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Foliage penetration Synthetic Aperture Radar (SAR)

Situation awareness and accurate target identification are critical requirements both for security forces engaged in counterterrorism operations. Ground, airborne and space-borne radars and EO sensors have proved to be of great utility in detection, tracking, and imaging of vehicles to high-speed fighter aircraft,   locating mortars and artillery,  over the horizon capability, and all-weather long-range surveillance.

 

However, these sensors are limited in capability to detect targets concealed in the foliage. Conventional forces and terrorists/insurgents have exploited this weakness by employing camouflage, concealment, and deception tactics like hiding in camouflage nets and forests for long.

 

Synthetic Aperture Radar SAR is a known technique for two-dimensional high-resolution ground mapping. The basic principle of any imaging radar is to emit an electromagnetic signal (which travels at the speed of light) toward a surface and record the amount of signal that bounces/echoes back, or “backscatters,” and its time delay. The resolution of a radar antenna is dependent on the antenna aperture—the larger the antenna, the better the resolution.

 

The geometrical resolution and sensitivity of radars are significantly improved using advanced signal processing techniques. Examples of the latter are synthetic-aperture radar (SAR) and ground moving target indication (GMTI) which enables both stationary and moving targets to be detected, positioned and classified at large stand-off distances.

 

A SAR takes advantage of motion of the antenna to achieve an apparent antenna length, or aperture, greater than its actual length. As the antenna moves along a flight path, successive echoes are received from the same target and may be processed to give spatial resolution equivalent to an antenna as long as the distance the antenna moved when receiving the target echoes. Thus, the terminology “synthetic aperture radar” is used to describe the radar system.

 

Atmospheric conditions such as clouds and rain do not significantly degrade the SAR signal. However, the presence of foliage (trees, brush, grasses) can greatly attenuate the signal.

 

Foliage penetration (FOPEN) SAR

The foliage penetration (FOPEN) SAR is an ultrawide-band system that uses lower frequencies to “see” through the foliage and achieve foliage penetration.

 

Static objects in forest terrain can be detected with low-frequency SAR, i.e. with a wavelength in the range 0.3-15 m. The low frequencies have the property of penetrating the vegetation layer with little attenuation and only causing a weak back-scattering from the coarse structures of the trees. Thus, static objects, such as stationary vehicles, can be detected also in thick forest by combining low frequencies with SAR technique which gives a resolution of wavelength size. This has been scientifically demonstrated in a plurality of experiments in recent years.

 

Low frequency ultra-wideband (UWB) SARs in particular uses low frequencies and a large bandwidth that provide them with penetration capabilities and high resolution. UWB SARs are typically used for near eld imaging applications such as foliage penetration, through the wall imaging and ground penetration.

 

The FOPEN SAR is an ultrawide-band (UWB) system that uses low frequencies to achieve foliage penetration. The system has a very high frequency (VHF) frequency range of approximately 20-70 MHz and ultra high frequency (UHF) range of approximately 200-500 MHz. Its basic operating principle involves transmitting of pulsed radio frequency waves and receiving the echoes scattered from targets and the ground surface. The echoes are subjected to analog preprocessing, digitized, and further digitally processed to produce the final imagery.
The UHF band is a fully polarimetric (HH1 , VV2 , HV3 ) side-looking radar. The VHF band operates with HH polarization.

 

Foliage penetration SAR has also been found efficient and cost-effective means of estimating the extent of contamination at UXO sites. The clearing of areas contaminated with unexploded ordnance (UXO) is the Army’s highest priority Environmental Restoration problem. The Department of Defense (DoD) currently spends millions of dollars annually on UXO cleanup efforts.

 

The primary advantage of using an airborne-based system is the ability to acquire a large amount of data covering a wide area in a relatively short period of time. Current methods for estimating the extent of a UXO-contaminated site are multiphase efforts, ground-based, and the site evaluation generally requires several weeks or months. With the airborne SAR, the time frame could be reduced to days. Although the FOPEN SAR system can rapidly gather data over a large area, it was designed to detect large tactical vehicles and its resolution limits the size of UXO that can be detected. Ordnance are typically found in clusters within the primary radius of a firing range, and a cluster of smaller UXO, which are not detectable individually, may be imaged. However, on the fringes of a range where the distribution of UXO is sparse, only the larger ordnance may be detected.

