P.-O. Frolind

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>>

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.

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.

Performance of the CARABAS-II VHF-band synthetic aperture radar

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 analysis. Spatial resolution, measured using 5-m trihedrals, is typically 2.5 m in both slant range and azimuth. The right-left ambiguity ratio, measured using 5-m trihedrals on both sides of the flight track, is about 10 dB for a single antenna element on receive. The noise level varies in the images and includes both multiplicative (integrated sidelobe ratio, right-left ambiguity ratio) and additive (radio-frequency interference, receiver noise) terms. Analysis of images from a recent campaign in northern Sweden shows that the additive noise term is less than -20 dB (noise-equivalent /spl beta//spl deg/) for slant ranges less than 14 km.

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°.

The experimental airborne VHF SAR sensor CARABAS. A status report

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, collecting a considerable amount of data often in parallel with other SAR sensors. The work on data analysis proceeds and results obtained so far seem promising, especially for applications in forested regions. The experiences gained are used in the development of a new upgraded system, scheduled for initial airborne tests in 1996.

A pilot experiment for stem volume retrieval based on VHF SAR data

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 environment conservation. Based on promising results obtained earlier in relating VHF SAR data to forest stand parameters a pilot project has been defined. The objective is to develop a fully integrated system for retrieval of forest parameters using VHF SAR images. A representative data set covering 625 km/sup 2/ of predominantly conifer-forested terrain was collected with the CARABAS-II sensor in November 2000 as an initial step.

Analysis of CARABAS VHF SAR data from BALTASAR-96

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 subsequent data analysis has resulted in a number of processed images with good image quality over a forested area in southern Sweden. The resolution of the imagery has been measured to 3/spl times/3 m/sup 2/. The present limitation before full performance can be achieved is due to uncompensated antenna characteristics.

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.

Measurements with the CARABAS SAR sensor during BEERS-94

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 ice. The flight programme focused on imaging passes parallel to the ERS-1 imaging orbit, but the area on ice defined for the ground truth activities was also imaged from orthogonal directions and some headings in between. One set of images was aimed for repeat-pass interferometry tests.