Per-Olov Frolind

Swedish Defence Research Agency · Sensor and EW Systems Department

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 this paper, experimental results for UHF wideband SAR imaging of humans on an open field and inside a forest is presented. The results show ability to detect the humans and suggest possible ways to improve the results. In the experiment, single channel wideband SAR mode of the UHF UWB system LORA developed by Swedish Defence Research Agency (FOI...

Low frequency synthetic-aperture radar (SAR) in combination with change detection techniques can be used to search for stationary ground targets. In particular, the method has shown a great potential when vehicle-size objects are obscured by foliage. However. one possibility is to improve the detection capabilities to increase the aperture angle of...

We present results from the LORAMbis bistatic VHF/UHF-band SAR experiment conducted in September 2010. The SAR flights were performed with the French SETHI system and the Swedish LORA system installed in two aircraft. Frequency band was 222-460 MHz and polarization was HH. Results from two tests sites (Kvarn and Remningstorp, both in southern Swede...

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 sy...

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 p...

We present results from a bistatic SAR experiment conducted with two airborne radars operating in the band 222–460 MHz using HH-polarization. The systems used a step-frequency chirp waveform and were synchronized using GPS 1PPS signals. The processed and analysed data were collected during four flight missions over a test site including forested te...

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 sy...

Sparse phased array antenna architectures that reduce the number of receiver channels are discussed and measured results of different architectures are presented. We show that by using multiple transmit antenna positions the receiver array can be sparse and hence the number of receiver channels reduced, still avoiding grating lobe effects. We also...

Time-domain back-projection is able to generate focused synthetic-aperture radar images for the most general geometries but at the expense of high computational burden. With the advent of the fast factorized back-projection algorithm, however, it is now possible to achieve computational performance in parity with frequency-domain algorithms and wit...

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 ill...

Multiple scattering is often a dominating scattering mechanism in VHF-band SAR, in particular in scenes including forested and urban terrain. Most important in this context is multiple scattering generated by double- or triple-bounce scattering from structures with orthogonal corners which often dominate for monostatic SAR geometries. Examples incl...

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 targ...

We describe a bistatic ultra-wideband VHF-band SAR experiment and present measurement results. The bistatic SAR system, operating in the band 28-73 MHz, is based on CARABAS-II as airborne transmitter and LORA as ground-based receiver. Monostatic SAR data can also be acquired simultaneously. In September 2007, a measurement campaign was conducted in...

Low frequency synthetic-aperture radar (SAR) in combination with change detection techniques can be used to search for stationary ground targets. In particular, the method has shown a great potential when vehicle-size objects are obscured by foliage. However, when only one single SAR image can be expected for the analysis another approach must be t...

SAR interferometry, requires the sensor separation to be below a critical baseline above which the coherency is lost as the ground projected frequency ranges do not overlap (frequency band shifting of one sensor -the delta-k system- is not addressed here). Indeed, as critical baseline increases with both wavelength and bandwidth, low frequency and/...

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 (< IGHz). FOI has been active in this part of the electro...

An implementation of a fast, factorised, back-projection scheme is presented which is based on interpolation of range-angle sub-aperture data. A method for determining the required angular sampling rate is proposed and the implementation scheme is evaluated for nearest neighbour and Cubic interpolation kernels against image quality and number of op...

Synthetic aperture radar (SAR) is a technology to generate high-resolution radar images for ground surveillance and mapping. By exploiting the lower VHF band, the attenuation reduces dramatically in forest areas compared to microwaves and new applications become feasible. Examples of these are detection of targets obscured by vegetation, stem volum...

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 det...

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 ran...

First Page of the Article

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 folia...

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 co...

A least-mean-square algorithm in the frequency domain, with amplitude normalisation, is proposed to remove narrowband radio frequency interference in low-frequency ultra-wideband synthetic aperture radar systems. The performance of the algorithm was tested on experimental data acquired with the CARABAS system

In Swedish forestry, reliable forest parameters are needed for both short and long term planning. The traditional methods used for acquiring forest data at reasonable accuracy are very expensive and labour intensive. Therefore, new cost-effective methods for data collection must be used to meet the requirements being imposed on timber supplies and...

