The United States Navy is proposing to construct one or more permanent, shallow water, acoustic tracking ranges along the coastal seaboard of the United States for the purposes of providing a highly technological water and beach training facility. The range will possibly encompass a range area of approximately 25 by 20 nautical miles and will be located along the continental shelf in a water depth of about 100 to 1,300 feet with a longitudinal axis of the range area extending parallel to the coastline. Within the range area, a number of hydrophones will be permanently installed so that acoustic wave energy may be measured and converted to electrical signals in order to electronically effectuate, orchestrate and observe military training, related exercises and ordinance tests. The hydrophones are generally wired together and then to shore. The hyrdophones, along with their housings and integral electronics, are generally referred to as nodes. Movement of ships, troops, aircraft and ordinance, for example, may be monitored within the range from a centralized command and control compound thereby providing interested parties with the ability to record and make improvements to the training exercise from actual observed events.
One problem, however, is that the hydrophone nodes, which are to be permanently installed along the ocean floor, are subject not only to a high corrosivity salt water, immense hydrostatic pressures and powerful underwater currents, but also to commercial fishing activities that surround or extend into the range. Thus, the nodes must be able to operate effectively in a tumultuous and dynamic environment.
Particularly disruptive fishing activities that require attention are those that utilize bottom trawlers and seafood dredges. Not only do such activities create upheaval of the seabed floor along the shelf but they potentially serve to rearrange installed nodes if contact is made between the trawlers and the installed nodes. In order to alleviate some of those problems, it is proposed that cables interconnecting the nodes will be buried in approximately three feet (one meter) of sediment below the surface of the sea bed, as such depth has proven acceptable in other applications, e.g. the international telecommunication industry. However, the problem of contact between trawlers and nodes is still not alleviated since the hydrophone cannot be buried.
A further problem of effectively establishing an underwater electronic range is that in order to determine the location of coordinated target objects, such as ships, troops and aircraft, for example, a reference grid, such as range and bearing from a known location, must be established. Generally, underwater targets to be tracked are outfitted with transponders which regularly transmit acoustic signals (pings) which are received on one or more bottom mounted hydrophones (nodes). In the past, however, bearing determinations were calculated by a quasi-triangularization method from information obtained from three or more hydrophones located at known widely separated locations. Yet having multiple sets of three hydrophones distributed along the ocean floor increases economic burdens and complicates installation procedures for such a relatively small geographic range area. Therefore, a need exists to minimize the number of node installations on the ocean floor.
Especially for shallow water operations, where acoustic propagation distances are reduced, significant savings could be realized if the bearing of the transmitted acoustic signal (ping) could be determined accurately from a single co-located set of hydrophones. By co-located, the inventors mean separated by less than a few feet such that they would occupy the same node. However, the arrangement must be inexpensive and therefore cannot have too many (say no more than six) hydrophones. To provide any real advantage over present systems, the bearing accuracy must be on the order of 1 degree.
Yet it should be appreciated that acoustic waves travelling through water are subject to many undesirable characteristics not found in other medium, such as the atmosphere, and the range equipment is therefore continually receptive to improvements that enable obtaining more accurate and reliable information. One of the most influential, but undesirable effect, is unwanted noise and therefore a further need exists that increases the signal-to-noise ratios of acoustic energy travelling underwater. Another undesirable characteristic, especially in shallow water, is decorrelation of the signal due to multipath and scattering. This reduces the ability to decode information contained in the acoustic signal. In order to achieve 1 degree bearing accuracy, and create a low error rate underwater communications system, a system would have to have significant gains against these unwanted propagation characteristics of the ocean water.
A further problem of range determinations exists when making range calculations for target objects that are located in shallow water. As before, the prior art attempted to improve difficulties by increasing the number of hydrophone arrangements which, again, adds costs and installation burdens associated with a small, shallow water range.