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In some antenna applications, it is desirable to introduce an interior surface that is absorptive at one frequency, and reflective at an adjacent frequency. Even a narrow band absorber, such as iron loaded Magnetic RAM, has absorption qualities far outside its optimal absorption band. The concept is to use a conductive-backed Radar Absorber Material (RAM) covered by a band pass Frequency Selective Surface. The FSS allows the frequencies to be absorbed to pass through to the absorber while reflecting frequencies away from the pass band. The example shown in this paper was designed to absorb energy in the 2-4 GHz band, and to be reflective below 500 MHz. Design considerations include: Overall thickness; Coupling between the FSS and RAM, and Size of the FSS elements relative to the internal antenna structure. Potential applications include: broad band antennas, scatter control, and cosite interference mitigation.
Recently, remarkable efforts have been spent to develop GPR (Ground Penetrating Radar) systems able to detect shallow anti-personnel mines. In order to achieve high resolutions, large bandwidths are necessary; furthermore antennas must operate detached from ground. The paper describes how an existing surface based antenna, developed for high resolution inspection of man-made structures, has been optimized following a combined measurementssimulation approach. The novel antenna is the basic element of a polarimetric array, composed of 35 elements, that will be part of a multi-sensors demining system under development in the frame of a European Union funded project (DEMAND). Measurements have been carried out in the frequency domain, by the means of an S-parameters modal decomposition. Results concerning bandwidth, leakage, impulse response of array channels and input impedance of the basic element are reported in the paper. Comparison between measurements results and simulations are presented.
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.
In order to better understand the capability and limitation of the radar in the VHF band, we present the results from RCS measurements on simple calibration objects of sizes from small to large. Though the uncertainty for measuring a small object is usually well behaved to within +0.2 dB, the greatest difficulty for a large object is the lack of knowledge on the distribution of the incident field. Some qualitative ideas may be obtained from fieldprobes along a few directions. Yet, a thorough investigation of the field in 3-D as a function of the frequency and polarization is generally beyond time and budget constraints. For the special cases of long and thin cylinders at broadside, we find that the difference in HH-VV is very sensitive to ka, which allows us to distinguish them apart.
The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
The National RCS Test Facility (NRTF) has a variety of unique test capabilities. Looking to further expand our testing options at the Mainsite test facility, the NRTF began developing a pit/pylon and rotator shroud test bed capability that would allow for radar cross section (RCS) measurement of test articles that are physically too small to accept a rotator. To reach the desired background RCS levels, the use of an expanded polystyrene foam column was not a viable option. In order to maintain the integrity of the calibrated system and enable the measurements of test articles with and without rotator bays on the same pit/pylon, a pit/pylon and shroud combination was required. Other important considerations that influenced the viability of a pylon system include cost effective mounting/dismounting of test articles, safety of the test articles and personnel, and the effective determination of backgrounds due to a stable and low observable pylon system. Our primary goal was to design and fabricate an inhouse system that met the needs of potential customers while satisfying our own clutter and background criteria. This paper documents the fabrication of the pylon and rotator shroud test bed. The results of an RCS characterization are also presented demonstrating the system’s ability to meet the desired RCS background goals.
Free space, coherent radar cross section measurements on a moving target trace a circle centered on the origin of the complex (I,Q) plane. Noise introduces only small random variations in the radius of the circle. In real measurement configurations, additional signals are present due to background, clutter, targetmount interaction, instrumentation and the average of the time-dependent system drift. Such signals are important contributors to the uncertainty in radar cross section measurements. These time-independent complex signals will translate the origin of the circle to a complex point (I0,Q0). Such data are then defined by the three parameters (I0,Q0), the center of the circle, and st, the radar cross section of the target. Data obtained when a target is moved relative to its support pylon can be separated into phasedependent and phase-independent components using the techniques of (1) three-parameter numerical optimization, (2) least-median-squares fit, (3) adaptive forward-backward finite-impulse response procedure, and (4) orthogonal distance regression applied to a circle fit. We determine three parameters with known and acceptable uncertainties. However, the contribution of systematic errors due to unwanted in-phase electric signals must still be carefully evaluated.
We present a methodology for the design of foam columns useful for the support of targets during static outdoor radar cross section (RCS) measurements. The methodology uses modal solutions along with genetic algorithms to optimize the design of a homogeneous column with resistive layers that provides minimal scattering over the design bandwidth. The methodology widens the design space, allowing for better design trades between electromagnetic and structural column performance. Results are presented for two representative design cases (broadband and spot-frequency narrowband), and the performance of the optimized column design is shown to be significantly better than that of the baseline foam column. Further design improvements are also suggested, including the use of the Born approximation for non-axisymmetric columns.
