Background
The Australian Institute of Marine Science (AIMS) operates and maintains a series of remote data stations situated on the Great Barrier Reef (GBR) in Queensland and in Western Australia. These stretch across 3,000km of World heritage marine park. AIMS recently upgraded its new Automatic Weather Station at Davies Reef, a coral reef approximately 70 km offshore from Townsville in North Queensland. Coral reefs all over the world are deteriorating, and in addition to remote sensing and monitoring, there is a need for installing in-situ monitoring, to provide long-term datasets and real-time monitoring. AIMS and James Cook University (JCU) are deploying a trial sensor-network at Davis Reef. The sensors will measure water temperature, solar radiation, salinity, pollution levels, etc in order to understand complex marine biological processes and predict future coral bleaching events.
The Challenge
The major challenges for a sensor network in a remote area are power and communications: how do you get reliable power and communications so far out to sea, without the need for expensive maintenance trips?
 |
|
Figure 1: Davies Reef: the old met tower (left) will be decommissioned later this year. The newly designed tower (right) is primarily for collecting meteorological data. It also is the first point of presence (POP) on the internet on the GBR and will host the “skipping microwave” link 70km back to shore. The SensorNet will be deployed around this tower. |
The Technology
With seed funding jointly provided by QCIF and AIMS, Dr Graham Woods from the department
of Electrical Engineering at JCU has developed a microwave repeater station with
a hybrid power system based on battery, solar and wind power for testing radio communications
over the horizon.
Hybrid Power
Total energy requirements to support a 60W microwave link and some instruments are estimated at 100W. The system should be at least triply redundant, given failures due to wind, birds and the salt environment. Solar powers are an attractive solution, however birds foul the panels decreasing the efficiency of the photovoltaic cells. Dr Woods is designing an innovative hybrid power system which will use both wind and solar power in order to provide enough capacity when there is no sun or when the panel is fouled by birds. The solar panel is already in place on Davies Reef and JCU and AIMS have set the wind turbine specifications for a marine environment. The challenge is in integrating the two systems together. The advantages of such a system are reliability and low maintenance.
 |
|
Figure 2: Stuart Kinninmonth deploying sensors at AIMS. |
Radio Communications
Streaming data from the reef back to the mainland is a challenge: high communication towers cannot be deployed at sea. A boat could be used as a relay back to AIMS but this entails expensive deployments to sea. An interesting possibility is to use evaporation ducting for radio transmission. Evaporation ducts are formed above the tropical ocean surface by strong vertical humidity gradients (Gossard 1981). Their heights are variable in space and time but in tropical waters they range from 5 to 25 m depending on humidity, wind speed and sea conditions (Kerans et al. 2002). Microwave signals trapped in the humidity duct are bent and spread horizontally allowing over-the-horizon propagation, just like in the ionosphere. Some loss happens when the waves are not reflected back into the duct and the link may drop out with diurnal variations in humidity.
 |
|
Figure 3: Data streaming via radio transmission in humidity ducts. |
Woods successfully demonstrated for the first time the use of evaporation ducting
in communications between Davies Reef and AIMS over a distance of 80km (Palazzi
et al. 2005). Propagation with a 10.5 GHz carrier yielded 20 megabits/sec communications
bandwidth - enough for the instrumentation that will be deployed. Numerical models
suggest that communications could even reach out to 200km, but with reduced reliability.
Benefits
Sensor networks reduce the need for expensive field deployments using marine research vessels. The hybrid system could be used as a way of reducing power use by reducing the load on diesel fuel. It is also an alternate source of power to remote areas with no access to power. Towers equipped with a communication link and the hybrid power could be used for video surveillance.
Where is the technology headed?
Further field testing will determine the optimum frequency range and antenna height needed for high speed microwave communications in the over-ocean environment. For this, more information is needed on how weather and sea conditions influence the height and stability of tropical evaporation ducts and its effect on communications.
Participants
Dr Graham Woods
School of Engineering, James Cook University
Industry Participant
Stuart Kinninmonth
Australian Institute of Marine Sciences (AIMS)
References
Gossard, E.E.. 1981. 'Clear weather meteorological effects on propagations at frequencies above 1 GHz', Radio Science, 16(5): 589-608.
Kerans, A., Kulessa, A.S., Lensson, E., French, G., Woods, G.S. 2002. 'Implications of the evaporation duct for microwave path design over tropical oceans in Northern Australia'. In, Proceedings of Workshop on the Applications of Radio Science (WARS02) Leura, Australia, 20 - 22 February 2002.
Palazzi, C.M., Woods, G. S., Atkinson, I., Kininmonth. 2005. 'High Speed Over Ocean Radio Link to Great Barrier Reef'. In, Proceedings of IEEE TENCON’05, Melbourne, Australia, 21-24 November 2005.
Reports
Progress Report - October
2006 (1.32 MB PDF)
Project Proposal (85
KB PDF)
Update: JCU recently participated in Indiana University's entry
in the SC06 Bandwidth Challenge, sending data from a diffractometer at
JCU. While the entry did not win, the entry received an Honorable Mention, for being the Spirit of the Competition.