AMTA Paper Archive
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Antenna Pattern Comparison Between and Outdoor Cylindrical Near-Field Test Facility and an Indoor Spherical Near-Field Antenna Test Facility
A new spherical near-field probe positioning device has been designed and constructed consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility located at Alenia Marconi Systems on the Isle of Wight, UK. As part of the nearfield qualification, testing was performed on a ground based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility also located on the Isle of Wight . These measurements included multiple frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program and includes discussions on the error budgets for the two ranges along with a discussion on the mutual error budget between the two ranges.
Far-Field Bistatic RCS From Near-Field Measurements
Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.
Phase-Dependent RCS Measurements in the Presence of Outliers
Coherent radar cross section measurements on a target moving along the line-of-sight in free space will trace a circle centered on the origin of the complex (I,Q) plane. The presence of additional complex signals (such as background, clutter, target-mount interactions, etc.), which do not depend on target position, will translate the origin of the circle to some complex point (I0,Q0). This type of phase-dependent I-Q data has been successfully analyzed. However, the presence of outliers can introduce significant errors in the determination of the radius and center of the IQ circle. Hence, we implement a combination of a robust and efficient Least-Median Square (LMS) and an Orthogonal Distance Regression (ODR) algorithm is used (1) to eliminate or to reduce the influence of outliers, and then (2) to separate the target and background signals. This technique is especially useful at sub-wavelength translations at VHF, where spectral techniques are not applicable since only a limited arc of data is available. We analyze data obtained as an Arrow III target moves relative to its supporting pylon. To demonstrate the effectiveness of the technique, we introduce rf interference signals into S band data and show that the uncontaminated parameters can be recovered with acceptable uncertainties.
Development of a MATLAB Toolbox to Assist in the Process of RCS Range Calibration
Over the past few years, range certification activities have become more commonplace, as industry, government and academia have embraced the process and acted to implement documented procedures at their facilities. There is now a significant amount of documentation laying out the process, as well as templates to assist ranges in developing their range books. To date, however, there have been fewer examples of useful tools to assist the ranges in better understanding how the process will affect their specific range. The authors have developed a first generation MATLAB toolbox designed to provide ranges a “what-if” capability to see the impact of specific range errors on the range’s operations. Included within the toolbox are several types of additive and multiplicative errors, as well as means of modeling various aspects of radar operation.
Cheetah PNA RCS and Antenna Measurement System
System Planning Corporation (SPC) is pleased to announce our new instrumentation radar measurement system denoted the Cheetah radar line. This radar system is based around the new Agilent PNA series of network analyzers. The PNA operates from 0.1 to 67 GHz and is utilized for making gated CW or CW RCS and Antenna measurements. The PNA has a built in synthesizer that allows the unit to be used without costly external synthesizers and external mixers. The PNA also has four identical receiving channels, two signal and two reference, that permit simultaneous co and cross pol measurements to be made. PNA IF bandwidth is selectable from 1 Hz to 40 kHz to optimize measurement sensitivity, dynamic range and speed. Using the segmented sweep feature of the PNA a single frequency sweep can be broken into segments, to further optimize the sensitivity, dynamic range, and speed. Each segment can have its own start and stop frequency, frequency step size, IF BW and power level. SPC has developed the high speed RF gating, low noise RF preamplifiers and high speed digital timing system, which allow maximum sensitivity, full up gated CW or CW radar measurements using the PNA. SPC has coupled the system to the CompuQuest 1541 RCS and Antenna Data Acquisition and Data Analysis Processing Software. This exciting new product line offers reduced cost and improved performance over current network analyzer based systems using the HP 8530, 8510, etc. Performance improvements are in the reduced noise figure, sensitivity, dynamic range and measurement speed. Measurement speeds are increased by at least a magnitude of order over the older systems and in some cases a couple of orders of magnitude.