 

Foliage penetration systems

The early history of FOPEN Radar was driven by interesting developments in radar technology that enabled our ability to detect fixed and moving objects under dense foliage. The most important part of that technology was the widespread awareness of the benefits of long dwell coherent radar and the advent of digital signal processing. Almost as important were the quantification of the radar propagation through the foliage, and the impact of radio frequency interference on image quality.

 

These systems were developed for both military and commercial applications and during a time of rapid awareness of the need and ability to operate in a dense signal environment. Finally, there is a clear benefit for use of polarization in the target characterization and false alarm mitigation.

 

New research in Multi-mode Ultra-Wideband Radar, with the design of both SAR and moving target indication (MTI) FOPEN systems. At common FOPEN frequencies, the systems have generally been either SAR or MTI due to the difficulties of obtaining either bandwidth or aperture characteristics for efficient operation.

 

Lockheed Martin’s High Resolution, Penetrating RADAR Detects, Geo-Locates and Communicates Threats

Ongoing counter-terrorism and counter-insurgency operations present tough challenges that our forces must face each day. They need surveillance and reconnaissance capabilities that provide a long-term stare at specific geographic locations so they can detect environmental changes, patterns, and asymmetric tactics. With Lockheed Martin’s Tactical Reconnaissance and Counter-Concealment (TRACER), customers have a long endurance surveillance capability for all operational environments.

 

TRACER, a lightweight, low-frequency synthetic-aperture radar (SAR), meets the challenges of seeing what was previously unseen by sensing with radio waves rather than light. This dual band (UHF/VHF) SAR can peer through foliage, rain, darkness, dust storms or atmospheric haze to provide real-time, high-quality tactical ground imagery, anytime it is needed, day or night. TRACER’s advanced penetrating SAR technology provides a unique ability to detect threats and illicit activities. Unlike most radars, TRACER uses low frequency radio waves in the UHF and VHF frequency ranges allowing detection through dense forest canopy and even below ground.

 

TRACER also incorporates data link technology that allows airborne processed results to be down-linked to ground stations immediately. The system includes a portable ground station to plan, collect, support missions, and exploit imagery. This penetrating detection capability on a persistent surveillance platform provides commanders at all levels with actionable intelligence in a tactical, useful timeline.

 

Proven Synthetic Aperture Radar for Manned or Unmanned Platforms

TRACER’s design is predicated on Lockheed Martin’s proven foliage penetration (FOPEN) technology, developed specifically to detect vehicles, buildings, and large metallic objects in broad areas of dense foliage, forested areas and wooded terrain. The TRACER system builds upon the FOPEN technology advancements by not only shrinking and modernizing the radar, but also by configuring it for unmanned endurance aircraft. The radar’s advanced detection capability suppresses background clutter and returns from stationary objects, while revealing the positions of mobile and portable targets. These technology advances, coupled with lessons learned from ongoing FOPEN operations, have contributed to new concepts of operations for the system. The system can be operated from low to very-high altitudes –– on manned and unmanned platforms.

 

Developed in the late 1990s under the sponsorship of the Defense Advanced Research Projects Agency, U.S. Army and U.S. Air Force, the FOPEN system has flown more than 1,000 successful missions. Over the course of hundreds of flights over the past decade, the FOPEN radar system’s detection and topography capabilities have proven extremely robust against a variety of targets and foliage environments.  Concealed targets obviously present a number of challenges to many different types of sensors. Both the FOPEN and TRACER systems are uniquely able to overcome many of these impediments to detect and accurately locate obscured targets. The change detection capabilities of FOPEN and TRACER not necessarily limited to large targets nor is the application of penetrating radar limited to forested environments. There are currently four qualified TRACER systems available for deployment on manned or unmanned platforms. TRACER was developed for the U.S. Army’s Intelligence and Information Warfare Directorate, based at Aberdeen Proving Grounds, Md.

