In-stride Compact Active-Passive Sonar _iCAPS_ Re-integrating A

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					    In-stride Compact Active-Passive Sonar (iCAPS): Re-integrating A Canadian
                       Technology for Littoral Applications

                               Linas Siurna & James Crawford
                                 Ultra Electronics Maritime Systems
                    40 Atlantic Street, Dartmouth, Nova Scotia, Canada, B2Y 4N2
               Phone: 902.466.7491, fax: 902.463.6098, email:

                                       James A. Theriault
                       Defense Research and Development Canada – Atlantic
                     P.O. Box 1012, Dartmouth Nova Scotia, Canada, B2Y 3Z7
                 Phone: 902.426.3100 ext. 376, email:

                                        Bruce Armstrong
                                 GeoSpectrum Technologies Inc.
                   30 Bel Ayr Avenue, Dartmouth, Nova Scotia, Canada, B2W 2E1
                        Phone: 902.434.3906, email:


The In-stride, Compact, Active-Passive Sonar (iCAPS) being developed by Ultra
Electronics Maritime Systems (UEMS) is built upon the foundation of several decades of
research, development and trials of active sonar concepts and technologies undertaken at
DRDC Atlantic. UEMS collaborated with DRDC Atlantic in the supply of active and
passive sensors for the recently completed Towed Integrated Active-Passive Sonar
(TIAPS) technology demonstration program. UEMS is presently re-integrating the
TIAPS Horizontal Projector Array (HPA) technology for littoral applications. The re-
integration of the HPA includes insertion of a new, proprietary projector technology that
will provide performance and reliability enhancements. The iCAPS sensor system
provides a small footprint, low mass, integrated active-passive system that is practical for
integration onto autonomous or manned, small surface platforms operating in the littorals.
This paper provides a summary review of the results of the TIAPS project, a profile of
the iCAPS system and its performance attributes and modeled and measured results for
the new HPA projector technology.


The In-stride, Compact, Active-Passive Sonar (iCAPS) system is built upon the
foundation of several decades of research, development and trials of active sonar
concepts and technologies undertaken at DRDC Atlantic, Canada’s naval research and
development establishment. Ultra Electronics Maritime Systems (UEMS) collaborated
with DRDC Atlantic in the supply of active and passive sensors for the recently
completed Towed Integrated Active-Passive Sonar (TIAPS) Technology Demonstrator
Program (TDP). One activity within the TIAPS TDP was a comparison between two
sonar source concepts, both supplied by Ultra1,2,3 (Figure 1). One source concept was a

 Theriault, J.A., Low Frequency Active Sonar in Shallow Water, Proc. of Submarines
and ASW conference, London, UK, September 2000.
traditional vertical array of Ultra model 28Fx1000 Free Flooded Ring (FFR) projectors.
The other was a novel Horizontal Projector Array (HPA) concept, developed by DRDC
Atlantic using Ultra Barrel Stave Projectors (BSP). The iCAPS technology profiled here
is an evolutionary re-integration of the DRDC Atlantic TIAPS HPA technology with an
emphasis on littoral applications and/or mass and real estate constrained platforms. The
re-integration of the HPA includes insertion of a new, proprietary Ultra projector
technology that provides performance and reliability enhancements.

Figure 1 – Canadian TIAPS TDP Sonar Concepts

The iCAPS system provides a small footprint, low mass, integrated active-passive system
that is optimised for integration onto autonomous or manned, small surface platforms
operating in the littorals. The in-stride, sprint and drift capability simplifies platform
operational constraints while the system is deployed. iCAPS comprises an active
“horizontal projector array” (HPA) integrated with an “activated-passive” digital receive
array. The HPA provides an azimuthal sector-scan capability with a feature that lets an
operator trade-off sector size and source level by using sub-arrays within the HPA.

