ENVIRONMENTAL VARIABILITY
OF THE LBVDS SEA TESTS
S. SUTHERLAND-PIETRZAK AND E. MCCARTHY
Naval Undersea Warfare Center Division, 1170 Howell St., Newport, RI 02842, USA
E-mail: sutherlandsa@npt.nuwc.navy.mil
The Lightweight Broadband Variable Depth Sonar (LBVDS) Demonstration Model
(LDM) Sea Test was conducted in September-October 2001 off the eastern coast of the
United States. The primary objective of the LBVDS program is to develop and
demonstrate a prototype sonar system that will improve the Navy’s capability to conduct
surface undersea warfare (USW) operations, particularly against low-Doppler threat
submarines in highly reverberant, littoral areas. One of the sea test objectives of the
LBVDS program was to evaluate the benefits of environmental adaptation. Studies of
shallow-water active sonar system performance have indicated that a large loss in
potential performance is often incurred because the sonar operating parameters are not
matched to the current environment. The LDM system can recommend waveforms and
processing parameters that are based on environmental measurements and performance
predictions. Acoustic performance predictions and measured results from this LBVDS
sea test will be presented.
1
Introduction
Improvement in tactical active sonar detection and classification in shallow water
environments is a critical USW need for the Navy’s future missions in littoral areas where
quiet, slow-moving, diesel-electric submarines are a threat. In order to provide the
improved performance for tactical platforms, the LBVDS Program, funded by the Office
of Naval Research (ONR), was established to develop both high-energy-density, lead
magnesium niobate transduction technology and environmentally adaptive, broadband
processing technology.
The Naval Undersea Warfare Center (NUWC) Division, Newport, Rhode Island, and
Lockheed Martin Corporation, Syracuse, New York, conducted the LBVDS FY01
demonstration system sea test (Sea Test C) to demonstrate the full capability of the newly
developed source, the real-time broadband processing, and the new receiver. The MultiFunction Towed Array (MFTA) was used as the receiver. Both the source and the
receiver had variable depth capability. In addition, the system had automated
environmental adaptation capabilities that characterized the environment and could be
used to continually optimize the system setup and processing to match the changing
acoustic conditions. These new capabilities are expected to improve detection,
classification, and tracking performance against a deep target (below the surface layer) in
shallow water. The sea test objectives were (1) demonstration of variable depth sonar
capability, (2) demonstration of broad bandwidth waveforms and processing, and (3)
demonstration of the value added of environmental adaptation.
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N.G. Pace and F.B. Jensen (eds.), Impact of Littoral Environmental Variability on Acoustic Predictions and
Sonar Performance, 579-586.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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S. SUTHERLAND-PIETRZAK AND E. MCCARTHY
Sea Test C was the third in the series of tests. The two previous sea tests, Sea Tests
A and B (completed in 1997 and 1998, respectively) were conducted with the Broadband
Active Sonar Testbed (BAST) which had a transmitter with a lower source level than the
LDM. Sea Test A was conducted off the Gulf Coast of Florida. Sea Tests B and C were
conducted in the Long Bay area, off the coast of South Carolina (Fig. 1).
The littoral environment of the Long
Bay area is a good site for testing sonar
performance under the duress of changing
environmental conditions because of its
oceanographic temporal fluctuations and
spatial variations. Sea Test C occurred in
the same general location as Sea Test B,
Sea Test B
but the operational area was expanded to
and C
include a moderately reverberant bottom
type (sandy) that is more common in the
littorals than the highly reverberant
bottom type (rocky) encountered in Sea
Sea Test A
Test B. Most of the testing for Sea Test C
occurred in this moderately reverberant
Figure 1. LBVDS Sea Test sites.
environment. Data were acquired with an
impressive array of waveform types—
varied pulse types, bandwidths, center frequencies, and Doppler sensitivities—at tactical
ranges in multiple geometries, i.e., varied source and receiver depths and a target at
different depths and aspects.
Detailed results and recommendations from Sea Tests A and B are available in the
LBVDS Sea Test A and B final test reports [1–3]. This paper briefly discusses the data
processing results from Sea Tests A and B as background and then presents the
environmental observations and measurements and acoustic modeling from Sea Test C.
2
Background
The first test, Sea Test A, was conducted in the Gulf of Mexico with a point source target
(echo repeater). The test focused primarily on evaluating the benefits of additional
bandwidth (BW) in a moderately reverberant environment. Gains were measured at 8
log(BW), and analysis confirmed that the measured gain could be attributed to the
increased bandwidth.
The second sea test, Sea Test B, revisited the issue in the highly reverberant acoustic
environment of Long Bay, South Carolina. This test focused primarily on bandwidth
effects on hyperbolic frequency modulation (HFM) transmissions. A complete set of
HFM measurements was made for a specified set of center frequencies and bandwidth
combinations. All of these measurements were made with the transmitter at a single depth
below the layer.
