Article published in Marine Technology Society Journal: Environmentally Adaptive Deployment of Lagrangian Instrumentation Using a Submerged Autonomous Launch Platform (SALP)

1. P A P E R
Environmentally Adaptive Deployment of
Lagrangian Instrumentation Using a Submerged
Autonomous Launch Platform (SALP)
A U T H O R
David M. Fratantoni
Woods Hole
Oceanographic Institution
A B S T R A C T
Satellite-tracked surface drifters, acoustically tracked subsurface floats, and ac-
tively ballasted profiling floats provide an effective and efficient means of describing
the ocean environment over a wide range of spatial and temporal scales. Many
coastal and blue-water process studies require the repetitive deployment of such
instrumentation over periods of days to years. At best, reliance on ships and/or air-
craft for serial instrument deployment can be expensive and logistically difficult. At
worst, such deployments may be impossible in remote locations, areas of unfavor-
able weather, or seasonal ice cover or in response to transient or episodic phenom-
ena such as spawning events or severe storms.
The submerged autonomous launch platform (SALP) enables serial deployment
of an arbitrary mixture of drifting instrumentation (surface drifters, subsurface
floats, profiling floats) from depths as great as 2,000 m on a standard oceanograph-
ic mooring. A single SALP magazine allows up to 16 floats to be deployed automat-
ically according to a user-defined schedule, interactively by real-time acoustic
remote control or adaptively in response to observed environmental conditions.
Here, we describe the design and implementation of the SALP prototype and eval-
uate its performance during extended field trials in the Atlantic Ocean near Bermuda.
During these trials, moored subsurface measurements of temperature, pressure,
and velocity were autonomously processed by the SALP and used to preferentially
deploy novel glass-encapsulated GPS/Argos surface drifters within mid-ocean me-
soscale anticyclones.
Keywords: ocean currents, instrumentation, autonomy
Introduction
From early drifter measurements
made during the 1876 Challenger ex-
pedition (Bowden, 1954), to drift pole
measurements of coastal tides (Haight,
1938), to basin-scale drift bottle studies
(e.g., Brucks, 1971), up to the present
global array of ARGO profiling floats
(e.g., Davis et al., 2001; Roemmich &
Owens, 2000), our understanding of
the mean and time-varying ocean circu-
lation has been strongly influenced by
the observed trajectories of drifting ob-
jects. The development of satellite and
acoustic tracking systems over the past
40 years has resulted in a modern suite
of surface drifters, subsurface floats,
and actively ballasted profiling floats
(Figure 1), which provide an effective
and relatively inexpensive means of ob-
serving the ocean environment over a
range of spatial and temporal scales.
These tools are widely applied to stud-
ies of the low-frequency circulation,
the evolution of mesoscale phenome-
na, the transport pathways of water
masses, biota and pollutants, and the
statistical characterization of turbulent
dispersion.
Regardless of the scientific applica-
tion, the cost of the apparatus, or the
complexity of the tracking system,
most modern drifting instruments
(collectively referred to as floats, here-
after) must be manually deployed at
sea from a ship or aircraft under direct
human supervision.1
Such manual
deployment is relatively efficient for
float studies coordinated with existing
oceanographic surveys or vessels of
opportunity. However, many coastal
and blue-water process studies benefit
from the repetitive deployment of
drifting instrumentation over periods
of days to years. Such serial float de-
ployments are necessary in order to
resolve transient circulation phenom-
ena associated with, for example, sea-
sonal wind or buoyancy forcing,
mesoscale eddies, upwelling events,
or deep convection. Even when ship
or aircraft resources are available, the
1
A notable exception is the “Float Park” con-
cept pioneered by Prof. Walter Zenk for
timed-release bottom-moored RAFOS float
deployments (Zenk et al., 2000) and recently
applied by Bower and Fratantoni in the Gulf
of Aden (Bower et al., 2005). In this system,
floats are individually moored to the ocean
bottom using burn-wire-controlled anchor
weights and are asynchronously released when
an onboard timer expires.