The TIAPS configuration shown in Figure 1 places the HPA near the front end and in-
line with the receive arrays. The HPA is used as the low frequency active sonar sound
  Cotaras, F.D., Towed Integrated Active Passive Sonar, TIAPS”, Proc. Undersea
Defence Technology Conference, Hamburg, Germany, June 1997.
  Bonin, Y.B., et. al., “A Horizontal Projector Array of Barrel-Stave Projectors for Low
Frequency Active Sonar”, Proc. Undersea Defence Technology Conference, Hamburg,
Germany, June 1997.
source to augment the passive detection capability of the receive array. A typical basic
streamer configuration for an iCAPS-based sonar system would comprise the tow cable
(TC), forward vibration isolation module (VIM), HPA module, after VIM and receive
array modules. The receive array modules can include a directional ‘active’-receive array
and an omni-directional active/passive receive array. The directional active-receive array
provides resolution of bearing ambiguity. With appropriate winch and handling system
design, the entire streamer is deployed and recovered as a single critical-angle tow.
Relative to a tow-body based Variable Depth Sonar (VDS), iCAPS permits a simplified
handling system that is easier to integrate, will install on smaller platforms, simplifies
automation of deployment and recovery and releases crew for other tasks.

In comparison to a VDS with a traditional vertical projector array that provides one-ping
coverage on all bearings, the HPA module performs similar to an antenna oriented with
its axis on the horizontal. This provides for an active ‘sector scan’ or azimuthal
directional steering capability. During the course of the TIAPS TDP, DRDC Atlantic
was able to conclude from both performance models and at-sea trials that the HPA will
provide ‘in-beam’ performance similar to that of a traditional vertical projector array. The
trade-off is the time taken to sweep in azimuth. There are some benefits to the azimuthal
directivity of the HPA in reverberation-limited environments and for scenarios where
there is a preferred look-direction, as discussed further below. Also, the sound field
produced by a distributed, multi-channel array of active transducers implies that the
maximum sound pressure level (SPL) that can be produced at shallow depths without
cavitation is greater than that of a single active transducer or an array containing a lesser
number of active transducers.

The amplifier and wet-end streamer winch used for iCAPS are standard military-grade
COTS technologies, packaged and optioned in configurations tailored for integration onto
a specific platform. Both components were demonstrated during the TIAPS TDP and
thus there are no developmental requirements for either, other than design adaptation for
mechanical and electrical platform integration.

HPA Historical Overview

DRDC Atlantic began development of the HPA concept circa 1985 as an offshoot of its
Barrel Stave Projector (BSP) development activities. The BSP is a Class 1 Flextensional
projector4; UEMS is licensed by the Crown (Government of Canada) to commercialise
the BSP for defence applications. The BSP is an air-backed design and because of its
particular design features, has a limited depth capability unless a depth-compensation
system is used. However, the BSP’s small size permitted a horizontal line array of
projectors to be contained within a streamer hose and deployed and recovered from a
winch. Circa 1990, DRDC Atlantic trialed a four-element BSP HPA. In 1992, a
prototype vertical array of FFRs was developed as an alternative source concept; the FFR
technology used is proprietary to UEMS. In 1994, DRDC Atlantic developed and began

 Jones, D. F., J. F. Lindberg, “Recent Transduction Developments In Canada and the
United States”, Proc. of the Institute of Acoustics, Vol. 17, Pt. 3, 1995.
sea trials of a depth-compensated, sixteen-element BSP HPA integrated with a UEMS-
supplied receive array. The TIAPS TDP commenced in 1997.

The main technology thrusts of the TDP were: (1) sonar source concepts: vertical versus
horizontal projector arrays, (2) directional and omni-directional ‘activated’-passive
receive arrays, (3) signal processing, data fusion and operator interfaces. The focus of
this paper is only on the technology descendants of the first thrust. During the course of
the TDP, DRDC Atlantic evaluated a vertical array of UEMS FFR projectors and a
horizontal array of UEMS BSPs (Figure 2) relative to the Canadian Navy’s objective to
integrate an active ASW sonar capability in its Halifax-class frigates and to replace the
CANTASS system. The TIAPS TDP included significant industry involvement that
resulted in technology transfer from DRDC Atlantic to industry to provide an industrial
capability to supply the navy’s needs.

