The L3 products are along-track subsampled and cross-calibrated products.
The Sea Level Anomalies (SLA) are used in oceanographic studies. They are computed from the difference of the instantaneous SSH minus a temporal reference. This temporal reference can be a Mean Profile (MP) in the case of repeat track or a gridded Mean Sea Surface (MSS) when the repeat track cannot be used. The errors affecting the SLAs, MPs and MSS have different magnitudes and wavelengths. The computation of the SLAs and their associated errors are detailed in Dibarboure et al, 2011 and Pujol et al, 2016. Both MP and MSS are referenced to the same reference period as specified in the QUID document. The methodology to change the reference period is presented in Pujol et al 2016.
Along track (L3) noise filtering
The filtering processing consists in removing from along-track measurements the noise signal and short wavelength affected by this noise. This processing consists in a low-pass filtering with a cut-off wavelength of 65 km over the global ocean. This cut-off wavelength comes from the study by Dufau et al. (2016) and is discussed in Pujol et al, (2016). It represents the minimum wavelength associated with the dynamical structures that altimetry would statistically be able to observe with a signal-to noise ratio greater than 1. The cut-off is reduced for regional products in order to preserve as much as possible the short wavelength signal. The different cut-off wavelength used are summarized in the QUID document.
The filtered along-track products can be subsampled before delivery in order to keep every second point along the tracks, leading to a nearly 14 km distance between successive points. Because some applications need the full resolution data, the non-filtered and non-sub-sampled products are also distributed in REP/DT mode and over some regions in NRT mode. The different subsamplings are summarized in the QUID document.