A 225 GHz polarimetric radar

An incoherent 225-GHz polarimetric radar capable of measuring the Mueller matrix of point and distributed targets is described. The transmitter employs an extended interaction oscillator that transmit 60-W pulses of 50- to 600-ns duration. Incoherent measurements of the Mueller matrix are achieved by transmitting four linearly independent polarizations and measuring the scattered wave using a dual-polarized receiver. A novel calibration technique that requires a single in-scene reflector is presented. Polarimetric measurements are presented of a dihedral corner reflector and foliage which are the first polarimetric measurements reported at this wavelength. The foliage measurements indicate a pronounced sensitivity of the polarimetric data to fine-scale surface structure     

A calibration technique for polarimetric coherent-on-receive radarsystems

A new technique for calibrating a coherent-on-receive polarimetric radar system is proposed. A coherent-on-receive polarimetric radar is capable of measuring the Mueller matrix of point or distributed targets directly by transmitting at least four independent polarizations and measuring the vertical and horizontal components of the backscatter signal simultaneously. The technique requires the use of two calibration targets, a target with known scattering matrix (such as a metallic sphere or a trihedral corner reflector) and any depolarizing target (for which knowledge of its scattering matrix is not required) to determine the system distortion parameters. The system distortion parameters, which include the channel imbalances, the cross-talk factors of both the transmit and the receive antennas, and the phase shifts and amplitude variations of the transmitter polarizers, are determined by measuring the calibration targets for four different transmit polarizations. The validity of the new calibration technique is examined by measuring the scattering matrices of spheres and cylinders as test targets using a coherent-on-receive radar operating at 34.5 GHz. Excellent agreement between the theoretical and the measured scattering matrices for the test targets are obtained     

Fully polarimetric bistatic radar scattering behavior of forestedhills

The bistatic radar scattering measurements of forested hills were performed at grazing incidence and at azimuth scattering angles from 28° to 66° from the forward scatter plane. Using pulse-to-pulse switching between orthogonal transmitted polarizations, the radar simultaneously measures two orthogonally polarized components of the scattered wave to obtain full polarimetric information about the scattering process. These are the first fully polarimetric terrain clutter measurements to be conducted at large bistatic angles. The complete Stokes matrix, computed by averaging successive realizations of the polarization scattering matrix, is used to examine the polarization sensitivity of the bistatic clutter. It is found that the polarization state of the EM wave scattered out of the plane of incidence strongly depends on the polarization orientation of the incident electric field. Unlike the monostatic case, these two incident wave polarization states are found to produce substantially different scattered wave behavior when trees are viewed at large bistatic angles. Scattered fields resulting from vertically oriented incident fields are found to be highly polarized and to produce bistatic clutter power levels that are strongly dependent on the polarization of the receiving antenna. In contrast, horizontally oriented incident fields are found to produce weakly polarized scattered waves with bistatic clutter power levels that are insensitive to the polarization of the receiving antenna     

Multipath scattering in ultrawide-band radar sea spikes

This paper presents sea scatter data collected with an ultrawide-band (UWB) polarimetric radar system that indicates that multipath scattering plays an important role in the generation of sea spikes. The radar system used in this study produces short pulses with a bandwidth of approximately 3 GHz centered at 9 GHz for a range resolution of approximately 4 cm. Pulse-to-pulse switching allows collection of the microwave echoes produced by all four combinations of linear transmit and receive polarizations [vertical-transmit vertical-receive (VV), horizontal-transmit horizontal-receive (HH), horizontal-transmit vertical receive (HV), and vertical-transmit horizontal-receive (VH)] each of which is collected by a sampling oscilloscope utilizing equivalent time sampling. In June 1996, upwind sea scatter data at grazing angles of 10°, 20°, and 30° were collected while the system was deployed on a research pier on the Outer Banks of North Carolina. An analysis of the strongest echoes (sea spikes) from this data set is presented and discussed. First, the cumulative distribution functions are presented. Second, an increase of approximately 5 dB is shown to occur in the polarization ratio (HH/VV) of the strongest echoes as the grazing angle decreases from 30° to 10°. Third, differences in the spatial and spectral characteristics of the VV and HH spikes are described. Through comparisons with laboratory results and a simple scattering model, these observations are explained by the presence of a multibounce scattering mechanism. The use of the model to extract wave height from the sea-spike frequency response is also explored     

A new system model for radar polarimeters

The validity of the 2×2 receive R and transmit T model for radar polarimeter systems, first proposed by H. Zebker et al. (1987), is questioned. The model is found to be invalid for many practical realizations of radar polarimeters, which can lead to significant errors in the calibration of polarimetric radar images. A more general model is put forward, which addresses the system defects which cause the 2×2 model to break down. By measuring one simple parameter from a polarimetric active radar calibration (PARC), it is possible to transform the scattering matrix measurements made by a radar polarimeter to a format compatible with a 2×2 R and T matrix model. Alternatively, the PARC can be used to verify the validity of the 2×2 model for any polarimetric radar system. Recommendations for the use of PARCs in polarimetric calibration and to measure the orientation angle of the horizontal (H) and vertical (V) coordinate system are also presented     

