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 a ...

and locating system. The system operated in the UHF 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 France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

by Sercel S.A., headquarters Carquefou, 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 Syledis has been replaced by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 The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

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 (short ...

s between mobiles and

radio stations Radio broadcasting is transmission of audio signal, 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-b ...

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 In navigation, a radio beacon or radiobeacon is a kind of beacon, a device that marks a fixed location and allows direction-finding equipment to find relative bearing. But instead of employing visible light, radio beacons transmit electromagnet ...

each placed at a known point. The beacons send the code back to the transmitter. The returned coded signal is placed in a

timeslot Broadcast programming is the practice of organizing or ordering (scheduling) of broadcast media shows, typically radio and television, in a daily, weekly, monthly, quarterly or season-long schedule. Modern broadcasters use broadcast automation ...

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. The transmitter computes the distances to the beacons and a computer, connected with the transmitter, computes the position of the vessel.

Possible causes for measurement inaccuracy

The Syledis system has a measurement sensitivity that can be expressed in centimeters. Due to weather conditions the wave propagation speed, for electromagnetic waves in air almost the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

, can change. In wet conditions, rain or snow, the wave propagation is a little bit slower than in dry conditions. So that gives an inaccuracy in determining the correct position of the vessel. Another important factor is the length of the cables used to connect the

antennas In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies a ...

with the radio beacons at the shore, for example placed on

lighthouse A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lenses and to serve as a beacon for navigational aid, for maritime pilots at sea or on inland waterways. Lighthouses mar ...

s, or on oil rig platforms and the cable(s) to connect the antenna with the transmitter at the vessel. Normally the antenna will be placed at the top of the mast and the transmitter will be placed in the wheelhouse, where the captain and the mate can see the displays of the transmitter. Those cables has to be

calibrated In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

in order to obtain the smallest possible measurement uncertainty.

References

* * Measurement Radio navigation Ubiquitous computing Wireless locating Geopositioning {{Radio-comm-stub