Tracking and Classification of Vehicles and Humans Using a Ground Wave Radar
This paper presents results from a tracking and classification radar that is contained in a coffee-can sized cylinder that sits directly on the ground. The 50 mW radar operates in the 3.1 to 3.6 GHz band using horizontal polarization. The results from earlier radar propagation channel studies will be discussed, including propagation characteristics as a function of polarization and frequency band. The design for this radar that exploits the channel propagation characteristics will be described. Data from tracking of vehicles and humans will be presented. Examples of the range profiles of groups of humans and of moving vehicles will be shown. We will also show a test of the capability of such a system to track humans through building walls.
A New Gated-CW Radar Implementation
This paper describes the new ORBIT/FR StingRay Gated-CW radar implementation that provides both performance and speed improvements over those previously utilized and fielded in RCS measurement systems. The radar is implemented using one or multiple pulse modulators used to provide gating of the transmit and receive signals, in conjunction with the new class of Performance Network Analyzer recently introduced by Agilent Technologies. The radar features an order of magnitude improvement in speed over that previously offered using implementations with the Agilent 8510 or 8530 network analyzer/receiver. In addition, base sensitivity improvements are realized, and the radar is more flexible with user selection among many IF bandwidth settings now available. The physical profile of the radar is also improved, meaning that additional performance gains may be realized by creating a more efficient packaging scheme where the radar may be located closer to the radar antennas, either in a direct illumination configuration or in a compact range implementation. These factors, when considered in aggregate, result in the new ORBIT/FR StingRay Gated-CW radar offering that provides a higher performance-to-cost value trade-off than was previously available to the RCS measurement community.
Reduction of Vertical Field Taper at a Ground-Bounce RCS Range
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.
An Approach to the Evaluation of Uncertainties for Complex RCS Measurement Data
The Radar Cross Section (RCS) measurement facility operated by the Stealth Materials Department of BAE SYSTEMS Advanced Technology Centre in the UK is an invaluable tool for the development of low observable (LO) materials and designs. Specifically, it permits the effect of signature control measures, when applied to a design, to be demonstrated empirically in terms of the impact on the RCS. The facility is operated within a 3m by 3m by 12m anechoic chamber where pseudo-monostatic, co-polar, stepped frequency data for a target can be collected in a single measurement run over a frequency range of 2- 18GHz, and for a range of azimuth and elevation angles using a Vector Network Analyser (VNA). The data recorded consists of the complex voltage reflection coefficients (VRC) for the chosen range of aspect angles. This includes data for the target, mount, calibration object, and the associated calibration object mounting where significant. All data processing is conducted offline using a bespoke post processing software routine which implements software time domain gating of the raw data transformed into the time domain prior to calibration. The significant sources of type A (random) and B (systematic) uncertainties for the range are identified, grouped, and an approach to the determination of an uncertainty budget for the complex S21 data is presented. The method is based upon the UKAS M3003 guidelines for the treatment of uncertainties that may be expressed by the use of real, rather than complex numbers. However, a method of assessment of the uncertainties in both real and imaginary parts of the complex data is presented. Finally, the uncertainties estimated for the raw VRC data collected are propagated through the calibration and the uncertainty associated with the complex RCS of a simple target is presented.
Analysis of Range Ambiguity Effects in a Gated Linear FM Homodyne Receiver
Radar systems that use pulsed waveforms for detection can be adversely affected by target returns whose round-trip time of flight is longer than the radar’s interpulse period. Unless techniques such as pulse repetition frequency (PRF) jitter or pulse phase encoding are employed, the receiver has no way of determining whether a target’s range is accurate. If this radar system is being used to collect radar cross section (RCS) data, the range ambiguities may exhibit themselves as clutter and cause unacceptable levels of data contamination. A Gated Linear FM Homodyne (gated LFMH) radar modulates its transmitted signal during the time of an individual chirp, or frequency sweep, which leads to two distinct PRFs; the chirp PRF and the interchirp pulse PRF. The chirp PRF is typically very low, on the order of tens to hundreds of chirps per second, and therefore insignificant with respect to range ambiguities. It is the interchirp pulse PRF that is typically of sufficient rate to factor significantly in the processing of data collected with range ambiguities present. This paper provides analysis of the effects of range ambiguities in a typical gated LFMH radar that occur during wideband RCS data collections. In addition, a method for optimizing the radar system parameters through the prediction of the range ambiguities will be shown.
