In response to Congressional tasking, the Office of the Chief of Naval Operations (OPNAV N85), in February of 1994, promulgated a draft Strategy and Priorities for unmanned undersea vehicles (UUVs), which identifies the following four basic mission areas for which the utility of UUVs has been substantiated:
- Mine Warfare and Mine Countermeasures (MCM)
- Intelligence Collection
- Tactical Oceanography
Mine warfare has been established as having the most immediate need for UUVs. “The proliferation of mines, and the willingness of nations to use them, challenges the free movement of U.S. and international shipping, and can impede or deny U.S. power projection in the littoral environment” (Navy Technology Needs Document, 9 September 1994).
Each UUV mission area requires a unique payload. For example, mine countermeasures might require a sophisticated synthetic aperture sonar with computer aided detection and classification. There are many technologies, however, which are common to all four missions. These include critical technologies for endurance, communications, precise navigation, low speed hydrodynamic control, command and control, stealth, and launch and recovery. The use of these technologies allows the Navy to use a common design for undersea vehicles while enabling the insertion of mission-unique payloads.
A UUV MCM mission scenario can be conceptualized to begin with the launch of the UUV from a submarine. Delivery of the UUV by an SSN will be covert, and will conserve energy by placing the UUV closer to its target area. The mission objective will be to determine a path or area devoid of mines. The UUV will communicate data and images to the host platform, and receive instructions, using either a fiber optic link or wireless acoustic communications now under development. At the completion of the mission, the UUV will return to the host platform and be recovered.
UUV systems feature a high degree of technology interdependence and changes to one technology area affect others. For example, the increased endurance resulting from progress in developing higher density energy storage/propulsor technology will tend to drive needs for longer-range communications, more sophisticated adaptive controllers/robotics, and more self-contained/independent navigation techniques/systems. Supporting disciplines and technologies, such as fault tolerance and signature reduction (magnetic and acoustic) must be incorporated as a UUV system is developed, and cannot be easily added on later. All technology candidates must be studied for system trade-offs before they are selected for incorporation into a UUV system.
The Near-Term Mine Reconnaissance System (NMRS) is a mine detection, localization and classification system for deployment from a fast attack nuclear submarine (SSN). It is expected to provide the fleet with an interim clandestine mine reconnaissance and surveillance capability for use during littoral warfare engagements. NMRS, which is currently being developed by Westinghouse Electric Corporation, is scheduled for its initial operational capability (IOC) in March 1998. NMRS includes two UUVs that are equipped with forward looking and side-scanning sonars, along with appropriate navigation, data processing and communications capabilities. The UUVs are being designed for launch and recovery from an SSN 688/6881 class submarine’s torpedo tube; they wilt be controlled and operated from the SSN through a fiber optic tether system. The SSN will also have associated data processing and communications capabilities to provide battle group commanders with a real-time assessment of the mine threat in the area surveyed by NMRS. It is expected the NMRS, which relies almost exclusively on the use of existing technology, will have a service life of about six years, and that it will provide an interim capability until the Long-Term Mine Reconnaissance and Avoidance System (LMRS) is developed and delivered to the fleet.
LMRS is currently in the conceptual development stage of definition. As with NMRS, LMRS will be deployed from an SSN, either via a torpedo tube or from a deck-mounted dry-deck shelter; LMRS may also have the capability for use by surface ships. It is expected that LMRS will provide very significant improvements in sensor performance (swath width, range, and probability of detection), vehicle endurance and control, and in data processing and communications capabilities. LMRS will constitute a major procurement action, and will be acquired through a series of competitive contracts, with the first contract to be issued in late fiscal year 1996. LMRS is scheduled for an IOC of early fiscal year 2004, and will have a life expectancy of about 20 years.
NMRS & LMRS are two present and future programs which demonstrate how the Navy is meeting UUV mission requirements, specifically the MCM requirement. Other mission requirements such as surveillance, intelligence collection, and tactical oceanography are the key drivers behind the UUVs for system capability requirements: covert launch and recovery; signature reduction; fault tolerance; and supporting technologies and disciplines. These mission and system requirements can be further defined by examining the UUVs critical enabling technologies.
The baseline for UUV energy storage is the rechargeable zinc-silver-oxide (Zn-AgO) wet-cell battery. This battery is currently used in the ASW Training Target Mk 30 Mod 1, which has been in the fleet since 1975. The mid-term goal is to increase UUV energy density to three times that of Zn-AgO, and the far-term goal is ten times that of Zn-AgO. In addition to energy density, other important attributes include affordability, safety, environmental impact (cleanliness), and rechargeability.