A complete error model for free space polarimetric measurements

The authors present a complete 12-term error model for the systematic errors in polarimetric radar and antenna free space measurements for test range and laboratory use. Errors are induced by the frequency response, the channel imbalance, the coupling between the transmit channels, the coupling between the receive channels, the coupling from transmit to receive and by the residual reflections of the environment. The errors are contained in three 2×2 matrices, the isolation matrix, the transmit matrix and the receive matrix. A full polarimetric calibration with the empty room, a sphere, a vertical dihedral corner and a 45° dihedral corner is proposed. The physical understanding is supported by a cubic signal graph for the error terms     

Design and performance of a 215 GHz pulsed radar system

A high-power 215-GHz pulsed radar system developed for remote-sensing applications is described that is capable of making backscatter measurements from terrain targets at ranges of several kilometers under normal atmospheric conditions. By using separate transmit and receive antennas, the amplitudes of the polarization matrix elements may be measured conveniently. A dedicated data acquisition system was developed allowing up to 15 range gates to be sampled at 100-ns intervals. Instrument stability of ±1.0 dB yields accurate scattering measurements of a variety of terrestrial targets. System performance and calibration, together with measurements of snow backscatter coefficients made during early 1987, are discussed     

Millimeter-wave bistatic scattering from ground and vegetationtargets

A 35-GHz bistatic radar system was used to measure the attenuation through trees and the bistatic scattering pattern of tree foilage. The data were found to be in good agreement with a first-order multiple scattering model. Measurements were also made to study the angular vibration of the bistatic scattering coefficient of a smooth sand surface, a rough sand surface, and a gravel surface. The measurements, which were made for HH, HV (horizontal transmit, verticle receive), and VV polarization configurations over a wide range of the azimuth angle and the scattering angle, provide a quantitative reference for the design and use of millimeter-wave bistatic radar systems     

Full polarimetric calibration for radar cross-section measurements: performance analysis

Full polarimetric scattering measurements are increasingly required for radar cross-section (RCS) tests. Conventional calibration fails to take into account the small amount of antenna cross-polarization coupling that will be present for any practical antenna. In contrast, full polarimetric calibration takes into account and compensates for the nonideal couplings in the transmit and receive channels and paths. We use an existing full polarimetric calibration procedure and a simulation-based performance to study how well the procedure improves measurement accuracy over conventional calibration under practical measurement conditions.     

瞬态极化雷达中极化测量与校准的数学原理及实验验证

瞬态极化雷达是一种具有两路正交极化通道独立收发能力的新体制雷达,可实现目标极化散射矩阵的瞬时测量。在对传统的瞬态极化雷达目标极化散射矩阵测量原理进行分析的基础上,详细介绍了基于模糊函数矩阵的极化测量新方法,给出了相应的信号处理流程;并提出了基于单个金属球定标体的极化散射矩阵测量结果校准方法;最后介绍了基于瞬态极化雷达试验系统开展的仿真实验、外场测量实验及实验结果。     

Polarimetric detection of objects buried in snowpack by a syntheticaperture FM-CW radar

Presents experimental results of polarimetric detection of objects buried in a natural snowpack by a synthetic aperture FM-CW radar. First, the principle of polarimetric imaging in the Co- and Cross(X)-pol radar channels is outlined based on the scattering matrix and the characteristic polarization states for a specific target. Then, polarimetric measurements were carried out to detect objects buried in a natural snowpack 230 cm deep. The targets included two orthogonally placed metallic plates, an ice layer within the snowpack, and a human body. It is shown that the polarimetric FM-CW radar clearly enhances target signatures and that it serves to significantly improve detection in snowpack. Several polarimetric detection results are displayed, demonstrating the potential capability of characteristic polarization imaging and the usefulness of FM-CW radar     

A general polarimetric radar calibration technique

A polarimetric radar calibration procedure is introduced and verified with experimental results. The procedure requires measurements of three known targets in order to determine the distortion matrices that characterize the effect of the measurement system on the transmitted and received waves. The scattering matrices for the known targets can be of any form, provided that a limited set of constraints is satisfied. A special case, wherein the transmit and receive distortion matrices are the transpose of each other, is considered. This case is useful for some single antenna systems and has the advantage that only two known targets are required     