66 Marine Technology Society Journal

2. cost, logistical complexity, and weather
dependence of these traditional de-
ployment platforms may compromise
the design, execution, and eventual
scientific dividend of float-based field
programs.
To address this problem, we have
developed a novel and cost-effective
approach to serial float deployment.
The submerged autonomous launch
platform (SALP) is deployed inline
on a conventional oceanographic
mooring at depths as great as 2,000 m
and is capable of automatically de-
ploying an arbitrary mixture of up to
16 floats of either positive or negative
relative buoyancy using a system of
burn-wire controlled clamps (Figure 2).
Hence, a single mid-depth-moored
SALP can be used to launch surface
drifters, mid-depth profiling floats
(e.g., Davis et al., 2001), and abyssal
RAFOS floats (e.g., Rossby et al.,
1986). Biofouling of floats during ex-
tended subsurface storage is mini-
mized by installing SALP at depths
well below the euphotic zone.
In the remainder of this article,
we describe the design and initial ap-
plication of the first SALP prototype.
In SALP Design and Implementation
and Low-Cost Surface Drifters for
SALP, we summarize the design and
implementation of the SALP and an
inexpensive satellite-tracked surface
drifter optimized for SALP deploy-
ment. Overall system performance
during open-ocean field trials is sum-
marized in the Field Trials and System
Evaluation section. Finally, in Dis-
cussion and Conclusions, we discuss
potential applications and future sys-
tem enhancements.
SALP Design
and Implementation
The design of the SALP2
was based
on the Data Capsule Magazine
(DCM) developed at Woods Hole
Oceanographic Institution (WHOI)
for the Ultramoor long-term mooring
system (Frye et al., 2000). The Ultra-
moor DCM is an inline mooring
element that stores compact data cap-
sules, which acoustically harvests data
from moored current meters, then in-
ductively transfers these data into peri-
odically released data capsules that
ascend to the surface and telemeter
data via satellite.
Ultramoor’s custom data capsules
were designed solely as a data relay
platform—their subsequent perfor-
mance as surface drifters was incon-
sequential to Ultramoor operation.
In contrast, the SALP was designed
to deploy well-characterized drifting
instrumentation that is generally com-
mercially available (e.g., RAFOS and
profiling floats) for the purpose of
initiating Lagrangian ocean measure-
ments. Transfer of subsurface mooring
data from the SALP controller to indi-
vidual floats via inductive telemetry
(in the manner of Ultramoor; see Frye
et al., 2000) has been considered but
not yet implemented. All floats must
use their own pressure, orientation,
and/or light sensors to enable power
FIGURE 1
Examples of drifting instrumentation commonly deployed for oceanographic research. (a) A 15-m
drogued surface drifter shortly after launch. (b) An isobaric RAFOS float. (c) A variable-buoyancy
profiling float (shown is an early APEX float produced by Teledyne Webb Research Corp.,
E. Falmouth, MA).
2
SALPs are pelagic tunicates often found
as clusters of gelatinous tubular segments.
Clusters of certain species (e.g. Pegea sp.) ex-
hibit an axial symmetry, not unlike floats stored
aboard the cylindrical launch magazine de-
scribed herein.
January/February 2014 Volume 48 Number 1 67

3. and initiate the float mission following
release from the SALP. In the case of
RAFOS and profiling floats, this is re-
duced to a relatively minor software
change since pressure sensors are rou-
tinely carried and regularly interro-
gated. The glass-encapsulated surface
drifter design discussed below incor-
porates a low-cost light detector and
the capability for addition of a pressure
or up/down orientation sensor for more
rapid system response during night-
time launches.
