CoAuthors

Christopher WATSON (University of Tasmania, Australia); Sam CARENTZ (CSIRO, Australia)

In view of the developing higher spatial resolution altimetry missions, we have been developing new in situ calibration/validation tools. In order to tackle the challenge of higher resolution radar altimeters, there sensitivity to wave or to sea surface height heterogeneity within the footprint, we develop instrumentation to investigate the more local and higher frequency SSH signals. The Bass Strait cal/val area is situated in the South West of the strait and is now equipped for the historical Jason site, but also for Sentinel-3A and Sentinel-3B. The classic method that was used in there over the past decades included, besides a reference coastal tide gauge, permanent moorings recording bottom pressure, temperature and salinity through the column. The accurate altitude reference is achieved by regular short-term occupation of the mooring sites with GPS buoys. This ensures both attaching the SSH measurements to the same reference frame as the satellite and also to correct for the bottom pressure sensor temporal drift. With time, current meters were added to the kit in order to validate the regional ocean dynamics.
Recently we added kits with bottom mounted 5 beam Acoustic Profilers. These measure current profiles, but also complete wave spectra and Sea Surface Height in addition to the bottom pressure, temperature and salinity.
In the first part of this presentation we show the results from the first years of development and operation of the system. The system was deployed at a Sentinel-3B crossover and includes the bottom pressure, temperature and salinity sensors on top of the ADCP. We analyze the ADCP data and its high frequency signals thanks to the high (>1Hz) sampling frequency. We also compare the measurements to the Bottom pressure and seawater density variations. We find that the acoustic system can track the surface at high frequency with cm accuracy. The drift of the bottom pressure sensor can be monitored. Changes in atmospheric pressure are also recorded by the system thanks to the inverse barometric effect difference between bottom pressure and acoustic surface ranging. Regular GPS buoy occupation is still needed to put the measurement in the satellite reference frame. The measurements are more accurate and much higher temporal frequency opens more possibility of usage numbers are given on the first months of Sentinel-3B operation in its nominal orbit.
We investigate the impact of various satellites sampling phases, from the 1-day repeat of the future SWOT-CAL/VAL phase to the 27-day repeat of Sentinel-3. For the SWOT fast sampling phase, we plan to have 2 to 3 of these within the swath in addition to a string of GNSS buoys.
We then investigate the impact this kit can have on the SWOT fast sampling phase analysis in Bass Strait by adding waves and currents fields. We particularly investigate the impact using a regional high resolution (400m) operational ocean model to test sampling strategies.