Enhanced accuracy by regional operation of Europe's new radiobeacon differential system

The maritime radiobeacon differential GNSS service in Europe has expanded very rapidly in the last two years. In September 2001, a new frequency plan was brought into effect across the whole of the European Maritime Area (EMA). This resulted in reduced levels of interference and enhanced coverage. There are now 162 maritime differential beacons positioned so that, as far as possible, all critical coastal locations are served by at least two stations. Along many coastlines, inevitably, three or more beacons can now be received simultaneously. Indeed, by day when coverage is greatest, more than 20 signals are available at some locations. This provides an opportunity to make use of multiple transmissions. With the ending of Selective Availability, spatial dilution of position has come to dominate the accuracy of radiobeacon differential fixes. We have proposed using these multiple sources of pseudorange corrections in a Regional Area Augmentation System (RAAS) to minimise spatial dilution. The approach would be similar to that demonstrated successfully on a larger scale with Loran-C in the Eurofix system. The paper presents the results of measurements made simultaneously on groups of radiobeacon stations under various receiving conditions. It demonstrates the degree to which RAAS processing of the results enhances position accuracy. In this work, the results from several receivers were combined. The same effect could be achieved with a multi-channel receiver, or by combining the data at a central point and re-broadcasting the result. Using recently-developed mapping techniques, the paper then analyses the availability of multiple beacon signals across the EMA and maps the areas in which enhanced performance is expected to be available using this new RAAS mode of operation by day and by night.