Zheng-Shu Zhou

Development of a ground-based polarimetric broadband SAR system for noninvasive ground-truth validation in vegetation monitoring

We have developed a ground-based polarimetric broadband synthetic aperture radar (SAR) system for noninvasive ground-truth validation in polarimetric SAR remote sensing of terrestrial vegetation cover. This system consists of a vector network analyzer, one dual-polarized antenna, and an antenna positioner. It can be operated in a frequency range from 50 MHz to 20 GHz, with a scanning aperture of 20 m in the horizontal and 1.5 m in the vertical direction. Tests carried out with standard reflectors showed that the polarimetric measurement capabilities of this system are satisfactory. Using the polarimetric ground-based SAR (GB-SAR) system, we carried out measurements on a specific vegetation cover pertinent to the remote sensing of forested regions within Sendai City, consisting of three different kinds of trees common within the Kawauchi Campus of Tohoku University. Measurements were collected in spring, summer, and autumn. Three-dimensional (3-D) polarization-sensitive images were reconstructed from the acquired data. Analysis of the 3-D polarimetric images of each measurement found differences (at times strong differences) among the polarization signatures. There were stronger reflections in all of the HH, VH, VV images in the second (summer) measurement, especially in the VH image, due to the substantial growth of branches and leaves in summer. This ground-truth validation system provided valuable information about the scattering mechanisms of the three trees selected for analysis in different seasons, which can be detected by broadband polarimetric ground-based SAR measurements. The experimental results demonstrate the excellent polarimetric performance of the newly developed SAR imaging system, which should find many useful and immediate applications in noninvasive ground-truth validation of diverse terrestrial vegetation covers.

Development of a hand-held GPR MD sensor system (ALIS)

ALIS (Advanced Landmine Imaging System), which is a novel landmine detection sensor system combined with a metal detector and GPR, was developed. This is a hand-held equipment, which has a sensor position tracking system, and can visualize the sensor output in real time on a head-mounted PC display. In order to achieve the sensor tracking system, ALIS needs only one CCD camera attached on the sensor handle. The new hand-held system ALIS is a very compact and do not require any additional sensor for sensor position tracking. The acquired signal from the metal detector and GPR is displayed on the PC display on real time, and the sensor trace can be checked by the operator. At the same time, the operator can visually recognize the signal on the same display. The CCD captured image is superimposed with the GPR and metal detector signal, therefore the detection and identification of buried targets is quite easy and reliable. Field evaluation test of ALIS was conducted in Afghanistan, and we demonstrated that it can detect buried antipersonnel landmines, and can also discriminate metal fragments from landmines.

Imaging algorithm of a hand-held GPR MD sensor system (ALIS)

We are developing a new landmine detection system, called advanced landmine imaging system (ALIS), which is equipped with metal detector (MD) and ground penetrating radar (GPR). Although this is a hand-held system, we can record the MD and GPR signal with the sensor position information acquired by CCD camera. Therefore, 2D MD image and 3D GPR image are possible after signal processing. But because ALIS is a hand-held system, the sensor position is random when it is operated in the field. So interpolation processing is used to deal with the problem and offer grid data set for both MD and GPR. Good MD image can be achieved after interpolation. Also, interpolation can prepare good data set for migration to get good horizontal slice image. After interpolation, 3D diffraction stacking migration with migration aperture is used to refocus the scattered signals and enhance the signal-clutter ratio for reconstructed good GPR image. The ALIS was tested in Afghanistan in December 2004 and could achieve good landmine image. Especially, GPR could obtain good image of anti-person (AP) mine buried at more than 20cm depth. Also MD image and GPR image could combine to distinguish mine from metal fragment.

Interferometric polarimetric synthetic aperture radar system

Polarimetric radar interferometry is a compound technique that has shown the ability to extract geophysical parameters from synthetic aperture radar (SAR) images and its usefulness in terrain classification and surface change detection. A three-dimensional image can be constructed by coherence integrating the backscatter data over the measured frequency band and the two spatial coordinates of the two-dimensional synthetic aperture. Accordingly, we have developed a vector network analyzer based three-dimensional synthetic aperture radar imaging system. The radar system consists of the two-dimensional antenna scanner, which are two double-ridged waveguide horn antennas. This test synthetic aperture radar system is used for monitoring radar targets in an anechoic chamber under ideal conditions. The system operates at frequencies between 50 MHz and 20 GHz. The antenna scanning aperture is about 2 meters by 2 meters. Being applied the rotation of polarization basis, the full-polarimetric images for several targets with various permittivities were acquired and some experimental results are demonstrated. When the radar target is a conducting cylinder, the polarization effect is clear. The differential interferometric SAR has shown the high resolution of less than one centimeter in near field condition. The interferometric polarimetric SAR provides abundant information about the radar target is identified. Moreover, the potentials of three-dimensional synthetic aperture radar system are discussed.

