"instrumenting the Great Barrier Reef"
Network Testing and Sensor Net DeploymentWireless communications were tested at Davies Reef in December 2004 by S. Kinninmonth and I. Atkinson. The signal to noise ratio was >20dB most of the time and up to 25 dB. 34 Mbps transmissions were achieved with CDMA.
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.
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. Their heights are variable in space and time but in tropical waters they range from 5 to 25m depending on humidity, wind speed and sea conditions. 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.
Wave propagation in humidity duct.
Woods successfully demonstrated for the first time the use of evaporation ducting in communications between Davies Reef and AIMS over a distance of 80km. Propagation with a 10.5GHz carrier yielded 20megabits/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.
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.
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
Future Benefits
This 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 such a communication link and hybrid power could be used for video surveillance.
