Autonomous Underwater Vehicles and Underwater Communication
Autonomous Underwater Vehicles (AUVs) reduce the need for large crews, divers and vessels in the operational area. AUVs are already operational for bathymetric and environmental mapping, pipeline tracking and mine hunting, and there is a trend towards their use for inspection and environmental monitoring. When using non-tethered solutions, underwater communication becomes crucial for data transfer and positioning. In this article you can find an overview of the latest technical developments.
Underwater Communication for Autonomous Underwater Vehicles
The best technology to set up long-range underwater communication links is acoustic communication, of which the performance is highly dependent on the environmental conditions. In the North Sea, for example, the combination of shallow water and strong winds complicates performance prediction for an underwater acoustic network.
Underwater acoustic communication using acoustic modems consists of transforming a digital message into sound that can be transmitted in water.
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Propagation Conditions
The following physical mechanisms can deform the signal and challenge the reception and interpretation of the contained message:
- Frequency-dependent attenuation: For frequencies relevant to underwater communications (1-100kHz), attenuation by water strongly depends on frequency.
- Geometrical spreading and multipath propagation: As acoustic energy spreads over larger areas the level diminishes with range. Furthermore, reflection from the bottom and sea-surface boundaries will cause distortion of the signal.
- Ocean surface variability: The movements of the surface due to wind and currents strongly affect the surface communication paths, causing Doppler spreading of the signal in frequency.
- Variable speed of sound: Sound bends towards regions where the sound speed is lower. In deep waters, this is the main factor affecting communication between two platforms due to the creation of ‘shadow zones’ where no acoustic communication is possible.
Acoustic Modem Properties for Autonomous Underwater Vehicles
An underwater modem translates digital messages into waveforms that can be transmitted acoustically. Digital modulation is the technique that allows a digital signal to be transferred over an analog channel and consists of mapping the information bits into analog waveforms that represent the data that we want to transmit. The main characteristics of an underwater modem are its communication bandwidth, its carrier frequency and the employed modulation method. The useful bandwidth is strongly dependent on the environment and the communication range.
Noise in Underwater Communication
AUV Noise
Noise from an AUV can interfere with the onboard modem and with the reception of acoustic messages from a receiving hydrophone. Noise sources at the AUV include hull vibrations and mechanical noise, propeller noise, electronic noise, flow-induced noise and payload cross talk.
Ambient Noise
Ambient noise is most prevalent in the low frequency band. However, anthropogenic noise originating from nearby sources can have a disruptive effect for communications
Conclusions
Many factors can affect underwater communication to and from AUVs. By having good knowledge of these factors in situ, it is possible to plan AUV operations more efficiently by adapting the bandwidth, communication protocol, network topology, and level of autonomy of the vehicle used.
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