GPS Workshop 2004 RTK-GPS Activity at the University of New Hampshire

GPS Workshop 2004 The UNH activity I am discussing today is primarily the work of LCDR Gerd Glang, NOAA who is enrolled in our MS Program. It deals with the practical aspects of conducting precision repeat hydrographic surveys with the aid of RTK-GPS.

GPS Workshop 2004 The objective was to showcase the present state-of-the-art in shallow water hydrographic surveying on an ONR funded Mine Burial Project. The goal was to achieve 10 cm repeatability in x,y, z.

GPS Workshop 2004 The survey area is near the MVCO, south of Martha’s Vineyard MA in approximately 12 meter depth. It is only 3 kilometers off the beach. The data acquisition is divided between in-house (UNH) and contract support (SAIC). The GPS processing has generally been done in-house, however Applanix post processed one set of POS-MV GPS data. D-GPS, RTK-GPS, and PPK-GPS are used.

GPS Workshop 2004 The shipboard equipment included: RESON 8125 multibeam sonar, POS-MV Ver2, Trimble MS-750, -860 and Ashtech Z-12. Z-12 data were captured using GRIM and Processed using Dynapos. Trimble and POS Novatel data were captured using in-house software and processed using WayPoint GrafNav.

GPS Workshop 2004 On one survey the vessel Z-12 data were captured using GRIM via an Iridium link and processed ashore as Inverse RTK using Dynapos. Trimble and POS-MV Novatel data were captured using in-house software and processed using WayPoint GrafNav.

GPS Workshop 2004 The intent was to: Determine x, y, z time series of the shipboard GPS antenna phase centers based on each base station and Transfer each of them into a time series of x, y, z at the POS-MV IMU mount.

GPS Workshop 2004 For each survey, locations of the reference antennas were computed using inverse variance weighting of multiple 3-hr OPUS solutions over a 48 to 72 hour period. Two of the three surveys, to date, used one vessel and the third used a different vessel. The physical separation of the antennas in the vessel frame were 8-10 m fore-aft, 2-4 m port-starboard, and 1-3 m in the vertical.

GPS Workshop 2004 The two base stations were separated by 3 km on a nominally NW-SE line. Antenna phase characteristics were employed when there was a difference between the base and rover. The rms solution residuals on each rover-base station pair were typically 1-3 cm.

GPS Workshop 2004 With this rich data set there were many independent comparisons that could be made. It was observed that the roll/pitch/heading rigid body analyses between the GPS and POS-MV were not as tight as were expected.

GPS Workshop 2004 The most probable causes of this problem are: The POS-MV and GPS antenna geometries were misaligned. The measured placement of the GPS antennas relative to the IMU were not quite right.

GPS Workshop 2004 It is important to recognize the fact that misalignments of the POS-MV in the vessel frame are not detected/corrected using the classical patch test. Only the relative misalignments between the POS-MV and the sonar head can be corrected based on the classical patch test.

GPS Workshop 2004 A Monte Carol technique was set up to find the “best” combination of POS-MV offsets in the vessel frame and x, y, z separations between each of the antennas and the IMU. Using that “best” combination reduced the rms of vertical differences between the antenna heights (transferred to the IMU) from 7.5 cm to 5 cm.

GPS Workshop 2004 The next major step in the project is to remove the speed dependent settlement that was embedded at acquisition and to “adjust” or replace the POS-MV heave data with the combined GPS vertical solution.

GPS Workshop 2004 The sounding solutions for repeat lines will then be compared as a function of the lapse time between the two lines. Results from a separate UNH effort to adjust relative navigation of two overlapping hydrographic surveys using an AC-component analysis of DEM’s, indicates that the statistical asymptote of that comparison should level out at approximately 5 cm for zero time difference.

GPS Workshop 2004 The final analysis of the repeatability that can be achieved using RTK-GPS at the MVCO shallow water site will be based on the repeat x, y, z’s of 5 special acoustic reflection markers that have been mounted to pipes that were jetted deep into the seabed. The fourth and presumably final repeat survey of the MVCO shallow water site is scheduled for April 2004.

GPS Workshop 2004 One lesson has been learned is that post processing the POS-MV significantly improved the navigation results.

GPS Workshop 2004 The major lesson that has been learned is that there is good reason to establish the true orientations of the POS-MV in the vessel frame (as well as, relative to the sonar head) and there is a need to improve the techniques for locating the GPS antennas and the IMU in the vessel frame.