What is Synthetic Aperture Radar (SAR)?
SAR is a type of radar that is used to create a 2D or 3D reconstructions of objects, such as terrain and infrastructure. The technology uses motion of the radar antenna over a target region to provide finer spatial resolution than conventional radars. SAR when used in Unmanned Aerial Vehicle (UAV) applications, is typically mounted on the underside of the UAV with 180-360 degrees field of view. The origin of SAR is based in the creation of an advanced form of Side Looking Airborne Radar (SLAR). The distance the SAR payload moves over a target in the time taken for the radar pulses to return to the antenna creates a large synthetic antenna aperture (the size of the antenna). Typically, the larger the aperture, the higher the image resolution will be, regardless of whether the aperture is physical or synthetic. This enables the SAR payload to create high-resolution images with relatively small physical antennas. Additionally, SAR payloads have comparatively larger apertures for more distant objects, allowing regular spatial resolution over a range of viewing distances.
The process of SAR image creation is successive pulses of radio waves transmitted to 'illuminate' a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of a meter down to several millimetres. As the SAR payload on board the UAV moves, the antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions. This process forms the synthetic antenna aperture and allows the creation of higher-resolution images that would otherwise, only be possible with a physical antenna. SAR when compared to traditional Forward Looking Infrared (FLIR) payloads that detect infrared radiation emitted from heat sources, offers a multitude of benefits, ranging from ability to see through obscurants to wide area surveillance.
Ground Moving Target Indicator (GMTI)
GMTI is a mode of radar operation that determines moving objects by eliminating clutter (unwanted echoes in the radar system). GMTI enabled SAR provides wide area surveillance and can detect moving vehicles, taxiing aircraft, hovering helicopters and even personnel. Some of these radars can also exploit fine Doppler modulations in the radar return to identify the vehicle class or type, and even rotating radar antennas. A radar which combines SAR with GMTI can accurately detect, recognise and identify most ground based targets when combined with qualified Image Analysts (IA) with appropriate image exploitation software. This is achieved with a comparatively large standoff range and a shallow slant angle when compared with typical Electro-optical (EO) and/or Infrared (IR) payload configurations. Due to the radar's nature of operation, this can also be achieved under all weather conditions, limited only by the weatherisation of the UAV being employed.
Current application and use
SAR with GMTI is currently utilised in Military organisations across the world for both ground and air based applications. SAR payloads have historically been utilised at the Strategic/Operational levels with Medium Altitude Long Endurance (MALE) and High Altitude Long Endurance (HALE) UAVs. Some recognisable examples of UAVs with SAR include the Grey Eagle MQ-1C in use by the United States Army, Watchkeeper operated by the British Army and the MQ-4C Triton which will be introduced into service in the Royal Australian Air Force in the near future. Its primary role is wide area surveillance, covering a much larger area than traditional UAV payload configurations. SAR in conjunction with MTI, also enables a cueing function, and when targets are detected, can cue either the inbuilt EO/IR payload or another ISR asset within the battlespace to further interrogate and/or commence the targeting cycle. This capability is enabled by qualified IAs, utilising advanced software for image processing, exploitation and dissemination. IAs then use the exploited product to disseminate the imagery, which is then used to provide the commander with enhanced situational awareness across a large area of the relevant theatre of operation. The capability is also utilised for topographical analysis, mapping, and change detection over a set time period focusing on infrastructure and topography.
Tactical Unmanned Aircraft Application
At this stage, there are currently no tactical level UAVs within the Australian Defence Force that are SAR enabled. SAR is a proven technology and the benefits of its application at the tactical level could potentially outweigh the induced liability of IA support on a larger scale than is currently utilised. Currently 20th Regiment, Royal Australian Artillery, operates the Tactical Unmanned Aircraft System (TUAS) in Army. The platform in use is the RQ-7B Shadow and the capability is currently under review in LAND 129 Phase 3, due for Government consideration in 2021. If the replacing system is considered with SAR Capability, this will substantially increase the Surveillance and Target Acquisition capability within the Australian Army at the tactical level. With the addition of SAR in Army TUAS, some of the current detractions of the system including inability; to provide wide area surveillance, reduced capability to see through obscurants such as smoke, haze, fog and cloud, and minimal change detection ability, would be fundamentally improved. With the ability to cross cue with SAR and GMTI in conjunction with standard payload sensors, the system would enable a much higher level of battlefield commentary over a substantially larger area with a higher standoff range than what the current system is capable of, enabling the covert surveillance on areas of interest. The only potential liability is the necessary addition of IA qualified Geospatial Technicians. Currently they are requested for major Exercises only; however, would be required on a semi-permanent, rotational detachment basis due to the analysis requirements of SAR imagery. The benefits that SAR can provide at the tactical level within Army are substantial. SAR is a rapidly advancing technology and noting that it is already being utilised down to the small UAV level in the civilian sector, it should be a priority within the Army’s only TUAS asset.