SHALLOW WATER UNDERSEA WARFARE TRAINING RANGES

Tactical training ranges provide the capability for measured performance and feedback: in a realistic environment. During a training exercise, the positions of all the participants are measured, time-tagged, and recorded in real-time. Training analysts review the data in both real-time and post-exercise modes. Additionally, real-time display of the track: data is used to provide range safety during the exercise. At the end of the exercise, the analysts provide debrief packages to the participant crews using a variety of methods. In some cases, such as for the air ASW crews training at the Southern California Offshore Range (SCORE), the analysts provide a live debrief to the crew-complete with playback of the exercise data. This is the most effective training, with the crews getting feedback immediately after the exercise while it is still fresh in their minds. On other occasions, only a debrief package containing plots or a videotape is mailed to the crew. The latter case is most typical of submarine crew debriefs.

An important part of making a training exercise realistic is to provide a target or an opponent for the crews. On undersea warfare (USW) training ranges, typically a submarine or a mobile target is utilized for this purpose. Exercise torpedoes are fired by the participants both to gain experience performing this function, and to allow evaluation of weapon employment. The tracks of the weapons and the mobile targets are measured and recorded by the range system to allow a complete picture of the exercise.

The decline in resources will have a direct impact on how these Navy exercises are conducted. Fewer mobile targets are going to be utilized, and the number of actual torpedo firings will decrease. Additionally, as the fleet decreases in size, submarines will experience increasing difficulty in providing live-target services. However, training-with its ability to serve as a force multiplier-will be more important than ever.

The integration of a simulation/stimulation capability into an underwater training range provides a cost-effective means to increase training realism in the face of declining assets. Additionally, other techniques to improve data collection and mission playback serve to further enhance the training value of an exercise.

The world, and therefore the types of conflicts in which the U.S. could potentially be involved, has changed in recent years. Previously, the Soviet Union was our primary adversary, and most of our naval operations occurred in open ocean, deep waters. As a result, the underwater training ranges that have been developed over the last 30 years have been in deep water, typically 3,000-15,000 feet. These ranges include the Barking Sands Tactical Underwater Range and its expansion (BSURE) off Kauai, Hawaii; the SCORE; and the Atlantic Fleet Weapons Training Facility Underwater Tracking Range in the Caribbean.

Today, the Navy is much more likely to be involved in regional conflicts that will require it to operate in shallow, littoral waters. Currently there are no shallow water ranges suitable for training on the same scale as provided by the current deep water ranges. In response to this shortage, the Navy has developed and approved a Mission Need Statement for a Shallow Water Undersea Warfare Training Range (SWTR).

The Navy currently has several programs in various stages of realization to address its future tactical training system needs. The three systems most applicable are the Battle Force Tactical Training (BFIT) System; the Joint Tactical Combat Training System (JTCTS); and the major focus of this paper, the SWTR. All of these systems utilize simulation techniques in a shipboard setting to enhance the training received by the crew.

The BFTT system supports pierside training for surface ships and submarines. Future onboard trainers (OBT) that provide simulated targets by stimulating the ships’ tactical sensor systems will have external control interfaces. Scenario generators located at the Fleet Combat Training Centers will be networked to the OBTs via the control interfaces, providing the ships with a common synthetic environment and threat scenario. Feedback from the ships (simulated weapon employments, evasive maneuvers, etc.) will be input to the scenario generators for realistic threat response and appropriate kill removal. Submarine shore-based trainers will also be integrated into BFI’T. The onboard submarine systems used to support BFTT will also be required for the simulation capability of the SWTR.

The JTCTS is the Navy’s and the Air Force’s premier program for integrating a simulation capability within a range system. This program is primarily focused on surface and air combatants, with the SWTR providing the real-time connectivity for the submarines at locations where the JTCTS supports littoral warfare on the east and west coasts. The JTCTS will provide radio frequency (RF) links to the participants, interfacing with the tactical systems to allow simulation of threat targets onboard the range participants. Conceptual operation is similar to that described for the BFIT, except that the participants are flying or underway as opposed to being pierside.

The SWTR will consist of a large area (500 square nautical miles) with underwater instrumentation providing tracking of range participants that may include submarines, torpedoes, mobile targets and other future undersea vehicles (such as unmanned underwater vehicles (UUVs)), as well as support simulation capability for the submarines. Water depths will typically be 120-1,200 feet. Two SWTRs are budgeted to be built, one on each coast.

