Radio Navigation - Satellite Navigation

Satellite Navigation

Since the 1960s, navigation has increasingly moved to satellite navigation systems. These are essentially DME systems located in space. The fact that the satellites are in orbit and normally move with respect to the receiver means that the calculation of the position of the satellite needs to be taken into account as well, which can only be handled effectively with a computer.

The Global Positioning System, better known simply as GPS, sends several signals that are used to decode the position and distance of the satellite. One signal encodes the satellite's "ephemeris" data, which is used to accurately calculate the satellite's location at any time. Space weather and other effects causes the orbit to change over time so the ephemeris has to be updated periodically. Other signals send out the time as measured by the satellite's onboard atomic clock. By measuring this signal from several satellites, the receiver can re-build an accurate clock signal of its own. Comparing the two produces the distance to the satellite, and several such measurements allows a form of triangulation to be carried out.

GPS has better accuracy that any previous land-based system, is available at almost all locations on the Earth, can be implemented in a few cents of modern electronics, and requires only a few dozen satellites to provide worldwide coverage. As a result of these advantages, GPS has led to almost all previous systems falling from use. LORAN, Omega, Decca, Consol and many other systems disappeared during the 1990s and 2000s. The only other systems still in use are aviation aids, which are also being turned off for long-range navigation while new differential GPS systems are being deployed to provide the local accuracy needed for blind landings.