The modern GPS receiver has a typical accuracy of an astonishing twenty feet, reducing the venerable 'Art and Science of Navigation' to little more than a simple read-out of the latitude and longitude, which can then be automatically displayed on an electronic chart or map.
But is it possible that the GPS signal might became unavailable for a time, or worse still, might the position obtained be inaccurate and misleading? To answer those questions it's helpful to know a little about how GPS works, and then to see what might cause such problems.
GPS Is Based on Satellite Ranging – Hence the Term 'Sat Nav' (Satellite Navigation)
The user's position is calculated by measuring distances from a group of three or four satellites (four if airborne) in space so as to obtain intersecting position circles by a process known as 'trilateration'. How is this distance measured?
The GPS signal travels down to earth from the satellite at the speed of light (186,000) miles per second and using the basic equation, Velocity x Time = Distance, the GPS receiver unit calculates the range from the satellite by measuring precisely the time taken for the signal to reach it from the satellite.
It's worth remembering that if the signal from the satellite is delayed by even one thousandth of a second, the distance will be out by a hundred and eighty-six miles!
Reasons for Errors in GPS Data
Provided there are no satellite or GPS receiver equipment errors and no reflection occurs, the GPS signal emitted from a navigation satellite orbiting in space may be delayed on its route to earth as it passes through:
- The ionosphere – a blanket of charged particles between eighty and a hundred-and-twenty miles above the earth
- The atmosphere – which carries varying amounts of dust, water vapour and other particles
It is in the ionosphere though that the greatest distortion of the GPS signal is likely, due to high-energy particles radiated from the sun, most of all during the 'solar cycle'.
GPS and The Solar Cycle
The solar cycle that reaches its climax in the years 2011/2012 occurs roughly every ten years causing at best, minor inconvenience to people who rely on the global positioning system. At worst, widespread disruption occurs, with even satellites themselves becoming disabled.
Imagine motorists in half the world lost and forlorn with their 'Tom Tom' or 'Garmin' GPS devices out of action and no map in the car's glove-box. Doubtless the very least of many other rather greater potential misfortunes.
The sun's solar flares and magnetic storms are intensified during the solar cycle, but at present scientists need to spot the flare-up on the sun before they can issue warnings, by which time charged particles are already on their way to earth.
However, NASA's Solar Dynamics Observatory was sent into space on February 11, 2010 on a mission to gather data on the sun's magnetic field activity and space weather, so that scientists may soon be better able to forecast the influence of solar flares and magnetic storms on the GPS.
GPS Reliability
All satellites are constantly monitored for errors in their position, altitude and speed by the USA's Department of Defense from ground stations around the world.
The NOAA Space Weather Prediction Center provides alerts and advisory bulletins to users of GPS devices, which it is always wise to check before putting absolute confidence in the reliability of the global positioning system, especially during the solar cycle.
References:
The Institute of Navigation Newsletter, Winter 2009-2010, pp 14
Exploring the Earth & The Cosmos, I. Asimov, Allen Lane 1993, pp 303
George Backwell - Merely a reporter who endeavors to provide a reliable compass for those who look for one.