基于简缩极化数据的三分量分解模型

基于部分极化波的二分理论,提出了一种针对简缩极化数据的三分量分解模型。该模型将简缩极化数据的Jones相干矩阵分解成表面散射、偶次散射和体散射三种散射机制Jones相干矩阵之和,进一步得到这三种散射机制各自的散射功率。选取德国根多夫市普拉特灵(Plattling)地区的TerraSAR-X全极化数据生成简缩极化数据,用提出的三分量分解模型对得到的简缩极化数据进行分解,并将分解结果与全极化数据的Pauli分解结果进行对比。实验结果表明:提出的简缩极化数据三分量分解模型能够很好地描述表面散射和体散射的散射行为。但是,相比全极化数据的分解结果,提出的模型高估了体散射分量,导致偶次散射分量偏小,这个问题需要进一步的研究来解决。     

Polarimetric backscatter measurements of deciduous and coniferoustrees at 225 GHz

Polarimetric radar measurements of foliage made in the 220-GHz transmission window are described. A noncoherent, 60-W-peak-power polarimetric radar was used to measure the target Mueller matrix directly by measuring the scattered polarization state for either four or six incident polarizations. Measurements of deciduous trees indicate that waves scattered from erectophil trees (vertically oriented leaves) are more highly polarized than waves scattered from planophil trees (horizontally oriented leaves) for near-grazing incidence angles. White pine trees, which have thin needles, where the least polarized of all trees studied. A comparison of the Mueller matrices of 11 independent footprints taken from seven white pine trees showed a high degree of consistency between measurements     

Low grazing incidence millimeter-wave scattering models andmeasurements for various road surfaces

Systematic characterization of the scattering behavior of traffic targets, clutter, and their associated interactions are required in order to design and assess the performance of millimeter-wave-based sensors for automated highway system (AHS) applications. In this paper, the polarimetric radar backscatter response of various road surfaces is investigated both theoretically and experimentally. In general, it is found that the overall scattering response of road surfaces is composed of volume and surface scattering components. Previously a hybrid volume scattering model was developed for predicting the backscatter response of smooth asphalt surfaces at millimeter-wave frequencies. There, only the volume scattering was accounted for, however, experimental results show that the surface scattering cannot be ignored when the surface roughness parameters become comparable to the radar wavelength. In this paper, the previous study is extended to include the radar backscatter response of concrete surfaces, snow-covered smooth surfaces, and rough asphalt or concrete surfaces. Radiative transfer (RT) theory is used to model the volume scattering and the integral equation model is used to describe the surface scattering. Asphalt and concrete mixtures are dense random media whose extinction and phase matrices are characterized experimentally. Ice and water over asphalt and concrete surfaces are modeled by homogeneous layers. Fresh snow is modeled by a sparse random medium whose extinction and phase matrices are obtained analytically. The University of Michigan 94-GHz polarimetric radar system was used to perform polarimetric backscatter measurements of the aforementioned road surfaces at near grazing incidence angles (70°-88°). Comparison of the measured and theoretically predicted backscattering coefficients and polarimetric phase difference statistics shows excellent agreement     

Millimeter-wave radars for remotely sensing clouds andprecipitation

Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds     

Deriving the linear depolarisation ratio of precipitation from copolar radar measurements

Usually, the polarisation-dependent scattering matrix is measured with the receive and transmit antennas alternately co- and crosspolarised. For scattering parameters with negligible differential phase, however, three copolar measurements can be used to obtain the complete scattering matrix. Less stringent demands on the signal-to-noise ratio are then necessary, which is especially convenient in the case of weak radar returns.<>     

Millimeter-wave backscatter measurements on snow-covered terrain

Backscatter measurements of snow-covered terrain have been made using a polarimetric frequency-agile experimental 94-GHz radar. The radar scanned snow surfaces at radar depression angles varying from 15 to 55°. Data collection using the polarimetric and frequency-agile radar along with concurrent detailed snow characterization measurements was performed. Typical results for the temporal and spatial variability of the snow reflectivity are presented and discussed     

Performance characterization of polarimetric active radarcalibrators and a new single antenna design

Several topics associated with the use of a polarimetric active radar calibrator (PARC), which is a high radar-cross-section transponder with a known scattering matrix, are addressed. The first involves experimental measurements of the magnitudes and phases of the scattering-matrix elements of a pair of PARCs that operate at 1.25 GHz and 5.3 GHz. The measurements were conducted over a wide range of incidence angles (relative to the boresight direction) in the azimuth, elevation, 45°, and 135° planes. The 5.3-GHz PARC, which consisted of two antennas placed several wavelengths apart, exhibited symmetrical patterns with no ripples and excellent isolation between orthogonal polarization channels. The 1.25-GHz PARC, whose antennas were in very close proximity to one another, exhibited unsymmetrical patterns as well as ripples in the phase patterns, thereby introducing errors in the elements of the scattering matrix. To avoid this problem, a single-antenna PARC is designed,, using an orthomode transducer. The single-target calibration technique is extended so that it applies to the use of a PARC as well as reciprocal passive calibration devices such as spheres and trihedral corner reflectors