CARABAS-II is an airborne SAR operating in the 20-90 MHz band. The low operating frequency enables detection of concealed objects in dense forests as well as mapping of forest stem volume. A number of calibration experiments have recently been conducted to evaluate system performance. In this paper, we report on some of the results from the analysi...

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 th...

A verification of DEM generation using ultra-wideband interferometry is presented. The processing is performed iteratively by first generating a coarse resolution DEM applying a stereoscopic technique, and then refocus the interferometric image pair based on this output. In the latter step the DEM heights are refined using the coherence phase. The...

The paper describes a new method to detect man-made objects hidden under foliage or camouflage. The method is based on change detection and thus multiple revisits of the same area. It uses SAR image data provided by the low-frequency and ultra-wideband CARABAS SAR system which operate in the 20 - 90 MHz frequency range. Experimental results show a...

Fully automatic geo-coding algorithms for CARABAS-II VHF SAR images have been developed. By using recorded flight parameters and a coarse scale (50×50 m) digital elevation model (DEM), complex CARABAS-II slant range images were gee-coded to a local orthogonal map projection and vice versa. The aims of this paper are to describe the algorithms and t...

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&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 succes...

A stream-lined precision processor has been developed for the CARABAS-II HF/VHF-band SAR system. The authors describe the basic system characteristics, the normal waveform used, and the signal processing techniques to form images. In particular, challenges related to the stepped-frequency waveform, the radio-frequency interference environment, and...

An algorithm for DEM generation using ultra-wideband interferometry is described. Results based on VHF-band data acquired by the CARABAS airborne SAR system is presented at a spatial ground pixel resolution of 16×16 m. The resulting DEM shows good agreement with topographic map. The results indicate good potential for topographic mapping of the tru...

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 r...

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 antenn...

An investigation of the image response change of a target placed at the nominal reference height due to motion compensation of ultra-widebeam VHF SAR is performed. Also, an approximate relationship between the image position, for a target out of the nominal height, and a certain actual flown track is derived

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...

CARABAS-II is a new airborne ultra-wideband and widebeam SAR which operates in the lower VHF band (20-90 MHz). Its design has been based on the experiences and insights gained from the earlier CARABAS system. The initial radar test flights were conducted during October-November 1996 which included a flight campaign as part of BALTASAR-96. The subse...

An investigation of target azimuth displacement due to motion compensation of ultra-wide beam VHF SAR data has been performed. The displacement is caused by a height deviation between the target and the assumed reference height. An approximate expression is derived for a line which the motion compensation inflicts on the range trajectory of the tar...

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...

The authors analyse different techniques for 3D SAR mapping. Interferometry using narrow-band SAR enables retrieval of topography but gives an ambiguous height. Stereoscopy can also be used but has degraded resolution due to speckle. For ultra-wide band SAR, i.e. wavelength-order resolution, these techniques converge and give similar vertical resol...

The CARABAS SAR has proven to be a significant contribution in the field of low frequency radar imaging. The wavelengths used have a potential of penetration below the upper scattering layer, in combination with high spatial resolution. The first prototype of the system has been tested in environments ranging from rain forests to deserts, collectin...

Ice mapping in the Baltic Sea using microwave synthetic aperture radar (SAR) images has been studied since the mid-eighties. In an experiment during the winter season of 1993/94 the airborne VHF SAR sensor CARABAS was used in parallel. Low frequencies mainly interact with the large-scale roughness, e.g. ice ridges, but may also penetrate into the i...

CARABAS, an acronym for `coherent all radio band sensing,' is an airborne, horizontal-polarization SAR operating across the frequency band 20 to 90 MHz, conceived, designed and built by FOA in Sweden. The original motivation for designing such a low frequency system was that a large relative or fractional bandwidth could be achieved at low frequenc...

CARABAS, an acronym for `coherent all radio band sensing,' is an airborne, horizontal-polarization SAR operating across the frequency band 20 to 90 MHz, conceived, designed and built by FOA in Sweden. The original motivation for designing such a low frequency system was that a large relative or fractional bandwidth could be achieved at low frequenc...

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 com...

CARABAS is an airborne synthetic aperture radar (SAR) system, operating in the lower VHF-band (20-90 MHz). Developed by the Swedish Defence Research Agency, CARABAS is a unique remote sensing sensor, in that the wavelengths used are extremely long (3-15 m), and hence can penetrate virtually unaffected through dense forest canopies. This means that...