Yu. N. Morgunov

Experimental studies of pulsed signal propagation from the shelf to deep sea

Results of an experimental study of low-frequency broadband pulsed signal propagation in a waveguide that includes the shelf zone, the continental slope, and the deep sea region are presented. Using phase-manipulated signals with central frequencies of 366 and 600 Hz, pulsed characteristics are measured at six points along the propagation track, the maximal distance from the source being 368 km. It is experimentally demonstrated that, in the presence of a negative sound velocity gradient in the near bottom layer on the shelf with a small bottom slope, the choice of the source position at the shelf bottom near the shoreline provides the formation of a continuous illumination zone in the deep sea near the USC axis and a stable pulsed characteristic with two main sound energy arrivals. The propagation velocity of the pulse that is last to arrive is identical (within the measurement error) to the velocity of sound on the USC axis at the point of reception. Possibilities for practical application of the results obtained from the experiment are discussed.

Acoustic tomography of dynamic processes in a sea shelf zone with the use of complex signals

Experimental data are presented on the use of single receiving and transmitting systems in acoustic tomography of dynamic processes in a shallow sea. The experiments are based on the use of the transmission tomography and opposite-direction sounding with complex phase-manipulated signals. The original data are those obtained by the authors in 1990–2000 on the shelf of the Sea of Japan near the Gamov Peninsula, in the vicinity of the acoustical-hydrophysical experimental site of the Pacific Oceanological Institute. A possibility of using combined transmitting-receiving systems (transceivers) for monitoring the temperature and fields of currents in the ocean is demonstrated.

Application of Complex Acoustic Signals in the Long-Range Navigation of Underwater Objects

The creation of modern technical means of research and exploration of the ocean based on submersibles of different designations is a priority field of science and technology development. Creation of intellectual autonomous unmanned submersibles (AUS), which provide multidisciplinary wide-scale study of sea basins and the ocean bottom and new basic knowledge in oceanography, marine geology, biology, and power engineering, is a pressing issue [1]. Significant progress in the development of electric power systems promoted the creation of an AUS with an operating coverage range of hundreds of kilometers. It has become reasonable to equip them with systems of transmitting and receiving low-frequency acoustic signals with a propagation range not smaller than the operating range of the submersibles as means for remote control of the operation and mutual maneuvering during their group operation. Up to the present, the development of such systems has been hampered by a lack of technical solutions for the measurement of the time of signal propagation from a sound source to a receiver over a distance of hundreds of kilometers with the required accuracy. In this work, we present the results of experimental studies of the propagation of low-frequency broadband pulse signals in an underwater sound channel (USC) using state-of-the-art technologies for synchronizing the receiving and transmitting channels. This allowed us to acquire unique data on the stability of acoustic wave propagation velocity in the USC for developing the technology of measurement of the distance between the sound source and the receiver. The main objective of the experiment was to discriminate the part of the signal propagating closest to the USC axis and to measure the time of its propagation from the source to the receiver. In order to do this, the method developed by the authors for sounding the sea medium by complex phase-manipulated signals was used. This technology allows us to distinguish and identify the arrivals of acoustic energy over different beam trajectories [2, 3]. The experiment was carried out in August 2006 in the Sea of Japan. Figure 1 shows the schematic geometry and methodological peculiarities of the experiment. According to the commands of the receiving vessel, complex signals (M-sequences, 511 symbols, and 4 periods of the carrier frequency per symbol) were transmitted with an interval of 5 min at a frequency of 600 Hz by the sound source stationary located near the bottom at a depth of 40 m and a distance of 450 m from the coast. The receiving vessel was represented by a yacht, which was used to deploy the radio hydroacoustic buoy with a hydrophone. The yacht was maneuvering under sail in the zone of reliable recording of radio signals. The hydrophone was lowered approximately to the USC axis, whose location was found from the measurement of the vertical distribution of sound velocity using the hydrological profiler from the yacht (Fig. 2). Signals were received at six points of the path at a distance from the transmitter ranging from 55 km to 368 km (Table 1). The buoy with the hydrophone was drifting in this process, and the coordinates were measured from the yacht using a GPS navigator when the yacht was passing close to the buoy. The signal information and marks of the common time system were recorded on a PC. The systems of common time based on thermally stabilized generators were included into the transmitting and receiving systems and were started before the beginning of the experiment. This allowed us to measure the times of signal propagation between the corresponding points with an accuracy not less than 10 ‐8 s. Processing of the information consisted in the calculation of the cross-correlation function between the received signals and the mask of the transmitted signal preliminary recorded by the PC. From two to four arrivals of acoustic energy, which propagated over different beam trajectories, were recorded in the pulse characteristics obtained using this method. The latest and maximal (in amplitude) arrival was identified as the one that passed near the USC axis, because it propagated near the minimum of sound velocity along the shortest path

