As described in the deployments section, moored oceanographic instruments form the backbone of the in situ data collected to determine the altimeter absolute bias. Central to the task of deriving a sea level record is the deployment of high precision pressure gauges (SBE26s) on the base of each mooring, and temperature and salinity instruments located at various locations through the water column (SBE37s).
For some deployments, an aquadopp current meter is used to aid in the assessment of sea surface topography. In aid of the SEA-IMOS program, the Storm Bay site has also supported 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. Other instrumentation in use includes coastal tide gauges, GPS equipped buoys and land based GPS reference stations.
Mooring Sensors and Schematic
The typical mooring configuration at each site is shown in the figure to the right. The instrumentation includes the following (see the deployments page for specific details of deployment dates and exact instrument specification):
Data from the tide gauges at Burnie and Spring Bay, both operated by the Australian Bureau of Meteorology, 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.
The tide gauges at Burnie and Spring Bay (in the south-east) are both equipped with Aquatrak acoustic-in-air sensors. Both gauges provide data at 6 minute intervals (comprised of the average of 1 Hz samples over a 3 minute period, centred on the time stamp). These field sites are calibrated every 18 months by the National Tidal Centre (NTC).
Data from tide gauges is also used in the production of the altimeter bias drift data stream. These data are obtained directly from the University of Hawaii Sea Level Centre.
The GPS buoys used for IMOS deployments were custom built by Dr Tony Sprent (University of Tasmania). The buoy consists of a central instrument capsule that houses a Leica dual frequency carrier phase GPS receiver and power sub-system.
The GPS antenna (Leica AR10) sits approximately 0.5 m above the mean water surface. Three external floats stabilise the buoy which is tethered horizontally to a surface float secured with an anchor for the duration of its deployment.