Nautronix has developed considerable expertise in acoustic signalling for its positioning products. In particular we have brought the application of Digital Spread Spectrum Signalling techniques to the acoustic environment. However, the technology can be equally well applied to data transmission with all the associated benefits.

The integrity of the transmission and associated data is extremely high and significantly better than is achievable with traditional acoustic signalling. With long range, high resistance to multipath and high immunity to interference, ADS² signalling can provide a fully reliable acoustic data link.

ACOUSTIC DIGITAL SPREAD SPECTRUM (ADS²) COMMUNICATIONS

Conventional narrow band acoustic signals have a frequency that does not change appreciably with time. There are several problems with this approach. First, narrow band signals are subject to catastrophic interference. This could occur when another signal of the same frequency is transmitted in the near vicinity of the desired signal. This could also occur due to poor immunity to propagation multipath or reflections of the original signal. Additionally, a constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information assurance is important.

To minimize problems with conventional narrow band acoustic communications, NASNet® acoustic transmissions between transponders and surface links incorporate mature ADS² corporate technology. This technology deliberately spreads the message over a comparatively large segment of the spectrum, typically more than 10 times greater than would be required for conventional narrow band communications.

This spreading is done according to an optimized, but complex, mathematical function in real time. In order to decode the signal, the receiver must know the exact spreading code employed.

Previous use of spread spectrum in other products has been met with limited success due to the severe effect of Doppler. The Doppler effect produces a positive shift in frequency when transmitter and receiver are moving toward each other, and a negative shift in signal frequency when moving apart. This Doppler effect is 200,000 times greater with acoustics in water than with RF in air. By solving this vexing problem Nautronix MariPro Inc (previously part of the Nautronix group) overcame one of the most challenging and important aspect of the ADS² technology implementation, now provided exclusively by Nautronix.

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