Mikael Lundberg

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.

A challenge problem for detection of targets in foliage

This paper describes a challenge problem whose scope is detection of stationary vehicles in foliage using VHF-band SAR data. The data for this challenge problem consists of images collected by the Swedish CARABAS-II system which produces SAR images at the low VHF-band (20-90 MHz). At these frequencies the electromagnetic energy from the radar penetrates the foliage of the forest, providing a return from a target concealed in a forest. Thus, VHF-band SAR technology transforms the foliage penetration problem into a traditional detection problem where the goal is to reduce the false alarm rate (FAR). Reducing the FAR requires suppressing the clutter in a VHF-band SAR image which is dominated by larger tree trunks, buildings and other man-made objects. The purpose of releasing the CARABAS-II data set is to provide the community with VHF-band SAR data that supports development of new algorithms for robust target detection with a low false alarm rate. The set of images supports single-pass, two-pass and multi-pass target detection.

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.

Fusion of multispectral and stereo information for unsupervised target detection in VHR airborne data

Very high resolution multispectral imaging reached a high level of reliability and accuracy for target detection and classification. However, in an urban scene, the complexity is raised, making the detection and the identification of small objects difficult. One way to overcome this difficulty is to combine spectral information with 3D data. A set of (very high resolution) airborne multispectral image sequences was acquired over the urban area of Zeebrugge, Belgium. The data consist of three bands in the visible (VIS) region, one band in the near infrared (NIR) range and two bands in the mid-wave infrared (MWIR) region. Images are obtained images at a frame rate of 1/2 frame per second for the VIS and NIR image and 2 frames per second for the MWIR bands. The sensors have a decimetric spatial resolution. The combination of frame rate with flight altitude and speed results in a large overlap between successive images. The current paper proposes a scheme to combine 3D information from along-track stereo, exploiting the overlap between images on one hand and spectral information on the other hand for unsupervised detection of targets. For the extraction of 3D information, the disparity map between different image pairs is determined automatically using an MRF-based method. For the unsupervised target detection, an anomaly detection algorithm is applied. Different methods for inserting the obtained 3D information into the target detection scheme are discussed.

SAR data collection over rain forests at VHF- and UHF-band

Radar imaging of tropical vegetation at VHF- and UHF-band has been performed using the airborne SAR sensors CARABAS-II and LORA, respectively. The acquired data set is limited to HH-polarized registrations only. The area mapped exhibits a rough terrain with dramatic topographic variations, mostly covered by dense tropical rain forests. Multiple illumination directions spanning 360° were adopted in the data collection for both sensors to overcome the shadowing due to the high relief topography. For each heading, the SAR images generated, adjacent in azimuth and from all imaging passes, were calibrated and geocoded separately and then merged into a mosaic representing the full ground coverage. A first output from the forest backscatter analysis indicates a 12 dB lower level at VHF-band at an incidence angle of about 70°. However, this preliminary result is based on one sample point only, where the investigated forested area was located on a fairly flat ground surface.