An investigation of bistatic calibration techniques

Several popular bistatic calibration techniques are investigated and comparisons made between the relative merits of the various techniques. The analysis addresses sensitivity to object alignment error, sensitivity to polarization impurity, and ease of implementation. Both theoretical concepts and practical considerations are discussed based on measurements accomplished at the European Microwave Signature Laboratory of the European Commission Joint Research Center, Ispra, Italy. This facility has the capability to produce far-field fully polarimetric precision bistatic measurements in a 30-cm-diameter quiet zone, suitable for comparing different calibration methods.     

Generation of digital terrain models with a ground-based SAR system

A novel technique for the retrieval of a digital terrain model (DTM) with a ground-based synthetic aperture radar (GB-SAR) system is described. A set of SAR images collected over a test site located at Salice d'Ulzio, Italy, is used to validate experimentally the proposed DTM retrieval technique. The topographic height is obtained using an algorithm specifically tailored for a GB-SAR system equipped with two receive antennas. The comparison with an existing DTM of the test site area shows that the retrieved DTM is accurate, with root mean square height differences between the two DTMs below 5 m.     

Interference from 24-GHz automotive radars to passive microwave earth remote sensing satellites

The automotive industry is currently considering the introduction of short-range radars (SRR) operating near 24 GHz for improving road traffic safety. SSRs are intended to observe the full azimuthal space cover around a vehicle using up to eight sensors. The sensors would operate in an ultrawideband (UWB) mode, occupying 3-5 GHz of bandwidth. Interference from SRR transmitters with passive microwave remote sensing satellites used for weather and climate monitoring could occur as the result of several coupling mechanisms, including direct coupling via the transmit antenna beam and scattering and diffraction of the transmitted signals from leading vehicles, buildings, and other nearby objects. In this study, we estimate the amount of coupling anticipated to occur from SRRs, including the direct and scattered contributions. The calculations are based on bistatic scattering measurements of a typical automobile and ray optical simulations of reflection and propagation in an urban environment. Using these calculations, the maximum allowable SRR transmitted power for interference levels acceptable for meteorological and climatological remote sensing applications are quantified. The study provides criteria for SRR operation with the Earth Exploration Satellite Service on a noninterference basis.     

A novel 3-D subsurface radar imaging technique

The problem of the formation of subsurface images using stand-off forward looking radar is by far more severe than that of forming the radar images in the free-space. A subsurface image needs to be accurately focused taking into account both the refraction and dispersion of the wavefield. This paper presents a novel imaging algorithm specially tailored for subsurface sensing. A simple and effective characterization technique for the retrieval of the dielectric permittivity is outlined. The proposed soil characterization and subsurface imaging techniques are validated experimentally. Results show that the geometric distortion in the subsurface images due to the refraction and dispersion of the wavefields is successfully corrected     

Indoor wide-band polarimetric measurements on maize plants: a study of the differential extinction coefficient

A study of the wide-band polarimetric backscatter of maize plants, measured in laboratory conditions, is presented. The backscatter slant-range profiles in both linear (H-V) and Pauli basis manifest a higher extinction coefficient in the vertical channel due to the dominant vertical orientation of the structure of corn plants. The difference between the horizontal and vertical range profiles as the wave penetrates into the volume is employed to retrieve the differential extinction coefficient. In addition, the polarimetric target decomposition, as proposed by Cloude and Pottier, is used to obtain range profiles of alpha, entropy, and anisotropy. All these results reveal important features to be accounted for in the retrieval of the biophysical parameters of agricultural crops by means of polarimetric synthetic aperture radar interferometry.     

Retrieval of biophysical parameters of agricultural crops using polarimetric SAR interferometry

An existing two-layer model for forest height estimation is adapted for agricultural crops in order to develop a retrieval algorithm based on polarimetric synthetic aperture radar interferometry. This new inversion scheme is specifically tailored for vertically oriented agricultural crops, with extinction coefficients dependent on the wave polarization. Physical parameters of the vegetation scene are estimated from the location of the measured coherences in the complex plane. The proposed inversion scheme is validated experimentally with indoor wide-band polarimetric measurements on samples of corn and rice fields. Results show that the estimates of the thickness of the vegetation layer and the ground topography are reasonably accurate for a wide range of frequencies and baselines. Moreover, some interesting results are also obtained when using only dual-polarized data, which brings up new applications for present and future spaceborne missions.     

