By Mark Radford, chief technology officer, Blighter Surveillance Systems
Recent conflicts in the Middle East and Ukraine have highlighted the challenge of radar detection of low-altitude and one-way attack drones, which fly ‘below the radar’ at altitudes under 100m, taking advantage of ground clutter reflections and radio propagation fading effects. These real-world issues conceal small air targets beneath the larger clutter returns and weaken the radio signal.
While low-frequency long-range air surveillance and counter-UAS radars are essential in a layered drone detection system and offer good capability for high-flying drones, their ability to detect low-flying and near-surface targets is often compromised. Why? Because they simply cannot avoid the dramatic fading effects of multipath when scanning the horizon and cannot consistently remove significant ground clutter.
This article examines how advanced Ku-band sensing, electronic scanning array antennas, micro-Doppler waveforms, and sophisticated clutter suppression technologies can minimize radio multipath fading and remove significant ground clutter, providing a COTS solution for detecting these low-flying and one-way attack drones.
Radar remains a detection system immune to weather, light, and sound conditions, and with careful design, can avoid detection by adversarial electronic sensing as well. With clever engineering and some degree of ingenuity, near-surface targets such as first-person-view (FPV), fiber-optic, and Shahed-type winged drones – even those flown at 50m or less while tracking the terrain – can be successfully detected, classified, and tracked by radar.
Ground Clutter Removal with ESA & Micro-Doppler Signal Processing
Ground clutter is the massive radar reflections from everything on the ground that is not moving. This can include mountains, hillsides, buildings, trees, vegetation, fences, signage, power lines, and other infrastructure. Anything that stands above the ground can reflect radar energy – often surprisingly efficiently – and it does not have to be metallic.
To detect a drone 5km away with a radar cross-sectional area (RCS) of 1 m² in an area with buildings or a hillside in view would typically require illuminating a total volume of over 1 million cubic meters. The radar must be able to discriminate between a 1 m² moving target and a radar reflection potentially 115,000 times larger.
Clearly, ground clutter is a major challenge for radars, which is why classic air surveillance radars and many C-UAS systems point upwards to illuminate air targets and avoid this problem.
Modern non-rotating electronic systems and digital signal processing (DSP) platforms allow the ubiquitous Fast Fourier Transform (FFT) to provide Doppler discrimination, the ability to distinguish different types of motion or targets by their frequency signatures.
A large ‘micro-Doppler’ FFT can segment the zero-Doppler (no motion) radar return from ground clutter within a Doppler bin equivalent to a tenth of 1km/hr. or less.
Electronic-Scanning Array Antennas Avoid Clutter Spreading
While modern DSP platforms allow this level of Doppler fidelity, mechanical rotation of classic single-antenna radars adds both incoming and outgoing Doppler components to the otherwise very narrow zero-Doppler ground-clutter measurement. This clutter-spreading can easily obscure a 1 m² target, such as a drone.
Electronic-scanning array (ESA) antennas provide a multitude of individual beam positions to deliver the required volumetric coverage, but each beam is static. No Doppler is spreading, so the radar can achieve remarkable levels of ground-clutter rejection, allowing the 1m² target in 115,000m² of clutter to be easily discriminated.
There are several ESA architectures, but they all produce symmetrical transmit and receive beams, with energy focused tightly on the area of interest. Other staring array technologies, such as holographic radars, also lack mechanical rotation but asymmetrically illuminate large areas of ground clutter, making clutter rejection considerably harder for near-surface detection.
The challenge of rejecting ground clutter while detecting low-flying drones requires an elegant, highly complex solution that leverages ESA antenna technology and Micro-Doppler signal processing. It also requires engineering excellence to ensure the fidelity of tiny target signals throughout the entire radar system, especially in the DSP.
Using Ku-band Radar to Reduce Radio Multipath Fading (RMF)
Those with a memory of driving down a long highway listening to FM radio slowly fading in and out, and eventually fading to nothing, have experienced Radio Multipath Fading (RMF). It affects almost all ground-based communications systems, and height is the best mitigating factor.
Radar is another communication system, with a twist. It suffers RMF from the radar to the target, and then the reflected signal is reflected to the same radar receiver. This results in a double-fading effect that acts like a brick wall and can severely limit the maximum detection range unless carefully managed.
While RMF is mostly recognized for its fading effects, it has one upside: the same direct-indirect phasing effect can enhance the signal under certain conditions. With careful design of radar deployment height, it can achieve some beneficial improvements in target detection range by allowing the radar signal to reach its normal signal-to-noise limit before the catastrophic effects of RMF overwhelm.
While RMF affects all ground-based communications systems, the operating frequency can have a significant effect on the immediacy of the fading problem.
Ku-band radars, operating typically in the 15 to 17 GHz dedicated radar bands, have the shortest wavelengths among typical longer-range government/military-grade radar systems. This short wavelength of just 2cm is easiest to manage, allowing RMF to be mitigated simply by elevating the radar a few meters above the ground. RMF will still occur; however, the fade periods are shorter, allowing target trackers to ‘join the missing plots,’ and the distance at which near-ground fading becomes terminal can be pushed beyond the signal-to-noise limitations of the same radar.
Blighter Surveillance Systems has implemented these techniques in its COTS ground-based radars, including its A400 and B400 2D radars and its A800 4D multi-mode radars, to protect against low-flying and one-way attack drones. These systems are deployed as part of a layered drone detection system by the US Air Force at several international airports and by multinational forward operating bases in the Middle East.
Mark Radford has worked in the radar industry since 1985, initially as a designer of high-performance signal processing solutions for naval radar systems and later as a system designer and development manager. Since joining Blighter in 2000, Mark has been responsible for the specification, design, and development of Blighter electronic-scanning array radars. It was the discovery of North Korean winged drones along the Korean demilitarised zone (DMZ) in 2014 that kick-started Blighter’s development of its Ku-Band ground radar system and later a complementary C-UAS system called AUDS.