The Navy is exploring advances in secondary battery systems in the areas of energy density as well as number of cycles and ease and speed of rechargeability. Secondary battery systems currently under development include improvements to Zn-AgO and advanced rechargeable batteries. The Mk 30 Mod 2 Target Program has set a battery improvement goal of reducing the lifecycle cost of Zn-AgO batteries by a factor of two through increased reliability, cycle life, and wet life. The advantage to improving the current Zn-AgO batteries is that they can be easily and immediately swapped into current systems. The most promising advanced rechargeable batteries include lithium cobalt dioxide (LiCoQJ, lithium ion, and molten salt. LiCoQ.i has been demonstrated to 100 ampere-hours and has a projected energy density of two times that of Zn-AgO. Other lithium and metal hydride rechargeables and molten salt chemistries are under development.
Candidate advanced primary batteries include lithium thionyl chloride (LiSOCI2, aluminum hydrogen peroxide (Al-H20:z) and zinc-oxygen (Zn-0,). These would have more energy density, but are not rechargeable. Low rate LiSOCl2 has been demonstrated to three times that of Zn-AgO, and developmental and commercial units are available. Al-H202 has been demonstrated on a laboratory scale, and has a projected energy density three to four times that of Zn-AgO. Zn-02 batteries are being developed for the portable electronics market and have been demonstrated on a small scale. When combined with dense solid oxygen sources, Zn-02 is expected to achieve two times that of Zn-AgO energy density in a UUV configuration.
The most work in development of fuel cells for UUVs has been accomplished under ARPA sponsorship. Their concentration has been on aluminum oxygen (Al-0,) semi-fuel cells and on proton exchange membrane (PEM) fuel cells. The ARP A program will culminate with a 15 KW land based demonstration of an Al-02 power plant, with approximately three to four times that of Zn-AgO energy density. The significant accomplishment of the PEM cell effort was demonstration of a 7 .5 KW, high reliability fuel cell assembly. The PEM fuel cell was run over 2,000 hours without a failure. Fuel cell energy density can range from four to ten times that of Zn-AgO, and is mostly dependent on the gas storage methodology.
The wick combustor, coupled with the Stirling engine, is being developed by ONR at the Applied Research Laboratory, Pennsylvania State University. This thermal energy system has a potential density of greater than ten times that of Zn-AgO. The wick combustor contains molten lithium, which is wicked up to an oxidant, sulfur hexafluoride (SFJ, where beat is generated. The combustor part of the system has been successfully run over 75 hours. The Stirling engine has a higher efficiency (40-50 percent) than Rankine systems (20-30 percent), but it has a higher mass per horsepower. When compared to the Stored Chemical Energy Propulsion System (SCEPS) power plant, the wick-Stirling is safer because the molten lithium is at a lower temperature and is separated from the combustion area. It is also more affordable, because the power plant can be stopped in mid-cycle and restarted, while the SCEPS cannot. More development work is necessary to marry the heat source with the Stirling engine. An in-water demonstration of a Wick/Rankine power plant aboard a UUV is scheduled for FY 1997-98.
High data rate, low bit error rate acoustic (wireless) communications with UUVs can eliminate reliance on fiber optic lines. The goal is to transmit data at a rate of 300 megabits per second. The Navy is approaching the development of this technology in two ways: by reducing the amount of data which must be transmitted, through preprocessing and compression; and by increasing the capabilities in acoustic transmission from 1 kilobit per second (kbps) at 1 km to 30 kbps at S km. Using this dual approach, an underwater modular network is being developed which is some-what similar to a cellular telephone system. The cells are oriented to independent transceivers which are the size of A size sonobuoys, so the loss of one node does not interfere with data transmission.
Using a low risk approach, the Navy has improved the data rate to five times that of the baseline, in real time. This has been accomplished by designing around multipath and reverberation, using frequency hopping, guard bands, noncoherent detection and averaging, and multiple frequency shift keying, and by designing around frequency smear and Doppler by sparsely populating the spectrum, leaving additional tonal spacing, employing Doppler sensing and tracking, and by widening spectral resolution. Temporal and spectral diversity are being used for redundancy. The ONR system with these features was tested during the summer of 1993 at Seneca Lake, New York. During this test, a data transmission rate of five kbps at five nautical miles was demonstrated. This technology development has stopped since there is little room for future expansion of capability.
The approach now being pursued was developed by the Woods Hole Oceanographic Institution, with funding from ARP A and ONR. This approach features coherent processing, instantaneous channel characterization and spatial diversity in the acoustic channel. Hydrophones are separated to maximize the potential for location outside the shadow zone. The power sum of all the elements (transducers) results in a greater signal-to-noise ratio. During deepwater testing off the California coast in 1991, a rate of 1000 bps over 100 nautical miles was demonstrated. Testing in Buzzards Bay, Massachusetts during 1993 demonstrated a rate of 30 kbps over 9 .5 nmi. These tests originally required a supercomputer for processing. Advancements in technology have reduced the required computer to the size of an A size sonobuoy container.
