Dear Editor-in-Chief,

In the October edition of this magazine you published our article entitled ‘Portuguese DGPS Network - Design and validation’. On page 29 we described a trial which had ‘the purpose of comparing the accuracy of GPS, DGPS and the EGNOS System Test Bed (ESTB)’. For that trial we used a different receiver to gather data from each system: a Trimble DSM 212H, to receive and record the GPS data, a Leica MX 9320, to receive and record the maritime DGPS service and a Garmin GPSMap 176 to collect EGNOS System Test Bed positions.

The final results were presented in a table the top two rows of which identified the system (in bold characters) and the type of receiver. Nevertheless, some readers did not notice the top row and understood the results as if they represented a comparison between three types of receivers, all collecting DGPS solutions. Therefore, we emphasise that the table represents the comparison between stand-alone GPS, maritime DGPS and the EGNOS System Test Bed.

Although three different types of receivers were used, the differences are mainly attributable to the differing performance of the three systems on that particular date (April 2003) and at that particular location (Lisbon, Portugal). The performance of the three receivers is comparable because all receivers were tested recording stand-alone GPS and the results were very similar. The text of the article, and especially the conclusions, clearly stress this issue by characterising the current performance of the three systems and not comparing receiver performance.

This article was intended to give only a very brief description of the results obtained with the three systems and was therefore relatively condensed. But those who attended the recent ION GPS/GNSS 2003, held last September in Portland will find a detailed description of the trial in the proceedings of that conference (session A6: ‘Comparative study between the accuracies of stand-alone GPS, maritime DGPS and the EGNOS System Test Bed’).