Introduction to satellite positioning

At the time of writing there are two operating Global Navigation Satellite Systems (GNSS). The best known of the satellite positioning systems is GPS (Global Positioning System) which is the satellite-based navigation system developed by the US Department of Defense under the NAVSTAR programme. The other is GLONASS, the Russian system. Although it had fallen into disrepair it is now restored to full operational status.

There are additional planned systems, including GALILEO, COMPASS and others. Galileo and Compass are both in their pre-operational phases and are expected to become operational in the time-frame 2014 to 2018.

GNSSs allow a receiver which can receive radio signals from four or more navigation satellites (in general) to compute its position within an Earth fixed reference coordinate frame such as WGS84. The key features are:

• It is a one-way system in which the satellites transmit navigation signals and receivers receive the signals and use them to compute their own positions (usually).

• The receiver must be able to receive signals from four or more satellites (for normal 3D positioning), and therefore it does not work in places where the signals cannot be received. The better the quality of the signals received, the better the positioning.

• The quality of the position fix is affected by the environment in which the receiver is operating: multipath, diffraction and interference all affect performance.

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