Syledis (SYstem LEger pour mesure la DIStance) was a terrestrial radio navigation and locating system. The system operated in the UHF segment of 420-450 MHz. It was manufactured in France by Sercel S.A. and was operational during the 1980s and until about 1995, providing positioning and navigational support for the petroleum sector in the North Sea and to other scientific projects. Syledis has been replaced by GPS.
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Functioning
Determination of the position of mobile vehicles, like f.e. vessels, using Syledis is accomplished by measurement of transit time of radio waves between mobiles and radio stations at known points. There are two modes of operation, active range mode and passive pseudo-range mode.
Further explanation of the functioning of the Syledis positioning system
A vessel is equipped with a transmitter that transmits a coded signal to at least three radio beacons each placed at a known point. The beacons send the code back to the transmitter. The returned coded signal is placed in a timeslot to determine the origin of the returned code. Therefore in an earlier stage a specified timeslot is connected to a specific beacon.
The elapsed time is proportional to twice the distance between the transmitter and the beacons. After the distance to the beacons is derived the position of the vessel can be calculated.