Per-Olov Frölind

Retrieval of forest stem volume using VHF SAR

The ability to retrieve forest stem volume using CARABAS (coherent all radio band sensing) SAR images (28-60 MHz) has been investigated. The test site is a deciduous mixed forest on the island of Oland in southern Sweden. The images have been radiometrically calibrated using an array of horizontal dipoles. The images exhibit a clear discrimination between the forest and open fields. The results show that the dynamic range of the backscattering coefficient among the forest stands is higher than what has been found with conventional SAR using microwave frequencies. The backscatter increases with increasing radar frequency. This work shows an advantage compared to higher frequencies for stem volume estimation in dense forests.

Ultra-wideband SAR interferometry

The authors introduce ultra-wideband synthetic aperture radar (SAR) interferometry as a new technique for topographic height retrieval. It is based on using a SAR system with large relative bandwidth that acquires data along two parallel tracks with a separation of the same order of magnitude as the flight altitude. The complex SAR image data are resampled onto a common reference surface, filtered, and followed by a Hermitian multiplication. The resulting interferogram is shown to have a finite depth-of-focus (DOF) in terms of phase coherence. The achieved height precision is controlled by the ambiguity height, which is shown to scale to the DOF as the relative bandwidth. This means that only one fringe is within the DOF as the resolution approaches the fundamental wavelength limit; i.e., the phase is unambiguously related to topographic height. The topography may thus be determined by changing the reference surface and retrieving the height at each step. The technique is successfully demonstrated to generate fringes based on VHF-band data acquired by the CARABAS airborne SAR system. Temporal decorrelation is not a problem due to the long wavelengths nor is the effect of tropospheric delay on the retrieved height.

Circular-Aperture VHF-Band Synthetic Aperture Radar for Detection of Vehicles in Forest Concealment

Circular-aperture synthetic aperture radar (SAR) imaging has been evaluated using the airborne very high frequency (VHF) band Coherent All RAdio BAnd Sensing (CARABAS)-II system (20-90 MHz). Images, as well as results, from detection of vehicles in dense forest concealment have been compared with linear-aperture SAR. Circular-aperture SAR imaging provides higher image resolution and increased object information, but complexity of signal processing and requirements on imaging geometry accuracy increases. The latter is, however, partly mitigated by using low frequencies in the VHF band. A high-quality digital elevation model is used to ensure high-quality image focusing and to avoid distorting object shape. Contrast optimization is used to reduce global focusing errors. The image resolution is observed to be about 1 m2 in agreement with theoretical predictions. Detection performance has been evaluated using image data from a full circular synthetic aperture, i.e., 360° of aspect angle variation. Results, both for single-pass detection (SPD) and change detection (CD), show a considerable advantage compared with detection based on linear-aperture SAR. The detection probability for SPD increases from 0.4 to 0.8 at a false-alarm rate (FAR) of 30/km2. For CD, the detection probability increases from 0.7 to 0.9 at a FAR of 2/km2.

The VHF/UHF-band LORA SAR and GMTI system

The paper describes design principles and presents first results for the airborne LORA (low-frequency radar) system. It covers operating frequencies in the VHF and UHF bands and has both synthetic-aperture radar and ground moving target indication modes. The main motivation for the system is to facilitate detection of man-made targets in a wide range of conditions, i.e. stationary or moving targets as well a targets in open terrain or in concealment under foliage or camouflage. The LORA system will operate in several configurations extending from 20 MHz to 800 MHz. Initial flight trials in 2002 were successfully conducted using the 200-400 MHz band. SAR images have been formed from the acquired data and are presented. A second band, 400-800 MHz, has also been completed but has not yet been tested in -flight. A third band, 20-90 MHz, is presently being added and will be completed during 2003. The paper also includes results from a recent experiment in northern Sweden which included an extensive target deployment to cover a broad range of operating conditions. VHF-band SAR (20-90 MHz) is compared with high-resolution Ku-band SAR. Results show the superior area-coverage rate of using VHF-compared to Ku-band for robust detection of stationary targets. The high-resolution images provided by the Ku-band SAR are, however, superior for classification and recognition purposes.

Performance of VHF-band SAR change detection for wide-area surveillance of concealed ground targets

VHF-band SAR used in conjunction with change detection techniques has shown promising results for wide-area surveillance of ground targets . By using VHF-band frequencies both targets in the open as well as concealed by foliage may be detected. These detections occur with high probability and with a low false-alarm rate. VHF-band SAR is able to detect hidden targets because both foliage attenuation and clutter backscatter is small. The clutter is further repressed through the use of change detection, thus significantly reducing the false-alarm rate. Change detection techniques are well suited for VHF-band SAR since temporal decorrelation is small at these large wavelengths. The CARABAS-II system performed a data collection during the summer of 2002. The primary goal of this collection was to gather data to evaluate VHF-band SAR change detection performance under various operating conditions. This paper reports the results obtained. In general, the results show a VHF-band SAR system employing change detection can reliably and robustly detect truck-sized targets hidden in foliage. The detection performance does deteriorate under certain conditions. A significant reduction is found for near-grazing angles. Additionally, a significant performance loss is found for smaller-sized targets when the radar bandwidth is reduced.

