3D-TDOA-CRFS
One of the applications highlighted for CRFS’ RFEye 3DTDOA is the detection of ADS-B spoofing, helping to enhance air safety.

CRFS takes wide area passive Radio Frequency (RF) detection a step further. The firm’s new RFEye 3DTDOA system offers impressive surveillance capabilities as Jon Bradley, the company’s Vice President of International Sales, explains.

“We are not a replacement for radars, but radar augmentation,” says Dr. Bradley. This augmentation, in the guise of CRFS’ RFEye 3DTDOA adds detection, geolocation and tracking capabilities similar to passive radar: “We developed the product in response to customer demands to be able to determine an aircraft’s altitude, as well as its longitude and latitude.”

Passive radar is in vogue. Put simply, it listens rather than transmits. Aircraft are electromagnetically noisy. They transmit Radio Frequency (RF) energy in the form of radio communications with Air Traffic Control (ATC); RF transmissions from weather, surveillance or fire control radars; Tactical Datalink (TDL) traffic, satellite transmissions, RF energy from radio altimeters and TACAN (Tactical Air Navigation System).

Detecting such emissions enables the geolocation and tracking of an aircraft. By using ground-based receivers positioned a set distance apart such systems can not only detect the emissions, but triangulate the position of the aircraft. This is done by measuring the very small difference in time that it takes the same RF transmissions to reach the antenna nearest the aircraft, compared to the one furthest from it. This is achievable because RF emissions travel at the speed of light; 161,875 knots-per-second (299,792 kilometres-per-second).

The key difference regarding the RFEye 3DTDOA and other passive radars is that it not only provides details of an aircraft’s position and altitude, but can “detect and track hundreds of aircraft,” including targets flying at supersonic speeds over 667 knots (360 kilometres-per-hour).

A recent demonstration of the technology to Armada Analysis showed an RFEye 3DTDOA system based in New York State comfortably tracking the multitude of aircraft in some of the world’s most dense airspace using these planes’ RF emissions, including their Automatic Dependent Surveillance/Broadcast (ADS-B) transmissions now increasingly used for ATC globally. Dr. Bradley adds that several current passive radar systems rely on ADS-B transmission on wavebands of 978 megahertz/MHz to 1.090 Gigahertz/GHz.

The RFEye 3DTDOA covers a wide waveband of ten megahertz up to eight gigahertz or even 18GHz. This encompasses the range of RF systems used by civilian and military aircraft. In the military sphere this includes an aircraft’s Link-16 TDL (960MHz-1.215GHz), transmissions from X-band (8.5GHz to 10.68GHz) fire control radars and voice radio communications using waveforms such as HAVEQUICK (225MHz to 380MHz) and SATURN (Second Generation Anti-Jam Tactical Ultra High Frequency Radios for NATO) for tactical communications between aircraft and with troops on the ground.

“We can detect any aircraft emitting multiple RF signals, and once we have those we have you,” says Dr. Bradley. Civil aircraft are almost always emitting RF and even military aircraft, which will try to preserve radio silence, still need to communicate with their comrades, and use their radars for fire control, surveillance or navigation

The RFEye 3DTDOA is now in service with several undisclosed customers. The system’s architecture includes four of the firm’s RFEye Node receivers; all of which can be positioned at ground level. These are linked to a standard laptop or personal computer. The computer hosts the firm’s signal processing software. This contains the all-important algorithms which enable the detection, location and tracking of aircraft: “We did not need to build a single new piece of hardware,” to realise the product, Dr. Bradley continued. An RFEye 3DTDOA weighs as little as six kilograms (2.7 pounds) making it highly portable and can be procured alongside training for under $500,000. Above all, the system is “simple, reliable and accurate,” Dr. Bradley emphasises.

by Dr. Thomas Withington