R. C. Spindel

An acoustic navigation system

Prepared for the Office of Naval Research under Contracts N00014-71-C0284; NR 293-008 N00014-70-C0205; NR 263-103 and the National Science Foundation/International Decade of Ocean Exploration Grant GX-36024 and the Applied Physics Laboratory of The Johns Hopkins University Contract 372111.

A high-resolution pulse-Doppler underwater acoustic navigation system

A high-resolution underwater acoustic pulse-Doppler navigation system has been developed and tested at sea. The system provides continuous, highly accurate tracking of underwater and ocean-surface platforms in a fixed 50-km2navigation net. Three reference buoys, moored 20 m from the ocean bottom, provide the navigation net used by shipboard processing equipment. Each reference buoy contains an acoustic transponder, used to obtain the acoustic travel times from the transponder to the platform, and a continuous-tone beacon, used to obtain the Doppler shift due to platform motion. The system is capable of determining the position of a platform with respect to the reference net with an error of 2-3 m. The relative position of the platform on a fix-to-fix basis can be determined within several centimeters over short time intervals (\approx 10min).

Signal Processing in Ocean Tomography

This paper addresses acoustic signal processing issues that have arisen in designing systems to measure the sound speed and current fields of the ocean using time-of-flight acoustic tomography. The systems are based on a scheme first proposed by W. Munk and C. Wunsch in 1979, who suggested that dynamic ocean processes could be observed by measuring the change in travel time of acoustic signals transmitted over a number of ocean paths. The travel time is subject to alteration in proportion to the magnitude of the sound speed and current inhomogeneities along each path. An estimate of these quantities can be obtained using inverse techniques.

High resolution acoustic navigation system

A high resolution underwater acoustic navigation system is provided by a combined pulse and continuous-wave or Doppler system. Locations of the object being tracked are periodically determined by the pulse subsystem and are used to initialize the Doppler subsystem. The Doppler Subsystem tracks the location from the fix obtained by the pulse subsystem. Both subsystems are interfaced with a central processing unit or digital computer and the data rate input to the computer is substantially reduced by the use, in the Doppler subsystem, of a phase angle quadrant change counter whose accumulated count is periodically provided as an input to the computer.

A mobile coherent low-frequency acoustic range

A portable system for the measurement of certain underwater acoustic propagation phenomena is described. Acoustic sources and receivers of special design are used. A precision tracking system enables coherent signal reception in the presence of source-receiver motion. The major elements of the acoustic range are described and examples of data are presented.

Long‐range sound fluctuations with drifting hydrophones

Phase fluctuations of underwater sound (406 Hz) transmitted between a fixed source and deep free‐drifting hydrophones have been obtained for transmission ranges of 200 km. Phase variations due to hydrophone drift are removed by a bottom‐moored, CW tracking system that corrects for motion‐induced phase variations of 0.06 rad or larger. Most of the energy traveled along refracted paths, eliminating much of the phase fluctuation due to bottom and surface scatter. Residual surface scatter effects are removed by narrow‐band filtering. Maximum observed phase fluctuations are 15 cycles over 3 h on the deepest hydrophones (1500 m). The mean‐square phase spectrum has a slope of −2 for frequencies between 0.4 and 40 cycles/h. The shallow hydrophone (300 m) data contain half the phase fluctuation of the deep hydrophones. Depth dependence of the fluctuations is attributed to internal gravity waves.

Acoustic‐internal wave interaction at long ranges in the ocean

Fluctuations in the amplitude and phase of low‐frequency sound propagated to long range in the ocean are predicted. Phase fluctuations are attributed to the passage of acoustic radiation through the internal gravity‐wave field; predictions are based on measured and modeled internal wave spectra. Ray theory is used to determine phase and amplitude variations as a function of time, space, and acoustic frequency. It is shown and experimentally verified that mean‐square phase fluctuations are depth dependent.

Acoustic phase tracking of ocean moorings

An acoustic tracking technique for monitoring the motion of deep ocean moorings is described. The system uses Doppler phase shifts from bottom-moored beacons to resolve 3-cm motions. The motion of two intermediate depth moorings is presented.

Survey technique for high‐resolution ocean navigation

We have reported on an acoustic Doppler navigation system for ocean use in several publications [R. P. Porter, R. C. Spindel, and R. J. Jaffee, J. Acoust. Soc. Am. 53, 1691–1699 (1973); R. C. Spindel, R. P. Porter, and R. J. Jaffee, J. Acoust. Soc. Am. 56, 440–446 (1974)]. The system can yield navigation fixes with respect to a bottom‐moored, continuous‐wave, beacon net with accuracies (on a fix‐to‐fix basis) of a few centimeters. The system has found use in generating synthetic space‐time acoustic apertures where it is necessary to track a receiver to within a fraction of an acoustic wavelength. In order to use the system to best advantage, a survey is required to determine precisely the relative positions of the beacons forming the net. We report here on a navigation system that combines a pulse transponder with a beacon to improve survey accuracy. It measures acoustic travel time between survey platform and mooring, as well as accumulated phase between survey points. Nonlinear regression techniques are...

Internal‐wave‐induced acoustic fluctuations in a horizontally stratified medium

Passage of an acoustic signal through an internal wave field results in the presence of amplitude and phase fluctuations at the receiver. The acoustic‐internal wave interaction has been discussed previously using ray techniques [R. P. Porter, R. C. Spindel, and R. J. Jaffee, J. Acoust. Soc. Am. 56, 1426–1436 (1974)]. We report here on wave techniques describing the interaction phenomenon. The Rytov perturbation approach is applied to a horizontally stratified medium which is perturbed by the internal wave field. Statistical expressions for the acoustic fluctuations are obtained based upon a postulated internal wave model [C. Garrett and W. Munk, Geophys. Fluid Dynam. 2, 225–264 (1972)]. It is shown that for internal wavelengths comparable to acoustic ray cycle distances a resonant condition exists leading to large acoustic fluctuations. The validity and limitations of the perturbation technique applied to the horizontally stratified medium are discussed.