Comparison of GNSS PPP-AR packages for post-processing seafloor geodetic data.

GNSS/A is a proven method to localize seafloor geodetic stations and, thus, to monitor seafloor tectonic phenomena. This approach combines GNSS positioning of a surface platform with acoustic ranging between this platform and acoustic beacons on the sea bottom, with the objective to localize the beacons in a global frame within a centimeter or so. As part of the FOCUS project, a network of seafloor stations was deployed offshore Sicily in Italy, at about 1900 meters depth. It was surveyed from a small autonomous surface vehicle (ASV) equipped with two GNSS receivers. Here we present the post-processing applied to the GNSS surface navigation data in order to reduce the uncertainty budget of the GNSS/A algorithm. We found significant discrepancies between GNSS receivers in terms of signal degrading effects (multipaths, cycle slips, signal interruptions). We also found that GNSS antennas are less subject to these effects when mounted on an ASV rather than on the main vessel mast. The correlation found between solution accuracy and signal degrading effects potency turned out to be inadequate to predict the former knowing the latter. Then, we compared the accuracy of kinematic solutions from three GNSS PPP-AR post-processing packages to a few/several centimeters level. From our analysis, we find that the best accuracy was reached using the CSRS-PPP service.