The effective operation of the SeaSonde High-Frequency (HF) radars managed by the IMOS Ocean Radar Facility requires a frequent, accurate and repeated calibration of the receiver antennas and their pattern. The antenna pattern can change over time due, for instance, to hardware problems or changes in the local environment surrounding the receive element. Far from being a problem, changes in the environment can be accounted for with proper calibration, thus frequently repeated calibrations are essential for ensuring that high-quality surface current measurements are collected.
Over the past years, the Ocean Radar Facility has adopted the standard calibration procedure recommended by the system manufacturer: a boat carrying a signal source, or transponder, tracks a semicircular path at a distance ranging between 1 to 2 km offshore at a constant speed not exceeding 2 m/s, under the ideal conditions of flat sea and no or light winds. While these are common conditions in closed seas, rough sea states, severe winds, submerged reefs and dangerous currents, as well as the remote launch locations for the boats, severely limit the calibration opportunities in Australia and represent dangerous conditions to boat operators and the Ocean Radar Facility staff.
Other calibration procedures have been developed recently to overcome these difficulties, including semi-automatic calibration approaches that exploit the radar echoes from ships of opportunity of appropriate size (such as tankers or container ships). This approach has proven to be very effective in the San Francisco Bay area, and a cost-benefit analysis for the IMOS Ocean Radars suggested that the budget usually allocated to the conventional boat-based calibration would have easily covered the capital costs required to upgrade the SeaSonde radar systems to this technology. However, shipping lanes offshore from the Western Australia systems are prevalently located on the outer edges of the radar operating ranges; consequently, an unambiguous identification of the ship echoes is potentially subject to significant biases due to the poor signal-to-noise ratio constraints.
The IMOS Ocean Radar Facility is now making use of a cutting-edge technology developed by Eduardo Romero, an HF radar technician at University of California at Santa Barbara, Marine Science Institute, in which a commercial-type quadcopter (“drone”) available at any hobby shop carries a compact signal source, in replacement of the more conventional type transponder, at a 300 m distance from the receive antenna. The calibration procedure is similar to the boat calibration, but faster, more effective and significantly safer for the operators: a mission is programmed and waypoints are uploaded to the drone’s internal GPS; the drone takes off, heads to the arc’s starting point and follows very accurately the loaded track; once the calibration is finished, it returns to its launching platform. Even in its autonomy, the drone is continuously under the operator’s control, and the mission can be interrupted at any time if problems arise, telemetry is lost or the battery level drops below 50%. In this case, the drone returns following the shortest path to its launching position.
Processing of calibration files and GPS tracks is then almost instantaneous, and a full calibration file can be generated in less than 30 minutes with minimum system downtime.
From a cost-benefit perspective, this methodology has reduced calibration costs well above any expectation. During a recent maintenance visit at the Bonney Coast systems, for instance, a total of 6 missions were performed with a 100% success rate at less than 1 / 10th of the budget normally allocated for the calibration purposes, again minimizing system downtime.
Calibrations at this site had been a priority for the Ocean Radar facility for a long time: several attempts had been made over the past three years, with most of them frustrated by severe weather conditions, dangerous sea states or failing transponders. The most recent calibrations performed at this node will now allow the reprocessing and the quality-control of historical data, as well as the output of calibrated and quality-controlled observations in near real-time.
Calibration is also very important for the WERA phased-array radar systems, and as such the Ocean Radar team is now investigating the possibility of extending and adapting the drone calibration to the entire radar network across Australia, with the added benefit of cost-effectiveness and greater safety for the technical staff.
Written by Simone Cosoli, Ocean Radar Facility Leader - Jim Pettigrew, Ocean Radar Technical Officer