Change detection of vehicle-sized targets in forest concealment using VHF- and UHF-band SAR

We describe an extensive data collection and analysis of change detection using VHF- and UHF-band SAR data. Two airborne systems (CARABAS-II: 22-82 MHz, LORA: 225-470 MHz) acquired data for multiple headings and incidence angles. Twenty one targets of five types were deployed in forest concealment. CARABAS-II gives the best performance for the target types investigated. Analysis of the data shows that performance degrades for increasing incidence angle, mainly due to reduced target radar cross-section. It is also shown that different change detectors give different performances. The best detector for one incidence angle is not necessarily best for another incidence angle.

Bistatic VHF and UHF SAR for urban environments

Bistatic synthetic aperture radar (SAR) enables new defense as well as environmental applications where the characteristics of the bistatic reflectivity can be exploited. Experimental results obtained with microwave systems have been reported but not much is published using lower frequencies (<1GHz). FOI has been active in this part of the electromagnetic spectrum for many years with the development and operation of two airborne SAR sensors, i.e. CARABAS-II (20-90 MHz) and more recently LORA (200-800 MHz). During 2006 experimental work was initiated to investigate the challenges of implementing a bistatic low frequency SAR system. Various synchronization tests were made in the lab as a preparation for the first bistatic VHF SAR data registrations. An area in the vicinity of Linkoping city was illuminated using CARABAS-II as the airborne transmitter and the LORA radar electronics as a stationary roof-top mounted receiver unit. The latter was reconfigured to be able to handle the frequency interval 20-90 MHz. The approximately 4.1 km by 4.1 km large common radar scene contains urban environments, open areas and forested parts. The CARABAS-II sensor simultaneously registered monostatic SAR data to facilitate the image interpretation by comparisons although the incidence angle on receive differs considerably.

Digital elevation map generation using VHF-band SAR data in forested areas

The paper investigates digital elevation model (DEM) generation based on data from the ultra wideband coherent all radio band sensing (CARABAS) very high frequency (VRF)-band synthetic aperture radar (SAR). The results show excellent capability to penetrate forest areas, i.e., the generated DEMs are found to be close to the true ground height. A conventional DEM, based on stereo photography and surveying, and additional phase differential Global Positioning System (GPS) measurements have been used for comparison. The results in heavily vegetated areas (stem volume up to 600 m/sup 3//ha) show a mean height difference of less than 1.5 m and a root-mean-square (rms) error of less than 1.0 in compared to the conventional DEM. Stable backscattering properties allows us to use large baselines in order to obtain high height sensitivity. However, the amount of poor data due to low coherence increases with the increase of the baseline. The optimum baseline which balances these two effects is found to correspond to an incidence angle difference of 4/spl deg/-8/spl deg/.

VHF/UHF bistatic and passive SAR ground imaging

Bistatic and passive radar are emerging technologies for covert ground surveillance based on cooperative or non-cooperative transmitters, respectively. The latter uses transmitters of opportunity, e.g. terrestrial digital video broadcasting (DVB-T). In the paper, we show results from two unique experiments conducted using airborne systems operating in the VHF and UHF bands. The first experiment investigates UHF-band passive synthetic aperture radar (SAR) in receive-only mode. DVB-T signals transmitted from a 300-m tall mast are used and SAR images are formed at ranges up to 15 km. The second experiment investigates VHF-band bistatic SAR with a transmitting airborne radar and a receive-only system on a small helicopter. Results show significantly higher signal-to-clutter ratio for man-made objects in forested areas at bistatic elevation angles of 20°.

Low frequency bistatic SAR measurements

Airborne bistatic SAR data have been collected at VHF- and UHF-band to investigate clutter suppression in forested and urban areas. The synchronization between the SAR systems is accomplished using the 1-PPS signal provided by the GPS system. The same signal is also used as input to a disciplined 10 MHz master oscillator integrated in both radar systems to maintain sufficient phase stability. Images have successfully been generated using the time domain fast factorized backprojection algorithm, modified for the bistatic case. Clutter suppression has been observed when comparing the monostatic and bistatic images acquired simultaneously by the two SAR sensors. Work is in progress to quantify and compare the obtained results.