Processing techniques for removal of target support contamination

The paper presents a new inverse synthetic aperture radar (ISAR) algorithm intended for radar cross-section (RCS) imaging and measurement from scattered fields. The method, based on a spherical-wave near-field illumination of the target, overcomes the requirement for an expensive compact range facility to produce a plane wave illumination. The formulation and the implementation of the algorithm are described. Some experimental results obtained in an anechoic chamber are presented to show RCS results similar to the conventional plane wave methods.

A brief overview of radar cross section (RCS) measurement techniques is presented. Parameters to be measured include: RCS versus target aspect angle; RCS versus frequency; high-resolution range profiles (one-dimensional images); and high-resolution two-dimensional inverse synthetic aperture radar (ISAR) images. In this review, the measurement radar is limited to ground-based radar systems. Targets include: operational full-scale ships and aircraft; full-scale aircraft mounted on pylons; scale model aircraft mounted on pylons; and scale model ships in water. The review includes compact ranges in anechoic chambers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Inverse synthetic aperture radar (ISAR) imaging is typically useful when there is a need to classify, recognize, or identify a moving target of interest. In fact, an ISAR image highlights two-dimensional (2-D) geometric features of a target, which can provide indications of target's type, size, and other salient information. Such information can be then used for target classification, recognition, and identification. In this chapter, we will provide five case studies on applications of ISAR imaging. Such case studies are chosen to diversify the type of application and to give additional indications about how to interpret ISAR images. Specifically, in the first case study (Section 12.1), we show an example of ground-based ISAR images of a noncooperative sailing ship. The ISAR techniques used to form the ISAR images are the range-Doppler (Chapter 3) and the ICBA (Chapter 5) techniques. Different time windows are chosen to show the effects of the coherent processing interval (CPI) length on the ISAR image. Since the radar is ground based, this represents a case where the ISAR image is formed by exploiting only the sea surface induced target's motions. The second case study (Section 12.2) refers to a scenario where the radar is carried by an aircraft and the target is a ship at sea. In this case, both the target and the platform motions concur to the ISAR image formation. Also in this case, the range-Doppler and ICBA are used to form the ISAR image sequence. The third case study (Section 12.3) concerns dual ground-based/satellite ISAR imaging of a noncooperative sailing ship. In this experiment two radars are used to image a sailing ship at the same time. Two techniques are here used to form the ISAR images: a refocus technique (see Chapter 8) to form the satellite ISAR image; and a range-Doppler with ICBA to form the ground-based ISAR image. In this case study, we show how a dual system is able to form ISAR images of the same target with different target's views and projections. This is a simple case of multiple-perspective ISAR images, which may be looked at as a way to improve a target's classification and recognition [1]. The fourth case study (Section 12.4) shows ISAR images of four aircrafts obtained by processing data acquired with a ground-base radar. This case study may be regarded as the classic ISAR imaging example. Four different airplanes have been selected to show how their characteristics are mapped onto ISAR images. Such characteristics can then be used by classifiers for recognition purposes.

Radar cross-section (RCS) is an important study parameter for defence applications specially dealing with airborne weapon system. The RCS parameter guides the detection range for a target and is therefore studied to understand the effectiveness of a weapon system. It is not only important to understand the RCS characteristics of a target but also to look into the diagnostic mode of study where factors contributing to a particular RCS values are studied. This further opens up subject like RCS suppression and stealth. The paper discusses the RCS principle, control, and need of measurements. Classification of RCS in terms of popular usage is explained with detailed theory of RF imaging and inverse synthetic aperture radar (ISAR). The various types of RCS measurement ranges are explained with brief discussion on outdoor RCS measurement range. The RCS calibration plays a critical role in referencing the measurement to absolute values and has been described.The RCS facility at Reseach Centre Imarat, Hyderabad, is explained with some details of different activities that are carried out including RAM evaluation, scale model testing, and diagnostic imaging.Defence Science Journal, 2010, 60(2), pp.204-212, DOI:http://dx.doi.org/10.14429/dsj.60.341