Estimation of ground surface topography and velocity model by SAR-GPR and its application to landmine detection

The height variation of ground surface and incorrect velocity will affect imaging processing of landmine. To eliminate these effects, ground surface topography and velocity model are needed. For effective detection of landmines, a stepped-frequency continuous-wave array antenna ground penetrating radar system, called SAR-GPR, was developed. Based on multi-offset common middle point (CMP) data acquired by SAR-GPR, we describe a velocity model estimation method using velocity spectrum technique. Also after pre-stack migration, the ground surface can be identified clearly. To compensate landmine imaging for the effect created by height variation, the ground surface displacement, a kind of static correction technique, is used based on the information of ground surface topography and velocity model. To solve the problem of incorrect velocity, we present a continuous variable root-mean-square velocity based on the velocity model. The velocity is used in normal moveout correction (NMO) to adjust the time delay of multi-offset data, and also applied to migration for reconstruction of landmine image. After the application of ground surface topography and velocity model to data processing, we could obtain good landmine images in experiment.

Development of an array-antenna GPR system (SAR-GPR)

SAR-GPR is a sensor system composed of a GPR and a metal detector for landmine detection. The GPR employs an array antenna for advanced signal processing for better subsurface imaging. This system combined with synthetic aperture radar algorithm, can suppress clutter and can image buried objects in strongly inhomogeneous material. SAR-GPR is a stepped frequency radar system, whose RF component is a newly developed compact vector network analyzers. The size of the system is 30cm x 30cm x 30cm, composed from 6 Vivaldi antennas and 3 vector network analyzers. The weight of the system is less than 30kg, and it can be mounted on a robotic arm on a small unmanned vehicle. The field test of this system was carried out in March 2005 in Japan, and some results on this test are reported.

Preparation of GPR+MD sensors evaluation tests in Japan and Afghanistan

Currently there are a few projects for landmine detection in Afghanistan, which is supported by the Japanese government. Some field test for landmine detection sensors have been carried out in Afghanistan and Japan. We introduce in this paper about the plan of these projects, and its evaluation tests. JST (Japan Science and Technology Agency) which is under the Ministry of Education, Culture, Sports, Science and Technology (MEXT)) is developing unmanned vehicles which are mounted sensors for AP landmine detection. The prototype of the sensors and equipments will be ready by February 2005 and will be tested in a test site in Japan by March 2005. Then, it is planned to be evaluated in Afghanistan in summer 2005. JICS (Japan International Cooperation System) which is under the Ministry of Foreign affairs (MOFA) has a project on Developing Mine Clearance related equipment in Afghanistan. In this project, we plan to evaluate mine detectors in Afghanistan until March 2005. The evaluation test of JICS project has already started in August-Deecember 2004, in Afghanistan,. In the evaluation the both projects, we are preparing test lanes. Most of the sensors to be evaluated is a combination of a metal detector and GPR, and as for GPR, there has been not many examples of such evaluation tests. In this paper, we introduce the outline of the evaluation test, and also discuss some technical aspects of the evaluation test for the combination sensors of a metal detector and a GPR.

3-D broadband ground-based polarimetric SAR data processing for the monitoring of vegetation growth variations

SAR is usually used for airborne or space borne remote sensing. It can also advantageously be exploited in a ground-based radar imaging system named Ground-based SAR (GB-SAR). We extended earlier approaches and developed an ultrawideband, ground-based, fully polarimetric SAR (Pol-GB-SAR) system for the monitoring of vegetation growth variations. Measurements on three type trees in different conditions were carried out by the developed SAR system. We proposed effective 3D broadband Pol-GB-SAR data processing algorithms in the paper. In situ polarimetric calibration obviously improved the features of the system. 3D images were reconstructed from the acquired data by a series of signal processing procedures based on a variety of wave equation migration methods. By implementing methods of radar polarimetry, the broadband GB-SAR system has possibility for monitoring changes in tree structure characteristics due to seasonal variations. Interpreted results demonstrated the target scattering characteristics in different vegetation growth situations showed good agreement with the ground truth

Ground-based polarimetric SAR systems for environment studies

Polarimetric SAR interferometry is a compound technique that has shown an ability to extract geophysical parameters from SAR images and is useful in terrain classification and surface change detection. SAR is a well-known technique for airborne or spaceborne remote sensing. It can usefully be exploited in a ground-based radar imaging system as well. By using radar polarimetry, a ground-based SAR system can be used for monitoring seasonal vegetation changes. We have developed a ground-based polarimetric SAR system for environmental studies. It is based on a vector network analyzer, polarized antennas and a scanning control unit. We describe the ground-based polarimetric SAR system and present some experimental results of monitoring trees with seasonal variation.