On-going development and evaluation of GNSS/INS equipped buoys for future altimetry validation: A case study from the Bass Strait validation site

Global Navigation Satellite System (GNSS) equipped buoys have a fundamental role in the validation of satellite altimetry. Requirements to validate next generation altimeter missions are demanding and call for a greater understanding and refinement of the systematic errors associated with the buoy approach. We report on recent progress in the development of the buoys used at the Bass Strait validation site, which now include both GNSS and Inertial Navigation System (INS) data collection. Based on the precision assessments of archived data collected from the existing UTas/IMOS Mk-IV buoy platforms, we explored potential improvements in buoy precision by addressing two previously ignored issues: changes to buoyancy as a function of external forcing, and biases induced by the platform dynamics. Our results indicate the precision of Mk-IV buoy against an in situ mooring is ~15 mm, with a ~8.5 mm systematic noise floor. From in situ mooring data and a regional numerical model, our investigation into the buoyancy displacement shows a strong correlation between surface currents, wind stress and buoy-against-mooring residuals. An empirical model correction reduces the residual energy by 50% over low frequency bands. Positioning effects associated with platform orientation derived from a modified Mk-IV buoy with INS integration show a range of 1-2 cm which is expected to be higher under rougher sea states. Our results indicate further enhancements are required with the introduction of our new Mk-V buoy platform to meet the stringent precision requirements for the validation of the Surface Water Ocean Topography (SWOT) mission. We identify areas for further development and highlight GNSS/INS equipped buoys will play an important role in the validation of new generation wide swath altimetry.