GNSS Based Hydrographic Surveying: clear advantages and hidden obstacles Thijs Ligteringen MSc., John Loog MSc, Dr. Leendert Dorst Hydrographic Service of the Royal Netherlands Navy P.O.Box 10.000; 1780CA Den Helder +31-70-3162826; ll.dorst@mindef.nl Abstract: To fulfill its task of publishing accurate and up-to-date nautical charts, the Hydrographic Service of the Royal Netherlands Navy employs two survey vessels worldwide: HNLMS Snellius and Luymes. In 2013, these vessels have been fitted with new Trimble DGNSS receivers in combination with the Fugro Marinestar SBAS service. The advantages are clear: accurate, reliable, and highly available 3D positioning in a realization of the International Terrestrial Reference System (ITRS), wherever the civilian and military hydrographic duties of the moment take place. As is widely discussed in literature, the advantages are especially prominent for vertical referencing of the vessel, and thereby for the measured depth values. The signal contains several corrections: water level, heave, draught, and squat, fully independent of the traditional sensors. This redundancy provides opportunities for quality control, efficiency improvement, and easy deployment out-of-area. Moreover, vertical referencing directly to the GRS80 ellipsoid of ITRS enables innovative data processing and storage independent of any realization of a tidal datum. This results in less ambiguous references, in historical and international contexts. Yet, there are some obstacles to usage for vertical referencing. They only appear after closer examination of the system. We will present the main obstacles that we found. We believe these are universal, and cause the limited operational use of ellipsoidally referenced surveying. The obstacles that have to be overcome are: 1. The vertical uncertainty of DGNSS systems in combination with a commercial SBAS is still rather large in relation to the IHO S-44 guidelines for survey uncertainty in shallow water. The higher the contribution of the vertical referencing component to the error budget, the lower the allowed uncertainties for other sensors are, translating into a smaller swath width of the multibeam echo sounder and eventually into more parallel survey tracks to fully ensonify an area. 2. Separation models between the ellipsoid and the required vertical datum (LAT, MSL, the geoid, or another chart datum) are still under development, mostly on the national level. As a consequence, there is no basin-wide standard, and uncertainties are strongly variable among and within separation models. 3. The implementation details of separation models in GNSS receivers are unclear and sometimes erroneous. This forces hydrographic offices to work with ellipsoidally referenced data until further into the processing chain. Although this is no disadvantage in itself, it hinders the gradual introduction of GNSS-based vertical referencing. 4. Although the SBAS signal may be highly available, the GNSS signals are weak, i.e. prone to be disturbed. Signal loss could happen due to common operations, like interference with communication devices. 5. In case the SBAS signal does get lost, the GNSS receiver switches to differential mode, introducing a change in horizontal reference frame to ETRS89 (for Europe). This implies an apparent shift in the ship track of about 60cm for the Southern North Sea. Based on these obstacles, we conclude that GNSS-based hydrographic surveying is a valuable additional tool, rather than a single system that would replace current sensors and procedures. BIOGRAPHIES Thijs Ligteringen is a Dutch Geodetic Engineer from the Delft University of Technology. He joined the Ministry of Defence as a navigation consultant. Curently, he is employed within the Department of Geodesy & Tides at the Netherlands Hydrographic Service. His main interests are navigation systems and the Law of the Sea (UNCLOS).