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Satellite navigation is in fact a sophisiticated and complex interaction
between satellites and ground receivers. This interaction produces
calculations which result in accurate data about a specific location. It
can be the location of person, a car or a ship. Let's have a look at the
main aspects of this interaction.

Every second a satellite broadcasts a time-stamped signal giving its
position. This is synchronised to an atomic clock and the signal
reception is dated by an on-board clock. The time difference is then
calculated and the difference, multiplied by the signal speed
(300 000 km per second), gives what is called a pseudo-range. The
term pseudo-range is used because the distance in metres between
the satellite and the receiver of the user is not calculated directly
through space measurement but through a calculation made using
the time measurement. The pseudo-range corresponds to the area
where the vehicle or craft is situated.

In theory, if the distance between a satellite and a receiver is
measured, the range of possible positions of the receiver is
anywhere on the surface of the globe within a radius centred on the
satellite. Measuring the distance to a second satellite narrows the
possible position to anywhere on the intersecting curve. With three
satellites and with reasonable assumptions the location can be
narrowed to a single point. In order to do this, however, the receiver
needs to have an extremely accurate atomic clock. As these are very
heavy and expensive, the signals of a fourth satellite are needed to
reduce timing errors and to give a more accurate position fix.

Determining precise location depends on measuring accurately the
distances between receiver and satellite, and that depends on very
accurate measurement of signal travel time. As signals travel at the
speed of light, travel times are tiny fractions of a second. 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 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 inaccurate
over longer periods. The solution is to reset the receiver’s time to the
satellite’s time continuously. This is done by the receiver 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!

Despite the predictability of the MEOs, it is still necessary for high
positioning accuracy to monitor the precise location of each satellite
constantly. This is done from a global network of reference stations on
the ground, whose positions are known to within centimetres. Each
satellite receives the reference stations’ data on its location, this is
called ephemeris data, which it then relays with its signal to receivers.

Numerous errors can degrade the accuracy of a positioning. 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 unusually long path because it is reflected off many
tall buildings or other surfaces before entering 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 by the side of runways.

Around the world there are two operating satellite navigation
systems, An American system and a Russian system. A third system
- Galileo, to be built and operated in Europe, is due to come on line
during 2008. The three systems will be fully interoperable, which
means that a user on Earth will be able to determine a position with
any receiver picking up signals from any combination of satellites
belonging to any of the three systems.

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