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: firstname.lastname@example.org 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: email@example.com Bruce Armstrong GeoSpectrum Technologies Inc. 30 Bel Ayr Avenue, Dartmouth, Nova Scotia, Canada, B2W 2E1 Phone: 902.434.3906, email: firstname.lastname@example.org Abstract 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. Introduction 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 1 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 2 Cotaras, F.D., Towed Integrated Active Passive Sonar, TIAPS”, Proc. Undersea Defence Technology Conference, Hamburg, Germany, June 1997. 3 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 4 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 controller). 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 cabinet. 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 5 Patent Application 526ca, B. Armstrong, January 2005. 6 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. 7 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 , 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 include: 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 array, 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 array. 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 sector. 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 receiver. 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 direction. 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. Summary 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 littorals.
Pages to are hidden for
"In-stride Compact Active-Passive Sonar _iCAPS_ Re-integrating A "Please download to view full document