GPS Daily says that the British Ministry of Defence (MoD) began working on an alternative Earth-bound navigation system because GPS doesn't work under water, which makes it difficult to track submarines after they've dived.
When a submarine is 20,000 leagues, or less, under the sea it calculates the distance from its last known GPS location with a set of accelerometers that monitors its movements. Unfortunately this system isn't terribly accurate. According to Neil Stansfield, who is working on the project, submarines tend to suffer navigation drift of around one kilometre every day that they're submerged.
The quantum compass uses lasers to hold and cool atoms to within a millionth of a degree of absolute zero (-273.15 degrees Celcius), making them the coldest objects in the known universe. At this temperature the atoms become very slow moving and extremely sensitive to any changes in the Earth's gravitational and magnetic fields. Replacing accelerometers with a quantum compass scientists have been able to reduce a submarine's daily navigational drift rate to around a metre.
Stansfield says that the current generation of the quantum compass occupies the space of a "metre-long shoe box". Commercial applications would need to reduce its size and price quite markedly to be successful.
As discussed in our in-depth look at how GPS works, a GPS device needs to be able to receive a time code from four different satellites in order to trilaterate its position on Earth. In consumer grade products GPS is accurate to within about three metres. The system begins to provide erroneous calculations when satellite signals are bounced around by tall buildings and fails completely when there's no clear line of sight to the sky, such as when you're under the sea or in a tunnel.
Being satellite based, GPS is susceptible to direct attack or malicious jamming; it can also be entirely or selectively shutdown at any point by the US government which runs the network. A future navigation system based on the quantum compass could overcome those hurdles.