Scientific Objectives
Research community, environmentalists, offshore oil companies and Navies are more and more asking for systems capable of monitoring the environment and/or to control equipments on large areas, either in quasi real time or episodically on the request of an end user.
Such systems comprise sensors located on the sea floor or in the water column whose data has be forwarded to a shore station; bottom settled underwater devices have to be remotely controlled from the shore by the end user. Up to now each of these underwater equipments is therefore linked to shore either by cable or via radio network. In this last case, several surface buoys have to be deployed above the underwater sensors. However it is often not possible to set surface buoys and cable on the sea floor because of associated cost, of environmental conditions (wave, current, …) or of human activities (fishing, trawling, navigation). In other cases such as critical situation (wreckage of oil tanker, dangerous container felts overboard, …) it is even quite impossible to set such cables or surface buoys.
In such conditions, the only way to transfer data from sensors to end user or to remotely control underwater devices from shore is to use an Underwater Acoustic communication link: acoustic modems (transmitter and/or receiver) are deployed at sea, most of them close to the sensors, to create an acoustic network. Specific protocol and data transmission schemes will allow data to be carried by the network from all points of measurement to a shore station, possibly for further radio dispatching.
However, creating a robust and reliable underwater acoustic communication link (so-called acoustic modem) in a shallow water environment is a complex task, and this is in fact an area of active research (in Europe mainly within the framework of MAST III). The environment in coastal areas, especially in or near shipping lanes is acoustically extremely difficult. It is characterised by severe multi-path propagation (the water depth is typically 10-100 m), rapidly changing conditions (turning of the tide, passing ships) and high noise levels (nearby ships). Moreover, the acoustic modems will typically have to be placed near or on the seabed, which will often prevent the existence of a direct sound path between two modems.
The overall objective of the project ACME is to design robust communication and protocol algorithms, which will be implemented and tested in a prototype of a shallow water acoustic communication network that can be deployed in shipping lanes or other coastal areas where data have to be conveyed acoustically. To achieve this objective, ACME will synthesise and exploit the scientific results of the three above MAST III projects. ACME has to add new algorithmic developments and ideas to overcome both the specific acoustic conditions and the problems posed by multi-node networking. Such a network must be autonomous which poses special demands to robustness. For example, the network must be able to cope with temporally poor communication between nodes. Robustness as defined in the ROBLINKS project is also required: the algorithms must be either self-adaptive or insensitive to changing environmental conditions. Operator intervention will be difficult, if not impossible.
The communication and protocol algorithms will be implemented in hardware, in a network of acoustic modems. The network will be based as much as possible on pieces of existing acoustic modems, already developed by partner 5 (ORCA). If needed, limited modifications to the hardware will be made. The fact that hardware modifications will be limited may pose a restriction on (the computational complexity of) the algorithms. If necessary, lower data-rates than desirable for the application will be accepted within the project, but in that case it will be made plausible that with hardware modifications sufficiently high data-rates are feasible.
During a sea experiment at the Bay of Brest (Trial 2), the network will
be thoroughly tested, in configurations needed to monitor current and/or
pollution. Bay of Brest (France), where many efforts are needed to prevent
pollution nitrates coming from the land or the rivers, is a good example
of the usefulness of an acoustic network to survey the environmental conditions
of a Bay. This is indeed a particularly safe and non invasive way to transmit
data of interest to the environmental people.
Another specific objective of ACME is to verify, within the last sea trial (Trial 3) in the Westerschelde shipping lane (access to Antwerpen harbour, Belgium), that the network can convey real time current profile measurements. In this shipping lane accurate knowledge of the current is of prime importance to safety guide the ships into the harbours. At this moment the current is measured using fixed poles located at the edges of the shipping lane. The current in the middle of the shipping lane is calculated using a simulation tool. In practice, however, this method is not accurate enough, and as a result ships regularly run aground. Real time knowledge of the current profiles in the middle of the shipping lane could prevent this from happening. To enable real time monitoring of the current, the acoustic communication network of ADCP sensors will be integrated with the existing measurement infrastructure. This application stresses the real need for the development of an innovative acoustic network.
Many other applications can be considered such as :
· Quick deployment of an acoustic network with sensors in the vicinity
of a critical zone : wreck of an oil tanker, container felt overboard
and containing dangerous material. Acoustic modems and associated sensors
are deployed quickly by ships from the surface. They all form an underwater
network to forward data to the shore or to ship cruising in the area.
· Permanent monitoring of zone around known source of pollution.
The scientific and technical objectives of ACME are:
1) To develop robust communication and protocol algorithms for an underwater
acoustics network,
2) To make a real time software implementation of the methods and algorithms,
3) To make a hardware implementation of a network prototype of acoustic
modems,
4) To test and evaluate the network in configurations needed for applications
of direct social interest (monitoring of pollution, measurement of current
and other water management related parameters ) in a realistic environment
as Bay of Brest,
5) To verify the value of a robust acoustic communication network in practice
by integrating the prototype with the existing measurement infrastructure
in the Westerschelde shipping lane, and to actually monitor the current
during a time span of weeks.
The final experiment in the Westerschelde shipping lane constitutes the
main benchmark problem for the ACME project. Taking into the consideration
the coastal water environment, the application of conveying ADCP data
near a shipping lane and the results of the earlier MAST-projects, quantified
objectives related to this benchmark can be formulated a priori:
- Minimum number of modems of the network: 3,
- Water depth: 10-60 m,
- Typical ranges between modems: 200 m to 2 km,
- Bitrates up to 1 kbit/s.