Once level 2 data (i.e data incorporating instrumental, atmospheric and geophysical corrections, and validated for quality assurance) have been processed, more advanced processing can be applied. This involves data qualification, pre-computing some parameters, re-projecting others, making a grid out of the along-track data, and even merging several different datasets from various satellites.
Selecting valid data
Processing high-level altimetry data begins with quality control, and validation of altimetry data and geophysical corrections. Only valid ocean data are selected. Editing criteria for each satellite is provided in the handbooks.
Applying altimetric corrections and computing physical parameters
Altimeter measurements have many potential sources of error. They need to be corrected for instrumental errors and geophysical influences (tides, atmospheric delays, etc). Then, by subtracting the range from the satellite’s altitude, the (corrected) surface height can be obtained. Other parameters can also be derived, such as sea level anomalies, etc.
Reducing orbit error: multi-mission processing
Multi-mission crossover differences are minimised by multi-satellite orbit error determination, or by adjusting less accurate orbits using a more precise mission as a reference. This is done for missions that do not have very accurate orbit determination (ERS and GFO) but also to remove biases and differences with the reference mission (T/P from February 2004, Envisat) [Le Traon et al., 1995], [Le Traon and Ogor, 1998]. Using the precision of the reference mission orbit, the orbit error can be estimated very accurately.
Mapping (or gridding) the along-track data
Altimetry data are, basically, along-track. For ease of use and compatibility with other datasets, they can be gridded, i.e. data along the track are interpolated and provided as points on a grid (e.g. data every 1/3° in longitude and latitude).
– Le Traon, P.-Y., F. Ogor, ERS-1/2 orbit improvement using Topex/Poseidon: The 2 cm challenge, J. Geophys. Res., 103, C4, 8045-8057, 1998.
– Le Traon, P.-Y., P. Gaspar, F. Bouyssel, H. Makhmara, Using Topex/Poseidon data to enhance ERS-1 orbit, J. Atm. Ocean. Tech., 12, 161-170, 1995.