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Full text: The Copernicus Surface Velocity Platform drifter with Barometerand Reference Sensor for Temperature (SVP-BRST)

P. Poli et al.: SVP-BRST: genesis, design, and initial results 
Ocean Sci., 15,199-214, 2019 
0.00 0.05 0.10 0.15 0.20 0.00 0.05 0.10 0.15 0.20 
SST uncertainty (K) SST uncertainty (K) 
(SLSTR 3-channel nighttime) (SLSTR 3-channel nighttime) 
Figure 5. SLSTR SST uncertainty validation plot for (a) all drifters 
and (b) a subset of HRSST-1 and HRSST-2 drifters, with uncer 
tainty bins of 0.001 K. An uncertainty of 0.05 K is assumed for the 
drifter SST. 
Also, when in that situation, the float is more likely to reach 
wave crests. There, the sky visibility is improved, reducing 
the GPS time to first fix (TTFF), which can serve as an addi 
tional indicator of drogue loss (Petolas, 2013). 
To investigate the influence of the drogue, the SVP-BS 
data record is revisited. These buoys used submergence sen 
sors, whereas drifters nowadays use strain gauges, e.g., as 
indicated by Rio (2012), who developed a advanced method 
to identify drogue loss using drifter currents, satellite altime 
try, and wind reanalysis data. The submergence (or tether 
strain gauge) readings are neither straightforward to inter 
pret nor fully reliable on their own (Rio, 2012). Flowever, 
the SVP-BS drifter data considered here (available from the 
Coriolis In Situ Thematic Assembly Center) are not found 
in the drifter dataset of Rio and Etienne (2018), which in 
cludes drogue presence flags. Consequently, for this analy 
sis, we use the submergence and GPS TTFF data. A visual 
inspection indicates that 10 of the 20 buoys in Table 2 have 
lost their drogues during their mission. For these buoys, two 
series of data records are extracted: (1) before drogue loss 
and (2) after drogue loss. 
During daytime, the median of the differences between 
the twin SST measurements is —0.04K in (1), whereas it is 
—0.03 K in (2). The reduction in differences may appear in 
significant, but it is consistent with the CT sensor being more 
often exposed to depths similar to the sensor integral to the 
hull when the drogue is lost than when the drogue is present. 
Similarly, the robust standard deviation of the differences be 
tween the twin SST measurements is 0.03 K in (1), whereas 
it is 0.01 K in (2). Again, this reduction is consistent with 
drogue loss for the same reasons. 
During nighttime, no influence of the drogue loss is ex 
pected if the temperatures are homogeneous just below the 
surface. This is indeed what is observed. The median of the 
differences is —0.04K in both (1) and (2), and the robust 
standard deviation of the differences is 0.03 K in both (1) 
and (2). 
In other terms, the SVP-BS data record confirms the ex 
pectation that once the drogue is lost, the SST probes on a 
drifter are more likely to be exposed to water immediately 
below the surface than when the drogue is present, and this 
effect is more visible in the presence of stratification (e.g., 
during daytime). To keep track of the drogue effect on SST 
measurements, it is important to monitor drogue loss as well 
the immersion depth and its variations. 
2.6 Limited traceability 
Adopting a more general point of view for SST observations, 
several works have already attempted to document the un 
certainties in the various in situ SST measurement methods. 
The present paper does not attempt to review all these efforts 
but cites relevant results from the comprehensive review of 
Kennedy (2014). While the focus of this earlier work was on 
the creation on long time series, with the largest issues iden 
tified at the time of World War II (transition on ships from 
bucket to engine-room intake), the quality of SST buoys was 
found to be the subject of several concerns. The first con 
cern is the spread in quality between buoys, depending on 
the source of the uncertainty estimate, with no reliable link 
to the actual metrological reference. The second concern is a 
suggested improvement in quality over time, though without 
quantified evidence or clear a priori reason for it that would 
be explained by metrological documentation. Both points 
stem from an insufficient knowledge of the sensor technol 
ogy, and of the calibration procedure that was actually used, 
for each drifting buoy deployed. The results shown earlier, 
showing differences in SST quality between general drifters 
versus HRSST drifters, reinforce the importance of enhanc 
ing the knowledge of drifter metrology and metadata. 
3 Design of the SVP-BRST 
The HRSST-2 efforts were initiated by the cal/val needs of 
AATSR SST retrievals. With the demise of this instrument 
after 10 years of service in 2012 (ESA Communications De 
partment, 2012), the HRSST-2 developments were put to a 
halt, until the replacement sensor (SLSTR on Sentinel-3) was 
launched. However, this gap gave time to finish all HRSST-2 
deployments and review the lessons learnt from them. Cou 
pled with the need to assert long consistent time series of SST 
at an accuracy level compatible with SLSTR requirements, 
sound bases were used to imagine a novel sensor package 
for reference SST. The result is the SVP-BRST, based on the 
SVP-B design (Sybrandy et ah, 2009), with a strain gauge 
to detect drogue loss. In addition, the HRSST-2 requirements 
presented earlier are included, as well as others, described 
The first additional requirement is to employ an addi 
tional HRSST sensor, in addition to the regular SST sensor. 
The HRSST sensor collects data within the 5 min before the 
round hour, when the position is updated by means of GNSS. 
The mean SST is to be computed from 1 Hz SST measure 
ments. In addition, the data can be relayed at 1 Hz frequency

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