T
he navigation device you might have in your car, or embedded in your phone, is just a receiver for information broadcast by the Global Positioning Satellite network, also called the Global Positioning System (GPS). It uses that information to locate your position on Planet Earth, and then does such helpful things as get you from point A to point B without getting you lost. The origins of GPS technology lie with the US military. In the 1970s, they developed and launched a network of satellites that had the ability to track the positions and movements of all military ships, planes and ground units, no matter where they were on the planet. It was only in the late 1980s that they allowed public access to this network, which sparked the GPS industry we know today.
How does it work? Today’s GPS devices use the radio signals broadcast from this satellite network to determine their latitude, longitude and height. In mathematical terms, these are known as X, Y and Z co-ordinates, and when used with a model of the Earth that has been mapped using these co-ordinates, it’s possible to calculate exactly where and how high above sea level a GPS receiver is. There are currently 29 GPS satellites orbiting the Earth, 20 000km above the planet’s surface, travelling at a speed of 12 000km per hour, fast enough to orbit the Earth twice in a 24-hour period. Of the 29 satellites, 24 form the core network, with 5 extras that serve as backups in case of hardware failure, so that a minimum of 4 satellites are “visible” from any point on the planet’s surface at any given time.
Trilateration While we try to steer clear of unnecessary jargon in Connect, this complicatedsounding word is needed to explain just how a GPS device works out where it is. Trilateration is a mathematical term for a calculation that uses at least 3 separate positions to determine the location of a 4th point. In this case, the 3 positions used are those of the satellites, and the 4th is the GPS device. Having 4 satellites visible when only 3 are strictly necessary simply serves to give the resulting calculation greater accuracy. Each satellite’s orbit is programmed such that every one of them knows exactly where the others are, and where they are going to be at any point in time. For such a high degree of accuracy, each satellite is equipped with an atomic clock – a device capable of the most accurate time measurement mankind is capable of. This position and timing information is useful because a GPS receiver – the device you find on store shelves – uses the radio signals sent out by the satellites containing that information to find out how far away each one is. That signal takes between 65ms and 85ms to travel from the GPS satellite to the device. Once received, the GPS receiver uses that information to perform the trilateration calculation that produces its final position in that moment. That result is then combined with highly-detailed mapping information to give the end-user a visual representation of exactly where they are located.
GPS data is only the beginning As amazing as this capability is, it’s the extras on any given GPS device that sell it. These range from live traffic updates, to a choice between guidance voices, to the depth of information available through the device, like points of interest, access to emergency services and others. Let’s take a look at each of these, and explain how they work. www.connectmag.co.za | 11