FEATURES OF COMMUNICATION EQUIPMENT SYNCHRONIZATION IN HYDROACOUSTIC TELEMETRY SYSTEMS
Introduction. The distinctive feature of a hydroacoustic communication line is the low speed of acoustic signal propagation in the aqueous medium (on the average 1500 m/sec). For this reason the time of signal transmission along a communication line is several seconds or tens of seconds, depending on the communication range, that causes considerable time-and-frequency instability of received signal parameters because of the rough sea and the inevitable drift and ship (buoy) roll. In communication equipment for hydroacoustic telemetry systems, the synchronous method of digital information transmission using discrete frequency or differential phase modulation is applied. Noise, connected with the Doppler effect, is the specific kind of interference in a hydroacoustic communication channel (HACC). The real situation in HACC is very difficult. So when transmitting signals, the general spectral shift along the frequency axis and spectrum rescaling occur because of the Doppler effect. Consequently, when constructing communication systems, one should take into consideration not only the change of transmitted message characteristics, but also possible interference immunity deterioration with range attenuation and with lower communication reliability. The purpose of the work is the identification of synchronization features in hydroacoustic communication systems. Results. During this work some communication equipment synchronization features for hydroacoustic telemetry systems were identified. Particularly, the special attention was paid to the Doppler effect problem, which occurs in such systems. In this work the analysis of literary sources on the specified problem was carried out. It is shown, that there are practically no Doppler effect compensation methods for multibeam communication systems and real HACC of oblique and horizontal orientation are multibeam. It is shown, that the application of systems with multiple phase or frequency modulation in the systems of hydroacoustic telemetry with variable signal propagation time is unreasonable because the phase store of such systems will be relatively low. The application of the systems of phase-locked loop frequency control is preferable for communication channels with long-term fading.