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This paper examines the primary differences between gated-c.w. and pulsed-i.f. instrumentation radar systems. Following a brief explanation of the fundamental theory behind each radar type, a performance trade study is presented. The impact of i.f. bandwidth on the operation and performance of the radar is presented by first briefly describing the major similarities and differences between the two radar types and the resulting impacts on performance. Differences in the gate performance, sensitivity, dynamic range, speed, and accuracy are summarized. To show the performance advantages and shortfalls of each radar type, benchmark test scenarios are presented. The resulting summary can be used as a guide in determining the optimal radar type for a specific range geometry and measurement requirement.
This paper considers hardware and software issues associated with accurate RCS, antenna, and near field antenna measurements. In particular we examine methods for making accurate measurements at high speed using existing network analysis equipment, such as the HP8510B. Techniques which allow for fundamental mixing are examined from the viewpoint of enhanced dynamic range and speed. Harmonic mixing techniques are also discussed and limitations related to IF bandwidth and harmonic locking are presented. The realtime requirements of software systems for these applications are presented and operating system considerations are analyzed. Interface attributes are examined with a view toward use with multi-tasking operating systems and the real-time requirements of high speed measurement systems.
In order to justify the expenditure for capital equipment such as a microwave receiver, it must be shown that the instrument provides the best value to the user. The best value for a microwave receiver for measuring today's complex microwave antennas dictates that the receiver be versatile enough to adapt and operate over a diverse set of applications and performance specifications. Some important characteristics to consider when evaluating a microwave receiver's value is measurement speed, frequency agility, number of measurement channels, remotability, dynamic range, ease of operation, and system integration. This paper addresses the development and important characteristics of a high speed microwave receiver that was designed to provide users with maximum productivity, and therefore, the best value for a microwave antenna measurement receiver. Receiver characteristics such as acquisition speed, frequency agility, number of measurement channels, controls, interfacing, and versatility are discussed.
Precise and complete measurements of advanced electromagnetic systems demand dramatically higher data acquisition speeds than those commonly attainable. Specific challenges include requirements for wideband measurements with arbitrarily spaced frequency steps. These types of measurements are often encountered in characterizing EW/ECM systems, radars, communications systems, and in performing antenna and RCS measurements. The Scientific-Atlanta Model 1795 Microwave Receiver offers capabilities directly applicable to solving measurement problems posed by highly frequency agile systems. These problems include: 1) timing constraints 2) data throughput 3) RF interfacing 4) maintaining high accuracy A technique is discussed which shows the application of the Model 1795 Microwave Receiver in its high frequency agility mode of operation. Measurement examples are presented showing the advantages gained compared to previous methods and instrumentation configurations.
This paper examines the economic justification process for a new Antenna or Radar Cross-Section (RCS) measurement system, and presents the techniques that can be used to determine the financial feasibility of a new system. Specific examples are given that will allow engineers to customize calculations to fit their company's specific accounting methods and labor rates.
This paper presents the productivity improvements that are possible in complex antenna measurements using state of the art instrumentation. The productivity improvement is calculated for a hypothetical antenna, and from this productivity improvement manufacturing cost reductions and payback times are derived.
The MUSIC (Multiple Signal Characterization) algorithm uses an eigenvector decomposition of measured data to classify signals in the presence of noise. It has been used for the angular classification of multiple radar signal emitters and ISAR imaging. Interest has grown in stray signal analysis in anechoic chambers. This paper will discuss the modification and use of the MUSIC algorithm for the decomposition of field probe data to angular spectrum. A brief discussion of the MUSIC algorithm theory will be presented. Modifications required for use in compact range angular spectrum analysis will be discussed in detail. Requirements on field probe measurements will be presented as well as their effects on the implementation of the algorithm. Both one way and two way measurements are considered for their relationship to the array manifold. Finally, some experimental validation generated on the Boeing range will be presented.
