Both Copernicus marine (CMEMS) and climate (C3S) services aim at answering the users’ needs for two main kinds of applications:

• The retrieval of mesoscale signals in the context of ocean modeling and analysis of the ocean circulation at global or regional scale. This requires the most accurate sea level estimation at each time step with the best spatial sampling of the ocean. Such dataset is produced within the Copernicus Marine Service (CMEMS) (previously distributed by CNES/AVISO).
• The monitoring of the long-term evolution of the sea level for climate applications and the analysis of Ocean/Climate indicators (such as the global and regional Mean Sea Level evolution). This requires a homogeneous and stable sea level record. Such dataset is produced within the Copernicus Climate Service (C3S).

Altimeter products	CMEMS	C3S
Recommended use	Ocean modelling, ocean circulation and mesoscale signals	Long-term evolution, climate applications, ocean monitoring indicators
Constellation	All-satellite merged	Two-satellite merged
Reference field	Optimised reference field for repetitive missions	Mean Sea Surface
Processing	Delayed-time (near real time also available in the catalogue)	Delayed-time
Distribution	CMEMS catalogue (previously distributed by CNES/AVISO)	C3S catalogue
Level 2 altimeter algorithms	Follow SL_cci recommendations when operationally compatible
Temporal resolution	Daily optimal interpolation
Spatial resolution	Gridded 1/4° (global) and 1/8° (regional products)
Time span and timeliness	Jan. 1993 – about 5 months delay
Temporal extension	3 – 4 times per year
Reprocessing	Every 3 – 4 years
Origin	E.U. Copernicus

Table: Main characteristics of the CMEMS and C3S DUACS altimeter sea level gridded products.

In the DUACS delayed-time products, the L2 altimeter standards compatible with the Copernicus operational constraints follow the ESA SL_cci recommendations. The main differences between the Copernicus Marine Service and C3S altimeter sea level products are the following:

• The number of altimeters used in the satellite constellation: all available altimeters are included in the Copernicus Marine Service products whereas a consistent number (two) of altimeters are included in the C3S products. Previous studies (Pascual et al., 2006 and Dibarboure et al., 2011) underscored the necessity of a minimum of a two-satellite constellation for the retrieval of mesoscale signals. Within the production process, the long-term stability and large-scale changes are built upon with the records from the reference missions used in both Copernicus Marine Service and C3S products. The additional missions (e.g. up to 7 additional missions at the same moment in 2019) are homogenized with respect to the reference missions. However, the total number of satellites strongly varies during the altimetry era and some biases may appear with the introduction of a new satellite flying on a drifting orbit. This may affect the stability of the MSL at global and regional scales. Even if the spatial sampling is reduced with less satellites, the risk of introducing such an anomaly is thus reduced in the C3S products and the stability is improved. In the Copernicus Marine Service products, the stability is ensured by the reference missions and the mesoscale errors are reduced due to the improved ocean surface sampling thanks to the use of all satellites available in the constellation.
• The reference used to compute the sea level anomalies is a mean sea surface (MSS) for all missions in the C3S products whereas a mean profile of sea surface heights is used for missions with a repetitive orbit in the Copernicus Marine Service products. These mean profiles increase the local accuracy of the sea level estimates, but the combined use of both MSS and mean profiles for successive missions in the Copernicus Marine Service merged sea level record can be at the origin of some biases affecting the sea level stability. Even if this has a minor impact at global scale (with respect to the uncertainty on an interannual timescale), the stability of the regional MSL can be affected, particularly in the Mediterranean Sea and Black Sea (see QUID and PQAR). So,
  the systematic use of the MSS for the sea level computation of all missions contributes to ensure the MSL stability in the C3S products,and the accuracy of the Copernicus Marine Service products is increased with the use of the mean profiles for repetitive missions.
• The temporal and spatial scales used in the mapping process (Optimal Interpolation) have been reviewed specifically for the Copernicus Marine Service production and are now different in C3S and Copernicus Marine Service sea level products.The spatial and temporal correlation scales have been adjusted (using an ensemble approach) to take into account the larger number of altimeters used in the Copernicus Marine Service configuration and to allow a better resolution of the mesoscale signal.Adjusting these scales yields significantly better results in areas of high ocean variability, especially at scales between 65 and 500km.