Ultramoor and SALP share several
fundamental design characteristics in-
cluding an aluminum magazine frame-
work and strength elements, liberal use
of plastic and titanium for long life and
low maintenance, acoustic telemetry
to avoid potential cable and connector
failures, and a simple float clamp/
release mechanism using low-cost, reli-
able burn-wire technology (see Fig-
ures 3 and 4). For flexibility, the SALP
was designed to use modular float trays
with float-specific clamping mecha-
nisms. The float tray concept was devel-
oped to allow a single SALP magazine
to deploy a variety of float types with-
out requiring modification to the mag-
azine structure or to the individual
floats. As built, the SALP can be con-
figured for up to 16 RAFOS (nomi-
nally 25 cm in diameter, 110-cm-long)
floats (or surface drifters built to this form
factor—see Low-Cost Surface Drifters
for SALP section below), eight of the
larger-diameter APEX/SOLO profiling
floats (10 cm in diameter, 220-cm long),
or any combination thereof.
SALP can be quickly reloaded in
the field with new floats in their asso-
ciated float trays by unbolting the old
tray and bolting the new tray in its
place. The burn-wire and spring actu-
ated release clamp are built into the
tray (Figure 4) so that all components
critical to the operation of the SALP
may be prepared ashore prior to ship-
ment and deployment at sea. A single-
conductor low-voltage underwater
cable connects each tray to the SALP
controller.
Floats deployed from the SALP are
larger and generally more buoyant
than the Ultramoor data capsules and
need to be held in place firmly for long
periods and under extreme stress dur-
ing mooring installation. Float clamps
FIGURE 2
Cartoon illustrating the SALP concept. Here, a single SALP is used to deploy surface drifters and
abyssal RAFOS floats. SALP enables floats to be launched at specific times, by interactive acoustic
remote control (here illustrated using a glider as a relay platform), or adaptively on the basis of
local environmental conditions. In this cartoon, SALP collects vertical stratification measurements
via acoustic telemetry from a modem-equipped moored profiler, processes and interprets these
data, and if specific user-defined criteria are met, deploys the appropriate instrumentation.
FIGURE 3
Major SALP mechanical components include
the welded aluminum magazine framework
and one of several interchangeable plastic
float trays with integral clamp assembly. The
SALP magazine is approximately 3.5 m in
overall length with a diameter of 1 m. Cylindri-
cal aluminum pressure vessels on the SALP
magazine hold the controller, batteries, and
acoustic modem.
68 Marine Technology Society Journal

4. and titanium springs were thus de-
signed specifically for each type of in-
strument to be stored aboard SALP.
Several at-sea tow tests of the SALP
magazine were conducted during ini-
tial development to improve and ver-
ify the clamp design. At launch time,
direct current is applied to a specific
float tray’s burn wire. When the burn
wire corrodes away (generally within
a few seconds in seawater), a titanium
spring forces the float clamp open,
simultaneously releasing the float from
captivity and pushing it out of its tray
and horizontally away from the maga-
zine to avoid fouling. The float then
rises (or sinks) to its preballasted mis-
sion depth.
In the simplest operational mode,
the SALP deploys floats according
to a predetermined schedule (e.g.,
weekly) programed into firmware on
a low-power single-board computer
(a Persistor CF2) contained within a
cylindrical aluminum pressure vessel
attached to the SALP magazine. In an
interactive command mode, an acous-
tic modem on SALP receives acoustic
commands from a nearby ship, surface
buoy, or AUV, which initiates float
release.
In an environmentally adaptive
mode, the SALP controller is fed data
from physically or acoustically linked
oceanographic sensors (e.g., CTD,
acoustic Doppler current profiler
[ADCP]) on a regular time schedule.
If certain user-defined threshold param-
eters are exceeded, the SALP controller
initiates float release based on these
inputs. For example, floats might be
launched in mesoscale rings on the
basis of velocity or temperature obser-
vations, surface drifters deployed in
coastal upwelling regions in response to
a temperature change or a fluorescence-
indicated phytoplankton bloom, or
profiling floats launched in convective
patches identified by vertical stratifi-
cation measurements. The SALP may
also be programed with various com-
binations of these operational modes.
It may be advantageous to target float
releases according to specific envi-
ronmental characteristics, with the
condition that a float is deployed at
least every X days with no more than
Y floats released in a Z-day period.
The prototype SALP system in-
cluded WHOI acoustic modems
(Freitag et al., 1998) on each of the
moored instruments and was attached
to the SALP controller. The acoustic
modem has a low-power detect mode
that allows it to always be on and lis-
tening for an acoustic message. The
sensor acoustic modems were defined
as transmit-only devices to minimize
their power drain and complexity.
