Syledis (SYstem LEger pour mesure le DIStance) was a terrestrial radio navigation and locating system. The system operated in the UHF segment of 420-450 MHz. Operational during the 1980s and until about 1995, it provided positioning and navigational support for the petroleum sector in the North Sea.
Syledis has been replaced by GPS.
Determination of the position of mobile vehicles, like f.e. vessels, is usually accomplished by measurement of transit time of radiowaves between mobiles and radiostations at known points.
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. Therefor 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. That is true.