ARKEN, A Measurement System for Dynamic Full-Scale RCS Measurements and ECM Evaluations in Operational Environments
To determine the radar cross section of full-scale objects in their operational environment, and for doing countermeasure evaluations, a radar measurement system has been developed. The system is mobile and flexible and can hence be placed in different surroundings. Its main objective is to make trustworthy and accurate measurements of the RCS of ground-, seaand air targets. This is achieved by a calibration procedure that is performed in connection to all measurements. The measurement system is well suited for RCS measurements in dynamic scenarios. The system can transmit radar signals that resemble the signals of existing threat systems. This property together with the fact that the system at the same time measures both the RCS of the target and the effects of ECM make the system well suited for ECM evaluation. Measurements have been made of many different types of targets on land, at sea and in the air. Different types of ECM, e.g. chaff, has also been evaluated.
Use of a Low-Cost Compact Measurement System for the Characterisation of Backscattering from Ship Superstructure Details
In this paper, the use of a low cost compact RCS measurement system is described, aimed at the characterisation of superstructure details. This system has been installed in a large room available within a shipyard, so that the measurement process is quite simple and efficient, even though under near-field conditions. Results are relevant to radar images and RCS, and can be used for the selection of details, for the optimisation of their backscattering and/or their installation process, and for the improvement of simulation codes. Comparison with simulations is also reported.
Thermal Sensitivity of a Compact Range
The ability to perform radar cross section (RCS) measurements, where background subtraction is applied, requires a measurement system that is very stable throughout the measurement time span. Background subtraction allows the measurement of low RCS components mounted in high RCS test bodies by permitting the scattering from the test body to be removed by coherently subtracting the test body (background) RCS from the target RCS measurement. Amplitude and phase variation of the illumination signal between the time that the target and background measurements are performed will limit the quality of subtraction achievable. Modern instrumentation radars can maintain extraordinary stability when exposed to controlled temperature environments, but controlling the temperature of a large compact range can be difficult. Other components of the measurement system, such as the reflector, can also be influenced by temperature fluctuations. Methods of controlling the thermal environment can have significant consequences. Lessons learned in the Advanced Compact Range at the Air Force Research Laboratory will be described.
Test and Assessment of a Direction Finding Antenna Measured on the Nose of an MH-47A Helicopter
One basic Direction Finding (DF) technique for Radar is Amplitude Based Comparison DF. Multiple directional antennas are placed around an aircraft to get a 360 deg view of the area. By placing these antennas on the aircraft, the antennas are subjected to reflections from the aircraft, which distorts the antenna characteristics. This antenna distortion causes errors in the measurement of the angle of arrival. The work presented here describes the measurement of the antenna characteristics of a cavity backed spiral antenna both by itself and attached to the nose of an MH- 47A helicopter nose measured in an anechoic chamber. The spiral antenna’s pattern was changed when it was measured on the helicopter. The effect this change in pattern has on the DF accuracy is discussed.
Extended Imaging Technique for the Investigation of Higher-Order Diffraction Centers
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.
Re-Qualification of the Optical Alignment of the Advanced Compact Range (ACR) Using Coherent Laser Radar Metrology
Originally installed in 1992, the Advanced Compact Range (ACR) at Wright-Patterson Air Force Base was completely aligned using a Leica multi-theodolite measurement system. The Coherent Laser Radar (CLR) System provides an automated precision measurement capability which can gather significantly more data permitting a more complete characterization of the range in a relatively unobtrusive manner. This paper presents the process and results of applying Laser Radar Metrology as an optical range re-qualification tool within the Air Force Research Laboratory’s ACR.
Enhanced Frequency Selective Absorber
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.
A Combined Measrurements and Simulation Based Design of a Novel Polarimetric Array for De-Mining Applications
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.
Algorithms and Mechanics Employed for Successful Portable Imaging Via the SCI-Xe Microwave Imaging System
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.
VHF Capability and RCS Measurements from Long Cylinders
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.
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