In 1998, under the TDP, industry (today Ultra) delivered a thirty-two-element HPA of
depth-compensated BSPs. The importance of blue-water missions drove of the need for a
system capable of deep-water operation and hence, the BSP was depth-compensated.
The TIAPS TDP is now complete. Relative to the over-board and off-board sub-systems,
the TDP concluded that the HPA provides similar ‘in-beam’ performance to the vertical
array of FFRs and permits a significantly simplified over-boarding and handling system.
The system trade-offs are that the vertical array of projectors provides one-ping coverage
on all bearings whilst the HPA sweeps azimuth with narrow beams, and thus requires
multiple pings over a period of time to complete a sweep. The implications of this are
discussed further below.

Notional Operational Requirements

In evolving the TIAPS HPA concept into iCAPS, UEMS was asked to target different
operational requirements than those defined for the TIAPS TDP. This resulted in
different sonar system attributes for the HPA and for the entire system. In general terms,
the operational needs that resulted in the iCAPS’ HPA and system design were:

• Capable of littoral or blue-water ASW
• Exhibit high system availability and coverage, achieved by:
          o Capable of in-stride, sprint and drift (“moving ASW”)
          o Capable of high speed moves while deployed
          o Capable of mono- or multi-static operation
• Achieve constrained foot-print and mass budget
          o Exhibit Nixie RL-272 winch footprint
          o Achieve 1800 kg mass budget for out-board units, over-board units, and
              transmit amplifier and sub-system controller of on-board units
• Suitable for integration onto Unmanned Surface Vehicle (USV) or surface-ship
• Capable of automated handling

One simple factor motivated interest in the HPA for these operational requirements and
defined the evolution path for iCAPS from the TIAPS HPA: in small platform
applications where operational agility is critical, simplifying the handling system and the
streamer is paramount; iCAPS accomplishes this.

Figure 2 – TIAPS TDP Sonar Concepts: HPA (foreground) and FFR Vertical
Array (background)

iCAPS – System Design

The iCAPS system design was driven by the notional operational requirements that
focused on small-platform littoral operations, thus constraining the mass budget and
footprint. In the particular application described here, the mass budget for off-board,
over-board and certain on-board equipment is 1800 kg (excludes on-board signal
processing and display but includes transmit amplifier, array receiver and sub-system

The iCAPS HPA design was enabled by the insertion of a new projector technology.
Over the past several years, UEMS has developed the Modular Projector System (MPS).
MPS exploits acoustic interaction between closely spaced underwater sound projectors to
produce a compact, low frequency, broadband source that is straightforward to
reconfigure for specific resonance frequency, bandwidth, and source level by simply
repositioning the basic building blocks from which it is composed. The MPS projector
technology, described further below, enables a reduction in the HPA hose diameter from
120 mm to 60 mm. The TIAPS HPA depth compensation system was deleted as the
native 300 m depth capability of the MPS design selected for iCAPS’ HPA is sufficient
to support littoral operations without resort to the complexity and reliability issues
associated with depth compensation.
The evolution of ICAPS’ HPA from TIAPS’ is a relatively straightforward re-integration
exercise, especially given DRDC Atlantic and UEMS experience with developing and
manufacturing sonar projectors and receive array streamers. As in the TIAPS TDP HPA,
the iCAPS HPA contains thirty two (MPS) projectors plus ancillary transformers and
non-acoustic sensors (NAS), all mounted onto a strength member and installed inside an
oil-filled hose. While configurable to specific operational needs, the baseline NAS
complement includes heading, temperature and depth sensors. An arbitrary operational
frequency range of 1300 – 1500 Hz was selected to provide inter-operability with legacy
systems, and a suitable MPS design configured to support this operational frequency
range. Alternate operational frequencies would entail reconfiguration of the MPS
projector’s building blocks and re-integration into the HPA module. Similarly, arrays of
greater or lesser number of projectors can be defined, 32 was selected here to achieve
certain acoustic performance metrics.

