Kebkal K., Bannasch R., Kebkal A.

If pulses of echo-sounders, acoustic telemetry systems or non-distractive control systems contain frequency constant carriers, then owing to multiple reflections from channel inhomogeneities every receive signal usually contains numerous, often time-varying multipath arrivals, which interfere so that both phase and amplitude of received signals fluctuate. This makes difficult or even impossible an accurate determination of receive signal parameters and timing. Exploitation of equalizing filters for resolution of multipath components and compensation for them comprises an unstable solution, especially if multipath interference becomes variable and extended in time. Some systems exploit the direct-sequence spread spectrum approach for combating the effect of multipath propagation and the influence of narrow-band noises. However, in order to achieve noticeable advantage, the frequency band (necessary for spreading the signal) must be unacceptably large. Alternatively acoustic pulses can contain a frequency-modulated carrier. Such signals are applied in radiolocation. Via matched filtering the sound-to-noise ratio as well as receiving stability can be substantially improved. Yet this approach has an essential disadvantage: practically used small values of frequency gradients (within the frequency modulated pulses) do not provide any possibility for realization of such beneficial feature as resolution of multipath arrivals. This paper shows that application of large frequency gradients can provide an opportunity of accurate resolution of multipath arrivals. Due to the property of acoustic signals to propagate with relatively low speed, each arrival comes to receiver with its individual (rather large) time delay and has its own instant frequency, which can significantly differ from instant frequencies of other multipaths. This provides an opportunity for accurate parameters determination of individual arrivals and, therefore, for accurate determination of characteristics of multipath-distorted received pulses. Obviously this can be used in digital acoustic telemetry for combating against multipath decay, as well as e.g. in echo-sounding for detailed evaluation of propagation medium properties.