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ABSTRACT Conventional radar imaging techniques combine information in angle and frequency to obtain the location of the scatterers which contribute to the radar cross section (RCS) of a target. From these information, supposing that the scatterers have a white and isotropic behavior, a high resolution 2D image can be built. However, in certain circumstances (for example low frequency), the narrowness of the available frequency band and/or the frequency dependence of the scatterers may limit the resolution of the produced images. To circumvent this difficulty, an imaging technique based on multistatic data at fixed frequency is proposed. The use of monochromatic data to image a target was already studied in monostatic configuration. In this case, even if the resolution is very fine, the presence of high sidelobe which decrease slowly limits this technique to target’s reflectivity produced by a limited number of reflectors. In multistatic configuration, the situation is more favorable because weighting functions can be applied to control the level of the sidelobes. To illustrate the performances of this imaging technique, images obtained from the response of various targets measured at low frequencies are presented. Keywords: multistatic RCS, monochromatic radar imaging,
A portable, handheld imaging verification radar (HIVeR) system is designed to verify the RCS integrity of a low observable (LO) platform. The HIVeR is the latest generation to a previously designed and field-tested system (SARBAR) that produced radar images of targets in real-time. For applications with LO aircraft, an objective of the present technology is to extend the first-generation SARBAR system performance to easier use, higher sensitivity, and effective pass/fail decisions for selected regions on the aircraft outer mold line (OML). A novelty of the HIVeR design is an automatic registration scheme incorporated into the radar set. The location and orientation of the HIVeR unit is continually recorded using a precision position and orientation monitoring system. This registration process locates the handheld radar antenna position and orientation with respect to a fixed coordinate system. Similarly, the region-of-interest (ROI) on the aircraft surface is registered in this fixed coordinate system. An important feature of the new HIVeR is its capability to form calibrated radar images along a surface defined by the OML of the LO aircraft. This enables the radar to produce images that can be related to the RCS integrity of the ROI. The image along the OML can be used for pass/fail decision-making by comparing the image with a “gold standard” image for the same region.
A strong interest exists in the commercial and military sectors for small and broadband antennas. For instance, in the automotive industry there is a need for a single antenna operating in the frequency range of 825-2500 MHz (AMPS, DAB, GPS, PCS, SDARS). For military applications, there is also a need to have a single aperture which permits operation in different communication bands and can be also used for imaging and guidance applications. These needs require wide band antennas, such as the miniaturized spiral antenna. In this paper we present the implementation of a spiral antenna situated on a ground plane that is fully functional at the size of 0.16 wavelength onward. Low profile (0.05 wavelength) and broadband operation design goals bring unique challenges, which must be confronted with multiple-front techniques. A combination of antenna geometry design and material loading results in the desired miniaturization effect. Further techniques, including the use of distributed resistors ensure good axial ratio and VSWR. Pattern uniformity and phase linearity of the antenna was also improved. In addition, we also examine the effectiveness of broadband spiral antenna miniaturization as a function of loading material’s dielectric constant.
A uniquely inexpensive solution to Frequency-Modulated Continuous-Wave (FMCW) radar was developed, using low cost Gunn oscillator based microwave transceiver modules. However these transceiver modules have stability problems causing them to be unsuitable for use in precise FMCW radar applications, when just one module is used. In order to overcome this problem, a unique radar solution was developed which uses a combination of 2 transceiver modules to create a precise FMCW radar system. This FMCW radar system was then used in a small Synthetic Aperture Radar (SAR) imaging system. The SAR imaging system was composed of a 12 foot long linear track to which the FMCW radar system was mounted. The FMCW radar system would traverse the linear track, acquiring data to be used for producing SAR imagery. The combination of the small aperture length, narrow bandwidth transmit chirp, and overall frequency instability of the FMCW radar system created a number of SAR imaging problems which were unique in this application. However, it was found that when these issues were properly addressed it was possible to create SAR imagery on a low budget.
This paper presents a Radar Crossed Section (RCS) time-domain near-field measurement and its Inverse Synthetic Aperture Radar (ISAR) imaging. The target includes a pyramidal horn and a metallic aircraft scale model. A pulse generator excites the transmit antenna and a digital sampling unit collects the data at the receiving side. A time gating window is subsequently applied to reject the multiple reflections. An efficient 3-D algorithm for ISAR based on time-domain near-field data is presented. The test results for six cases demonstrate excellent ISAR images. In particular the geometry of 3-D reconstructed target can be displayed in perspective manner. The advantage of using time-domain near-field measurements is three-fold. First, it reduces measurement time in the order of one-tenth compared to frequency-domain measurements. Second, it mitigates the multiple reflection effects via time gating. Third, near-field measurements require relatively little real estate which reduces the cost tremendously since a compact range is not needed.
