Abstract:

An adaptive beamformer was developed for use with existing underwater acoustic modems and for measurement of the underwater acoustic communication channel. The output of the beamformer is made up of many independent channels, each derived from the application of orthogonal vectors to the array elements. Each channel represents a vertical beam that is focused in the direction of correlated energy arriving at the array of vertically spaced transducers. Within each beam, nulls are created in the direction of interference. Experiments were conducted in shallow water to characterize the acoustic communication channel at many frequencies and ranges so as to evaluate modem performance using various encoding methods and waveforms. It was found that high-reverberation levels exist at our test site in the shallow water near the shore environment. In this environment, adaptive processing reduced reverberation and transmission errors significantly. Wideband PSK was used to evaluate channel characteristics. In the process of testing the adaptive beamformer, waveforms were developed for a low-cost single-channel modem for use on AUVs. Results of the adaptive beamformer and the single channel modem are presented. [Work supported by ONR.]