MERCATOR OCEAN JOURNA: 
SEPTEMBER 2021 
2. STATUS AT THE END OF COPERNICUS 1 
In 2021, the SL-TAC is managing 4 product lines in NRT; 9 
9roduct lines in REP/DT, including the redistribution of C3S 
sea level products; 6& product lines for auxiliary and 
Intermediate data. This represents nearly 125 different 
datasets. 
The SL-TAC NRT processing system acquires and processes 
measurements from 6 different altimeters, including the 
Sentinel-3A/B tandem. They improve the mesoscale signal 
sampling and also ensure the continuity of climate signal 
thanks to the cross-calibration processing using the Jason 
refence mission. The high number of altimeters available 
also ensure the system resilience in case of failure of one 
mission. 
Procedures used for the different system evolution and 
Implementation were consolidated and allows the SL-TAC 
to face important necessary emergency evolutions, as the 
temporary absence of the reference mission encountered 
in 2019 and 2020, minimizing the impact to users and 
applications. 
S5L-TAC continuously worked on the evolution of products 
and their quality improvement, using outcomes from 
different R&D projects (e.g., CNES DUACS-RD; ESA CC). The 
content of SL-TAC products was completed with different 
fields, auxiliary or derived from altimeter measurement, 
that can be used for different applications. Products quality 
was also refined, as well as its description for users. Six 
additional scientific peer-reviewed publications were 
issued (Pujol et al., 2016; Juza et al., 2016; Heslop et al., 
2017; Taburet et al., 2019; Ballarotta et al., 2019, Sanchez- 
Roman et al., 2020) and QUID documents have been 
Jpgraded with up-to-date quality results. 
S5L-TAC products, based on conventional altimeter 
measurement, now reached their maturity for the 
>bservation of the large mesoscale signal in open ocean. 
Along-track a/o0 gridded products are currently used by all 
MFCs, for assimilation a/o0 validation purpose and are also 
a useful upstream product for the Multi Observations TAC. 
The gridded altimeter product defined for the global ocean 
is currently the second most downloaded product by 
external users, with about 2000 distinct users identified in 
NRT and nearly 3500 in REP/DT. 
2.1 Post 2021 perspectives 
In the coming years, the SL-TAC system will first have to be 
updated following the evolution of the altimetry 
constellation to ensure a robust sampling with a minimum 
of 4 satellites. These changes will first include the 
integration of new altimetry missions: S6A (2021); J3-EOL 
(2021), HY2C (2021), HY2D (2022), SWOT's nadir (2022). 
53C (2023). The Interleaved SAR/ Low Resolutior 
Measurement (LRM) radar measurement modes on S6A 
Will allow to continue the 30-year LRM timeseries. Although, 
a longer-term objective will be to use the SAR processing 
an the Sentinel altimeter missions. S3C (SAR) and HY2C 
and D (LRM) will be integrated to complete the coverage. 
Then, strong efforts need to be done to improve the 
resolution of sea level products, today limited to 
wavelengths larger than -70 km (Dufau et al., 2016, 
Vergara, 2019). This will be possible thanks to the 
improvement of instruments (SAR on board S3A/B vs 
historical LRM) and the associated processing. They wil 
enable a reduction of the posting of the along track product 
to -1 km and the provision to regional MFCs improve 
ıbservation and associated error for assimilation. 
After 2022, the launch of SWOT mission will most probably 
pen a new area with its unprecedented spatial resolution 
with 2D swath of sea level. The availability of high resolution 
along-track and sSwath products combined with new 
mapping methods, such as dynamic mapping (Ballarotta et 
al., 2020) or multiscale calculation, will enable a higher 
resolution of L4 mapped products at global and regiona 
scale. Data driven method has also emerged these last 
years and could be an alternative in the coming years. 
Local improvement will be performed in areas where the 
coverage and quality of the sea level observations are 
strongly degraded, notably in polar and coastal areas 
Finally, regular reprocessing will be crucial on all altimeter 
missions with up-to-date geophysical corrections 
(atmospheric fields, barotropic and internal tides, MSS, 
etc.) to improve and homogenize the processing to reach 
the accuracy needed by climate applications