Wikipedia
Syledis
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 Syledis has been replaced by GPS.