The far-field radar cross section (RCS) measurement of a large scatterer is generally difficult due to the limitation of available measurement range. One way to avoid this difficulty is to adopt a scale model by compact range measurement. However, there are some problems in this measurement: it is labour intensive and expensive to construct the scale model, and the frequency characteristics of the surface impedance of the scatterer may not be reproduced at shorter wavelength measurements, so real target measurement is necessary to obtain accurate data. There exists a near-field antenna measurement technique, however a direct application of this technique for RCS measurement fails because the RCS depends on the locations of both the source and the observation points. Falconer (1988) has overcome the above difficulty and successfully estimated the far-field RCS by the near-field RCS measurement. We have also proposed a far-field RCS prediction algorithm which uses cylindrical or planar near-field RCS data. However these algorithms are applicable only for the monostatic RCS. We report novel prediction algorithms of the far-field bistatic scattering cross section (SCS) by extending the proposed algorithms. These are presented in cases of spherical, cylindrical and planar scanning. We also show simulation results of the far-field SCS prediction in order to validate our methods.

A measurement method and system based on conventional radars has been proposed to address the problem of radar cross-section (RCS) measurement of ship targets in open seas. Calibration errors for RCS measurement on ground plane range (GPR) are analysed, and the error elimination method and accurate RCS calculation formula are given. An RCS measurement system is designed by improving conventional radars. These systems measure two Luneburg lens reflectors of different sizes on the lake. The average RCS of the big Luneburg lens reflector is obtained, and the error is calculated to be 2.73% compared with its actual value, which verifies the rationality of the proposed measurement method and system. The system measures the RCS of a ship in the open sea and obtains its circumferential RCS distribution. The effect of the error in the ship's position calculation on the RCS measurement results is analysed, and the uncertainty range of the error is obtained.

In this paper, we propose and investigate a model based on asymptotic methods, designed to compute the radar cross section (RCS) of complex targets. A combined method of physical optics, geometrical optics, and the equivalent current method is applied to establish an effective backscattering analysis procedure. The scattering model includes shadowing effects, multiple bounce, and diffraction by edges. This model is used for RCS estimation and to generate inverse synthetic aperture radar (ISAR) raw data for imaging applications. We review the theoretical aspects and describe the proposed model and experimental setup in detail. Numerical results are provided and compared with the experimental results obtained in the anechoic chamber of ENSTA Bretagne to validate the approach through the computation of the RCS for canonical objects and a generic boat. Finally, a parallelepiped representation of the RCS is presented to be used for ISAR imagery, and the first results concerning the ISAR imaging of a generic boat are provided.

Radar cross section (RCS) measurements are widely employed for the target identification of objects such as aircraft. The RCS of large, complex geometries is usually determined in a well defined, scaled experimental setup. In such a setup, the measuring frequency equates the frequency of interest multiplied by the scaling factor of the model. For high scaling factors, frequencies well above 100 GHz result so that conventional microwave equipment reaches its technological limits. In this paper, we discuss the use of a fiber coupled terahertz time domain spectroscopy system for angle dependent RCS measurements. Several measurements on simple objects like metal plates with corresponding simulations are performed. We also investigate how angular misalignment and displacement of the scatterer effects the measurement results. Moreover, we discuss the influence of non planar incident wavefronts and compare experimentally obtained data to simulations using an enhanced physical optics model.

This communication deals with radar cross section (RCS) measurement of a horn antenna within a reverberation chamber (RC). RCs recently became a viable alternative to anechoic chamber to perform RCS measurement. Indeed, one can benefit from the RC statistical properties to retrieve the line of sight wave among the multiple reflections that occur within the cavity. It is the first time that this RC setup is used to measure the RCS of an antenna. More specifically, it is shown that it is possible to distinguish the antenna RCS measured for two different load conditions (short and match). As a consequence, it enables retrieving the antenna gain from RCS measurement performed in RC.

