D. S. Strobykin

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

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.

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.

Experimental studies of seasonal variation for temperature fields on the Sea of Japan shelf by acoustical methods

The results of experimental studies are given for the pulsed characteristic of an underwater channel on the Sea of Japan shelf depending on the seasonal variation of a temperature field.

A hardware and software system for measuring the angular structure of the acoustic fields in acoustic tomography

A hardware and software system for measuring the angular structure of acoustic fields using the vector-phase methods for processing the complex M-sequence probing signals is described. The results of testing the system are given. A unique relationship between the change in the temperature condition on the stationary acoustic route and the times and angles of arrival of the acoustic energy is determined under controlled hydrological conditions. It is shown that, using vector receivers in tomographic schemes, it is possible to obtain an independent additional parameter of the pulse response of the diagnosed waveguide and increase thereby the efficiency of reconstruction of hydrophysical fields from acoustic probing data.

An Electromagnetic Towed Hydroacoustic Source with a System for Hydrostatic Pressure Control and Compensation

A hydroacoustic towed emitting system has been developed based on an electromagnetic source with a system of hydrostatic pressure control and compensation. The frequency range of emitted signals is 140–270 Hz, the sound pressure is as high as 3700 Pa, and the depth of immersion is 1–25 m. The system has been designed for carrying out targeted routine investigations in different seasons of the year in shelf water area using a small vessel. The system has been tested successfully.

Peculiarities of the Formation of the Interference Structure of Scalar-Vector Acoustic Fields on the Shelf of the Sea of Japan

The results of experimental study of the spatial structure of the scalar-vector acoustic field formed during towing of a tone low-frequency emitter over the shelf of the Sea of Japan are discussed. The experiment was accomplished by towing the source of a tone signal with a frequency of 134 Hz at a depth of 20 m over various acoustic tracks at distances up to 10 km from an integrated receiving system consisting of a receiver of acoustic pressure and three orthogonal components of the acoustic pressure gradient. Special attention has been focused on study of the interference structure of the scalar and vector fields with provision of the technical reliability of the method and the results of the experiment under controlled hydrological conditions. We discuss the quantitative characteristics and peculiarities of the interference formation along tracks that differ in depth. The unique results of comparing the horizontal and vertical components of the fields are most interesting of all. They allowed us to reveal the existence of eddy structures in the acoustic field of the source over several tracks. We analyze the possibility of practical application of the results of our research.

Acoustic-hydrophysical experiments in the coastal zone of the Korean Peninsula

This paper presents the results of a pilot experiment on determination of the possibility for effective application of the navigational systems of autonomous robots and remote tools of water structure and dynamics monitoring in shallow water areas. The experiment was based on investigation of the regularities of formation and interaction between hydroacoustic and hydrophysical fields when sounded by complex phase-shift keyed signals with the central frequency of 2500 Hz. The possibility of monitoring of temperature and current fields by acoustic tomography methods in the water area of several square kilometers in size and no more than 10 m in depth, at depth changes up to several meters due to tidal effects, was studied. Additionally, the possibility and precision of solution of navigational problems for underwater objects at such shallow depths was studied. For this purpose a hardware-software complex, equipped with two sources of navigational signals and an imitator of the submersible receiver path, was used.

Experimental studies of the characteristic features of vector receivers in application to ocean tomography

Experimental data on the angular structure of the sound field on the shelf of the Sea of Japan are discussed. The data are obtained by using the vector-phase processing of complex signals in the form of M-sequences with a central frequency of 2500 Hz. With the parameters of the water bulk being monitored, an unambiguous relation is established between the variability of the temperature regime on a fixed acoustic path and the arrival times and angles. It is shown that the use of vector receivers in ocean tomography provides an additional independent parameter of the pulse response of the underwater waveguide and leads to an increase in the efficiency of solving the problem of reconstructing the hydrophysical fields from the data of acoustic sensing.