Gunnar Stenström

Synthetic-aperture radar processing using fast factorized back-projection

Exact synthetic aperture radar (SAR) inversion for a linear aperture may be obtained using fast transform techniques. Alternatively, back-projection integration in time domain can also be used. This technique has the benefit of handling a general aperture geometry. In the past, however, back-projection has seldom been used due to heavy computational burden. We show that the back-projection integral can be recursively partitioned and an effective algorithm constructed based on aperture factorization. By representing images in local polar coordinates it is shown that the number of operations is drastically reduced and can be made to approach that of fast transform algorithms. The algorithm is applied to data from the airborne ultra-wideband CARABAS SAR and shown to give a reduction in processing time of two to three orders of magnitude.

The airborne VHF SAR system CARABAS

FOA has designed, built and tested a new airborne synthetic aperture radar system, CARABAS, which operates in the lower part of the VHF-band. This frequency region gives the system a good ability to penetrate vegetation and also to some extent ground. Furthermore, the resolution cell and the smallest scatterers influenced by the frequencies are comparable in size and a reduced speckle level in the final SAR image is obtained. Critical parts in the development have been the antenna, the receiver and the signal processing algorithms. A number of test flights have been carried out during 1992. The first major SAR campaign was conducted in October 1992 at two different test sites and included deployment of different reference targets in parallel with a ground truth programme.<<ETX>>

LORAMbis A bistatic VHF/UHF SAR experiment for FOPEN

LORAMbis is the experimental part of a joint research program between Sweden and France. The objective is to evaluate the performance of low frequency bistatic SAR for clutter suppression in various applications, e.g. under-foliage target detection or mapping of urban scenes. The airborne bistatic SAR data are acquired using the VHF/UHF component of the ONERA SAR system SETHI and the VHF/UHF LORA system operated by FOI. The first data collection campaign was conducted in December 2009. Focused bistatic SAR images have been formed and indicate that the implemented synchronization method is sufficient. It is based on a GPS disciplined 10 MHz reference signal that is generated in both systems.

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.

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.