Abstract

GNSS (in fact it usually means GPS) is at the moment the fundamental means of positioning for any variants of navigation. However some attributes of it can be not sufficient for every users or in some tasks, so there are any cases when this system needs support. From the mariner point of view pure GPS properties perfectly fulfils expectations of sailors at the open sea, however there are lot of cases, when accuracy of the system are not sufficient. Besides of accuracy other performance characteristics, for example availability, risk of jamming etc. are important. As the “medicine” for this, differential version of GPS is suggested. At the moment hundreds of DGPS reference stations exist all over the world, but simultaneously other augmentation systems provide services for example European overlay for GPS (European Geostationary Navigation Overlay Service - EGNOS). But over 30 years of DGPS use at sea causes that sailors prefer it than EGNOS. At the moment as EGNOS is fully operational, it is time for evaluation of pro and cons of both services in the maritime context. As it was mentioned, at the open waters there is no doubt about GPS usability and in fact DGPS as well as EGNOS are not necessary. In real situation this is anyway impossible because of limited coverage of DGPS. It is truth that EGNOS is accessible on all the seas around Europe, however seamen do not see so far need of such high accuracy of the fixing. However on restricted waters, on approaches to harbours and on river navigation both systems seem to be indispensable, especially because their accuracy. On the other hand observation made with AIS over accuracy of ships’ position on Gdanska Bay suggest that problem is not clear. In these waters two DGPS reference stations are available aside from EGNOS. Observation of AIS messages shows, that on the some leg of the route some ships’ positioning data are changeover on DGPS when other one DGPS is switched on GPS, and other one does not change accuracy at all. From the other side positions fixed with DGPS at different locations but at a similar distance from Reference Station have noticeable different accuracy. The location of these places suggests the influence of the signal propagation over land on the fixes accuracy. In the paper analysis of different aspects of both systems work are presented as well as their usefulness and limitations are analysed.

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