Study of how hydrological conditions affect the propagation of pseudorandom signals from the shelf in deep water

The paper examines how hydrological conditions affect manifestation of the acoustic “landslide” effect, which consists in focusing of acoustic energy in the near-bottom layer on the shelf and its transition to the axis of an underwater sound channel in deep water. We compare the results of experiments performed in the Sea of Japan in April 2014 and August 2006 on the same acoustic track, where the distance between corresponding points was more than 100 km. In April, the hydrological conditions in the shelf region of the track and in the upper layer of the deep-water part of the sea were characterized by the presence of a relatively weak (~0.35 s–1) negative vertical sound velocity gradient, whereas in August 2006, it was ~1.5 s–1. Experimental and numerical studies showed that the acoustic landslide effect also manifests itself under conditions of a weak negative sound velocity gradient, but the structure of the acoustic field trapped by the underwater sound channel has a more complex character with a time-expanded pulse characteristic. Nevertheless, its ordered, stable, and well-identified structure at all track points chosen for measurements make it possible to reliably create an efficient (with accuracies to hundredths of a percent) underwater navigation systems like GLONASS and GPS for the spring hydrology season.

Specificities of applying pseudorandom sound signals to measuring impulse responses on the shelf of the Sea of Japan

Solving the problems of underwater acoustic communication and navigation for controlling underwater objects greatly depends on a correct estimation of the hydrological and acoustical environment in the region. Analysis of the domestic and foreign experience in the field of navigational support of self-contained underwater devises shows that, to solve the problem, it is technically and economically advantageous to deploy a set of fixed sources of navigation signals in the region with a range of coverage that is at least not less than the size of the region of interest. At long distances and, especially, in a shallow-water sea, the key factors in solving the problem of navigation are correct determination of the efficient sound speed and the time of signal propagation for each path connecting sources and receivers.

A mobile measuring complex for studies in the field of acoustic navigation of remote submersibles

A measuring complex has been designed for experimental investigations of the effects that hydrophysical processes in a marine environment have on the quality of navigation using on-board equipment of submersibles in the process of signal reception from hydroacoustic beacons installed near the shoreline. The main systems and characteristics of the measuring complex are described.

Processing of acoustic signals and computer modeling instrumental and programming measuring complex for acoustic navigation investigations

This paper describes the main systems and results of testing a mobile instrumental programming complex developed for experiments on investigation of the influence of hydrophysical processes in the marine environment on the quality of solving the navigation problems using stationary acoustic beacons.

Acoustic remote sensing of currents at the shelf of the Sea of Japan

Results of experimental studies of the field of currents in the shelf zone of the Sea of Japan are discussed. The studies were carried out in 2001–2002 near the Gamov Peninsula, in the region of the acoustical-hydrophysical site of the Il’ichev Pacific Oceanological Institute (Far East Division, Russian Academy of Sciences). The purpose of the studies was related to the problems of developing the systems for long-term remote sensing of the climatic variability of the sea medium and for acoustic tomography of the dynamic processes in a shallow sea. In the experiment, combined acoustic transmitting and receiving systems (transceivers) and complex phase-manipulated signals with a carrier frequency of 2500 Hz were used. The velocities of currents calculated from the acoustic data agree satisfactorily with the velocities measured by standard methods of oceanography.

A study of how temperature field variations affect accuracy in measuring the distance to underwater objects

The paper presents the experimental testing results for an acoustic method that increases the accuracy of measuring underwater object positioning systems; the method is based on application of temperature field measurement data in a water area where an underwater object functions. Measurements conducted in September 2011 in Vityaz Bay of Possiet Bay have shown that application of the method permits a two- to threefold reduction in the positioning error related to the change in the sound propagation conditions.

Acoustic-hydrophysical testing of a shallow site in coastal waters of the Korean Strait

The paper describes the results of testing experiments for solving problems of thermometry and positioning of an autonomous underwater vehicle (AUV) in the Korean Strait in a shallow sea with less than 10 m of water. The studies were conducted on acoustic tracks up to 615 m long, sensed with complex phase-shift keyed signals with a centeral frequency of 2500 Hz. Under field experiment conditions, it was shown that the resolution of the structure of pulse responses makes it possible to sense temperature changes less than one degree and to secure positioning of the AUV with an accuracy better than 1 m when operating in the near-bottom layer.