The acoustic link frequency is centered
on 15 kHz, and the data transfer rate
during field-testing was 100 bits per
second net throughput after error cor-
rection coding. This conservative data
rate could be increased if high data
throughput were required. Inductive
communication between instruments
and SALP, using the mooring wire as
a communication medium, could be
used as a drop-in replacement for the
acoustic modems. However, a key ad-
vantage of acoustic communication is
the lack of dependence on the physical
connection between the sensor and
the SALP platform. Thus, an acousti-
cally linked SALP can communicate
with instruments on a mooring with
a synthetic rode, with instruments on
FIGURE 4
Photographs of the SALP during deployment at sea. (a) Partially loaded SALP being deployed
for the first 3-month test period southeast of Bermuda. (b) SALP on deck prior to the second
test deployment. For this test, 12 of 16 deployment positions were populated with LCD drifters.
(c) Detail of the SALP float tray clamping mechanism and the LCD drogue constructed of mussel
sock. (d) Close view of the SALP clamping mechanism in the open position.
January/February 2014 Volume 48 Number 1 69

5. adjacent moorings, or with a mobile
platform (e.g., AUV, glider) that is
either collecting data or serving as a
satellite relay platform.
The SALP control electronics are
powered by a lithium battery (∼50 D
cells) providing a minimum of 3-year
endurance. The primary energy user
is the low-power acoustic detector,
which has an average energy require-
ment of 50 mW. The system control-
ler remains in a sleep state more than
98% of the time, which makes its con-
tribution to the overall energy budget
almost negligible. The energy needed
to actuate the burn wires is small. In-
dividual sensors (e.g., CTD, ADCP)
deployed with or near the SALP
are powered by independent internal
batteries.
Low-Cost Surface
Drifters for SALP
To take advantage of the new ob-
servational capabilities enabled by the
SALP, we developed an inexpensive
GPS-navigated surface drifter pack-
aged within a standard RAFOS glass
tube (e.g., Rossby et al., 1986) (Fig-
ure 5). The low-cost drifter (LCD) is
capable of multiyear subsurface storage
onboard SALP at depths as great as
2,000 m and a drifting endurance of
6 months. This device is not intended
to compete with the standard WOCE/
SVP drifter for basin- or global-scale
observing programs (e.g., Fratantoni,
2001; Lumpkin & Johnson, 2013).
Rather, the LCD is a tool for focused
regional process studies requiring a
large number of inexpensive surface
drifters with moderate endurance.
Commercially available surface drift-
ers, while offering greater endurance
due to a larger hull volume, cannot
be easily adapted for use on SALP as
their spherical plastic/fiberglass hulls
are incapable of deep submergence,
and the compact and reliable packag-
ing of a 1-m holey-sock drogue would
be a substantial challenge.
The mechanical design of the LCD
utilizes the major mechanical and elec-
tronic components of a typical RAFOS
float with the addition of a GPS receiver
and a subsurface drogue (see Figure 5).
The pressure housing is a large (90 ×
2,200 mm) test-tube-like section of
glass rated to a nominal 4,000-m depth.
The GPS and Argos antennae are
mounted to a polycarbonate rod, which
is attached to the controller/lower end
cap assembly. The aluminum lower
end cap is a two-part assembly. One
half is permanently affixed to the
glass forming the primary glass to
metal seal. The other half utilizes an
O-ring seal permitting the case to be
opened and closed repeatedly without
compromising the seal on the glass. A
slight vacuum is drawn on the housing
to enhance this seal. Once at the sur-
face, the glass housing acts as a spar
buoy with the Argos and GPS anten-
nae exposed. The CLS/Argos satellite
tracking and data telemetry system
has been proven to be reliable, rela-
tively inexpensive, and well suited for
low-throughput applications such as
transmission of drifter position and
status information.