Radar cross section analysis essentially rely on classical spectral analysis methods. By inverse Fourier transforming the scattering coefficients, one can deduce the amplitudes and localizations of the scatterers. Unfortunately, such methods suffer from a lack of resolution since it is tied to the inverse of the extent of the data domain of interest. The use of high resolution spectral analysis can help to overcome these difficulties. Nevertheless, the expected gain of resolution is due to the enrichment of the model that is fit to the data (usually a sum of complex exponentials). One of the key point is then the order of the model, which can usually be found with appropriate criteria (MDL, AIC,…). The amplitudes and positions of the scatterers are finally estimated. The algorithm proposed here performs the detection and estimation tasks at the same time, which turns out to be more robust than conventional sequential algorithms.
This paper is designed to illustrate the technical advances in Network Analyzers and how they can be effectively utilized in an RCS test range. The Hewlett-Packard 8530A [1 - 4] has been utilized in antenna test ranges since the 1980’s and will be used as a reference comparison. Advances in network analyzer hardware and software provide increased functionality, speed and accuracy for RCS measurements. A typical RCS full polarization matrix imaging measurement will be used to illustrate these advances in technology. Range gating, digital and down-range resolution and alias-free range topics will be discussed illustrating the technical advances that can be utilized in an RCS test range. Flexibility of network analyzer hardware will also illustrate the effectiveness of reducing measurement hardware complexity resulting in an increase in measurement speed and accuracy.
This paper will compare and evaluate the results of two imaging techniques on near-field ISAR data. The two techniques are polar reformat and back propagation. The back propagation technique is a wave number technique that accounts for wave front curvature. The techniques are evaluated on simple targets at various image distances and aperture extents. Finally a suggestion is made to when the more computationally complex back propagation technique should be used.
New RCS data on two-sphere in rotation are presented. From the simple geometry, the results allow us to verify both the cross-range and down-range distance scales in imaging. With the known RCS of the individual spheres, we find that it is feasible to calibrate the image RCS scale to dBsm, provided when care is taken to mitigate the shadowing and sidelobe effects.
The task of performing reliable RCS measurements in complex environments under near-field conditions is gaining more and more interest, mainly for a rapid assessment of RADAR performance of constructive details. This paper describes a low-cost compact measurement system fully developed by IDS, that allows fast and effective acquisition of diagnostic images under nearfield conditions and far-field RCS estimation in a nonanechoic environment. The hardware of the system is composed of a planar scanner, two horn antennas, a Vector Network Analyzer and a computer. The two axes scanner allows 2D scanning of antennas in a vertical plane. For each point of a predefined grid along the scanned area, the Analyzer performs a frequency scan. The acquisition software synchronizes scanner movements with data acquisition, transfer and storage on the computer’s HDD. The software has post-processing capabilities as well. A number of focusing algorithms permit to produce 2D and 3D diagnostic images of the target as well as 2D backprojection. It is moreover possible to reconstruct the RCS starting from near-field images. Along with system features, a summary of performances and some simple targets images are presented.
A technique to identify failures in an antenna array using corporate feeding is presented. This technique combines near-field imaging close to the radiating elements and the printed transmission lines forming the feeding network. Planar near-field probing is done with a loop and a miniature dipole probe less than 0.1 free-space wavelength above the antenna under test. Two cases are considered, one where the feed lines would be separated from the radiating elements by a metal ground plane and another where the lines and the elements are on the same metal layer. In the first case, precise diagnostic based on extraction of vectorial forward and reverse wave complex coefficients on each line segment is possible. In the second case, it is not possible to extract these coefficients. However, defect localization is still possible by relying on symmetries present in the near-field maps of selected line segments.