Analysis of Sea Test B data confirmed that broadband waveforms offer increased
detection and tracking performance. Figure 2a shows the gains obtained in Sea Test A
against reverberation and ambient noise. Notice that, as expected, increasing the
bandwidth does not provide additional gain against an ambient background and may
ENVIRONMENTAL VARIABILITY OF THE LBVDS SEA TESTS
581
actually degrade system performance. However, in reverberant environments, in both Sea
Tests A and B, increasing bandwidth produced significant gains. In Fig. 2b, the signal
excess gains against reverberation are also plotted for three different center frequencies
(fc): low, medium, and high. In all three cases, the pulse length (T) remained constant,
and only the bandwidth for each center frequency changed.
Figure 2a. Sea Test A signal excess.
Figure 2b. Sea Test B signal excess.
The ocean bottom types for Sea Tests A and B were very different and consequently
produced very different reverberation backgrounds. Reverberation may also change
because of the center frequency of the waveform. In the Sea Test B environment, as can
be seen in Fig. 2b, the lower end of the band produced less reverberation. The
background levels were closer to ambient. The higher end of the band produced greater
reverberation levels, and the performance for this portion of the band was not as good.
However, this was not the case for Sea Test A, which was conducted in a more moderate
reverberation environment. The reverberation levels for Sea Test A were relatively
consistent across the band.
Determining the best center frequency and bandwidth for a specific environment is
important for optimizing system performance. The ability to measure the characteristics
of the ocean environment as it changes, both spatially and temporally, is therefore
essential to sonar system optimization. The Instrumented Tow Cable (ITC) was
developed under the LBVDS program to provide a more accurate measure of a key
environmental characteristic—the temperature profile of the water column.
The ITC design is based on an optical fiber embedded in the tow cable for the
source. Light transmitted in the fiber is used to measure the temperature of the ocean
water in situ. An example of the variability of ocean temperature as measured by the ITC
is shown in Fig. 3. The measurements were made every 3 minutes in the Hudson Canyon
[4]. Given the variation in temperature as a function of range and depth, the
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S. SUTHERLAND-PIETRZAK AND E. MCCARTHY
measurements provided a more complete environmental picture to support more accurate
acoustic modeling and performance prediction. Without the ITC, Fig. 3 would have
required over 60 expendable bathythermographs (XBTs).
0
Kilometers
30
Deg C
Figure 3. Instrumented Tow Cable (ITC) ocean temperature.
3
Environment
LBVDS Sea Test C required an acoustic environment that was shallow, reverberation
limited, and downward refracting, with a flat or gently sloping bottom. The test also
required a site with a low occurrence of endangered marine mammals. The site which best
met these criteria was Long Bay, South Carolina. This operational area (OPAREA) had
been studied in the previous LBVDS test (Sea Test B), and it was the site for two Towed
Active Receiver System (TARS) sea tests (June and September 1998) [5] and a Littoral
Warfare Advanced Development experiment (LWAD SCV 97) [6].
The bottom is characterized by a sand wedge that slopes gently down from northwest
to southeast, with the depth changing from 100 to 1500 m. The sand wedge gives way to
a rough, deeply scoured, limestone plateau at 200 to 250 m in depth before the
continental shelf plunges into the ocean depths beyond. The plateau has been observed to
be predominantly sand covered in the northeast and rocky in the southwest. All test
geometries were executed between the 100- and 400-m isobaths. This area is strongly
influenced by the Gulf Stream current and the “Charleston bump,” which diverts the
current and can form a stationary eddy that is almost perfectly centered in the OPAREA.
This eddy, consisting of warm Gulf Stream water surrounding a cool, coastal parcel, was
observed during the first phase of both LBVDS sea tests. The eddy appeared to travel
north (Fig. 4) during the second half of the test, so that fairly homogeneous Gulf Stream
water dominated the area during this phase.
The coastal waters of the Long Bay OPAREA were found to be highly variable in
both time and space. Figure 5 shows four XBT temperature profiles that represent the
basic profile types. Most of the coastward profiles exhibited the strong midcolumn
temperature gradient as characterized by profiles #14, #17, and #49. The upper
isothermal layer very often formed a deep surface duct, and the deeper isothermal layer
had a tendency to form another acoustic duct offset somewhat from the bottom. The
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ENVIRONMENTAL VARIABILITY OF THE LBVDS SEA TESTS
fluctuations in depth of the steep temperature gradients between the layers in sea test B
were found to follow a Garrett-Munk spectrum for internal waves; continuous
temperature data were not available for a similar analysis on Sea Test C. The uniform,
downward refracting temperature profile shown by XBT #103 represents the more
homogeneous waters of the Gulf Stream. In this case, reflection from the bottom allowed
both the downward refracting and layered profiles to form a bottom channel.