The LCD controller functions as
the interface between the GPS receiver
and the Argos transmitter. Its primary
tasks are power management and com-
munications. While attached to the
SALP, the LCD sleeps in a low-power
mode. A light detector mounted within
the glass hull senses when the released
drifter reaches the surface. This sig-
nals the controller to activate the GPS
and satellite transmitter and begin the
drifting mission.
The design target for the LCD was
a drifting endurance of 6 months with
GPS positions every 3 h. The amount
of energy available to power the LCD
is constrained by the buoyancy of
the pressure housing. Experience has
FIGURE 5
(a) Schematic summarizing the main constituents of the LCD developed specifically for SALP stor-
age and deployment. The LCD makes use of the same internal structure and glass pressure vessel
as a typical RAFOS float (e.g., Rossby et al., 1986). Photographs of the mussel sock material used
for the drogue are shown (b) at close range to show the scale of the mesh and (c) in a 15-m coil
ready for attachment to a SALP LCD.
70 Marine Technology Society Journal

6. shown the standard glass hull adequate
to support a 30 AH battery of alkaline
C cells at the surface while maintaining
a useful antenna height. Assuming
an Argos transmitter repetition rate of
90 s for 8 h per day (1/3 duty cycle),
the alkaline battery pack should provide
a service life of 180 days at a 3-h GPS
acquisition rate. Greater endurance or
more frequent positioning will require
the use of lithium batteries.
Without an attached drogue, the
water velocity measured by a drifter
may be contaminated by the direct in-
fluence of wind on the exposed surface
buoy (downwind slippage), by verti-
cally sheared near-surface currents, or
by the nonlinear rectification of surface
waves. Several studies (e.g., Geyer,
1989; Krauss et al., 1989; Niiler
et al., 1995) have investigated the ef-
fectiveness of various surface drifter
drogue designs. The characteristics of
a “successful” drogue design depend
largely on the scientific objective of
the drifter program. In coastal regions,
or when the emphasis is on short-term
mixed-layer processes, relatively small
drogues (less than 1 m2
in frontal
area) can be useful (e.g., Manning
et al., 2009). In contrast, the standard
drogue design of WOCE/SVP drifter
has a cross-sectional area exceeding
5 m2
. Several drogue designs were con-
templated for the LCD. The final ap-
proach utilized a 15-m-long flexible
plastic mussel sock clamped around
the lower circumference of the glass
tube (see Figures 4 and 5). This mate-
rial is lightweight, strong, and easily
rolled; provides a reasonable drag-to-
weight ratio; and is extremely inexpen-
sive. A 15-m length of mussel sock
provides a cross-sectional area of ap-
proximately 1.5 m2
. The mesh drogue
is deployed by the LCD’s controller
once the drifter reaches the surface by
means of a burn wire attached to an
aluminum weight mounted on the
drifter’s end cap. This system serves
to retain the drogue in a compact
bundle while aboard SALP but allows
the drogue to unfurl under minimal
tension once the LCD is released.
Field Trials and
System Evaluation
To verify the performance and reli-
ability of the SALP and LCD, several
short-term tests were performed at
the WHOI dock. These tests enabled
evaluation of the acoustic links, the
burn-wire release mechanisms, and
the various SALP operational modes.
A 5-knot tow test of a loaded SALP
float tray was completed to verify its
ability to withstand the rigors of de-
ployment. Local tests were followed by
two deployments in deep water south-
east of Bermuda near the Bermuda At-
lantic Time Series site (Figure 6). The
circulation in this region is dominated
by mesoscale eddies of O (100 km)
diameter, which move slowly west-
ward with time. This site (nominally
31o
40N, 63o
59W in a water depth
of 4,609 m) was chosen for its easy ac-
cessibility and long observational re-
cord (e.g., Michaels & Knap, 1996)
including time-series data from the
Bermuda Testbed Mooring pro-
gram (BTM; Dickey, 1995). Histor-
ical data from the BTM were used
to define the logic and thresholds
for environmentally adaptive drifter
deployment.
