During the last two years, work has been under way on the use of satellite communications for the transmission of data from instrumentation placed in the ocean bed. The set-up of the transmission link has required the development of several new systems, including an underwater vehicle capable of successfully emplacing instrument packages within deep ocean sediment formations, a transmitter-receiver system able to send acoustic signals through both 50 metres of sediments and a 6 km water column, sensors which will give information on sediment characteristics, and a satellite communications system for the automatic quasi-real-time relay of data from the emplaced instrumentation to the laboratory in Ispra (Europe).

BACKGROUND

With the increasing need to investigate and understand processes controlling the distribution and dispersion of materials in coastal and deep water zones, the development of instrumentation that is capable of working autonomously has occurred over the decade. With the advent of more powerful (and cheaper) electronics using little energy, the range of applications has increased as well as the length of time which is required before recovery of the instrumentation. This last point is particularly important because the need to study long-term mechanisms requires the collection of data over comparable periods of time The advent of satellite transmission systems such as ARGOS (Argos, 1978; Andunson and Fossum, 1980) and METEOSAT (Houet, 1986) have opened up the possibility of obtaining data from remote areas, in real time. Not only has this allowed the development of networks covering large areas it permitted the control of the correct functioning of the system and the replacement of components as required, thus lessening the loss of valuable data.

Murray et al (1987) described work carried out to demonstrate the feasibility of setting up an acoustic link for data transmission between instrumentation emplaced within deep oceanic sedimentary formations and a mobile shipborne surface receiver-transmitter which can then relay data to a land-based European ground station via the European Space Agency's METEOSAT geostationary satellite

On the basis of these studies, the Commission of the European Communities, Joint Research Centre, Ispra Establishment (Italy), decided to develop an experimental operating system as part of its research on long-term environmental processes. The aims of the project are to demonstrate that the investigation of slow environmental processes, either within the deep water column or in sedimentary formations, can be studied using specialized instrument systems with the possibility of controlling the investigation and receiving in-situ data in real time The communications link uses a mix of satellite and high-frequency radio techniques to communicate with an autonomous surface platform which incorporates a two-way underwater acoustic link for the control of deployed autonomous instrumentation A laboratory control and data collection centre have been set up at the Joint Research Centre in order to monitor the relay system and the deployed sub-bottom instrumentation Each of these components is described below

Fig. 1 Instrumented transmission platform (available in full paper)

The METEOSAT Network was chosen as it allows the transmitted data to be received directly at the Institute, so that real-time modelling can be carried out.

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