Björn Larsson

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.

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.

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.

Observations of clutter suppression in bistatic VHF/UHF-band synthetic-aperture radar

The paper presents results from a bistatic SAR experiment conducted using two airborne SAR systems operating in the high VHF- and low UHF-band. The Swedish SAR system LORA operated together with the French SAR system SETHI and collected data in different bistatic geometries in the frequency band 222-460 MHz and using HH-polarization. The two SAR systems were synchronized using the 1PPS GPS-signal. Data were collected during four flight missions over the main test site with forested terrain and buildings as well as controlled target deployments. A fifth mission was included over a second test site with an extensive data base of forest parameters but without target deployments. The bistatic radar data have been processed to SAR images and first analysis completed. Results show significant suppression of strong forest clutter and that the effect increases with bistatic elevation angle. The clutter reduction is observed in areas with dominating double-bounce scattering. Data analysis shows that forest clutter can be suppressed by 10 dB for a bistatic elevation angle of 10°.

Trans-ionospheric distorsion in satellite VHF-band SAR systems

VHF SAR imagery has proven to be a powerful tool in several civilian and military applications. For a spaceborne instrument the ionospheric effects become significant. The impact of the VHF signal distortion on the SAR image formation process is an open question. The CARABAS-II radar system was configured on ground to collect VHF data at some of the occasions when the space shuttle was passing by during the SRTM experiment. The registrations at 84 MHz using a dual-polarisation antenna array are now in the phase of evaluation, with the objective to detect the target and examine the stability for a trans-ionospheric signal path.

A production line for forest stem volume measurements from VHF SAR data

Retrieval of forest stem volume has been the main civilian application investigated for VHF SAR data during recent years. Based on promising results obtained from previous studies, a project was defined with the objective to develop a fully integrated system for retrieval of stem volume at high spatial resolution from CARABAS-II VHF SAR images. Using the developed production line, stem volume can easily be retrieved within predefined forest stands or within homogeneous areas defined by segmentation of optical satellite images. In order to evaluate the accuracy and efficiency of the production line, CARABAS-II images have been collected and analyzed over a test area covering 25/spl times/25 km/sup 2/. The major part of the area is owned by the Swedish forest company Holmen Skog AB. For coniferous forest stands, exhibiting normal forest production conditions, the accuracy in terms of root mean square error (RMSE) for estimation of forest stem volume is about 44 m/sup 3/ ha/sup -1/ (22% of the average stem volume) in the range of 70-400 m/sup 3/ ha/sup -1/, using the developed production line. In comparison to earlier research tools, the developed production line speeds up the analyses approximately 10 times.

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.

The French-Swedish multi-frequency SAR campaign RAMCAR98

The joint French-Swedish SAR experiment RAMCAR98 was carried out in two predominantly forested regions of France. The test site in Les Landes is very flat, whereas the area of Loze/spl grave/re exhibits a variability in the terrain topography of hundreds of metres. Good ground truth data are available for both areas. A comprehensive data set has successfully been co-registered at VHF-, L-, and X-band, consisting of fully polarimetric radar scenes for the two microwave bands as well as single-polarized interferometric acquisitions at X-band (single-pass) and VHF-band (repeat-pass). The main objective with the campaign was to investigate the use of radar imaging techniques in heavily foliated geographies and address application issues such as retrieval of the woody biomass, generation of digital elevation maps, and detection of man-made targets located in deep hide.