While common means for positioning in the hydrographic and marine industry are NAVSTAR Differential GPS, users of these positioning systems are not limited to using differential submetre accuracy GPS. DGPS receivers with Real-Time Kinematic (RTK) technologies are being used widely nowadays, providing centimetre-level accuracies in X and Y, and in some receivers also in Z. These accuracies are obtainable even over considerably long ranges from a reference station.

These receivers are also the only system that can achieve complete repeatability. All this precision comes at a cost. So, not surprisingly, RTK receivers are considerably more expensive. However, if your work requires this precision, these receivers will be cost-effective. This product survey reviews RTK-enabled receivers only and reports on a total of nineteen products provided by nine manufactures. Although RTK generally utilises a fixed, nearby ground station within line of sight distance of approximately 20km, the CNav, Fugro and Trimble receivers are able to use satellite-broadcasted corrections providing high accuracies over much longer distances. These signals are available through subscriptions. With a view to the future, the Fugro, Topcon and Leica receivers are equipped with the L5 band and prepared for Gallileo. Thanks to the vertical accuracy of the positions, the hydrographic surveyor is able to correct the depth data online for water level changes due to tide, wind, swell or even waves, providing easy online depth reduction to a survey reference level. In addition, offshore construction works or any other work requiring stable vertical centimetre-level accuracy benefit from RTK technology. Hydro international is much indebted to all the manufacturers who contributed to this product survey, and exposed their products to our scrutiny.