Moored oceanographic instruments form the backbone of the in situ data collected to determine the altimeter absolute bias. At the chosen comparison points (Bass Strait and Storm Bay), a pair of moorings are deployed by the CSIRO Coastal Moorings Team. Instrumentation on the moorings is designed to enable the computation of a precise and continuous SSH time series. Central to this task is the deployment of high precision pressure gauges (SBE26s) on the base of the mooring, and temperature and salinity instruments located at various locations through the water column (SBE37). In aid of the TasIMOS program, the Storm Bay mooring also incorporates a WQM instrument.

The use of two moorings at each site enables important redundancy and the ability to assess drift in the pressure sensors and possible settlement of the mooring platforms. The service, download and redeployment of the moorings alternates between each mooring every six months

Data from tide gauges at Burnie and Spring Bay, both operated by the National Tidal Centre (NTC), are used in the production of the altimeter absolute bias. Both of these gauges are part of the Australian Baseline Sea Level Monitoring Project (ABSLMP). Further information, including raw data can be obtained from the ABSLMP page on the Bureau of Meteorology website: http://www.bom.gov.au/oceanography/projects/abslmp/abslmp.shtml

Data from tide gauges are used in the production of the altimeter bias drift data stream. These data are obtained directly from the Permanent Service for Mean Sea Level (PSMSL).

Land based continuously operating Global Positioning System (CGPS) data is fundamental to the operation of this sub-facility. Analysis of CGPS data provides the absolute coordinates of key reference sites of interest relative to the International Terrestrial Reference Frame (ITRF). The ITRF is the same reference frame used for the orbit determination on the satellite altimeters, and therefore this enables the direct comparison of our in situ data with that derived from the altimeter. CGPS data collocated with tide gauges also provides vital information on the vertical rate of motion of these tide gauge sites. This information is vital in the computation of altimeter bias drift. This sub-facility relies heavily on the geospatial research infrastructure installed as part of the NCRIS AuScope program.

Episodic deployment of GPS equipped wave rider buoys enables the computation of SSH on a continuous basis in an absolute reference frame (the ITRF). The custom built GPS buoys are deployed over the mooring oceanographic instrumentation typically for durations up to 48 hours. Estimates of SSH are determined at rates of 2 Hz over this period. The GPS buoy SSH is filtered to remove the effects of swell and other high frequency effects before comparison with the mooring derived SSH. This comparison, achieved using data from several GPS buoy deployments effectively enables the transformation of the mooring SSH onto the absolute datum defined by the buoy SSH. At this point, the mooring SSH can be directly compared to the altimeter to derive the altimeter absolute bias.

The GPS buoy used has been custom built by Dr Tony Sprent (UTAS). The current evolution (Mk IV) consists of a central instrument capsule that houses a Leica dual frequency carrier phase GPS receiver and power sub-system. The GPS antenna sits approximately 0.5 m above the mean water surface. Three external floats stabilise the buoy which is typically tethered to a mooring for the duration of its deployment.

If you would like to use the derived GPS buoy SSH data, contact the PI