Topic of the Moment – Geolocation
Geolocation is about much more than finding your way to obscure pubs down backalleys using your phone. The Guardian has reported on the use of GPS data to help reduce childbirth-mortality in Nigeria, while the BBC tells of a new, high-tech bracelet that employs satnav to try to protect aid workers at risk of being kidnapped. But how does technology know where in the world we are?
The development of the Global Positioning System followed the launch of the first artificial satellite, Sputnik 1. US observers listening in to Sputnik’s signal realised they could use the Doppler shift of the radio waves being transmitted to work out both the satellite’s speed and its closest approach to their antenna – so they could calculate its position.
A GPS receiver uses a constellation of satellites in Earth orbit to work out its position on the ground.
A network of base stations tracks the satellites’ positions in orbit. The satellites themselves continually broadcast messages containing their location and the time of transmission.
When messages from several satellites are picked up by a receiver, which needs to contain an accurate clock, its distance to each satellite can be worked out from the time taken to receive the message.
Corrections must be made to account for the slowing of time in a gravitational field as predicted by general relativity – without them, GPS would quickly become wildly inaccurate.
Knowing the satellites’ locations and the distances to them allows the receiver to pinpoint its position on the Earth’s surface.
GPS needs to have a line of site to four or more satellites to work. If that’s not possible – for example if you’re indoors – then your smartphone can still work out where you are using local Wi-Fi signals.
It works in a similar way to GPS, but makes use of the strength of the received Wi-Fi signal – because the power received decreases with distance in a particular way, it’s possible to use the signal strength to work out the distance to a Wi-Fi access point.
Using several access points of known location, your position can be determined by simple triangulation, though it’s less precise than GPS.
By communicating with a nearby base station, a smartphone can determine its location by using one of two methods similar to those above. Positions derived from base stations are less precise than the other methods and their accuracy is further limited by the density of such stations in a given area.
Most simply, the power of the signal is used to work out the distance to base stations whose locations or identities are included in their transmissions, just as with Wi-Fi identification.
Or for slightly greater accuracy, the phone and base station can exchange signals similar to those broadcast by GPS satellites, containing a time of transmission, which is then used to work out the distance to the base station.
This is still less precise than Wi-Fi, but, as far as we know, no mobile phone base station has ever sent a driver 800 miles out of their way.