The primary function of a training range is to provide ground truth position of all participants. All current ranges, both deep and small-area shallow, use a tracking technique referred to as multilateration. Three or four receive hydrophones are required to be within the hearing radius of the acoustic pinger mounted on the undersea vehicle to be tracked. The distance between the pinger and each hydrophone is calculated based on a time of arrival measurement. The position of the vehicle is then calculated based on the known position of the hydrophones. This technique provides high accuracy in deeper water for a reasonable investment in hydrophone quantities.

The major cost driver of any undersea range is the in-water hardware that consists of the sensor nodes, the cable, and the installation associated with the hardware. This cost factor is particularly acute in the harsh shallow water environment where the sensor node structures and cable are typically more expensive because of their required ruggedness, and the installation is more costly because of the potential need to bury the cable. The number of sensors required for a particular sized area is greater in shallow water than in deep water because of the shorter acoustic propagation paths that are supported in shallow water.

The goal is to minimize the number of sensors, and therefore the overall cost of the SWTR. The design of the SWTR will require only one sensor node to be within hearing range of the vehicle to obtain its position. There are a variety of techniques that are currently under development for single sensor tracking.

One technique requires the acoustic telemetry of positional data from the underwater vehicle to the hydrophone. Almost all undersea vehicles have some type of navigation system, spanning from simple dead-reckoning schemes to complex inertial systems. Data from these systems would be tapped into and transmitted by the pinger to the hydrophone. Combining these data with single-axis range to the hydrophone using a Kalman filter, results in an accuracy that meets the training range requirement.

Another technique that can be employed is the use of a multimode hydrophone that provides a measurement of bearing angle. That measurement, combined with the single axis ranging and telemetered depth (measured onboard the vehicle by the pinger), yields a position.

In Figure 1, the equipment onboard the submarine, an OBT that is an integral part of the submarine, outputs the proper stimulation signal to the front end of a particular tactical system based on the target being simulated. The OBTs are typically capable of generating a number of canned scenarios. The crew operates the tactical systems, and reacts based on the target’s actions.

The status of the tactical systems is recorded by a software module within the Submarine Fleet Mission Program Library (SFMPL), which resides on the submarine’s TAC-3 computer. These data include sensor contacts, navigation information, fire control solutions, and weapon firing presets. Another module within SFMPL provides the capability to replay the data on a debrief display that is nearly identical to those used at the current undersea training ranges.

The data collection and the replay functions are currently being added to 688/6881 submarines as part of another training range program initiative. In this program, the problem of timely debrief for the submarine crews has been addressed. It is very impractical to get the crew off the submarine and into the Range Operations Center (ROC) for a debrief by the training analysts after an exercise. Typically the crew gets a debrief package containing plots and, possibly, a videotape several weeks after the exercise. The training impact is largely lost at that point. The onboard debrief capability within SFMPL, coupled with a data link to the ROC, allows the training analysts to construct a debrief package using both data from the submarine and from other range sources. The package is transmitted back to the submarine for replay within hours of the events and shows the submarine’s tactical picture overlaid with ground truth from the range. This more timely approach provides meaningful feedback to the training participants. This capability will be an important part of the training experience both on the SWTR and on existing training ranges.

The BFTT interface subsystem will serve as the external control point for the submarine’s OBTs. This subsystem will allow the OBTs to simulate targets based on offboard generated scenarios, instead of only on the canned runs contained within the OBTs. When the system is used pierside in the BFIT mode, the scenario input and the tactical system output will be provided via a land-based linkage such as a fiber-optic cable or a RF data system.

The data link to a submarine exercising on the SWTR will be an acoustic telemetry link. A number of bidirectional acoustic transducer nodes will be located throughout the range area so that at least one node will be within the hearing radius of the submarine at any one time (Figure 2). The nodes are connected together and to the ROC using undersea fiber optic cable. To minimize the amount of cable required, and therefore the expense associated with the in-water hardware, a number of the nodes will be multiplexed on the same fiber optic cable using time-division multiplexing techniques similar to those used in the telecommunications industry.

The data transmitted from the ROC to the submarine will consist of the messages necessary to queue the BFTI’ interface to control the OBT. For example, the queue message might provide the target type and initial parameters for range, bearing, course, and speed. The BFTI’ interface subsystem would run that scenario based on the initialization, plus any updates that are received. The OBT is then responsible for generating the high-fidelity target signal required for the simulation. In this way, the data-rate requirement for the acoustic telemetry link, with its inherently narrow available bandwidth, can be minimized. The acoustic transmit capability of the nodes will support other functions, such as range safety information, (i.e., the ground truth positions of other live participants on the range) and cost-effective underwater telephone (WQC) voice communications.

