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"IGURE 2 | A schematic drawing of a Sea-Bird Scientific CTD unit. [Source: 
om fRiser et al.. 2018). Copyriaht 2018 American Geophysical Union (AGUM. 
measurements differ from the ensemble averages are considered 
anomalous and are thus returned to the manufacturer. 
At the University of Washington (UW), pre-deployment 
sensor checks consist of checking the pressure sensor and the 
conductivity cell. The pressure sensor is checked against a highly 
accurate Type T Hydraulic Deadweight Tester. Two sets of data 
are collected: the first at room temperature and the second at 
cold temperature, to simulate what the floats would experience 
in the ocean. At each of these two temperatures, measurements 
are collected at 5 different pressure levels spanning 0-2,000 dbar. 
At each pressure level, 3 consecutive measurements within 1 
dbar of each other are taken and a median filter is applied to 
get the final value. If any of the final values from the test CTD 
differ from the standard pressure by more than 2.5 dbar, the 
test CTD is returned to the manufacturer for recalibration. The 
Commonwealth Scientific and Industrial Research Organization 
(CSIRO) at Hobart, Australia, also conducts a similar deadweight 
pressure test at 20,000 kPa and 20°C. For the conductivity cell 
check, UW uses an in-house reference SBE-41 CTD that is 
calibrated regularly at Sea-Bird Scientific. The reference CTD and 
the test CTD are then plumbed to sample from the same batch of 
rontiers in Marine Science | www.frontiersin.orı 
Argo Data 1999-2019 
standard seawater with the aid of a peristaltic pump. The mean 
values from 100+ samples are compared. If the mean values differ 
by < 0.005 PSS-78, the test CTD is accepted for deployment. The 
Argo group at NOAA Pacific Marine Environmental Laboratory 
(PMEL) does a similar conductivity cell check. 
At the Japan Agency for Marine-Earth Science and 
Technology (JAMSTEC), shipboard CTD casts are conducted as 
often as possible when floats are deployed from research vessels. 
By evaluating float temperature values at deployment against 
shipboard CTD casts over 300 floats, JAMSTEC found that the 
differences were mostly within + 0.005°C. 
Satellite Communications 
At the beginning of Argo, almost all floats transmitted their 
data via the Systeme ARGOS> location and data transmission 
system (Argos-2), operated by Collecte Localization Satellites 
(CLS/Argos in Toulouse, France, and CLS/America in Maryland, 
JSA). These are one-way, low-bandwidth satellites, with an 
effective data throughput of no more than 1 bit per second. 
The data transmission rates are such that in order to guarantee 
error free data reception and location in all weather conditions, 
the float must spend between 6 and 18h transmitting at the 
sea surface. 
Other satellite transmission systems have been used on 
profiling floats over the history of Argo. The Argos-3 satellite 
system was implemented on ARVOR floats for evaluation of 
its interactive low- and high-data-rate modes, both offering 
bidirectional transmission and a higher throughput than the 
older Argos-2 satellite system. While the low-data-rate mode 
offered faster data transmissions, the high-data-rate mode 
suffered from electromagnetic noise around Europe (Andre et al., 
2015). At regional scales, non-global transmission systems such 
as BeiDou in Asia and Orbcomm in North America have also had 
limited use on profiling floats. 
At present, the majority of Argo floats (77% of the active 
fleet as of March 2020) use the Iridium satellite system for 
data communication (Figure 3). There are two methods by 
which data can be transmitted using Iridium (Riser et al., 2018). 
The first method uses a Circuit-Switched Data (CSD) channel 
and is usually routed via the Router-Based Unrestricted Digital 
Internetworking Connectivity Solutions (RUDICS). The second 
method uses Short-Burst Data (SBD) and is analogous to sending 
an SMS text message. In general, the CSD method is used when 
a large quantity of data needs to be transmitted, while the SBD 
method is used when the volume of data being transmitted is 
relatively small or when the data have been compressed. 
The first Argo float that used Iridium communication 
was deployed in 2005. Since then, Iridium has become the 
preferred means of satellite communication because it is a 
two-way, higher bandwidth system, with transmission rates 
of around 300 bits per second. Iridium allows more data to 
be transmitted in a shorter period of time than via Systeme 
ARGOS. The transition to Iridium dramatically reduced the 
”Throughout this paper, the term “ARGOS floats” refers to profiling floats that use 
the Systeme ARGOS (Argos-2) satellite system as the means of data transmission, 
while the term “Argo floats” refers to profiling floats in the Argo Program. 
Qanteambear 2020 1 Valııme 7 1 Article 701