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Know It All: GPS -- Position, Position, Position

Published in Geare magazine, Issue #34, 2005

I look with great envy on those with GPS-equipped cars. I'm still stuck with a paper map.

GPS -- the Global Positioning System -- is a system developed for the US military, but has since become far more valuable for civilian use. Yet until 2000, that use was limited.

A car GPS system consists of two parts: the GPS receiver and a set of electronic maps. GPS is truly global. The maps are partial, covering major cities in some detail, major streets in smaller communities, and the main trunk routes. We can expect the maps to improve over coming years both in coverage and providing finer detail. But how does the GPS component work?

It all hangs off the GPS satellite array. To provide full global coverage, at least 24 satellites must be in operation. Since the first satellite launch in 1978, 52 have been placed in orbit. Not all of these are still operational. They wear out and have to be replaced from time to time; part of the system's $US400 million per year operating cost.

The satellites are orbit at an altitude of a little over 20,000 kilometres, so each circumnavigates the planet twice per day. The space shuttle, unreliable white elephant that it is, most definitely is not used to launch these satellites. Instead they're flung aloft using conventional booster rockets, typically a Delta II.

The satellites are distributed over six different orbits, each of the six at a different angle to the others. The idea is to ensure that at least four, and preferably more, satellites are in direct line of sight from just about any point on the Earth's surface. Accurate GPS operation depends on receiving the signals from at least four satellites.

A GPS receiver is sometimes said to work out where it is by a process of triangulation. That's where you can calculate your position by measuring the angles between yourself and a couple of fixed locations. In fact, they don't work quite that way. Instead, a GPS receiver detects the distance between itself and each of four (or more) satellites and uses this to determine its position. You can get a sense of how this works by looking at the four corners of a wall in your room. Imagine, if you know precisely the distance between yourself and each of those corners, plus the locations of the corners themselves, you can determine precisely where you are in the room.

It order for this to work, the satellites must be masterpieces of precision. Each has an incredibly accurate atomic clock on-board, and its time signal is beamed towards us. The distances between the GPS receiver and the satellites are calculated from the different times taken for the signals to reach the receiver. The clock within the receiver needn't be accurate, because it works out the precise time from the four satellite signals it receives.

Also transmitted is the precise location of the satellite, so the receiver has everything it needs to work out exactly where it is.

Even atomic clocks can drift, and even satellites can wander slightly in their orbits, so there are a number of ground stations that track the satellites and update them on their locations and the time.

Basic GPS units are accurate to within twenty metres. That accuracy is determined not by any imprecision in the satellite system, but such natural causes as the slightly variable delay to the radio signal as it punches through the ionosphere. The satellites transmit on several frequencies, and military receivers (like those on smart bombs) can access one of the other channels, compare it with the first one, and calculate a more precise location after cancelling out problems caused by the ionosphere. These can theoretically achieve an accuracy of within 30cm.

But until 2000, only a 'course' signal was available to civilians, good for only a few hundred metres of accuracy. The encrypted high precision signal was restricted to the military. If that hadn't been eased up, no one would have GPS in their car. It just wouldn't be accurate enough.

© 2002-2009, Stephen Dawson