It is challenging to perform the near-field to far-field transformation with a high precision for the multiple-scattering targets using the conventional monostatic radar cross section (RCS) measurement. In this paper, a novel near-field to far-field transformation method with bistatic RCS measurement is proposed. First, the RCS measurement is perceived as a double radiation model. Doing this, the relationship of near-field data and far-field data at different angles is derived, and the convolution function is given under the cylindrical wave. Second, the near-field scattering data are acquired using the proposed bistatic RCS measurement method that combines one transmission and multiple receptions with the rotation of a revolving table. The near-field data are then extrapolated twice with the convolution function, and the far-field RCS are obtained accordingly. Moreover, this paper also analyzes the truncation angular and sampling intervals for the measurement consideration. Finally, the effectiveness and the high precision of the proposed transformation approach for the multiple-scattering target are demonstrated using the numerical simulations and the experimental characterization with an example of dihedral structures.

We propose a target identification scheme exploiting the temporal dependency and spatial structure of radar cross-section (RCS) measurements in low-frequency, passive, or long-range surveillance radar systems. We employ target RCS modeling integrated into a hidden Markov model (HMM) with state-duration modeling. Assuming that the radar system ensures a sufficient sampling time, it is possible to use the spatial characteristics of airborne targets, because there is consistency between successively sampled RCS measurements. In addition, to exploit the whole temporal characteristic of the sequence of RCS measurements,which has rarely been considered in the literature, we adopt the HMM and target RCS modeling. To accomplish this task, we accurately develop target RCS models and establish the relationship between the target-sensor orientations, which are the hidden states of the HMM, and the correspondingRCSmeasurements. The proposed target identification scheme, which only uses the sequence of RCS measurements, is demonstrated with simulation results and an analysis for various signal-to-noise ratios and target-fluctuation models.

A linear-scan-based wideband near-field radar cross section (RCS) measurement technique for achieving wide effective-angle-coverage is proposed in this letter. Both the data preprocessing and linear near-field to far-field transformation (LNF2FFT) techniques are used to ensure the accurate measurement of wideband far-field RCS in a wide azimuthal angle range. With the linear-scan-based near-field sampling strategy, the object under test (OUT) keeps stationary during the measurement process, and the equipment of turntable is not required. Moreover, based on the 2D plane wave expansion, LNF2FFT is proposed to translate the linearly sampled near-field data into far-field RCS pattern. An adaptive time gating strategy is also proposed to deal with the varied background property in liner samplings. Serval important practical factors relating to the measurement accuracy, such as phase center calibration, background subtraction and calibration, are also discussed in detail. Two illustrative test examples are presented, in which RCS measurement with wide effective-angle-coverage of ±35° and the average wideband measurement errors less than 0.4 dB are achieved in 1 m limited measurement range.

The single parabolic cylindrical compact range based on a linear array feed achieves a high-quality quiet zone through optimization of the array excitation. Its capability for testing automotive antennas has been demonstrated. As a plane wave generation device, Radar Cross Section (RCS) measurement is considered an important potential application. Furthermore, the high aperture efficiency offered by the linear array excitation optimization provides cost advantages, particularly for RCS measurements of large equipment. However, to date, there are no reported RCS measurements s specifically using the single cylindrical compact range with a linear array feed. Therefore, this paper presents RCS scattering tests using the line array compact range with optimized amplitude and phase excitation. The scattering test method is detailed, employing MIMO technology for interleaved transmit and receive operations with the linear array. The measured RCS results demonstrate strong consistency with simulation results, thereby confirming the feasibility of this approach for RCS measurement.

A free-space measurement system at 220 GHz ∼ 300 GHz has been built with stepped frequency continuous wave (SFCW) for Radar cross section (RCS) measurements. The system consists of vector network analyzer, Millimeter-Wave Converters and target support structure. The measurement techniques such as background subtraction, synchronous calibration and range distance compensation are verified by RCS measurement data. The system measurement accuracy is control within about ±3 dB at −21 dBsm.

Spatial filtering is a useful method to suppress undesired reflection from unwanted scatters in Radar Cross Section (RCS) measurements. Actually, it is difficult to prepare an ideal field which satisfies the far-field criterion for RCS measurements of large targets. We applied the filtering method to a bistatic RCS measurement in a near field and investigated the validity of that method by varying the scanning angular span. Electromagnetic simulations show that predicted RCS profiles from near-field data with unwanted scatters closely matched far-field reference data of the test target. In conclusion, the results show that the method is effective for bistatic RCS measurements in practical field enviroments.