 

Swedish FOI’s CARABAS & LORA systems

The Swedish Defence Research Agency – FOI (formerly known as FOA) – has performed research since the mid-80´s in the area of airborne ultra-wideband VHF-band synthetic aperture radar (SAR). The work has resulted in two airborne CARABAS systems operating in the 20-90 MHz band. The prime application is for detection of man-made objects concealed by foliage or camouflage. Results have also shown that CARABAS is capable of accurately mapping forest stand volume (m3 /ha), or biomass (ton/ha), up to about 1000 m3 /ha, which is of high interest for environmental and commercial applications

 

CARABAS

CARABAS, an acronym for “Coherent All Radio Band Sensing“, is an airborne, horizontal-polarization SAR operating across the frequency band 20–90 MHz, conceived, designed and built by FOA in Sweden. The original motivation for designing such a low frequency system was that a large relative or fractional bandwidth could be achieved at low frequencies. For reasons to be explained, a large fractional bandwidth was considered to be of potential benefit for radar detection in severe clutter environments. A feasibility study of a short wave ultra-wideband radar started at FOA in 1985. Actual construction of the CARABAS system commenced 1987, aircraft integration took place during 1991 and the first radar tests were conducted in early 1992. From the fall of 1992 onwards, field campaigns and evaluation studies have been conducted as a joint effort between FOA and MIT Lincoln Laboratory in the US.

 

LORA

LORA (low-frequency radar) is FOI:s new airborne radar which will succeed CARABAS. It will operate from 20 MHz to 800 MHz and will be used for demonstrating new defence and civilian applications. The main application is expected to be detection of man-made objects in a wide range of operating conditions, i.e. both stationary and moving objects located in the open or under concealment.

 

LORA has been designed as a multi-function VHF/UHF band radar system which can simultaneously operate in both SAR and ground moving target indication (GMTI) modes. It operates in two basic configurations: 1) Ultra-wideband SAR/GMTI 200-800 MHz, and 2) Ultra-wideband SAR 20-90 MHz. The latter will be a replacement for the CARABAS system which will be completed during 2003.

 

 

Foliage penetration technology

A SAR system consists of a transmitter, a receiver, an antenna (including a pointing or steering mechanism), image processor and display unit. An imaging radar system must distinguish between single and multiple scatters located in close proximity. Resolution is the minimum distance needed between adjacent scatters to separate them in the imaging map. Fine resolution provides the capability to image a complex object or scene as a number of separate scattering centers. The bulk of the development effort in radar imaging is at improving the radar resolution. Generally, the range resolution is inversely proportional to the bandwidth of the transmitted signal. A wide bandwidth means finer range resolution. In conventional radar, resolution in the azimuth direction improves as the antenna beamwidth becomes smaller. Antenna beamwidth becomes smaller as antenna aperture size or radar frequency increases. Hence practical constrains such as antenna size and transmit frequency will limit the azimuth resolution of the conventional aperture radar.

 

The development of advanced processing algorithms solved this problem, leading to a new generation of imaging radars called Synthetic Aperture Radar. A Synthetic Aperture Radar (SAR), or SAR, is a coherent mostly airborne or spaceborne sidelooking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. Over time, individual transmit/receive cycles (PRT’s) are completed with the data from each cycle being stored electronically. The signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined (taking into account the Doppler effects inherent in the different transmitter to target geometry in each succeeding cycle) to create a high resolution image of the terrain being over flown. The SAR works similar of a phased array, but contrary of a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are result of the moving platform now.

 

Detection performance of a radar system is directly related to the target-to-background backscattering statistics evaluated for the specific operating conditions. In general, radar backscattering is a complicated function of target geometry and its electromagnetic properties. Backscattering from the target background also contributes and competes with the target backscattering in the radar resolution cell. The coherent combination of target and background backscattering results in a statistical variability that reduces detection performance.

