USS Asheville
Leads the Way
in
High Frequency Sonar


by LTJG Leonard Moreavek, USS Asheville,
and T.J. Brudner, Applied Research
Laboratories, University of Texas

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As the mission area of the Submarine Force expands to embrace the shallow water littorals, the advantages of using high frequency (HF) sonar in tactical operations as an adjunct to more traditional medium frequency systems are becoming more apparent. The high range and bearing resolution that can be achieved with active HF sonar makes possible many new capabilities that are essential to success in shallow water operations of all kinds. USS Asheville (SSN-758) has been a pioneer in demonstrating the value of HF sonar ever since she was selected as the operational test platform for the High Frequency Sonar Program (HFSP), and had her basic HF system installed in March 1995.

High Frequency Sonar – Tailored for Shallow Water
The higher frequency and shorter wave lengths of HF sonar yield greater range and bearing resolution compared to medium frequency systems, and these in turn enable better discrimination between undersea targets and bottom and surface reverberation. In addition to facilitating mine avoidance and active ASW in littoral areas, high range and bearing resolution make possible precise ice canopy and seafloor profiling for under-ice and shallow water navigation and mapping. The increased bandwidth available at higher frequencies and the greater absorption of HF signals in the water allow active operation with reduced counter-detectability, and the large bandwidth available in HF systems can also be used to conduct high data-rate acoustic communications.

The faster absorption of high frequency sound does not permit the long passive detection ranges of lower frequency systems in deep water. However, in shallow water, passive detection ranges are often limited by environmental noise and adverse propagation – rather than absorption. In these noise-limited environments, the narrow beamwidths achievable in HF passive surveillance modes allow excellent discrimination between multiple contacts and biologics, which is key to success under littoral conditions.

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With Asheville in drydock, the chin receiver and
projector arrays are visible beneath the bow dome.


The Asheville Installation

HFSP was initiated by the Advanced Systems and Technology Office of the Program Executive Office for Undersea Warfare (PEO USW). The program was implemented by a team that included industry and Navy and university laboratories. The initial installation was coordinated by the Naval Underwater Systems Center (NUWC) and included a large diameter chin-mounted receiver array developed by Northrup Grumman, transmitter arrays mounted on the sail and chin developed by the Applied Research Laboratories of the University of Texas at Austin (ARL-UT), and inboard processing hardware and software from Lockheed Martin. This installation, referred to as the Advanced Mine Detection System (AMDS), provided capabilities that included active volumetric search for bottom and moored mines, ASW, and seafloor contouring. Further upgrades installed in November 1997 included a large HF sail receiver array and advanced processing algorithms and displays developed at ARL-UT, as well as upgraded AMDS processing developed at NUWC and implemented by Lockheed Martin. Data collected aboard Asheville during sea trails have been used to refine and enhance high frequency sonar capabilities and processing algorithms for transition into future HF systems.

The Advanced Development Model (ADM) arrays on Asheville comprise both projector and receiver arrays on chin and sail. Each array provides a specific aperture characteristic to support multiple missions. The large horizontal aperture of the chin receiving array generates very narrow beams in azimuth for significant reverberation rejection and improved shallow water performance. Thus, the chin array is ideal for active mine and ASW detection, and its passive capabilities have opened up new operational possibilities. The chin projector can ensonify the region below the submarine without interference from the bow, and the low position of the chin arrays on the hull enables them to be operated with the boat at or near the water surface.

The large vertical aperture of the sail receiver array yields narrow beamwidths in elevation, again reducing multi-path reverberation and suiting it well for detecting ASW and mine targets – especially when the latter are close tethered – plus seafloor and canopy profiling and under-ice navigation. The location of the sail arrays also allows them to provide unobstructed obstacle avoidance for the submarine during surfacing and under-ice operations.

COTS: Rapid Development of New Capabilities
In conjunction with the new projectors and receiving arrays, commercial off-the-shelf (COTS)-based signal processing hardware and associated software support advanced computer algorithms to perform the multiple sonar functions described above. This modular COTS approach allows for expanding processing power and adding new features simply by replacing computer chips or installing new software, much as one might upgrade a personal computer by installing a new CPU, adding memory, or updating the operating system. The processing equipment is located in several areas throughout the ship, and a separate rack of equipment is used to configure and drive the projector arrays. The system is controlled and operated from the sonar room using a workstation with two color display surfaces and a touch panel. The operator is able to select mission-specific modes, which automatically determine the combination of projector and receiver transducers called into play.

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A 3-D perspective view of a volcano crater off the
coast of Lanai as it appears on the HFSP display.

As a demonstration of the flexibility inherent in the modular approach, a key change was made during the initial at-sea testing, when Sonarmen on-board Asheville suggested creating a High Frequency Passive Broadband (HFPBB) function to supplement the existing active features. Most Sailors are familiar with the time and effort involved in developing and installing major changes to conventional hardware systems. Entire banks of equipment are removed or gutted, new gear is installed by the vendor, and installation times are often measured in weeks. Taking advantage of the modularity of Asheville’s COTS HF system, HFPBB was developed, installed, and tested over a three-day period. "It was incredible," said STSCS(SS) Donald Jernigan, the LPO at the time of the install. "These software engineers thought about it for a minute and said, ‘Yeah, we think that’s doable.’ Next thing we knew, we had the only HFPBB system in the fleet!"

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A sailor operates the HFSP system display, which
shows both plan and perspective views of the bottom.