For the purposes of this engi-
neering field trial, we targeted drifter
deployments within anticyclonic
(clockwise-rotating, warm-core) meso-
scale eddies (e.g., McGillicuddy et al.,
1998). The core of a warm eddy is
characterized by a deeper-than-average
thermocline depth or, alternatively,
a warm anomaly on a particular
depth surface within the main ther-
mocline. Circulation about the warm
eddy core is considerably faster than
the background mean flow and may
extend to depths of several hundred
meters.
The SALP test mooring (Figure 6)
was designed to provide temperature
and pressure time series at two depths
using Sea-Bird SBE-37 Microcat
CTDs at 145 and 500 m. Velocity
was measured over the upper 120 m
using a 300-kHz RDI ADCP at 136-m
depth. One CTD was installed on the
SALP magazine frame and directly
wired to the SALP controller. A second
SBE-37 and the ADCP were equipped
with WHOI acoustic modems and
programed to send hourly measure-
ments to the SALP controller. The
SALP magazine itself was installed at
a depth of 500 m to avoid the strongest
near-surface currents and to minimize
biofouling.
Using BTM data, we developed
drifter launch criteria based on the de-
viation of recent temperature, velocity,
and pressure measurements from run-
ning 240-h means. This strategy en-
abled us to target rapidly developing
anomalous environmental characteris-
tics (such as passage of an eddy over the
mooring) without reference to specific
criteria (such as a particular measured
temperature), which might reflect
other time-varying phenomena such
as internal waves, meteorological
events (e.g., tropical storms), or sea-
sonal variations.
Specifically, we programed the
SALP controller to compute 24- and
240-h running means of raw hourly
measurements of temperature, pres-
sure, and velocity.3
Prior to mooring
3
Velocity information acoustically telemetered
from the ADCP was limited to three vertical
bins centered at 50-, 80-, and 110-m depth.
January/February 2014 Volume 48 Number 1 71

7. deployment, the BTM data were used
in conjunction with satellite altimetry
to define thresholds for these anoma-
lies consistent to the occurrence of a
mesoscale anticyclone at or near the
mooring site. The following useful
thresholds for 136-m temperature
(T), 110-m velocity (V), and 136-m
pressure (P) were found.
jT24 À T240j > 0:3°C
jV24 À V240j > 15 cm=s
jP24 À P240j > 50 m
where the subscripts denote moving
averages over the previous 24 and
240 h, respectively. Once deployed,
the SALP controller performed these
tests hourly following receipt of new
data from the CTD and ADCP instru-
ments. The next drifter in sequence
was launched if any one of these
three conditions were met. In addi-
tion to these environmental cues,
the SALP controller was programed
to launch at least one drifter in each
30-day period, while allowing no
more than two launches within a
14-day span.
The pressure criterion was moti-
vated by an examination of BTM
mooring dynamics, which indicated
substantial mooring blow-down dur-
ing periods of strong upper-ocean ve-
locity such as during passage of a
mesoscale eddy. The vertical excursion
of the moored CTD at 136 m can be
thought of as a proxy for upper-ocean
velocity integrated over a several-
hundred-meter vertical extent. This
was found to be a more sensitive indi-
cator of substantial velocity anomalies
than a single near-surface ADCP bin.
An initial 3-month deployment
(July 10, 2003-October 2, 2003) of
SALP with four LCD drifters aboard
demonstrated the ability of the SALP
controller to collect data from acousti-
cally and directly linked sensors on the
subsurface mooring and to perform
scheduled drifter releases. On recov-
ery, it was determined that the SALP
controller had experienced multiple
unexpected hardware resets, which
resulted in the loss of release-decision
diagnostic information. The system
was returned to WHOI, carefully eval-
uated and improved, and prepared for
a longer-term deployment.
A second SALP test mooring (Fig-
ure 6) with 12 LCD drifters aboard
was deployed in the same location
from April 12 , 2004, until May 14,
2005. System performance over this
13-month period was excellent with
several instances of environmentally
triggered drifter launches associated
with mesoscale eddy passage (Figure 7).