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
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.
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000 [1]. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z- 540) Range Certification Demonstration Project. The scope of the RCS uncertainty analysis for the demonstration project was limited to calibration targets. Since that time we have initiated a detailed RCS uncertainty analysis for a more typical target measured in the ACR. A “more typical” target is one that is much larger with respect to wavelength than the calibration targets and characterized by a wide dynamic range of RCS scattering levels. We choose a 10’ ogive as the target due to the fact it is a large target, exhibits a wide dynamic range of scattering, and the scattering levels can be predicted using readily available CEM codes. We will present the methodology for the uncertainty analysis and detailed analyses of selected component uncertainties. The aspects of the uncertainty analysis that are unique to the “typical target” (i.e., a non calibration target) will be emphasized.
In real life, most radar targets are located outdoors. Here we present results from outdoor broadband RCS measurements at the X-, Ka- and W-band of “Holger”, a metallized model-scale aircraft with cavities. RCS vs. angle data in the wing plane (0° elevation) were recorded at discrete frequencies (9, 35 and 94 GHz) in both horizontal (HH) and vertical (VV) polarizations. ISAR data at 7-13, 32-38 and 92-97 GHz were acquired. Results from a 104.1 m ground range and a 162.7 m free space range will be compared.
This paper presents the design and performance characteristics of a novel active stability control capability that Aeroflex Incorporated has developed and implemented in the élan-2000 pulsed-IF instrumentation radar. The real-time technique incorporates an internal power reference loop that continuously monitors and compensates for phase and amplitude drifts within the radar RF analog circuitry through high-speed processing of the streaming data collections. Vector corrections are applied to each recorded data point, using internal loop samples of the transmit pulse from a common RF channel and digitizer, without degrading other overall system performance capabilities. Demonstrated stability levels exceed –50 dB over the full operational RF bandwidth, for periods of several hours, with environmental temperature variations of several degrees. This measurement mode provides ~30 dB of improvement over conventional instrumentation radar systems under similar test conditions, which consequently enables significant improvements in measurement applications incorporating background subtraction or where extremely stable system parameters are required.
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.
The true RF spectral response represents In- phase and Quadrature (I and Q) data in the frequency domain, and is identical to that mea- sured in many anechoic chambers including the one at Mission Research Corporation. Given a Linear FM (LFM) response, a method is derived that extracts the true RF spectral response. In the process some basic features of LFM systems are explained. The derivation depends on the assumption that the received signal is zero outside a de¯ned interval. Validation consists of applying the extraction process to both sim- ulated and measured LFM data from the ERIM DCS radar system.
The Range Commanders Council Signature Measurement Standards Group (RCC/SMSG) Performed a Demonstration program with three DOD Radar Cross Section Ranges to evaluate and improve their documentation and evaluation process and criteria documented in what is known as a "Range Book". After a successful Demonstration Program, The RCC/SMSG has embarked on the evaluation of Industry RCS Range Calibration and measurement processes and procedures and compliance with the RCC/SMSG ANSI-Z540 (ISO-25) evaluation criteria. The Lockheed Martin Helendale RCS Range was evaluated by a committee of industry volunteers appointed by the RCC/SMSG after a review of their experience and credentials. The Lockheed Martin Orlando RCS Range requested an evaluation of their "Range Book" shortly after the completion of the Helendale evaluation. Each review committee is made up of three RCC/SMSG approved reviewers, at least one of which has participated in a previous review either as a review requester or a review committee member. This paper will put forth the process used by this review committee and the lessons learned from this and previous reviews. This paper will also discuss the RCC/SMSG process for obtaining an RCC/SMSG review.
This paper refers to a special type of antenna, called frequency independent antenna, used in Stepped Frequency Continuous Wave (SFCW) radar employed for humanitarian demining. The radar transmits 128 frequencies within the frequency range from 400 MHz to 4845 GHz, in groups of 8 simultaneously transmitted frequencies. It has been built at the International Research Center for Telecommunications transmission and Radar (IRCTR), Delft University of Technology. Two Archimedean spiral antennas with opposite sense of rotation, in order to decrease coupling signal below –55dB, have been chosen. Precise antenna behavior characterization is needed because SFCW radar is phase sensitive. The paper is focused on antenna footprint measurements, translating data from frequency domain to time domain and gating in order to remove any unwanted signals. Some phase and amplitude pattern using gating measurements are presented.
Antenna tracking systems are an important part of practical system designs. The goal of antenna tracking for communication applications is to provide sufficient accuracy to limit pointing loss, while for radar applications, to determine the target’s position as accurately as possible. Antenna tracking systems are reviewed describing both open and closed loop designs. Corresponding measurement techniques to quantify system performance are described.