| Popis: |
M sequences have long served as enabling technology for numerous basic research experiments to observe the fluctuations of acoustic propagation and for many acoustical oceanographic studies including ocean acoustic tomography. They provide coherent processing gain without loss of temporal resolution. Simply stated M sequences combine the energy of CW with the resolution of a pulse. In addition to processing gain, the M sequences can eliminate ping stealing and allow for environmentally friendly transmissions (lower peak pressure levels). Yet, early attempts (circa 1960) to apply M sequences to monostatic active sonar were unsuccessful. In hindsight, the inability to process Doppler and possibly an inappropriate application of a matched filter correlation are the reasons. Two approaches for M‐sequence applications to sonar are discussed here, one for passive, whereby clandestine background experiments can evaluate sonar conditions, and the other for a multisource bistatic active. Ultrafast Hadamard transforms make possible full range Doppler processing for both applications and the combination of orthogonal sampling and cancellation by coordinate zeroing are applied to reduce the direct arrivals and some of the stable reverberation components for the active application. The same approach can be used to process and separate simultaneous transmissions from more than one source that transmit nearly orthogonal signals. |
