The main sources of errors

The quality of the merged L4 products directly depends on the quality of the L3 products used in input of the L4 processing. Nevertheless, the main source of error comes from the sampling capability of the altimeter constellation. The more altimeters are available; the best is the mesoscale sampling. Another source of error for L4 products is directly linked to the methodologies and parameters applied for SLA interpolation on a regular grid. Optimal Interpolation (see DUACS processing) used in DUACS processing does not allow the restitution of the full dynamical spectrum limiting the capability of retrieving small mesoscale in L4 products (Chelton et al, 2011 and 2014).

Quality of the global gridded products (MAJ ?)

Quality of the regional gridded products (MAJ ?)

NRT vs REP products

The NRT gridded L4 products are usually less accurate than the DT one. Different factors explain this degradation:

• The reduced accuracy of the different parameter and corrections applied for SLA computation as discussed before
• The availability of altimeter measurements potentially reduced in NRT condition due to problem on platforms or ground segment, usually retrieved in DT conditions.
• The uncentered temporal window used in NRT L4 processing since measurements in the future are not available, reducing the amount of data by a factor 2 compared to the DT conditions.

The last one is considered as the main sources of errors for mesoscale signal reconstruction in L4 NRT products as discussed in Pascual et al (2008). The authors showed that at least 4 altimeters are required in NRT conditions to retrieve the same accuracy of the DT products generated with only 2 altimeters.