort all that out inside your GPS. Also recent advances in chip sensitivity, has allowed some manufacturers to catch very weak satellite signals, making them able to get a fix in urban canyons as well as under dense tree cover. One in particular is from a manufacturer named SiRF. For more on their chipset and why it’s important see our article – What is SiRF star III?

Getting a Fix, Accuracy and What is WAAS?

So, when your handy GPS can see a signal from a minimum of four Satellites, it can set a precise location on the globe. Essentially, it could do it with signals from three, but to deal with some of the errors the fourth satellite is needed. The satellites send down signals to the earth and your GPS receiver calculates how long it took to get that signal, which when divided by the speed of light and when adapted for a few travel errors for coming through the atmosphere, it can see where it is. Sometimes, these signals come with errors that can be corrected. So, the satellite signals come encoded with correction signals. It essentially says, “Hey, it took me 45 nanoseconds to get down here from the satellite, but I am having trouble today with the atmosphere and it should have only taken me 44.5 nanoseconds.” This error correction helps drive accuracy up. To figure out this error, satellites transmit to a ground station of known location, and that ground station analyzes the data and send back the error amount. So when the satellite transmits to you, it already knows what types of errors it is making, and compensates for it. This is Called Wide Area Augmentation System or WAAS. When a GPS unit is WAAS enabled, it listens for these error correction messages and gives you a more accurate fix on where you are. Most GPS units sold today come equipped with WAAS or a WAAS mode.

May 31, 2006

Basics of GPS