-£ Menn et al. 
SVP-BRST Fiducial Reference Network 
Barometer port 
air intake 
FIGURE 1 | Photo of SST sensor (left) and of MoSens device with HRSST 
sensor (right) 
CONCEPTION OF THE REFERENCE 
BUOYS 
A drifting buoy with a novel sensor package has been 
developed, called SVP with Barometer and Reference Sensor 
for Temperature (SVP-BRST, see Poli et al., 2018). It is a 
spherical drifter of 40 cm diameter made of high pressure molded 
polypropylene (see Figure 2). A 12.5 m-long line (including 
an elastic section) is attached below the buoy and linked to a 
stainless bracket. A holey sock drogue centered at 15m depth 
is suspended to the line. It is 0.8 m in diameter and 6m in 
length. The drogue loss is detected by a strain gauge, instead of a 
submergence sensor (Lumpkin and Pazos, 2006). 
A GPS receiver is included in the buoy to provide position 
estimates, and various GPS quality parameters. The strain gauge 
reading and the GPS Time To First Fix (TTFF) are transmitted 
as indicators of drogue loss. The transmission is made hourly, 
via a 30-bytes iridium Short-Burst Data (SBD) message, in a new 
dedicated format (Blouch et al., 2018). 
The buoy is based on the SVP-B design (Sybrandy et al., 2009). 
It is equipped with a Vaisala PTB 110 BAROCAP sensor featuring 
an accuracy of +0.6 hPa (according to Vaisala documentation) 
for temperature variations from 0 to 40°C (40.3 hPa for the 
temperature range from 15 to 25°C). It is delivered with a 
NIST traceable calibration certificate. The measurement of SST 
is made by two sensors: a regular SST sensor with an initial 
trueness superior to 0.1°C and a new High-Resolution SST 
sensor called HRSST. As recommended by best-practices, both 
are protected from solar and buoy radiations by a cap. The regular 
sensor for SST (called analog sensor thereafter) is made with 
two cupronickel bolts of diameters 1.4 and 1.9cm, protecting 
a 6mm tube in which is inserted a thermistor (see Figure 1). 
This configuration is used on nke Instrumentation SC-40, and 
is similar to that of other SVP buoys. 
The HRSST sensor is composed of a thermistor inserted in 
a small stainless steel needle of 0.9 cm length L and 0.12 cm in 
diameter D. The resolution is 1 mK and its trueness is expected 
to be better than 0.01 K. 
The HRSST sensor is associated with a hydrostatic pressure 
sensor in a removable cylindrical housing containing the 
electronic board of the two sensors. The cylindrical housing 
is necessary to calibrate these instruments in thermo-regulated 
baths, but it is removed when these modules are integrated in the 
buoys. This assembly is called MoSens by nke Instrumentation. 
The MoSens module hydrostatic pressure sensor presents a 
theoretical trueness of 0.05% on a range of 0-30 dbar and a 
rontiers in Marine Science | www.frontiersin.orı 
Digital HRSST sensor 
and associated 
hydrostatic pressurı 
sensor 
Analog SST sensor 
atrain gauge 
FIGURE 2 | Schema of the buoy with its sensors (the line, the stainless bracket 
and the holev sock droque are not represented}. Doc ® pke Instrumentation 
resolution of 0.05 dbar. Data are transmitted to the buoy mother- 
board by a serial link Modbus. 
THEORETICAL CONSIDERATIONS ON THE 
TEMPERATURE MEASUREMENTS 
de Podesta et al. (2018) demonstrated that “the radiative error 
for an air temperature sensor, in flowing air depends upon the 
sensor diameter and air speed, with smaller sensors and higher 
air speeds yielding values closer to true air temperature.” HRSST 
and SST analog sensors are protected from direct solar radiations 
by a cap but one part of the sunlight enters also the ocean. 
Seawater is close to a blackbody in the infrared part of the 
spectrum and the blue-green radiations can be reflected at depths 
as great as 50m (Le Traon, 2018). In the ocean, light radiations 
are reflected by particles or phytoplankton, and absorption and 
scattering decrease strongly their intensity, and hence their effects 
on exchanged heat flux. This exchange is therefore secondary 
compared to the impact of convection or conduction. However, 
one part is backscattered to the surface and can be detected 
by satellites to measure ocean color. This part can also be 
detected by sensitive sensors and it is interesting to evaluate 
the error induced by radiations on the measurement of the true 
temperature of seawater. 
It is therefore interesting to see if de Podestas affırmation is 
also true in seawater. He establishes the balance equation between 
the heat flux exchanged in steady state and the fluxes due to 
irradiation and self-heating as follows: 
hA(Ts — Tsw) = [P”R+ es LDE] + 40€es AT? (Tw — Ts) 
(1) 
where h is the heat transfer coefficient, A = xLD is the surface 
of exchange of the sensor considered as a cylinder of length L 
and diameter D, Ts is the temperature of the sensor’s surface, Tıw 
Qantembear 2019 I Valııme A 1 Article R7£