We have investigated a method that reduces the vertical field taper at a ground-bounce radar crosssection range using a vertical antenna array. An experiment was designed were the coherent data from two measurement channels were independently recorded and stored for post processing. The two datasets were weighted and added in the postprocessing to form the extended zone with improved vertical field taper. Vertically distributed point scatterers on a special test object were used to aid in optimizing the method using imaging techniques. The method is evaluated using simulations and measurements. The usefulness of this method for RCS measurements of full-scale objects such as vehicles and aircraft is discussed. We find that the method can be used to reduce the vertical field taper over a wide frequency band in the way that theory predicts.
This paper presents an approach to experimental identification and investigation of the higher-order diffraction effects. The proposed technique allows one to determine parameters (particularly coordinates of the attachment and launching points) of the higher-order diffraction centers and can be considered as an extension of the Inverse Synthetic-Aperture Radar (ISAR) imaging technique.
This paper explores the possibility of constructing the interior field distribution of a Luneburg lens antenna through a brute-force implementation of microwaveholographic imaging. Images of the interior fields are constructed at various depths within the lens. Results from measured data, direct simulation, and the Fourier transform of simulated near-field data are compared in order to perform an in-depth comparative study.
Sensor Concepts, Inc. has developed the SCI-Xe Portable Microwave Imaging System prototype for use in the assessment of the low observable (LO) characteristics of fielded military platforms in their native environments. The SCI-Xe is a single man deployable suitcase-size system that employs a small linear rail in order to acquire Linear Synthetic Aperture Radar (LSAR) data in the 8-18 GHz frequency range. Data collections are performed via a single button push and the data is stored on a removable harddrive for comparison to an existing database for analysis. Recent deployment of the SCI-Xe prototype has provided excellent feedback on the viability of performing repeatable field measurements using alignment techniques that do not significantly affect the overall system size and weight. The SCI-Xe employs a video camera and uses video image algorithms such as edge detection, thresholding, and overlay masks to provide a simple coarse alignment to a stored baseline position. Once positioned, a single LSAR collection is performed to provide the radar data necessary for analysis, which includes a robust image registration algorithm to first, perform a quantitative assessment of the positioning accuracy and second, align the data for further image filtering and statistical processing.
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
A Ground-Based Synthetic Aperture Radar (GB-SAR) interferometer system operating at 17 GHz is used to monitor the movement of an active landslide. The selection of the optimal image formation technique for such an imaging system is addressed. The algorithms considered in this study are those previously developed for spaceborne and airborne SAR. A near-field algorithm that forms the image in the time domain is selected as the optimal solution. Furthermore, example results obtained in a measurement campaign in Schawz (Austria) are shown.
A portable handheld antenna array system (SARBAR) capable of generating high-resolution two dimensional spotlight radar images is designed and built. The design goals were to build a portable device with maximum sensitivity, that can generate zonal images of a target at close range, and produce live updates of the scene (goal of 10 image per second). To achieve the design goals, an array antenna setting with separate transmit and receive elements have been used. The radar system is based on conventional FM-CW homodyne radar. The novelty of the design, however, is that for each FM CW waveform, the signal is successively routed through all the transmit elements and received from the designated receive elements. The transmit/ receive switching is such that a complete scan over the entire frequency and aspect interval is obtained in less than 80 msec. This allows image update rates that make the SARBAR resemble a video camcorder.
Sensor Concepts Inc. has prototyped a fast, lightweight, dechirp-on-receive radar called the SCI-Lr to provide the capability of a range instrumentation radar in a highly portable package. The small weight, size and power requirements of the SCI-Lr allow a variety of new deployment options for the user including in a small general aviation aircraft or on a mountaintop that is accessible only by four wheel drive. Pulse rates up to 20 KHz enables investigation of high Doppler bandwidth phenomenon such as ground vehicle microdoppler features. The dual integration from dechirp-on-receive matched filtering in fast time and Doppler processing in slow time provides high sensitivity with low output power. Planned enhancements of waveform bandwidth up to 2 GHz , frequency operation between .2 and 18 GHz and pulseto- pulse polarization switching will provide high information content for target discrimination. The flexibility provided by the hardware is augmented by software tools to examine data in near real time to monitor data quality and sufficiency. A variety of applications are being investigated including RCS measurement, SAR and ISAR imaging, Ground Moving Target Indication, and signature collection for ATC.
Although the theory is straightforward, practical implementation of spherical near-field scanning for evaluating test chambers presents some significant challenges. Among these are the requirement for accurate probe positioning and the difficulty in minimizing support-structure blockage. We report on recent NIST efforts to mitigate these difficulties and present our most recent results.