Recent Developments In Accurate Navigation At Distances Of 300 To 400 Kilometres Using UHF Radio Positioning
Authors: Nard Georges P. , Pieuchot Maurice P. , De Vault John L. 
Published: October 2008

INTRODUCTION
Since 1973, SERCEL has developed a radio positioning system operating in UHF and taking advantage of the most advanced techniques of signal encoding and processing; this is the SYLEDIS. The main characteristics of the SYLEDIS were disclosed in 1974 (OTC 2005); namely, its accuracy (a few meters), its range beyond the horizon (100 to 200 km), and the reliability of the all solid-state hardware.
Over the past years, the good results achieved and some technological improvements have led to an ever-increasing interest by the users. Today, some 50 systems are in intensive use in more than 20 countries over the five continents. The most frequent applications are in connection with geophysical surveys, especially when they involve high resolution and accurate positioning of drilling platforms. Some applications deal with aerial surveys.
Field reports and results have confirmed the operating performances announced in 1974 and especially the range, the accuracy, and the responsibility of the system.
Users have obtained a good positioning around the clock with an accuracy of a few meters up to ranges of 100 to 200 km. But users always want more, and they have expressed the wish to reach a range of 300 to 400 km with an accuracy in the same range, with the same responsibility, and preserving the flexibility of the system.
For 4 years, SERCEL has investigated problems related to increasing the range. These problems deal with (1) increasing the power in the transmitters and the sensitivity in the receivers; (2) reducing the bandwidth to meet regulations concerning the occupancy of frequencies and to avoid interferences with other radio communications; (3) improving the quality and the reliability of measurements in the presence of disturbances observed at long ranges such as abnormal tropospheric refractions; and (4) maintaining, everything that contributes to the flexibility of the system, such as use of nondirectional aerials on board the ships, automatic operation excluding any manual coincidence adjustment or human interpretation, possible operation either in an active mode for a few users in the same network (With an onboard transmitter) or ill a passive mode with an unlimited number of uses (Without onboard transmitter) or in a combination mode.
After reviewing briefly the principle of SYLEDIS and the performances obtained, we shall explain how the technical problems related to the increased range have been solved and what the result has been on performances.

REMINDER OF CHARACTERISTICS AND OPERATIONAL CAPABILITIES OF SYLEDIS
As with most electronic distance-measuring devices the SYLEDIS uses the general principle of a mobile onboard interrogator operating in conjunction with an array of beacons spread along the coastline at very well known locations. The location of the interrogator is at the intersection of two or three circular loci obtained by direct measurements of the propagation time between the interrogator and two or three shore-based beacons.