SSN-774 Virginia-class Mission
The program design goal is to produce a submarine flexible enough to carry out seven very different missions:
- Covert Strike by launching land-attack missiles from vertical launchers and torpedo tubes;
- Anti-Submarine Warfare with an advanced combat system and a flexible payload of torpedoes;
- Anti-Ship Warfare, again, using the advanced combat system and torpedoes;
- Battle Group Support with advanced electronic sensors and communications equipment;
- Covert Intelligence, Surveillance and Reconnaissance, using sensors to collect critical intelligence and locate radar sites, missile batteries and command sites as well as to monitor communications and track ship movements;
- Covert Minelaying against enemy shipping; and
- Special Operations, including search and rescue, reconnaissance, sabotage, diversionary attacks, and direction of fire support and strikes.
Operating in the shallow waters of littoral areas imposes a different accoustic environment for which previous submarine classes were optimized. As reported in ONR Ocean Science and Engineering Newsletter # 2 (Feb. 1997) it is well known that as a result of the selective frequency effect of the shallow-water sound channel, a band of frequencies exist in which the propagation is enhanced (i.e., the transmission loss is relatively small). This "optimum frequency" regime arises from the combined effect of the volume attenuation at the higher frequencies and the loss due to interaction with the sea bottom at the lower frequencies.
Because of the proximity of the boundaries in shallow water, multipath transmission and multi-angle scattering from the sea bottom are concomitant characteristics of shallow-water acoustic reverberation. Consequently, long-range reverberation in shallow water is far more complex than the deep-water case. Because of interaction with the bottom, long range sound propagation in shallow water is characterized by separation of the constituent modes as a result of the differences in modal group velocities. This results in elongated, low amplitude signals. Further, because of the non-uniform effects of the interaction--e.g., the higher-angle modes suffer greater attenuation--only several modes may be needed to characterize the sound field. Hence, mode filtering is a useful approach for investigating multipath fields in shallow water.
The spatial structure of the accoustic signal in the waveguide formed by the surface and bottom in shallwo water is significantly different from the that in the free field of deep water. Hence, due to modal interference in a waveguide, conventional beamforming techniques cannot be used. Several on mode filtering methods are possible source ranging and depth estimation in the shallow water wave guide. Signals of several modes may be separated, and after correction for arrival time and phase, these filtered normal modes may be recombined to obtain a compressed and enhanced signal.
In some shallow water regions very strong and sharp summer thermoclines exist, and are accompanied by conspicuous internal waves. Anomalous attenuation of sound between 300 Hz and 1200 Hz is associated with these conditions, with very large variations (as much as 30 dB at some frequencies) in the frequency response of the transmission loss. These abnormally large attenuation can be attributed to internal wave-induced acoustic mode coupling. In particular, the internal waves cause a transfer of energy into the higher-order modes, which, since they interact more with the lossy bottom, leads to a frequency-dependent energy loss (or attenuation) in the sound wave.
Reducing acquisition and life-cycle costs is a major objective of the New Attack Submarine design and engineering process. Cost avoidance is anticipated through the application of concurrent engineering design/build teams, computer-aided design and electronic visualization tools, system simplification, parts standardization, and component elimination. These innovations are intended to ensure that the ship is affordable in sufficient numbers to satisfy America's future nuclear attack submarine force level requirements.
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