Technology : Military precision for civilian GPS

2019-02-27 02:05:07

By Barry Fox AIRLINES would dearly like to use the Global Positioning System for navigation. But the signals are not accurate enough. This week a new Inmarsat satellite starts transmitting signals that will allow planes to use GPS during takeoff and landing. There are 24 GPS satellites orbiting the Earth, transmitting very precise time signals. A receiver picks up signals from three or four satellites simultaneously and decodes the time differences to calculate its position on the ground. The satellites broadcast two versions of these signals. One signal, encrypted for military use only, is transmitted at 1228 megahertz and can give a position to within a few metres. The second signal, transmitted at 1575 megahertz is not encrypted and is used by ordinary civilian receivers on ships and planes. But the military deliberately introduces errors into this system to cut its accuracy to around 100 metres. The signal can be further distorted as it travels through the ionosphere. Military users get a signal that compensates for this error. But this is also encrypted to prevent civilians using it. So, for about 5 per cent of the year, the accuracy of civilian GPS falls to 300 metres. George Kinal of Inmarsat says: “Five per cent of all flying time adds up to an awful lot of aircraft.” There is also no mechanism to warn civilians if one GPS satellite develops a fault, as warnings are also encrypted. Inmarsat’s new satellites will provide communications for ships, aircraft and vehicles. They will also transmit signals at 1575 megahertz. These transmissions will tighten the timing accuracy of the GPS signals, compensate for distortion caused by the ionosphere and warn of any faults. The military has no control over these signals. They are generated at ground stations run by the Federal Aviation Authority and beamed up to Inmarsat’s satellites for retransmission. Slightly modified GPS receivers will use these correction signals to achieve accuracy and reliability close to military standards. The first of the new generation of satellites was launched a month ago and should start transmitting from over the Indian Ocean on 11 May. Four more will be launched over the next 18 months. The US government recently promised to allow civilian users to tune in to the accurate military signals (This Week, 13 April, p 6). However,