Where are the navigational satellites?
It is only possible to determine a location on Earth if you know the location of the navigational satellites very precisely. This is achieved by placing the satellites in highly stable Medium Earth Orbits (MEOs) at an altitude of about 20 000 kilometres. MEOs are the orbits of choice for a number of reasons: their stability enables exact orbit predictions; the satellites travel relatively slowly and so can be observed over several hours, and the satellites can be arranged in a constellation so that at least four are visible from any point on the Earth’s surface at any time.
Despite the predictability of the MEOs, it is still necessary to monitor the precise location of each satellite constantly to achieve high positioning accuracy. This is done from a global network of reference stations on the ground, whose positions are known to within centimetres. The ground system sends data derived from the reference station measurements to each satellite. This precise location information, called ephemeris data, is then relayed by the satellite to the user receivers with the navigation signal.
The receiver measures travel times by comparing ‘time marks’ imprinted on the satellite signals with the time recorded on the receiver’s clock. The time marks are controlled by a highly accurate atomic clock on board each satellite.
These clocks, however, are too expensive to incorporate into standard receivers, which have to make do with small quartz oscillators like those found in a wristwatch. Quartz oscillators are very accurate when measuring times of less than a few seconds, but rather inaccurate over longer periods. The solution is to re-set the receiver’s time to the satellite’s time continuously. This is done by the receiver’s processor using an approximation method involving signals from at least four satellites.
For this system of measurement to work, all satellites need to be synchronised so that they can start transmitting their signals at precisely the same time. This is achieved by continuously synchronising all on-board atomic clocks with a master clock on the ground. These super-accurate clocks can keep time to within one second in 100 million years!