LDM representative XBTs
0
20
40
60
80
100
Depth (m)
120
140
160
180
XBT-14
200
XBT-17
220
XBT-103
XBT-49
240
260
280
300
10
12
14
16
18
20
22
24
26
28
30
Te mpera ture
Figure 4. Sea surface temperature, 17 October.
4
Figure 5. Selected temperature profiles.
Acoustic modeling of environmental variations
At sea, the acoustic modeling was performed by the Comprehensive Acoustic System
Simulation/Gaussian Ray Bundle (CASS/GRAB) and was used to determine sonar setups
in real time. The modeling determined the best waveform, processing parameters, and
source and receiver depths for a specified scenario. Post-test results presented in this
paper are full bistatic SNRs modeled by CASS/GRAB.
Figures 6, 7, and 8 show the results of modeling performed to analyze the potential
system performance as a function of source and receiver depths. The SNR plots are based
on a medium center frequency, 2-s pulse length, 400-Hz bandwidth, and omnidirectional
sensors. Each pair of plots shows a shallow source and receiver (top) and a deep source
and receiver (bottom). For consistency, the profiles were run for the same location (XBT
#49 location) but with different XBTs.
Figure 6 shows the SNR for XBT #14. The shallow source and receiver depths are
200 and 300 ft, respectively; the deep source and receiver are at 500 and 600 ft,
respectively. This figure shows clearly the presence of a deep duct, whereas XBT #49, in
Fig. 7, is dominated by a surface duct. The source and receiver in Fig. 7 are at the same
depths as in Fig. 6.
Figure 8 is an example of the predicted SNR for XBT #103. This environment
clearly shows a strong surface layer for a shallow source and bottom bounce returns for
the deeper source. The source and receiver are at 25 ft in the top plot and at 400 ft in the
bottom plot. The source and receiver depth configuration is one of the important
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S. SUTHERLAND-PIETRZAK AND E. MCCARTHY
environmental adaptation parameters. Clearly a sensor placed deep provides a detection
advantage for profiles such as #14 and #103 but does not add significant coverage for the
deep layered profiles such as #49. A more detailed analysis of the acoustic modeling will
be performed on the Sea Test C data and presented in follow-on analysis efforts.
Figure 6. Signal to noise ratio for XBT #14 – shallow (top) and deep (bottom) sensors.
Figure 7. Signal-to-noise ratio for XBT #49 – shallow (top) and deep (bottom) sensors.
ENVIRONMENTAL VARIABILITY OF THE LBVDS SEA TESTS
585
Figure 8. Signal-to-noise ratio XBT#103; S/R at 25 ft (top) and at 400 ft (bottom).
For this environment source and receiver depth significantly affect the system
performance. Optimum sonar performance depends on choosing the appropriate depths,
as well as the appropriate bandwidth, center frequency and pulse length.
5
Discussion
Both accurate measurement and analysis of the acoustic environment are critical to realtime adaptive setup and operation of sonar systems to achieve optimal sonar performance
in littoral waters because these environments change so rapidly and dramatically over
both time and space. The sonar performance of waveforms of a given center frequency
and bandwidth may vary considerably because of the environment. The results from Sea
Tests A and B showed the variability of system performance in reverberation-limited and
ambient-noise-limited environments due to bandwidth and center frequency. Against
reverberation, the performance improves as a function of increasing bandwidth. In
ambient-noise-limited conditions, the coherent gains from increasing the bandwidth will
not provide any performance improvement. However, using the bandwidth in a
noncoherent method may provide improved performance.
The source and receiver depths are also critical for optimal sonar performance. The
acoustic modeling results using the measured XBTs from Sea Test C show that, in certain
environments, placing the source and/or receiver below the layer, or closer to the bottom
to reduce the grazing angle, should provide improvement over a shallow source and
receiver configuration.
The LBVDS data sets will contribute significantly to the effort to determine how to
automatically adapt and optimize the search performance of active sonar systems on a
USW-capable platform, using in-situ knowledge of the acoustic environment.
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S. SUTHERLAND-PIETRZAK AND E. MCCARTHY
Acknowledgments
This work was sponsored by the Office of Naval Research (ONR), program officer
Kenneth Dial, and managed by the Naval Sea Systems Command, Program Executive
Office for Mine and Undersea Warfare (PEO(MUW)). The Surface Ship USW Combat
System Program Office (PMS411) program manager is Greta Conde (PMS411U). The
project manager for NUWC Division, Newport, is Maurice Simard (Code 3112).
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