Satellite - Altimetry calibration and validation

High precision satellite altimeter missions including TOPEX/Poseidon (T/P), Jason-1 and now OSTM/Jason-2, have contributed fundamental advances in our understanding of regional and global ocean circulation and its role in the Earth's climate and regional applications. These altimeter satellites essentially observe the height of the global oceans – as such, they have become the tool of choice for scientists to measure sea level rise – both at regional and global scales as well as giving information about ocean currents and large- and small-scale variability. The determination of changes in global mean sea level is of fundamental importance in understanding the response of the ocean to a continuing warming climate – both through thermal expansion of the ocean, melting of the major ice sheets of Greenland and Antarctica, and mountain glaciers, and redistribution of water over the continents and atmosphere. As with all scientific observations, it is vital that the measurement tool is operating within its specifications – calibration and validation is therefore an important component in this regard.

This IMOS sub-facility provides the sole southern hemisphere in situ calibration site to provide an ongoing calibration and validation data stream directly to the international (NASA and CNES sponsored) Ocean Surface Topography Science Team (OSTST). This contribution, importantly, spans multiple altimeter missions, thus enabling the assimilation of multi-mission satellite data to determine ongoing changes in sea level with flow-on effects to other uses of this data. The OSTST collectively oversees the operation and calibration of the precision altimetry missions, and ensures each of these missions is performing as accurately as possible in order to meet mission objectives. The IMOS supported calibration site in Australia is one of four primary in situ calibration/validation sites that contribute to the OSTST. The remaining sites include Harvest (USA), Corsica (France), and Gavdos (Greece). The Australian calibration site includes two comparison points where in situ data is compared against the altimeter – Bass Strait and Storm Bay. These two locations both lie on descending (N -> S) pass 088 of the satellite altimeter, and thus share similar satellite orbit characteristics. The use of these two sites allows detailed investigation into the accuracy of the altimeter over two different wave climates. The average significant wave height at Storm Bay is approximately double that observed at the comparatively sheltered Bass Strait location. One of the ongoing issues with satellite altimeter missions is the “Sea-state bias” correction which is related to wave shape and height. We plan to use the different wave climates at the two locations, coupled with the fact that some other things (such as orbit errors) are the same at both locations to improve the quality of this correction.