The main deployments associated with this sub-facility are located at altimeter comparison points located offshore in Bass Strait and Storm Bay. The primary sites at each location are located along the descending track of pass 088 of Jason-series altimeter missions.
The water depth at the Bass Strait site is ~52 m, and ~96 m at the Storm Bay site. For the Bass Strait site, a second location was instrumented in August 2015 in order to address the calibration and validation of the European Space Agency Sentinel-3A mission.
The calibration approach adopted within the sub-facility is based on the direct comparison of in situ versus altimeter-derived sea surface height at each comparison point. In order to derive a continuous in situ time series, data from repeated deployments of moored oceanographic instruments is used.
Pressure sensors (SBE26s) combined with data from temperature and salinity sensors (SBE37s) enable the computation of a precise but relative sea level record on a 5 minute time base. Current meters are also used to determine along and across track current velocity that aids in understanding sea surface topography.
Each site consists of two separate moorings, each typically deployed for ~1 year duration before servicing and re-deployment (with servicing for each mooring typically staggered by ~6 months). The complete inventory is shown in the Figure and Table below. The mooring deployment and servicing is undertaken by the CSIRO Coastal Moorings Team.
The datum of the sea level record derived from the moored instruments is effectively the electrical zero of the pressure gauge – this differs for each deployment of the moored arrays. In order to place the data on the same absolute datum, coastal tide gauge data as well as absolute sea surface height derived from GPS equipped buoys is used. GPS buoys are deployed at each site every ~3 months for ~50 hours duration. See the calibration and validation section for further information.
The tide gauge data used is obtained from the Australian Bureau of Meteorology’s Burnie and Spring Bay tide gauges that are part of the national baseline array . GPS buoy sea surface height is derived by the sub-facility by processing the buoy data against land based reference stations deployed simultaneously at Rocky Cape and Stanley (in support of the Bass Strait deployments) and Bruny Island and Tasman Peninsula (in support of the Storm Bay deployments).
The comparison between the filtered GPS buoy sea surface heights and the mooring relative sea level record yields some information on the expected precision of the in situ data. As shown in the Figure below, the standard deviation of this difference is ~24 mm.