An investigation of bistatic calibration objects

Several popular metallic bistatic calibration objects are investigated, including a sphere, long and short cylinders, dihedral, trihedral, circular disk and wire mesh. Comparisons are made between the advantages and disadvantages of various objects for calibration. The analysis addresses sensitivity to object alignment error, availability of accurate radar cross section (RCS) calculations and bistatic RCS levels. Both theoretical concepts and practical considerations are discussed based on measurements accomplished at the European Microwave Signature Laboratory (EMSL) of the EC Joint Research Center (JRC) in Ispra, Italy. This facility has the capability to produce far-field fully polarimetric precision bistatic measurements in a 30 cm diameter quiet zone, suitable for comparing different calibration objects.     

Evaluation of a Near-Field Monostatic-to-Bistatic Equivalence Theorem

This paper presents the results of an investigation to quantitatively determine the limits of Falconer's monostatic-to-bistatic equivalence theorem (MBET). Falconer developed two extensions to Kell's MBET: one that is applicable to near-zone data and one that is valid in both the near- and far-zone regions. This paper encompassed collecting and analyzing both monostatic and bistatic radar cross-section data for perfect electrically conducting objects. Specifically, the authors analyzed the effects of varying the transmission frequency, scattering object complexity, and receiver bistatic angle. Objects ranged in geometric complexity from simple canonical objects to multifaceted shapes that produce multiple reflections. Empirical data collected in the far zone were compared with the analytical predictions produced by a commercially available method-of-moment (MoM) code. The code was run at X-band through K-band frequencies for a comparison with the measured data. The empirical bistatic data were then compared with the estimate produced by the MBET to ascertain the region in which the MBET approximation is applicable. Finally, the MoM code was used to produce near-field scattering predictions to facilitate the evaluation of Falconer's near-field MBET. It is shown that the complexity of the scatterer restricts the region of validity for the MBET, where shadowing and multipath interactions prevail. The disparity between the MBET accuracies for the different test objects used clearly illustrates this point.     

3-D radar imaging using range migration techniques

An imaging system with three-dimensional (3-D) capability can be implemented by using a stepped frequency radar which synthesizes a two-dimensional (2-D) planar aperture. A 3-D image can be formed by coherently integrating the backscatter data over the measured frequency band and the two spatial coordinates of the 2-D synthetic aperture. This paper presents a near-field 3-D synthetic aperture radar (SAR) imaging algorithm. This algorithm is an extension of the 2-D range migration algorithm (RMA). The presented formulation is justified by using the method of the stationary phase (MSP). Implementation aspects including the sampling criteria, resolutions, and computational complexity are assessed. The high computational efficiency and accurate image reconstruction of the algorithm are demonstrated both with numerical simulations and measurements using an outdoor linear SAR system     

Extension of the 3-D range migration algorithm to cylindrical andspherical scanning geometries

A near field three-dimensional (3-D) synthetic-aperture radar (SAR) algorithm specially tailored for cylindrical and spherical scanning geometries is presented. An imaging system with 3-D capability can be implemented by using a stepped-frequency radar which synthesizes a two-dimensional (2-D) aperture. Typical scanning geometries commonly used are planar, cylindrical, and spherical. The 3-D range migration algorithm (RMA) can be readily used with a planar scanning geometry. This algorithm is extremely accurate, preserves the phase, and corrects for the wavefront curvature. The RMA cannot be directly applied with nonplanar scanning geometries. However, as an alternative solution, we propose to backpropagate the backscattered data onto a planar aperture in the vicinity of the measurement aperture and then apply the 3-D RMA. The proposed imaging algorithm is validated both numerically and experimentally