MICHELLEHAYWOOD
Are we there yet ?
Michelle Haywood digs into the history and science behind today ’ s ubiquitous GPS systems , which amazingly were not created to find shipwrecks
THE GLOBAL POSITIONING SYSTEM ( GPS ) has become a routine method of locating position . A couple of decades ago , handheld GPS units started to come to market but they were still a specialist bit of kit . Now that technology exists in smart phones and we all carry it around with us every day .
GPS is based on time and the known position of GPS specialised satellites . It was first launched in 1978 by the United States Department of Defense in 1978 , with 24 satellites being in orbit by 1993 . Originally it was limited to military use , but after a Korean Air Lines flight was shot down by the USSR in 1983 because it had strayed into prohibited airspace , President Reagan issued a directive that GPS should be available for civilian use to .
Initially , the civilian signal was intentionally degraded , a policy known as Selective Availability . This led to the development of Differential GPS systems being developed particularly by the US Coast Guard , which used ground stations to send a correcting signal to DGPS units . In 2000 , President Clinton directed that civilians should get the same level of accuracy and that Selective Availability should be permanently turned off . The Russian GLONASS and EU Galileo ( and soon the Chinese BeiDou-2 system ) are all operational as well .
GPS satellites carry very stable atomic clocks that are synchronised with each other and with the ground clocks . Each GPS satellite continuously transmits a high frequency , low power radio signal travelling at the speed of light . This signal contains the current time and data about its position . The speed of radio waves is constant and independent of the speed of the satellite .
There is a time delay between when a satellite transmits the signal and when it is received , and this is related to the distance between the satellite and the receiver . A GPS receiver monitors multiple satellites and , at a minimum , needs four satellites to calculate its position . The four Time-of-
Flight ( TOF ) values are computed to give a latitude , longitude and height . But there ’ s a problem with the clocks , as predicted by Albert Einstein many years before launching satellites was even possible . The whole system depends on very accurate timing of the signal from the satellite to the receiver . Satellites in orbits high above the Earth experience less gravity than the atomic clocks on the ground . This is because the curvature of spacetime is greater at the Earth ’ s surface than it is at high orbit . Einstein ’ s theory of General Relativity predicted that clocks closer to a massive object will seem to tick more slowly than those further away . This means that clocks in the satellites appear to be ticking faster than identical clocks on the ground .
And then there ’ s the effect of Special Relativity , which deals with the relative motion between the receiver and the satellites . Special Relativity predicts that the receivers see the clocks in the satellite ticking slower because of the time dilation effect of their relative motion .
So , we have two opposing effects on the atomic clocks in the satellites . The effects aren ’ t balanced though . General relativity would mean the satellite clock gains about 45 microseconds per day and the Special Relativity effect means they would lose about 7 microseconds per day . Overall , satellites appear to tick 38 microseconds per day faster than ground clocks .
So , would such a tiny amount of time make a difference ? Well , yes it does . After two minutes of this error , GPS positions would no longer be correct and after a day the error would be 10km . Absolutely no use for finding a wreck site !
Happily , the engineers who designed the GPS system were aware of these relativistic effects when the system was designed . Satellite clocks are actually set to tick slower than ground clocks , so that once they are in orbit , they appear to match the reference clocks at the GPS ground stations . Each satellite is also capable of receiving ground data to provide corrections if needed . �
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