In Asheville’s case, the upgrades were performed by an Engineering Change Team, consisting of one technician and two programmers. This small-team concept is key to operational flexibility, since it allows modifications and corrections to be made anywhere in the world. The teams can fly to the ship’s current location with parts and software (a new pre-programmed hard drive or CD-ROM for on-board loading) and get right to work. Installation and testing usually take less than two days. "Maintaining this COTS hardware and unique software presents a lot of challenges," said STS3(SS) Neal Bedersen, Asheville’s HFSP expert, "but with the support we get from NUWC and ARL, and their ability to deliver software and hardware support, it’s been great. And the capability gained is phenomenal!" For minor system updates, ship’s force Sonarmen can download changes straight to the system’s hard drive from a floppy disk mailed to the ship. In the future, major research and development upgrades could well be installed remotely via modem and secure phone lines.

At Sea Testing
The baseline performance of each of the HF sonar functions was demonstrated during two sea trials, one conducted off the coast of Washington in 1995 and the other off Hawaii in 1997. The chin array performance baseline was established during the 1995 tests, and the sail array was assessed in two phases during the more recent trial. The first phase of the sail array testing demonstrated the system’s seafloor profiling and mine detection capabilities. During minefield testing, high detection probabilities and long ranges were achieved with very few false calls, even against bottom mines. These excellent results were due largely to a three-dimensional Computer Aided Detection (CAD) algorithm, which alerted the operator to mine-like targets. According to STSC Steven Graham of the Operational Test and Evaluation Force (OPTEVFOR), who provided an operational assist during testing at Hawaii’s Shallow Water Training Minefield, the new arrays and processing algorithms "have the potential to provide a dramatic and much-needed increase in mine detection and avoidance capabilities along with precision bottom mapping and navigation."

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An electronic technician works on the
installation of Asheville's sail projector array.

The second phase of the testing was conducted in deep water to determine the active and passive ASW performance of both arrays. Active detection ranges were achieved out to the limits predicted for the environment. In addition, notable passive detection ranges were seen using the chin array. Since real sea ice was in short supply near Hawaii, the target ship for the exercise was also positioned to simulate an ice keel for verifying the under-ice performance of the sail array in that scenario.

Operational Experience
The HFSP equipment also been used successfully during Asheville’s operational deployments since 1995, and its performance has been above expectations from the start. Under many circumstances, often in shallow water, observed passive detection ranges of submarines have met or exceeded those of the AN/BSY-1. Typical HF propagation characteristics yield a sharply defined maximum detection range. While transiting certain well-known "trawler-infested" waters in WESTPAC, this cutoff range provides a valuable "tripwire" for avoiding contacts, since anything detected on HFPBB is close enough to be a collision threat. HF’s extremely high bearing resolution even allows early detection of pairs of trawlers. "Using the HF Broadband in conjunction with our BSY-1 system allowed a significant reduction in cross-track TMA maneuvers," says Asheville’s Captain, CDR Bruce Grooms. "We were able to significantly improve our search effectiveness and more easily detect quiet trawlers that might otherwise be a collision threat."

In the ASW mode, HF active not only detects and tracks actual targets, but also occasionally detects wakes from surface ships and submarines. These wake detections provide not only contact bearing and range, but also an excellent indication of course and speed, all in one ping. Many of the modes use transmissions that are designed to be undetectable to modern acoustic intercept devices, thus maintaining the boat’s inherent stealth while yet improving her sonar capabilities. "ASW is still our meat-and-potatoes mission, and in shallow water against a quiet contact, HFSP has given us the edge," says LT Jack Shriver, Asheville’s Combat Systems Officer. "Passive or active, one whiff on HFSP and he’s a sitting duck."

The profiling mode uses the active reverberation from seafloor features to "paint" a three-dimensional map of the bottom contour ahead of the ship, a feature Asheville first demonstrated during her 1996 WESTPAC deployment. During the 1998 deployment, while operating in poorly charted, extremely shallow waters, Asheville used this real-time data extensively for navigation safety. "Using profile mode is like flying on a clear day, instead of having to use instruments," according to ETC(SS) Larry Wood, Asheville’s Assistant Navigator. "You can see exactly what you’re getting yourself into. Of course, I’ll still hang on to my trusty fathometer, but I can see the wave of the future for littoral navigation." Another rapid turnaround COTS development was a real-time recording system to log profiling data. Developed at the ship’s request just weeks before the 1998 WESTPAC deployment, the recorder electronically tags bottom features with on-board navigation data, so the observations can be used for correcting charts.

Future HF work
Upgraded production versions of the sail projector and receiver arrays on Asheville will be installed on all 688I-class submarines as part of the HF Upgrade that will be done in conjunction with Phase IV of the Acoustic Rapid COTS Insertion (ARCI) program. In addition to the sail arrays, next-generation versions of both the chin projector and receiver arrays are planned for the sensor suites of the Virginia class. The processing baseline for the two applications will use refined versions of the algorithms tested on Asheville, implemented on ARCI hardware.

The addition of a Precision Underwater Mapping (PUMA) capability will be provided as a future upgrade through the Advanced Processing Build (APB) framework. The PUMA algorithms rely on high precision, sonar-derived navigation, accurate profiling, and advanced CAD algorithms to generate high-resolution terrain and target maps of the seafloor in real time. The APB will deliver improvements to both active and passive ASW capabilities as well, with Initial Operational Capability currently scheduled for 2002.

Conclusions
Has the HF Sonar Program been a success? If one considers that AMDS was originally installed on Asheville only as a temporary system to test feasibility and functionality, the answer is clear. The ship has fully integrated AMDS and HFSP into all aspects of her operations. The ease with which the system has been adapted and updated to meet new demands demonstrates the value and versatility of COTS-based modularity. Relatively inexpensive to build and install, and easily upgraded, the HFSP system has demonstrated significant operational advantages. As noted above, it is planned for integration into Virginia-class new construction and for installation on selected Los Angeles-class boats. HFSP is moving submarine warfare into the littorals with an innovative approach to system development and a growing body of at-sea experience – and Asheville was there first.