The data received via acoustic telemetry from the submarine will include position data and portions of the tactical system data collected within the SFMPL module. The data will be used to support simulated weapon firings. When the submarine is operating against a simulated threat, a real weapon firing will probably not occur. Instead, a water slug will be fired, and then the weapon preset data will be downloaded to the ROC via the acoustic telemetry link. At the ROC, a torpedo simulation will be run, and appropriate hit/miss criteria will be applied. A torpedo simulation is currently in use at the Atlantic Undersea Test and Evaluation Center using a similar method; however, the weapon preset data is passed to the ROC post-exercise on a RF link. Again, to minimize acoustic data rate requirements, tactical system data not necessary for real time range operations will be stored and transmitted post-exercise via the ultra-high frequency (UHF) satellite communications/RF link to the ROC. The tactical system data will be used by the training analysts to help them assess what occurred during the exercise. Similarly, the debrief package will be sent via the UHF link, not the acoustic telemetry link.

There are many sources for the simulated target data. The easiest one to conceive of {although probably the hardest to implement with realism) is a computer-generated simulation located at the ROC. The simulator would generate the scenario, and would react to the on-range submarine’s actions in a realistic manner. However, a much more versatile solution would be to implement a ROC interface with the Defense Simulation Internet (OSI). This would be accomplished by making the computer systems in the ROC compliant with the Distributed Interactive Simulation (DIS) communication protocols. DSI/DIS allows simulation systems at diverse locations to operate in a common synthetic environment as depicted in Figure 2.

The use of DSI/DIS literally opens up a whole world of target/opponent sources to be used by the SWTR. The computer-generated simulation discussed previously could reside anywhere; for example, at a Navy laboratory like the Naval Undersea Warfare Center Division Newport {NUWCDIVNPT). More realism could be obtained by using shore-based trainers connected to the DSI; a submarine crew at the trainer in Groton, Connecticut could oppose a crew operating a submarine on the SWTR. The realism would be enhanced because there would be a man-in-the-loop on both sides, with the simulation serving only to collocate the crews in the same environment. The submarine’s sensor systems would indicate the presence of the trainer’s submarine based on the initial offset parameters, and the actions that the trainer crew would execute. Similarly, the trainer’s sensor inputs would be based on the submarine’s movements and actions on the SWTR.

Ships and submarines participating in BFIT exercises could also participate in a manner similar to the land-based trainer example. Another scenario might have two submarines on separate SWTRs, one on each coast. The simulation capability would allow them to operate in the same environment with any initial parameter configuration desired. The two ranges would be overlaying each other in the virtual world. The combination of live vehicles (underway on the same range, different ranges, pierside in BFIT, land-based trainers, etc.) that could exercise in this synthetic environment is almost limitless.

There are several advantages to using simulation in a training range system. This concept combines the best features from each type of system. The SWTR in itself supports training in a realistic environment typical of future threat locations. The range allows real submarines to operate in conjunction with other live assets, and to fire exercise torpedoes. The added capability to provide for simulated targets/vehicles further enhances the training experience. Simulation not only helps to make up for the shortfalls caused by declining resources, but also has additional benefits.

Simulation in conjunction with the training range allows exercise scenarios that cannot be accomplished exclusively with real participants. For example, simulation provides the ability to increase threat densities to realistic levels that are too costly to implement using real targets. Additionally, simulation allows the creation of scenarios that may be too dangerous to execute with only real participants. With a simulated target, there is no restriction as to how close it can come to the submarine on the range. Finally, simulation can supply targets/threats that may be otherwise unavailable, such as Kilo class submarine.

Simulation onboard the submarine while underway on the SWTR can provide a much higher-level of realism and stress than would be available strictly using a shore-based trainer. It can provide the crew with experience using their ownship equipment configuration. The SWTR will allow a mix of real and synthetic participants to maximize the value of the training received.

The NUWCDIVNPT is currently conduction investigations, developing prototypes, and demonstrating and validating the concepts necessary to construct the new SWTRs. Production of the first SWTR will commence in FY97 for the East Coast, with an Initial Operational Capability (IOC) of FY99. This range will be located in the Onslow Bay area off Camp LeJeune, North Carolina, in support of the Littoral Warfare Training Complex. Installation of the West Coast SWTR will begin in the Southern California area in FY98, with an IOC of FYOO. The IOC for both of these ranges will occur with an initial instrumented area of 125 nmi² each. Expansions scheduled through FY01 will increase the area at each location to 500 nmi².