A major concern for any user of a compact range for RCS or antenna measurements is the quality of the wavefront over the quiet zone and background chamber levels at the desired frequency band. Amplitude and phase ripple in the quiet zone is an indicator of how well the electromagnetic energy is collimated coherently by the reflector system. The amount of ripple depends on the reflector system, reflector edge treatment to reduce diffraction, frequency band and chamber interactions. Edge treatment techniques such as serrations on the reflector edge helps to reduce diffraction of unwanted energy into the quiet zone. Constructive and destructive interference of diffracted of energy in the quiet zone causes the amplitude and phase ripple. The goal is to reduce the ripple to a minimal amount. Previous studies by the author have compared two-way and angle transform field scanning techniques. The results strongly indicate that both techniques provide good agreement. The two-way method has the disadvantage of strong dependence on the scanning target directivity. A directive target will tend to disregard diffraction from the reflector edges because of its low sidelobes. Its advantage is that there is no need for external mixing equipment for the microwave receiver. The angle transform is simple in configuration consisting of a narrow flat plane or bar mounted on an azimuth positioner and rotated. The disadvantage is a summing of energy in the zero-doppler cell yielding an artifact ripple. Both of these methods also depend upon software gating algorithms including gate shape and width which directly influence the amplitude and phase ripple. The aim of this study is to compare the two-way, one-way field scanning techniques and the angle transform method. Can comparisons be made between the methods? Can a fairly good agreement be made? Are multi-path considerations addressed in one-way scan techniques? Hughes Aircraft will use one of the compact ranges at the Antenna Test Facility of Motorola GEG at Scottsdale, Arizona with the March Microwave (Vokurka) dual reflector system. Field scans will be measured using both the two-way and one-way techniques. The two-way method will use the 8 cm diameter disk as the scanning target, mounted on a horizontally traversed scanner. The one-way method will use a standard gain horn mounted on the same scanner. The angle transform method will use an 8 ft narrow flat plate rotated in the quiet zone. The field scans will be measured and studied at 10 GHz.
Accurate calibration methods are of essential importance in RCS measurements. First, absolute RCS determination (in dBsm) can be carried out accurately provided a correct algorithm is used describing the RCS dependence of some reference target at all frequencies. Unfortunately, this technique gives error-free calibrated data at one position only. In this paper a new technique for qualifying of RCS ranges will be described. A reference target with well-known RCS response is used during the calibration measurement. The amplitude and phase distributions are then computed for all required positions within the test zone. Finally, an error estimate in measured RCS responses can be made by using two other application programs.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. The planewave spectral (PWS) probe technique is one proposed method for identifying the location and magnitude of range scattering. This paper presents the results of a PWS probe of a compact range. The interpretation of the PWS plots is discussed in comparison with the range geometry. Nine separate scattering centers are identified. The meaningfulness of the PWS picture was tested by introducing a known dipole source.
In this paper the initial construction and validation phase for a new elevated outdoor antenna range is described. The facility is designed to provide excellent pattern, gain and reflection measurements in the 20 MHz to 40 GHz frequency range for apertures and arrays up to D = 16 feet in length. Shown in detail is a physical description of the facility and equipment, an error budget and the results of field probing and antenna measurements. A discussion of the results shows a facility capable of antenna measurements at S/N levels of 60 dB providing a dynamic range of over 40 dB with error levels less than plus-or-minus 0.44 dB. Throughout the discussion, special attention is given to the full automation of the range in Phase 2 and its possible use for radar cross section measurements.
In an industrial park, a clear area sufficient for antenna measurements is very hard to find and even more difficult to justify. The solution to this problem was to use the roof of a large industrial building. To avoid the reflections from industrial stacks used for air conditioning and spray booths and to provide a flat surface sufficient for good ground reflection operation, an elevated ground reflection antenna range was constructed. The range consists of a ground screen 40 feet by 110 feet mounted on a metal framework 14 feet high. A telescopic source antenna tower is located at one end of the range, and an azimuth-over-elevation antenna positioner with a model tower is located on a raised platform at the other end of the range. The range was evaluated using a lightweight field probe and the experimental data compared to calculated data derived from the NEC-BSC2 computer code. An analysis was made of the probe data defining the sources of extraneous energy and their possible reduction. Pattern comparison data is given to illustrate the correlation between the field probe data and the actual uncertainty experienced in making UHF antenna pattern measurements on the elevated ground reflection range. Finally, planned physical improvements to the range are discussed.
Georgia Tech Research Institute has designed, developed and installed a large outdoor compact range for the U.S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. Some of the unique hardware and software developed as part of the instrumentation and computer control tasks for the compact range are described.
