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In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software. Accurate alignment of the range is enabled by a tracking laser interferometer. This composite nearfield scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coodinate systems. Comparison data are presented.
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.
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 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.
This work is part of the UK Ministry of Defence initiative to examine causes of uncertainties in RCS measurements and to establish a network of certified facilities. Having developed a ‘best practice’ guide where causes of uncertainty were listed, the effect of polar diagram was selected as a priority topic. Correction algorithms for RCS measurements require knowledge of the beam shape and resolution in crossrange of the significant scatterers. Accordingly, the accuracy of polar diagram measurement, the effect of amplitude ripple and the applicability of the correction algorithms to near-field data were addressed. Measurements were made on two targets, a long cylinder and a small aircraft. Two antennas and two ranges were used to achieve 1dB, 3dB and 6dB illumination tapers across the cylinder. The 6dB taper situation was modelled for three different numbers of points. The work demonstrated that polar diagram effects are significant for point scatterers or simple targets, like the cylinder; however, for the small aircraft with a large number of distributed scatterers, the overall effect is less significant.
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.
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.
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.
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.
The RFDM (Radio Frequency Development Model) of the PLANCK satellite has been tested in the Alcatel Space CATR (Compact Antenna Test Range) in 2002. The antenna was constituted by a telescope designed by the University of Minnesota for the Archeops balloon borne payload, and corrugated horns manufactured by electroforming process. At the beginning, characterization of the quiet zone of the Compact Range with a planar scanner is presented. A full amplitude/phase/co-pol and amplitude/cross-pol discrimination probing of a 5m x 5m quiet zone at 100 GHz is displayed. Then, we focus on measurements of the antenna response at 100 GHz performed in 4ð steradian with a dynamic range better than 100 dB. We cross-validate the measurement results with the RF predictions of the numerical model using the GRASP8 software developed by TICRA.
SOLANGE is a large RCS indoor measurement facility operated at SHF and V/UHF frequencies. In the V/UHF band, couplings between the target and the walls can be exhibited. These perturbations due to non-directive transmitting/receiving antenna, and non-absorbing walls must be eliminated to derive the intrinsic response of the target. To reduce their levels CELAR introduced smart methods («SAV »: Site Altitude Variable and « EAV »: Environnement Altitude Variable): the transmitting/receiving antenna (and also the target in the EAV method) is translated along the elevation axis, and the acquired data are averaged. CELAR and CEA collaborated to qualify the chamber in the U/VHF band. The aim of the study is to identify and quantify the error sources, and to suggest some improvements. The analysis, based on RCS measurements of canonical targets, includes data processing (clutter reduction) and evaluation of the effects of SAV and EAV on the couplings. A theoretical algorithm is used to assess the performances of the processing, and to optimize measurement altitudes. It introduces an analytical model for the antenna and its images with respect to the walls, and calculates the scattered near field. This study enabled us to suggest improvements in the parameters of the processing, as well as in the RCS facility configuration.
Undesirable antenna to antenna coupling has caused EMI problems between the WSC-6 SATCOM system and various systems in many shipboard installations. Long term solutions are currently being explored to resolve this EMI problem, which include adaptive interference cancellers and redesign of the WSC-6 feed and subreflector. However, these solutions are expensive and require several years to develop. An intermediate solution using RAM shrouds around the main reflector and subreflector edges of the WSC- 6 antenna has been proposed. The RAM shrouds were designed to reduce the spillover and diffraction of the antenna while having minimal impact on the antenna performances. A lightweight RAM was chosen to minimize the weight increase of the antenna. A prototype unit with the proposed modifications has been fabricated, assembled and tested in a tapered anechoic chamber, a near-field range, and a compact range. Significant reductions in the WSC-6 antenna sidelobes and backlobe have been verified via these measurements. Highlights of these modifications are described. Measured data (near field, compact range, tapered chamber, and shipboard) are presented.
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.
An advanced method of phased-array antennas (PAA) performance monitoring is presented in this paper. It allows receiving more accurate results for PAA with binary phase shifters. The special attention is paid to such practical aspects of design and application of built-in performance monitoring systems, as the algorithm of controlled channel signal isolation, choice of the location of probe antennas and transfer matrix calibration which is necessary for PAA performance monitoring at system level.
Wide frequency bandwidth feeds are used in compact ranges when multi-octave bandwidth operation of the range is desired. Dual-ridge or quad-ridge horns have been widely used in RCS applications as well as in antenna measurement applications to achieve wide band operation. This selection is made to take advantage of the lower cost of quad-ridge horns vs. other options. In designing a compact range, one primary concern is the beamwidth of the feed over the operating band. This affects the amplitude taper across the quiet zone of the range. Another primary concern is the movement of the phase center vs. frequency of the feed. This directly affects the phase taper across the quiet zone as a result of de-focusing of the reflector. Here we present measured data of the beamwidth and phase center movement vs. frequency of a wide-band quad-ridge feed designed to operate from 2.0-18.0 GHz. Measured and predicted quiet zone performance data over this bandwidth are presented with the feed installed in a Model 5751 compact antenna test range having a 4-foot quiet zone.
This paper describes a fully automated test set-up for obtaining spacecraft payload end to end system performance including EIRP, saturation flux density, transponder group delay, phase noise, carrier to noise ratio and G/T in a Compact Antenna Test Range (CATR). The RF signals are routed to and from the appropriate antennas by RF switches under the control of the test software. Spacecraft architectures vary widely often requiring adaptation of the test software. To enable this, it was decided to use an interpreted public domain language (Tcl/TK and IncrTcl) as the basis for the test programs to achieve easy customization without the need to recompile the test code. The test set-up is being implemented in the ESTEC Compact Payload Test Range.
Transverse electromagnetic (TEM), time-domain antennas are designed to reproduce accurately the time characteristics of received impulsive fields. For minimal distortion, the antenna response to the fields must be constant in amplitude and linear in phase. We are using numerical techniques are employed to improve previous designs by simulating modifications to the original design without costly, time-consuming hardware alterations.
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.
ORBIT/FR has implemented several antenna receiver systems covering a wide range of frequencies using low frequency Network Analyzers (NWA). The low frequency NWA has been utilized for low frequencies in the NWAs design frequency range (typically up to 4 GHz) as well as for higher frequencies using external transmit and LO sources, external multipliers and fundamental or harmonic mixers up to 75 GHz. Such configurations can be much more cost effective in comparison to preconfigured high-end systems. The technical advantages are user defined IF frequencies, integrated low frequency operation, simultaneous dual channel operation and narrow band high speed frequency switching. Through the right selection of external components, performance parameter as frequency range, dynamic range, sensitivity and measurement speed can be optimized in relation to cost. Also a very flexible upgrade path is possible from basic to complex systems.