The resulting MPS HPA module is 23 m long by 60 mm diameter and has an in-air mass
of 120 kg. Peak source level is 227 dB//1 μPa⋅m with a minimum 220 dB available over
the operating band. For the particular application described here, the severe system mass
budget constraint required significant system-level trade-offs. The first trade-off was to
limit the HPA and activated-passive omni-directional array to forward-look operation
only (Figure 3). The receive array length was also constrained such that only limited
pure-passive performance would be achieved while fully supporting the activated-passive
mode in the forward look direction. As well, incorporation of a directional receive array
was precluded. UEMS’ Quad digital directional receive array has a 90 mm module
diameter. The volume this represents would have driven the system to a larger capacity
winch drum, resulting in excessive mass.

The HPA module with 32 active channels is fully integrated with (optional) vibration
isolation modules (VIMs) and an ‘activated’-passive omni digital receive array module.
The VIMs and digital receive arrays are derivatives of production UEMS military arrays
that are in fleet use in the U.K. and Canada. The iCAPS wet-end configuration
comprises: 300 m unarmoured tow cable, 23 m HPA module, 60 m intermediate cable
section, a 60 m omni-directional receive module and short drogue tail.

The HPA module is powered by a multi-channel switch mode pulse-width modulation
(PWM) amplifier manufactured by Instruments Inc. Instruments Inc. supplied the transmit
amplifier for the 32-element BSP HPA used in the TIAPS TDP. The particular amplifier
model selected for the iCAPS HPA provides 2 kVA per channel and is built into a single

The resulting system mass budget is: (1) 450 kg for out-board units, (2) 700 kg for over-
board unit and (2) 600 kg for the on-board transmit units.
Figure 3 – Forward-look beam pattern for 32-element MPS HPA with Side-lobe
Levels -13 dB From the Main Lobe (simplified graphical rendering)

Modular Projector System (MPS)

Bruce Armstrong invented the MPS. UEMS has been developing the MPS and has
applied for patent protection5. Various MPS projectors have been developed and
delivered to naval customers. MPS is a versatile method for manufacturing a low-
frequency underwater sound projector6,7. As its name suggests, a MPS projector is
assembled from a number of small sound projectors that are mounted in close proximity
to each other. By exploiting the acoustic interactions amongst the projectors, the
transducer designer can choose the resonance frequency, bandwidth and output power of
the system within wide limits.

In principle, many projector types such as rings or barrel-stave projectors could be used
as the “building block” for a MPS, but for most applications the best choice will be a
flexural disk projector, also known as a “bender”, as shown in Figure 4. A bender is

  Patent Application 526ca, B. Armstrong, January 2005.
  Yeatman, P. and B. Armstrong, “Constructive Use of Acoustic Interactions in a Multi
Element Projector Array”, Proc. Conference of Institute of Acoustics Sonar Transducer
and Numerical Modeling in Underwater Acoustics, London, UK, 2005.
  Crawford, J. C., et. al., “A Modular Projector System: Modeled Versus Measured
Performance”, Proc. Undersea Defence Technology Conference, Amsterdam, 2006.
inexpensive, reliable and easy to build, but its most important advantage to MPS is that
its pancake-like shape permits milli-λ separations. As an illustration of the versatility of
the MPS concept, eight identical benders, each with an 1800 Hz resonance in the free
field, can produce a MPS with a 600 Hz resonance and source level greater than 200
dB//1 μPa at 1m. This cylindrical MPS would have a mass of 8 kg, a length of 20 cm, a
diameter of 14 cm, and an operating depth of 300 m without pressure compensation. A
different quantity and spatial arrangement of the same 1800 Hz benders could be used to
produce a MPS with resonance frequencies ranging from 450 to 1600 Hz with source
levels exceeding 210 dB. The MPS projector shown in Figure 4 has 24 bender elements
of 125 mm diameter, weighs 22 kg and has a resonance of 400 Hz. This transducer is
used within a towed, low frequency sonar system.