 

For SAR systems operating in the UHF and VHF bands, backscatter phenomenology is quite different from microwave frequencies. Target sizes are often in the resonance region, i.e. of wavelength size, and the angular variation of the backscattering is much smaller than at microwave frequencies. Another important effect is the interaction between the target and the ground surface, i.e. the coherent combination
of the direct and ground-reflected backscattered waves. This effect reduces target backscattering for lower frequencies since the direct and reflected waves tend to cancel each other. The effect becomes more pronounced for grazing angles and for small target heights above the ground compared to the wavelength.

 

A number of experiments have been performed in order to investigate the optimum choice of frequency band.  The main conclusion is that foliage becomes increasingly transparent below 1 GHz, and below 100 MHz the two-way attenuation is most often less than 3 dB. In terms of the foliage backscatter, it is only below 100 MHz that backscattering decreases when the tree stems enter the Rayleigh scattering regime. However, even below 100 MHz it still significantly affects detection performance.

 

Typically, stems have a diameter up to about half a meter which implies that their backscattering drops significantly when the radar wavelength is larger than about five meters. This mechanism suggests that the optimum radar wavelength for detection of a vehicle-sized
target under foliage is in the VHF-band with a weak dependence on stem diameter.

 

FOPEN SAR is the synthetic aperture radar for foliage penetration application, it is used to image targets concealed by foliage areas. For achieving the higher range resolution, UWB waveform is utilized into FOPEN SAR. To further achieve high resolution in the azimuth as well as range, long synthetic apertures or large integration angles are required. However, these large integration angles lead to severe range migration, or motion through resolution cells (MTRC). Scatterers at different locations in an imaged scene experience different levels of MTRC. The variation in MTRC makes selection of proper image formation algorithms critical. Moreover, the large integration angle, together with UWB waveform, brings about new complexities and challenges to the traditional SAR imaging techniques

 

Receivers with large analogue and digital dynamic range have a rather narrow bandwidth. LORA’s  solution to this  problem is to use a stepped-frequency waveform so that the instantaneous bandwidth is much smaller than the full bandwidth. The latter is reconstructed by stitching together frequency bands in the signal processing. Each pulse in the waveform has a large time-bandwidth product (typically, a chirp) to meet average power requirements.

 

One of the most problematic issues when designing an ultra-wideband radar below 1 GHz is the challenging radio frequency environment. The radar system must be able to share the frequency bands with a large number of other services, i.e. without causing harmful interference.
Furthermore, the system must not saturate due to external interference which requires a receiver subsystem with very large dynamic range and out-of-band suppression. The dynamic range requirement is in direct conflict with the large bandwidth in order to achieve high resolution SAR.

 

The system is operated in one of two modes—spot or strip. In spot mode, the radar is focused on a single point and data are gathered at different angles as the aircraft flies over the area. An image 3 km by 3 km is typically obtained in spot mode. Strip mode differs from spot mode in that the radar viewing angle is held fixed and a swath of ground is imaged along the flight path. Strip mode produces a 2 km by 7 km image. Image resolution varies but is typically less than 1 m in the UHF band for both modes. The data acquired during this test were collected in strip mode. Spotlight SAR has an advantage over the strip-map SAR in terms of its resolution.

 

In particular, VHF-band SAR provides a robust means for detecting truck-sized targets concealed in foliage. Targets concealed in foliage are most often visible in CARABAS-II SAR images but the backscattering from tree stems cause false alarms. The false alarm rate may, however, be significantly reduced by applying change detection. The latter relies on collecting images at several occasions and detects targets using classification methods.  Change detection is applied to VHF-band SAR images and significantly improves target detection performance

 

References and resources also include:

https://www.researchgate.net/publication/4064888_Development_of_the_ultra-wideband_LORA_SAR_operating_in_the_VHFUHF-band/link/5a3b5da9458515a77aa9ea02/download

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