IEEE Robotics & Automation Magazine - September 2013 - 44

Depth

Figure 1. Depth values range from 50 m up to almost 1,800 m
in this area. From the oceanographic point of view, this region
is located in a frontal zone that divides the warmer and lighter
coastal waters from the colder and denser waters of the central zone [13]. Although this structure is rather persistent, it
has a significant seasonal and interannual variability and
intense mesoscale activity related to the strong frontal system.
A main current system, the so-called Northern Current (NC),
shown in Figure 1, flows along the continental slope with
speeds of 0.3-0.4 m/s−1. Synoptic time scales are in the order
of three days in this region.
A fleet of three coastal Slocum gliders [14] (Figure 2) was
deployed on 20 August 2010 at the southwestern boundary of
the access-restricted area, aiming to characterize this hypothetical battlespace. Deployments were equidistant, with two
gliders at the northern and southern boundaries and the last
one on the center line of the region. Glider trajectories at the
northern and southern boundaries were defined as straight
lines toward the coast and perpendicular to the continental
slope. The objective of these gliders was to characterize the
boundaries by means of perpendicular cross sections of the
frontal system associated to the continental slope. The central

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Figure 3. Temperature sections in °C between 20- and 85-m
depth collected by the fleet of gliders on 21 and 22 August. The
coastline is represented by the black line.

IEEE ROBOTICS & AUTOMATION MAGAZINE

september 2013

Robotic Marine Environmental Characterization
The data set collected by the robotic fleet was merged with
the SST map following a similar methodology as proposed in
[12] to generate an integrated picture of the volumetric temperature distribution in the region. Hereafter, this method
will be referred to as the "data-driven assessment method."

Depth

Figure 2. A Slocum glider used by NURC in the REP10 field
experiment.

glider was also directed toward the coast but its trajectory was
modified depending on local current information. The gliders
transited the access-restricted area until 22 August. The fleet
collected a total of 419 vertical profiles of water conductivity
(salinity), temperature, and depth (pressure) (CTD) between
20- and 180-m depth and with a horizontal interval of
~400 m (Figure 3) using an unpumped Seabird 41 CTD sensor with an accuracy of 0.002 °C and a resolution of 0.001 °C.
The gliders surfaced every 3 h to transmit the collected data.
In this work, the environmental assessment is focused on the
3-D thermal field. Characterization of the underwater temperature field is typically the foremost requirement in MILOC
for antisubmarine warfare operations as it provides the basis
for determining the range and performance of sonar systems.
In addition, the spatial distribution of this quantity at the sea
surface is routinely measured from remote sensing platforms.
In this field experiment, in situ measurements from the fleet
of gliders were merged with an ultra-high Mediterranean resolution SST analysis map with 1-km resolution obtained on
21 August (Figure 1). This map is based on infrared measurements collected by radiometers on various satellites with statistical interpolation to fill cloudy areas [15]. In addition, a
towed vehicle, ScanFish MK II, equipped with a pumped SeaBird 49 CTD sensor was used to sample the restricted area on
21 and 22 August to provide sea-truth data for the validation
of the environmental characterization procedures. The temperature resolution of the sensor is 0.0001 °C with an accuracy of 0.002 °C. The towed vehicle undulated between 5- and
80-m depths at a horizontal speed of 3 m/s−1, following the
trajectory displayed in Figure 1. The corresponding temperature profiles are illustrated in Figure 4.

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Figure 4. Temperature profiles in °C between 5- and 80-m depth
collected by the towed ScanFish on 21 and 22 August. The
coastline is represented by the black line.

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013