Figure 4 – MPS Projector using 24 Flexural Disks of 125 mm Diameter

The acoustic performance of a conventional underwater sound projector is fixed at the
time the projector is designed. If this performance exceeds the specification, the
projector will be heavier and larger than is optimal resulting in negative system-level
consequences. For instance, if this projector is part of a towed system, the tow body and
its handling system must also be larger and stronger. On the other hand, if the
performance of the projector does not meet the specification, the customer must either
sacrifice a portion of the specification or pay for a costly and time-consuming redesign of
the projector, if indeed a single projector can be made to meet the specification. Thus the
versatility of the MPS benefits both the end-user and the transducer manufacturer.

What Distinguishes MPS from a Conventional Array

Projectors have been used in conventional arrays for decades with typical separations
between projectors a good fraction of a wavelength, typically on order of λ/2. In contrast,
the projectors in MPS typically have milli-λ spacing. In the MPS patent application [5],
two criteria are listed that must be satisfied before an array can be considered a MPS.

   1. The separation between projectors must be less than or equal to the characteristic
      size of the projector, and in preferred embodiments must be less than one-half the
      characteristic size.
   2. The projector must be small compared to the wavelength of the acoustic wave at
      the resonance frequency of the system. At a minimum the characteristic size of a
      projector must be less than λ/8.

The iCAPS system comprises an array of 32 MPS projectors with each MPS projector
assembled from sixteen benders of 50 mm diameter and separated by a closely controlled,
milli-λ separation to achieve the target acoustic operating frequency range. The
individual benders resonate at 3610 Hz. Assembled into the iCAPS MPS, the resulting
projector’s resonance permits operation at 1500 Hz and below with each projector having
a transmit voltage response (TVR) of 145 dB.

iCAPS Versus The Notional Operational Requirements

iCAPS yields many advantages related to handling of the wet-end streamer. These

   1. iCAPS comprises a fully integrated towed array sonar wet-end,
   2. The ship handling system requirements are minimised: one winch system,
      simplified array handling, minimum operations personnel are required for
      streaming and recovery, automation is simplified,
   3. The dimensions of the towed active transmit array are compact, the HPA can be
      wound on the same drum as the acoustic towed array receiver and tow cable,
   4. Streaming and recovery in high sea states are readily achieved; deployment,
      recovery, towing and storage are no more limited than with any single-tow towed
   5. Transmit array and receive array depth may be altered simultaneously by one tow
      cable scope adjustment (critical angle tow), and
   6. Ship emergency maneuvers are no more limited than with any single-tow towed
There are tactical advantages to using a low frequency active system in combination with
a passive system, and further advantages where the low frequency active sonar is based
on a horizontal projector array. The passive sonar may be used to detect and determine
the line of bearing to sound sources in the ocean. Active sonar may then be used to
detect and localise targets in the vicinity including range determination. The HPA is well
suited to this scenario because there is no requirement to transmit energy in directions
other than the direction of the suspected target. This concept may be attractive to ship
commanders because the active sonar insonification at high power is limited to a narrow

In some environments, the continuous use of active sonar may be required if search and
area sanitization procedures are to be implemented. In such cases the required number of
HPA transmissions depends on the source level required and the size of the sector for
which coverage is required. It is seldom considered necessary to provide 360° coverage
when the tow ship speed is relatively high.

Intense reverberation fields are often set up by the margins of deep ocean basins,
coastlines, and seamounts, or by the azimuthal anisotropy in the scattering process and/or
propagation conditions. Using a HPA transmitter can enhance reverberation suppression
in such environments. The HPA transmitter may be used to insonify only a small sector
in the direction of interest. Thus the intense scattering in azimuthal space away from the
direction of interest is reduced within the limits afforded by the side lobe response of the
transmitter. This transmission rejection may then add to the side lobe rejection of the
towed array receiver for an improved total suppression in the presence of such scatterers.

