Numerous errors can degrade the accuracy of a position measurement. For example, errors in satellite to receiver distances can creep in if conditions within the ionosphere, the electrically charged outer layer of the atmosphere, slow down the signal. Conditions within the ionosphere are influenced by the level of activity on the surface of the Sun. Inaccurate distance measurements will also occur if the signal takes an abnormally long path because it is reflected off tall buildings or other surfaces before reaching the receiver.
There are various ways of overcoming such inaccuracies. The best known is called differential satellite navigation, which uses a fixed receiver in a known position as a reference.
The time taken for the signal to travel from the satellite to the fixed receiver can be calculated precisely because the positions of the fixed receiver and the satellites (and hence the length of the travel path) are known precisely. Any difference between the calculated travel time and that actually measured reflects inaccuracies introduced by disturbances in the ionosphere.
If a moving receiver, attached to an aircraft for example, is within a few hundred kilometres of the fixed receiver, then it is fair to assume that the errors experienced by the signal in reaching both receivers will be roughly the same, as variations in ionospheric conditions tend to be similar over large areas. The timing errors determined by the fixed receiver can then be used to eliminate similar errors in the moving receiver. Major users of satellite navigation, such as large airports, may decide to use the differential technique by installing their own fixed receivers.
Another technique, which makes use of two positioning signals at two different frequencies, does away with the need for differential satellite navigation. It works on the principle that each frequency will be slowed down by a slightly different amount when travelling through the atmosphere. By sending the two frequencies at the same time and recording the time difference between their arrivals, it is possible to build up a model of ionospheric conditions. This dual-frequency technique can increase positioning accuracy to better than one to two metres and will be standard for Galileo.
Finally, it is possible to determine positions to within a few tens of centimetres using a technique called Three Carrier Ambiguity Resolution (TCAR). TCAR eliminates inaccuracies by looking at the wavelike structure of the signal and determining how much it has been pushed along during its passage through the atmosphere. Such phase shifts can be measured to within a fraction of the wavelength, which is typically 20 centimetres.
The calculations needed to do TCAR are presently beyond the capabilities of standard hand-held receivers and this technique is used mainly for long-term monitoring of movement in buildings, oil pipelines, changes in sea level and even changes in the shape of the Earth’s crust.
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!