Syledis (SYstem LEger pour mesure la DIStance) was a terrestrial

radio navigation Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination. The basic principles ...

and locating system. The system operated in the

UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...

segment of 420-450 MHz. It was manufactured in

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan ar ...

by Sercel S.A., headquarters

Carquefou Carquefou (; ) is a commune in the Loire-Atlantique, department in the region Pays de la Loire, in western France. Population Economy The headquarters of the geophysical research company Sercel is located in Carquefou Sport In 2008, the loca ...

, and was operational during the 1980s and until about 1995, providing positioning and navigational support for the petroleum sector in the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

and to other scientific projects. Syledis has been replaced by GPS.

Functioning

Determination of the position of mobile vehicles, like f.e. vessels, using Syledis is accomplished by measurement of transit time of

radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (sho ...

s between mobiles and

radio stations Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...

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