The SALP magazine functioned as ex-
pected with no major issues. The plas-
tic (PVC) components of the clamping
mechanism were found to swell slightly
at depth resulting in occasional stick-
ing. This caused several drifters to
be released later than intended even
though SALP controller made a cor-
rect release decision and triggered the
appropriate burn wire. The relevant
moving parts were subsequently rede-
signed with additional clearance.
All moored sensors recorded high-
quality full-length records, and 98.9%
of acoustically transmitted CTD data
and 97.5% of acoustically transmitted
ADCP data were received by SALP
error free. Acoustic communication
performance depends on environmen-
tal conditions including transmission
loss (spreading and absorption), rever-
beration, and ambient noise.
All 12 LCD drifters were released
due to appropriate environmental cues
or elapsed time intervals (Figure 7).
During the last 3 months of the deploy-
ment, the system correctly attempted
to release drifters from four unpopu-
lated bays on the SALP magazine, re-
sulting in a total of 16 release events
over 11 months. In total, SALP made
11 autonomous launch decisions
based on environmental cues. When
subsequently cross-checked with satel-
lite sea surface height fields, all of these
FIGURE 6
(Left) Sea surface height anomaly for September 29, 2004, derived from gridded multiplatform
satellite altimetry observations (http://www.aviso.oceanobs.com/duacs/). Note the substantial
number of mesoscale vortices of O (100 km) diameter, including an anticyclone coincident with
the location of the SALP test mooring. (Right) Mooring diagram summarizing the construction of
the SALP test mooring deployed southeast of Bermuda. The SALP was moored at a nominal depth
of 500 m. Hourly acoustic communications were maintained with an RDI ADCP at 136-m depth
and a Sea-Bird SBE-37 CTD at 145-m depth.
72 Marine Technology Society Journal

8. releases were consistent with occur-
rence of a mesoscale vortex near the
mooring site.
The novel LCD drifters functioned
well with several collecting data for
more than 9 months following deploy-
ment. A composite view of the 12 LCD
trajectories resulting from the second
SALP field test demonstrates the re-
markable variability in particle path-
ways that can result from serial drifter
deployment (Figure 8). Approximately
one half of the drifters meandered
slowly to the southeast of the mooring
site, while the remainder moved north-
ward and encountered the strong,
meandering Gulf Stream. While most
drifters remained within the western
subtropical gyre, one drifter with a par-
ticularly long lifetime (13 months)
crossed the Mid-Atlantic Ridge and
drifted as far east as the Azores. As
mentioned above, several drifters
were temporarily held aboard SALP
following burn-wire activation due to
excessively close tolerances in the
clamp design and inferred swelling of
plastic components due to long-term
deployment at depth. Because of
these delayed releases (sometimes as
long as a few months), only a few drift-
ers show compelling evidence of cir-
culation around an eddy.
Discussion and
Conclusions
Although the idea of launching
floats from a moored platform may
be unusual, quasi-Lagrangian and Eu-
lerian measurements are actually quite
complementary. While a moored cur-
rent meter provides unmatched tem-
poral resolution and easy access to
flow statistics and spectral character,
information regarding transport path-
ways and the characteristics of coher-
ent features are often easier to extract
from a float or drifter trajectory. Mul-
tiple floats launched from a single loca-
tion also provide rare access to basic
problems in geophysical fluid dynam-
ics. For example, LaCasce and Bower
(2000) have studied the relative dis-
persion of pairs of subsurface floats
in the North Atlantic with the goal of
statistically characterizing the effective
horizontal diffusivity. Unlike single-
particle statistics, measurements of rel-
ative dispersion can provide insight
FIGURE 7
The SALP controller compared 24- and 240-h moving averages of acoustically telemetered tem-
perature and pressure to determine when to release drifters. (a) Pressure and (b) the difference
between 24- and 240-h averaged pressure. The dashed line indicates the threshold (50 m) used for
a release decision. (c) Temperature and (d) the difference between 24- and 240-h averaged tem-
perature. The dashed line indicates the threshold (0.3°C) used for a release decision. In both (a) and
(c), the thin gray lines depict the raw hourly data, the bold line depicts the 24-h moving average, and
the thin black line depicts the 240-h moving average. (e) Timeline of release decisions. Note that,
while 16 release decisions were recorded, only the first 12 positions on the SALP magazine were
occupied with drifters.
