The L3 products are along-track subsampled and cross-calibrated products.

SLA computation

The Sea Level Anomalies (SLA) are used in oceanographic studies. They are computed from the difference of the instantaneous SSH minus a temporal reference (se FAQ for details). This temporal reference can be either a Mean Profile (MP), in the case of repeat track, or a gridded Mean Sea Surface (MSS), when the repeat track cannot be used. Details of the reference surface used in the DUACS processing is given in the dedicated FAQ page. Both MP and MSS are referenced to the same reference period as specified in FAQ page. The methodology applied to shift the reference period is presented in Pujol et al 2016.

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 the Copernicus Marine Service  QUID document.

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.

For the 1 Hz SLA construction, this processing consists in a low-pass filtering with a constant 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 to preserve as much as possible the short wavelength signal. The different cut-off wavelength used are summarized in Table 2. No subsampling is applied in the SLA 1 Hz processing.

Table 2: Filtering and subsampling parameters used for L3 1 Hz products.

Region	Product considered	Filtering cut-off wavelength (km)	Distance between two points (km)
Global	Filtered and not-subsampled	~65	~7
	Not-filtered and not-subsampled	–	~7
Europe	Filtered and not subsampled	~40	~7
	Not-filtered and not-subsampled	–	~7

For the 5 Hz SLA construction, two different filtering/subsampling are applied on the upstream 20 Hz measurements:

• For the raw (“unfiltered”) SLA 5 Hz computation, a filtering on the raw 20 Hz measurement is applied in order to filter the measurement noise at the 5 Hz Nyquist frequency (i.e., ~2,5 km). Then a subsampling is applied to extract the 5 Hz positions.
• For the filtered SLA 5 Hz computation, as for the 1 Hz processing, the short wavelength dominated by the measurement noise is reduced by a low pass filtering. Then, a subsampling is applied to extract the 5 Hz positions. The cut-off wavelength used for the filtering varies from one altimeter to the other, according to their observing capability estimated on the upstream full rate (i.e., 20 Hz) measurement.
  • For the processing over the European area, it was estimated over the North Atlantic area over a reduced temporal period (April-June 2022) and thus could be revised in the future. Currently, the cut of wavelength varies from 40 km for the Sentinel-6A HR mission to 55 km for the Jason mission (Table 3).
  • For the processing over the global ocean, the observing capability of the different altimeters was estimated over a nearly 1-year period (October 2022-August 2023), considering possible geographical variations. Figure 1 illustrates the results obtained for Sentinel-6A (SAR mode measurement) and Jason-3 (LRM mode measurement). The cut-off wavelengths can vary from 20 km (Sentinel-6A, West Pacific, to 120 km (HaiYang-2B, South-Est Pacific area). The mean observable wavelength over the global ocean is close to the one observed in the North-East Atlantic area (see Table 3).

Table 3: Effective mean spatial resolution of the DUACS NRT L3 5 Hz: Mean value for the North-Eastern Atlantic Area (estimated over the period April-June 2022) and global ocean (estimated over the period October 2022-August 2023).

	Sentinel-6A HR	Sentinel-3A/B	Jason-3	Swot-nadir	HaiYang-2B
Spatial Wavelengths observable in the North-Eastern Atlantic Area (km)	40	48	55	55	55
Mean spatial Wavelengths observable in the global ocean (km)	40	48	57	57	63

Figure 1: Spatial distribution of the cut-off wavelength used for the 5Hz “sla filtered” processing. Sentinel-6A (left) and Jason-3 (right).

Derived variables generation (for L3 5 Hz)

The L3 derived variables consist of geostrophic currents (absolute and anomalies) in the across-track direction. This processing is applied only for the final L3 5 Hz production.

The geostrophic current is computed using a finite difference of maximal order 5 in open ocean. The order is progressively reduced when approaching the coast or a gap with no valid measurement. This processing is applied on SLA, after filtering of the short wavelengths dominated by noises (see section bellow: Along track (L3) noise filtering), to retrieve the anomaly of the geostrophic current. It is also applied on MDT, to retrieve the mean component of the geostrophic current. This processing is applied on high resolution upset (20 Hz) measurements. The estimation of the geostrophic current in the equatorial band is not available.

The MDT field is obtained by interpolating the Mean Dynamic Topography (MDT) on along-track positions. The MDT used in the DUACS reprocessing is described in section Homogenization and cross-calibration

Measurement subsampling

For the L3 5 Hz production, a subsampling is applied on along-track measurement in order to reach the final 5 Hz sampling. Indeed, the previous processing steps are applied using the upstream measurement sampling, i.e., 20 Hz sampling. This subsampling is thus applied on the different variables delivered in the L3 5 Hz products (e.g., SLA, currents). No subsampling is applied in the L3 1 Hz processing.