At Southwest Research Institute, an automated antenna performance evaluation system has been developed for evaluation of mast-mounted direction finder antennas. This system utilizes a dual-channel receiving system and IF processor with off-line antenna pattern analysis software. Antennas are mounted on a test range which includes computer-controlled antenna positioners, test frequency transmitters, and a data acquisition equipment group. Amplitude and phase data is digitized and recorded for automated off-line antenna performance evaluation. The evaluation software provides a Fourier analysis of the antenna patterns which characterize distortion, alignment, relative phase relationships, amplitude mismatch, and bearing deviations (from theoretical values) for each antenna array.
The Vulnerability Assessment Laboratory (VAL) anechoic chamber at White Sands Missile Range, New Mexico was reconfigured and refurbished during the last part of 1988. This paper will be a facility description of the state-of-the-art Special Electromagnetic Interference (SEMI) investigation facility. Electromagnetic susceptibility and vulnerability investigations of US and, in some cases, foreign weapon systems are conducted by the EW experts in the Technology and Advanced Concepts (TAC) Division of VAL. EMI investigations have recently been completed on both the UH-50A BLACKHAWK and AH-64A Apache helicopters in the chamber. The paper will cover the facility's three anechoic chambers, shielded RF instrumentation bay, computer facilities for EM coupling analyses, and the myriad of antenna, antenna pattern measurement, amplifier, electronic, and support instrumentation equipment for the chambers. A radar cross section measurement and an off-line RCS data processing station are also included in the facility.
An overview of non-destructive real-time testing of missiles is discussed in this paper. This testing has become known as hardware-in-the-loop (HIL) simulation because it involves the actual missile hardware.
Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.
The purpose of this paper is to discuss the accuracy requirement of a generic measurement system for in-flight antenna pattern evaluations. Elements of the measurement technique will be described. An attempt is made to distinguish the measurement requirement for a narrow beam radar antenna in contrast to that for broad beam communication antennas. Major elements of the measurement technique discussed include the flight path geometry, the multipath propagation problem, and the measurement errors. Instrumentation requirements consist of the ground segment, the receive and the tracking subsystems, and the airborne equipment, the radar components and the navigation and attitude sensors. Considering the in-flight antenna pattern testing as a generalized antenna range measurement problem, various sources of measurement errors are identified. An error budget assumption is made on each error component to estimate the overall expected accuracy of the in-flight antenna pattern measurement.
This paper presents the results of a measurement program conducted to evaluate antenna coverage from a constrained-space airborne platform. Two levels of coverage are reported: Level 1 details coverage without the aircraft, but over a small groundplane, while level 2 includes aircraft effects. Coverage and gain for each polarization was evaluated by quantizing the data and computer processing to obtain a measure of goodness vs frequency. In particular, horizontal polarization showed considerable changes, as expected, compared to level 1 performance. Vertical polarization showed smaller, but significant changes.
Ray Proof has recently completed the construction of a shielded anechoic chamber in the Air Force Anechoic Facility at Edwards Air Force Base in California. Measuring 250 feet by 264 feet x 70 feet high, it is believed to be the largest anechoic chamber in the world. The facility will be used for EW testing of full-scale aircraft such as the B-1 B and B2 and will be operated for the Air Force by Rockwell International, the prime contractor for the project. This paper discusses parameters, statistics, and design features. The shielding was designed and quality controlled during construction in order to meet the NSA 65-6 specification, modified to extend to 18GHz. Layout of pyramidal anechoic material, varying from 12 inches to 24 inches in thickness with 36 inch around lighting fixtures, was designed to meet a return loss specification of 72 dB at 500 MHz, and up to better than 100 dB in the 3-18 GHz region. The chamber features a sliding pocket door 200 feet long and 66 feet high. To meet the stringent NSA 65-6 requirement, a threefold inflatable-bladder/ fingerstock seal was used around the door. The other feature of the chamber is an 80 foot turntable with a separately shielded control room suspended beneath. The table can rotate a 250,000 pound load through plus-or-minus 190 degrees, positioning to an accuracy of plus-or-minus 0.1 degree. A number of innovative procedures such as locating a portable factory to manufacture the absorber near the construction site enabled Ray Proof to complete and test the chamber ahead of schedule.