Even when the reverberation field is azimuthally isotropic, selective azimuthal
insonification with a HPA permits enhanced performance when appropriate transmission
waveforms are used. The reverberation spectrum for CW pulses at moderate to high
Doppler is determined by the side lobe response of the sonar to reverberation arriving
outside the beam of interest. The amount of frequency spreading is determined by own
ship's speed. The reverberation spectrum levels are thus reduced within the limits
afforded by the side lobe response of both the HPA transmitter and the towed array

Shallow Water Operations

At moderate to long ranges in shallow water, mode stripping often renders the
reverberation field nearly two-dimensional such that reverberation arrives at very shallow
grazing angles. The vertical directivity of the projector array is consequently irrelevant.

At short ranges in shallow water, vertical directivity may be useful. The relatively
narrow vertical beam width of the end fire beam of the HPA is expected to enhance target
detection capabilities in such environments. Although this enhanced performance is only
available at or near end fire with the HPA, forward end fire is often an important
In shallow water operations, sonar operation at shallow depths is required. The
distributed nature of the HPA transmitter provides a favourable cavitation threshold
relative to a smaller array of projectors capable of generating the same SPL.

In the environment of interest, if sonar coverage over a wide sector is desired and less
than the maximum source level of the HPA is acceptable, the HPA can be operated with a
subset of the transmitter elements. The source level and length of sub-arrays can be
traded off against sector size (in azimuthal degrees) and the number of required
transmissions. For example, in a 32 active channel system, a 12 dB reduction from
maximum source level would permit a 180° sector coverage with one transmission. This
mode of operation will reduce the reverberation suppression discussed above.

Deep Water Operations

Vertical directivity may be desirable in deep water to suppress Fathometer returns and
reverberation arriving at steep angles with respect to the horizontal. As noted above, the
HPA provides vertical directivity at steering angles off broadside.

Environmental Considerations

As indicated above, the HPA provides for directionality of transmission. This readily
allows the operator to minimise the area in which any transmission may have potentially
adverse effects on marine life.

Handling and Reliability Implications

The ability to operate, deploy and recover a towed receive array in high sea states from
naval vessels has already been demonstrated by many naval forces. The HPA
transmitter, when part of the same streamer, can be considered to provide the same
capability. This is also accomplished without complex and expensive crane or A-frame
over-board equipment typically required to recover and deploy a transmitter configured
in a tow body. The winch, a level wind and a suitable over-stern fairlead are all that is
required; these are the same basic over-boarding components as for a standard towed
array receiver.

The HPA, being a linear array of individually powered projectors, allows for graceful
degradation of the transmitter in the event of projector or electrical malfunction. In other
words, the failure of a single projector channel has minimal impact. Moreover, if a
complete HPA module malfunction or significant degradation occurs, replacement of the
transmitter module in a towed array is a simple and relatively straightforward on-deck
operation (even while at sea). Change-out of the HPA can be accomplished with simple
reel handlers or deck hands; no crane is required. The HPA module is 23 m long by 60
mm diameter and has a 120 kg mass in-air.

Two decades of Canadian naval research laboratory and industry experience developing
sonar systems based on a novel, horizontal projector array were brought to bear in
developing iCAPS. The evolution of the TIAPS TDP HPA, developed to meet the
Canadian Navy requirements of-the-day, to meet a notional operational requirement for a
small platform, mass-constrained littoral system required re-integration of the HPA to
reduce mass and volume. This evolution was enabled by the insertion of new projector
technology and removal of the depth compensation system. The iCAPS HPA is
inherently capable of azimuthal steering and a Canadian directional digital receive array
is available. However, the 1800 kg mass budget constraint provided to UEMS as a target
demanded significant trade-offs such that these capabilities could not be incorporated into
this particular iCAPS model. For applications with lesser mass constraints, these
capabilities can easily be re-integrated into the iCAPS system. The iCAPS system
provides a small footprint, low mass, integrated active-passive system that is optimised
for integration onto autonomous or manned, small surface platforms operating in the

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