In November 1994. at the American Defense Preparedness Association semi-annual symposium held at the NUWC, Division Newport, high data rate acoustic communications was demonstrated live using the Large Diameter UUV (LDUUV) as its platform. The demonstration acoustically transmitted prerecorded object detection data from the LDUUV, which was located in Narragansett Bay at the Gould Island shallow water test facility. The signal was transmitted via a RF link to the presentation at Spruance Hall at the Naval War College. The acoustic signal was transmitted through the water over a distance of2.5 kyds at a rate of 30 kbps.
Using these high data rate acoustic communications techniques, images such as sonar displays, laser linescans, and television images can be transmitted to all players in a mission. The system would allow two-way communications so that, in addition to receiving data, stations could transmit instructions to the UUVs.
The next generation of UUVs must be able to interact with the environment using robotics. UUVs with on-board robotic mechanisms will be able to perform such tasks as tagging objects, taking soil samples, hooking up cables. and performing other undersea work. Early robotics demonstrations will involve wire or fiber optic connections, but data transfer will be limited to acoustic communications parameters for realism. This will include limitations on bit rates and delays due to propagation of sound waves. Later demonstrations will utilize actual acoustic transmission of data and commands, and still later, instructions will be carried out autonomously. The script for early demonstrations will include simple tasks, such as moving an object. More advanced scripts will require complex work such as connecting cables, object recovery and sampling.
The baseline accuracy of autonomous navigation is contained in the Target Mk 30 Mod 1, which uses a ring laser gyroscope (RLG) Guidance and Control System for an accuracy of 22,250 meters circular error probability (CEP) (worst case). The UUV goal is 50 m CEP, irrespective of length of run. A more accurate traditional system, consisting of an inertial navigation system (INS), a correlation or Doppler velocity sonar (CVS/DVS), and a Kalman filter, wiII have significant error with long endurance. For example, the Large Diameter Advanced Test Vehicle (LDA-TV), during demonstration runs in FY 1992, exhibited a projected 2,800 m CEP for a six hour period based on observed one hour real data. The LDATV was equipped with a RLG-based INS and a simple Kalman filter.
The Large Diameter UUV (LDUUV), now being used by NUWC for demonstration runs, wiII use the LDA TV system, improved with a better Kalman filter and DVS. It is projected that it will attain an accuracy of 150 m CEP over a six hour run time. More accuracy (down to 50 m CEP) during longer runs will require updates of the system during a mission. These system updates can be obtained through the Global Positioning System (GPS) or through non-traditional techniques. GPS requires getting an antenna out of the water, possibly compromising stealth or taking time away from the mission to get a fix. Non-traditional techniques may overcome these limitations. These techniques include: terrain/ contour following, bottom-mapping/map matching, geophysical (magnetic or gravity), video, zero velocity update, or acoustic communications.
Many UUV missions require platform stability in very shallow water to ensure proper operation of sensors and payloads. Certain sensors and payloads require specific speeds for optimum operation. During recovery by a host platform (particularly a submarine), the UUV will require fine control. The basic UUV hydrodynamic control system includes an adaptive, nonlinear controller with advanced effectors, including tunnel thrusters, fins, variable ballast, and propulsor. Thrusters work best at speeds from zero to three knots, and fins at speeds over three to four knots. The low speed control system will have to operate the vehicle over all speeds, zero to twelve knots, including the transition zone.
Control of the vehicle, its components, subsystems, and payloads is coordinated in the control computer. While early autonomous systems had unique designs, the current emphasis is on standardization. UUVs under development at NUWC contain a high percentage of COTS items, including the control computer, the rack, and the interfaces. ONR and ARPA have adopted standard industry interfaces for the UUV systems, including ethemet, RS-232, and -422, small computer standard inteterface (SCSO, and fiber optic links.
As the endurance of UUVs grows, control systems must be improved to include fault tolerance and planning and replanning of missions based on unforeseen events. Intelligent controllers will be developed which can preserve the mission and withstand system faults. UUV systems must be designed for robustness and fault tolerance from inception.
Covertness, reliability, and self-preservation require that particular attention be paid to the reduction of acoustic and magnetic signatures of UUVs. Many sensors, such as magnetometers, are degraded by the presence of magnetic interference. Excessive acoustic or magnetic signatures can cause detonation of mines. Remote enemy sensors may be alerted to the presence of UUVs if they detect magnetic or acoustic energy. NUWC has been designing systems which incorporate acoustic signature reduction for some years. The Torpedo Silencing Research Vehicle and the LDATV served as testbeds for silencing hardware and techniques. Noise reduction hardware designed for the LOA TV include composite, coated bulls; vibration dampening mounts; and methods of decoupling machinery from the outside water column.
A 21 inch UUV, now in the design stage, will demonstrate, in water in a tactical size vehicle, the more advanced critical technologies discussed in this paper, including advanced energy storage, acoustic communications, robotics, navigation, low speed hydrodynamic control, vehicle and system control, and signature reduction. These ONR developed technologies will be available for Navy acquisition programs.