GPS – An Overview

GPS Satellite via Wikipedia

The system sounds very complicated. More than thirty satellites orbit the earth and dozens of monitoring stations are operated internationally by the sinister-sounding National Geospatial-Intelligence Agency. The system can predict earthquakes, plot nuclear explosions and keep the world’s clocks accurate to within ten nanoseconds. The whole network cost billions to develop and many millions a year to maintain…

… But you’ve probably used it on a daily basis for half a decade.

GPS actually follows a fairly simple principle. If you have enough satellites in the right orbit around the Earth, you’ll be able to contact several of them from any point on the planet’s surface. This makes the GPS system a pretty accurate tool for determining where you are on the planet.

Each GPS satellite is constantly transmitting a message. This message contains three items of information which are of interest to the receiver on Earth:

The time. This time of transmission is accurate to within ten nanoseconds. It’s the most reliable “clock” other than the nuclear clocks on which it is based.

The ephemeris. This is the precise orbital information, outlining where the satellite is in the sky to a mind-bogglingly exact degree.

The almanac. The satellite will transmit the conditions and rough orbital values of the other satellites, basically reporting the health of the system.

The last one isn’t much use to most of us – we’re more interested in where we are and how we’re going to get there. The time at which the message from the satellite was sent combined with the precise location of the satellite is enough information for a GPS receiver to plot its location on earth to within a few paces. This is the technology which can detect tectonic activity, take ships on world cruises, take you on a cross-country drive or simply introduce you to every pub within walking distance.

The receiver will use at least four satellites to be absolutely sure of what’s going on. It’s at this point that the receiver will also take other things into account: dead reckoning involves calculating movement from a fixed point, and a car’s on-board computer may be able to make accurate assumptions based on the car’s speed and direction. Planes will also have altimeter readings. This means that on some occasions, only three satellites are required (what with space being three-dimensional and all) but generally speaking, you’ll be using at least four.

This seems like a lot of effort for a little screen on the dashboard of your Mondeo. But GPS hasn’t always been about helping ordinary folk like us scoot around the countryside in comfort. In fact, its civilian uses were the last things on the minds of the inventors when GPS was concocted way back in 1972.

The tests that year took place over White Sands Missile Range, New Mexico. This huge tract of desert had seen nuclear explosions by then, and would later be home to space shuttle landings, but in 1972 it was where the first GPS receiver prototypes were tested. At this point, its uses were solely military – guiding planes and bombs to their targets in Russia. In 1978 the first GPS satellite was launched but it wasn’t until 1983 (after an airliner was shot down because it strayed into the wrong airspace) that the system was opened up for civilians as well. In 1998 Al Gore threw some money at the project to make it more civilian-friendly and since then its use has rocketed. The oldest satellite is more than 20 years old, having been launched in 1990. The youngest (at the time of writing) was launched in July this year. Its name is GPS IIF-2.

Of course, the US Department of Defense is not giving us civvies unbridled access to their system. After all, it could be very dangerous in the wrong hands. GPS receivers capable of working at an altitude higher than 11 miles or faster than 1,152mph are classified as “munitions” and are therefore subject to strict rules. This prevents anybody but the US Army from using GPS to guide their ballistic missiles, although you could use it in missiles which travel slowly enough.

GPS isn’t the only player when it comes to this sort of thing. Galileo is Europe’s answer to the Americans’ GPS, and it’s expected to be operational in 2014. China has come up with Beidou and Compass, while GLONASS is the characteristically Russian global system. It’s very likely, however, that GPS will remain standard for a long time to come.

By Dee Mason

Author: range

I'm mathematician/IT strategist/blogger from Canada living in Taipei.

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