Superimposed training low probability of detection underwater communications.

This paper proposes a superimposed training method for low probability of detection underwater acoustic communications. A long pilot sequence was superimposed to the message for equalization and synchronization purposes. A fast Hadamard transform (FHT) estimated the channel impulse response and compressed the pilot energy. A Wiener filter performed equalization. The interference signal was removed using hyperslice cancellation by coordinate zeroing. An inverse FHT decompressed the remaining sequence energy and the message was retrieved. Results from a shallow water experiment presented bit error rates <10-2 for signal-to-noise ratios <-8 dB.

Horizontal coherence of low-frequency fixed-path sound in a continental shelf region with internal-wave activity.

Sound at 85 to 450 Hz propagating in approximately 80-m depth water from fixed sources to a joint horizontal/vertical line array (HLA/VLA) is analyzed. The data are from a continental shelf area east of Delaware Bay (USA) populated with tidally generated long- and short-wavelength internal waves. Sound paths are 19 km in the along-shore (along internal-wave crest) direction and 30 km in the cross-shore direction. Spatial statistics of HLA arrivals are computed as functions of beam steering angle and time. These include array gain, horizontally lagged spatial correlation function, and coherent beam power. These quantities vary widely in magnitude, and vary over a broad range of time scales. For example, correlation scale can change rapidly from forty to five wavelengths, and correlation-scale behavior is anisotropic. In addition, the vertical array can be used to predict correlation expected for adiabatic propagation with cylindrical symmetry, forming a benchmark. Observed variations are in concert with internal-wave activity. Temporal variations of three coherence measures, horizontal correlation length, array gain, and ratio of actual correlation length to predicted adiabatic-mode correlation length, are very strong, varying by almost a factor of ten as internal waves pass.

Observations of low-frequency temporal and spatial coherence in shallow water.

Measurements of temporal and spatial coherence are most always confounded by the presence of multipath interference. Here, two sets of data are presented that allow separation of arrivals from individual propagating modes and then unambiguous computations of temporal and spatial coherence free of multipath effects. A consistent finding for surface reflected bottom reflected (SRBR) modes is that lower order mode are more coherent in both time and space than higher order (steeper) modes. Also, SRBR paths are much more coherent than refracted bottom reflected mode groups that cannot be separated in time and are affected by multi-mode interference.

Temporal coherence of mode arrivals.

Temporal coherencies are compared for individual pulse arrivals of 100 and 800 Hz centered broadband signals propagating through identical shallow channels during periods of low and high internal wave energy. All 100 Hz modes are clean, distinct in time, and remarkably coherent (t(coh)>1 h). A near continuum of modes is observed for the 800 Hz signal with much shorter coherence times. Higher order modes are less coherent. Internal wave scattering appears to dominate the 100 Hz signals, whereas bottom scattering also randomizes the 800 Hz signal. Observations are consistent with similar measurements for this and other sites for intermediate frequencies.

Observed limiting cases of horizontal field coherence and array performance in a time-varying internal wavefield.

Using a moored source and horizontal/vertical line array combination, horizontal coherence properties of high signal to noise ratio (> or =20 dB) 100-1600 Hz signals have been measured. Internal waves in the area of the measurement created moving episodic sound-speed anomaly structures, influencing coherence length. Measured horizontal coherence scales for 100 Hz ranged from 5 to 20 acoustic wavelengths, and were inversely related to the sound-speed anomaly strength. Horizontal field properties were compared with fields computed using modal decompositions of the vertical signals. The comparison allows azimuthal field coherence properties to be studied apart from normal-mode interference effects.