Optimum Choice of Beacon in Maritime DGNSS Systems

Abstract

Marine radiobeacons are widely used by the maritime community worldwide as an efficient means of broadcasting differential GPS data to users at sea. In Europe and North America, large numbers of these beacons now serve coastal regions, waterways, and some inland areas. Frequently there is overlapping coverage and a choice of stations. But users receive little guidance as to how to select the beacon that gives the highest quality service. Receivers that choose a beacon automatically generally select either the nearest station or the strongest signal. But the performance of the data-link is optimised by choosing the station received with the highest ratio of signal to either noise or interference. With Selective Availability set to zero, spatial dilution and time-to-alarm have become key factors. This paper compares four beacon selection strategies by means of a computer model based on well-established coverage analysis and system design techniques. We recommend a new ‘post-SA’ beacon selection method that chooses the nearest station that meets the time-to-alarm requirement. This strategy has been used to identify the ‘best beacon’ throughout the European Maritime Area, with stations operating in accordance with the new band-plan adopted last September. We also identify the alternate beacon to use if the preferred station should fail.