FIGURE 8
A composite of all trajectories resulting from SALP-deployed LCD drifters during the year-long
second test deployment. The location of the SALP mooring southeast of Bermuda is shown.
The nominal location of the Gulf Stream north wall (with a one-standard-deviation uncertainty
band) is shown based on monthly mean observations since 1966 (courtesy of Fisheries and
Oceans Canada/MEDS).
January/February 2014 Volume 48 Number 1 73

9. into a variety of physical processes
operating simultaneously on different
scales (e.g., Er-El & Peskin, 1981). Be-
cause floats are not often launched in
pairs or clusters, the uncertainties asso-
ciated with relative dispersion mea-
surements are generally quite large.
One novel approach to reducing this
uncertainty might employ a SALP to
release multiple independent pairs of
floats into flows of known spectral
character as measured by a co-located
current meter.
Recently, two SALP magazines
loaded with profiling floats were suc-
cessfully utilized on a mooring in the
northeastern Labrador Sea (Furey
et al., 2013). Similar to our earlier
Bermuda field trials, the goal of this ap-
plication was to use environmental
cues to autonomously deploy floats
(profiling APEX floats in this case)
within Irminger Rings along the west-
ern coast of Greenland. In addition to
studies of mesoscale phenomena, we
envision several general scenarios
under which SALP could provide sub-
stantial benefit to ocean research.
Circulation Studies in
Geographically Remote or
Environmentally Hostile Regions
As shown by Furey et al. (2013),
SALP can facilitate intensive float
studies in regions previously inaccessi-
ble due to environmental constraints
or logistical complexities (e.g., high
latitudes, mid-ocean ridges, hydro-
thermal vents, politically unstable re-
gions, areas of seasonal ice cover).
The unit cost of a SALP is low enough
to obviate the need for recovery from
extremely remote locations. The im-
plementation of inductive transfer
of data from moored instruments to
drifters (in the manner of Ultramoor)
could make such an approach even
more beneficial, as subsurface mea-
surements could be retrieved even
if the subsurface instrumentation
could not.
Studies of Episodic
Ocean-Atmosphere Phenomena
Many important ocean/atmosphere
phenomena occur sporadically but with
great intensity (e.g., severe storms, deep
convection). A SALP could facilitate
remote investigation of the oceanic re-
sponse to strong atmospheric forcing
without requiring ships or aircraft to
remain on alert for extended periods
or to operate under extreme weather
conditions. A SALP could also be
deployed beneath a surface meteoro-
logical buoy to enable real-time remote
control and/or environmentally adap-
tive float deployment.
Maintenance of Large-Scale
Float Arrays in Regions of
Persistent or Divergent Flow
A small number of SALP moorings
could facilitate replacement of ARGO
profiling floats in rapidly flushed, in-
frequently visited regions such as the
Southern Ocean and the equatorial
Atlantic and Indian Oceans.
In summary, we believe that the
SALP concept of environmentally
adaptive drifter deployment has the
potential to provide broad benefit to
the oceanographic community by fa-
cilitating intensive and cost-effective
studies of ocean circulation and by en-
abling investigators to address difficult
research problems that are presently
financially or logistically untenable.
Acknowledgments
The substantial contributions of
Dan Frye, Jim Valdes, Don Peters,
Jon Ware, Peter Koski, and Ed Hobart
to the design and implementation of
SALP are gratefully acknowledged.
Mooring operations were led by John
Kemp and Will Ostrom with the assis-
tance of John Lund. We thank the cap-
tain and crew of the R/V Weatherbird II
for their able assistance at sea. SALP
development was supported by the
National Science Foundation through
Grant OCE-0136255.
Author:
David M. Fratantoni
Autonomous Systems Laboratory
Physical Oceanography Department
Woods Hole
Oceanographic Institution
Woods Hole, MA 02543
Email: dfratantoni@whoi.edu
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