MERCATOR OCEAN JOURNA: 
SEPTEMBER 2021 
biological communities. These products have mainly 
consisted of directly usable and easy to manipulate Level 3 
(along-track cross-calibrated Sea Level Anomalies (SLA)) 
and Level 4 products (multiple sensors merged as maps or 
timeseries) and have been available in global and regional 
versions over European Shelves. In 2015, the SL-TAC 
started the Copernicus Marine Service in demonstration 
mode, with only 6 main product lines. During Copernicus 1, 
the SL-TAC has been completely transformed to a fully 
operational system. This transformation has been carried 
out according three main axes. 
1.1 Insure a robust production ingesting 
from 4 to 6 altimeters 
The first achievement has been the efficient and robust 
service provided along the 6 years, without interruption of 
the NRT production and with regular temporal extensions of 
the REP/DT series, nominally on a quarterly basis since 2015. 
To ensure this service and maintain the system’'’s resilience 
and product quality, the SL-TAC implemented 22 different 
versions within the 2015-2020 period corresponding to 
different evolutions implemented. These versions handled 
various upstream product versions changes and platform 
incidents of the altimeter constellation. Then, the 
consolidation of the upstream data ingestion has been a 
major priority. 
Early 2015, measurements from 4 different altimeters were 
used. The SL-TAC has integrated measurements from new 
altimeters as soon as they were available and nowadays, 
Neasurements from 6 different altimeters are operated. The 
capacity to manage different product versions changes and 
platform incidents of the altimeter constellation, made the 
SL-TAC able to maintain the system resilience and product 
quality. Main achievements in terms of constellation 
management are: 
ı‚everaging measurements from Jason-3 mission in NRT 
conditions since September 2016. Jason-3 is the so-called 
eference altimeter: it guarantees the large-scale accuracy 
af SL-TAC products in NRT and it is the backbone of the long- 
*erm stability of REP/DT products for Climate research. The 
atroduction of these measurements in the system was 
'hus particularly challenging. It was indeed necessary to 
‚ake into account the global and regional sea level biases 
abserved between Jason-3 and its predecessor Jason-2 
neasurements in order to ensure the continuity of the 
sea level and avold any impact on users and applications. 
A specific procedure was defined to properly manage the 
*emporal variability of these biases. It included a strong 
coordination with space agencies ensuring data avallability 
or both altimeter missions during the CalVal phase. This 
collaboration has been necessary to estimate the sea level 
lases that SL-TAC has needed to manage, 
reinforcing and using the method and procedure 
previously defined to manage emergency situations 
like the different prolonged data outage of the reference 
altimeter mission that occurred in 2019 and 2020. The 
interruption of the measurements for a long period of 
more than 8 days affected the NRT product generation of 
the SL-TAC. To secure the NRT service continuity and to 
avold significant offsets and biases in ocean topography 
products, Sentinel-3A was used as a temporary anchor 
in the SL-TAC system. While not a reference altimeter, it 
whas been stable enough over a few weeks to ensure 
a seamless transition over the Jason-3 safe hold mode 
period once NRT regional bias maps were estimated, 
introducing the Sentinel-3A mission In the system 
early 2017, in collaboration with Eumetsat. In the same 
way, the Sentinel-3B mission was introduced in early 
2019. The global Synthetic-Aperture Radar (SAR) mode 
coverage of Sentinel-3A and B enables more accurate 
observations of small mesoscale signals and make 
them valuable measurement for the SL-TAC production. 
Additionnaly, the tandem leads to a better signaı 
sampling thanks to the optimal 140° orbit phasing 
(as requested a few years ago by the Copernicus 
Marine Service). Since 2019, with 6 different altimeters 
available, the Sentinel-3 tandem contributes to more 
than 37% of the gridded (L4) SL-TAC global product. This 
contribution can reach more than 50% when the number 
of other altimeters available is reduced (Figure 1). An 
OSE showed that the Sentinel-3 constellation provides 
a significant gain of energy especially in high varlability 
regions and it reduces errors by 50% in in these areas 
(Figure 2). Thereby, the Sentinel-3 constellation appears 
to be crucial in the restitution on the ocean mesoscale 
eddies in Copernicus Marine Service systems, 
- assessing the quality of the Sentinel-3A mission thanks 
to an experiment in the Algerian Basin. It operated 
an ocean glider and a ship mission, along the same 
track and in synchronicity with an overpass of the 
Sentinel-3A mission. This provided three independent 
views of the ocean velocity field, along a section that 
encompassed three different oceanographic regimes. 
The results demonstrated the capacity of Sentinel- 
3A to retrieve fine-scale oceanographic features (>20 
km). The comparison with measurements from in situ 
platforms showed significant improvements, about 
30% In resolution and 42% In velocity accuracy, using 
a SAR mode with respect to lower-resolution mode of 
conventional altimetry (Heslop et al, 2017), 
- introducing the Chinese HaiYang-2B (HY-2B) mission in 
2020. It improves the mesoscale sampling over the globaı 
ocean and reinforces the SL-TAC system resilience when 
ıncidents occur on other altimeter missions