B. Flood

Development and operation of an airborne VHF SAR system-lessons learned

The interest in using VHF/UHF ultra-wideband SAR for the purpose of target detection and recognition has grown in the past few years. Low operating frequencies are required to penetrate an obscuring canopy or soil. Several crucial design issues can be identified for a system to be operated in the lower VHF-band (20-90 MHz), in particular the antenna system, the receiver chain and the waveform generator. The low directivity of the antenna implies that the synthetic aperture will be long, and this sets requirements on the accuracy and temporal stability of the navigation system to be able to apply the motion compensation properly. The large amount of data will also have an impact on the SAR processing schemes to be used, trading off processing time versus some image quality parameter. A lot of experience has been gained from the realization and operation of the airborne CARABAS I system and has now been invested in the upgraded system, CARABAS II, operational since 1996, and with considerably improved performance figures. CARABAS II has so far acquired data appropriate for studies of applications such as foliage penetration, biomass estimation and SAR interferometry.

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.

Development of the ultra-wideband LORA SAR operating in the VHF/UHF-band

LORA (low-frequency radar) is a new airborne VHF/UHF-band radar which has both synthetic-aperture radar (SAR) and ground moving target indication (GMTI) modes. The main motivation for the system is to facilitate detection of man-made objects in a variety of conditions, i.e. stationary or moving, located in open terrain or in concealment under foliage. The LORA system will operate in several configurations extending from 20 MHz to 800 MHz. Initial flight trials during 2002 were successfully conducted using the 200-400 MHz band. SAR image examples are shown including both forested areas and man-made objects. A second band, 400-800 MHz, has also been completed but not yet flight tested. A third band, 20-90 MHz, is being added and will be completed during 2003.

ALOS PALSAR Calibration and Validation Results from Sweden

In 2006 calibration activities for ALOS PALSAR were conducted in Sweden. Four five-metre trihedral corner reflectors and three smaller dihedral reflectors were deployed and operated during eight months. 23 PALSAR scenes were acquired over the calibration site allowing an evaluation of the quality and temporal stability of the data. Results show that the co-polarized data have been stable during the whole calibration period with variations in the trihedral responses lower than 0.7 dB. The measured resolution in azimuth was 4.4 m and in slant range 4.7 m for single polarization images and 9.5 m for polarimetric data. For the cross-polarized data large variations in the dihedral responses were found. It is assumed that this is caused by a larger sensitivity to pointing errors. For the polarimetric data, estimation of Faraday rotation gave values ranging from 0.1deg to 3deg.

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.

First airborne tests with the new VHF SAR CARABAS II

The first airborne experiment with a new upgraded VHF SAR system was carried out in October 1996. A very flat island was selected as the test area to minimize the influence from the topography and facilitate the calibration and system analyses. Data acquired over the area with this new sensor, CARABAS II, have successfully been processed. The major problem encountered concerns large paired-echo sidelobes. To improve the image quality, a careful system analysis has been carried out to obtain correction coefficients for the signal processing to compensate the overall amplitude and phase ripple. The results presently available indicate resolution figures somewhat lower compared to the theoretical values. It is believed that the main explanation for the found performance degradation is insufficient information of the antenna characteristics and the measures currently taken for the radio frequency interference filtering.

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.