Peralex has installed two passive radar stations in the Northern Cape to monitor air traffic for the Square Kilometre Array (SKA) radio telescope in what is one of the first commercial applications anywhere in the world for the technology.
Francois Louw, Product Development Manager at Peralex, said that his company was contracted to supply a commercial passive radar system as the South African Radio Astronomy Observatory (SARAO) has a requirement to monitor their airspace. Requiring minimal electromagnetic interference is a co-requisite for this, making passive radar an ideal solution. (Peralex has supplied other components to the SARAO, including computing hardware, storage pods and radiation shielding.)
The SKA project is being co-hosted by Australia and South Africa, with South Africa building an initial 197 dishes, of which 64 are currently operational as the MeerKAT radio telescope near Carnarvon in the Northern Cape. The site was chosen for its relative lack of radio frequency interference. However, the sensitivity of a radio telescope is such that it is still able to detect some interference, in particular radio interference from transponders, communication equipment and weather radars that may be carried by aircraft flyingin close proximity to telescopes at the SKA site.The risk of damage to receivers (but not antennas) should an aircraft fly too close to an antenna is one that needs to be managed.
The SARAO therefore needed a way of monitoring air traffic without radar signals affecting its radio receivers. It chose to use passive radar, since passive radar does not emit any signals and only relies on transmitters of opportunity. Several techniques are used to detect objects, including phase interferometry to determine the angle of arrival of a return and doppler for better estimation of location and velocity. By using several transmitters of opportunity, accuracy is greatly improved.
In the case of the passive radar at the SKA site, Peralex used several FM radio broadcast transmitters in the area. As the FM signals are outside the operational frequencies for the radio telescope antennas, their signals do not pose a problem. Peralex subsequently established two receiver nodes northeast and west of the SKA core, which pick up the FM transmitters in Prieska, Calvinia, Carnarvon and Upington. The Visserskloof node was commissioned and accepted in October 2018 and the De Hoek node was commissioned and accepted in February 2019. Both nodes are ‘off the grid’, being powered by solar panels. They use low bandwidth VSAT links for network connectivity. All electronics are shielded to prevent the leakage of electromagnetic interference. Peralex is in the process of installing two additional nodes at the SKA site.
Before full deployment, Peralex ran several trials with aircraft including Albatros, King Air, Bosbok, Bushbaby, Cessna 172, Cessna 206, RV7, Ravin, Whisper and Jabiru light aircraft as well as microlights and gyrocopters. Chartered PC-12 aircraft can be detected at 160 km from the core site, although when the aircraft flies at low altitude (50 metres above ground), detection range is about 20 km.
When aircraft landed at the SKA site as part of scheduling operations, the telescopes were typically shut down for two hours to accommodate incoming and outgoing flights. With Peralex’sComRad passive radar, the operators are able to track flights and reduce antenna downtime to 30 minutes, thus saving around 15 hours a month of downtime.
The passive radar system can primarily be utilised for the detection of scheduled traffic, although is effective in detecting unscheduled and general aviation traffic. Utilisation of this system is intended to form part of an integrated solution to mitigate the potential radio interference risks associated with aircraft in the area, whilst still enabling the continuation and co-existence of aviation activities.
Louw said the main benefits of passive radar are its cost effectiveness, spectral efficiency (it does not require additional bandwidth use), low power requirements (since it only receives), low maintenance (no moving parts), counter-stealth capability and covert operation. Some challenges exist, however. For instance, when radio stations broadcast extended periods of silence, these are unsuitable for the radar to use, although the radar uses multiple frequency channels and multiple transmitters to help mitigate this.
Louw concluded that the ComRad passive radar system has provided the South African Radio Astronomy Observatory with an enhanced air surveillance capability that is successfully tracking air traffic, measuring radio frequency interference and reducing telescope antenna downtime due to scheduled fly-ins.
Peralex has been collaborating for a number of years with various institutions in South Africa to develop the emerging technology that is passive radar, including the University of Cape Town and the Council for Scientific and Industrial Research (CSIR). Other entities involved in passive radar development include Lochtron and the University of Pretoria.
Multiple passive radar nodes have been rolled out across South Africa, with five main sites in the North-Eastern part of South Africa being used to test and refine the technology. One site is on the roof of the CSIR’s buildings in Pretoria, another is at the University of the North West (Potchefstroom campus), a third at Emalahleni (Tshwane University of Technology campus), one at Hartebeeshoek and another at OR Tambo International Airport.
The CSIR has been collating data from all these sites to provide a comprehensive picture of South Africa’s airspace around OR Tambo International Airport, with data being fed from the sites to a central node located at the CSIR. It has also been adding target reference data from the Air Traffic Navigation Services (ATNS) company to allow for a direct comparison between the performance of primary and secondary radars at OR Tambo and the passive radar. This enables derivation of the relative detection probability and accuracy of the passive radar and also allows for the identification of false alarms. In the area close to OR Tambo, passive radar detection and tracking results shows it within 100 m of the reference tracks obtained from ATNS.
The node at the CSIR has been running for over two years and the node at the South African National Space Agency (SANSA) at Hartebeeshoek some two years. Passive radar data from the OR Tambo site has been logged for over a year.