Performance comparison - gated-C.W. and pulsed-I.F. instrumentation radars
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.
High speed, multi frequency measurements
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.
Design of a short range for testing large phased arrays
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.
Requirements for accurate in-flight pattern testing
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.
The World's largest anechoic chamber
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.
Accuracy in RCS calibration techniques
An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.
Methods of transforming antenna Fresnel region fields to far region fields
For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.
A Low cost portable near-field antenna measurement system
Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer. The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50 dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.
Automated multi-axis motor controller and data acquisition system for near-field scanners
The National Institute of Standards and Technology (NIST) has developed a multi-axis controller and software data acquisition system that has improved probe position accuracies in near-field scanning. This extends the usefulness of the NIST planar near-field scanner to higher frequencies. This system integrates programmable power supplies into an existing planar measurement system with new software that controls the power supplies and the data acquisition. It provides the higher positioning accuracy required for millimeter wave measurements at a reasonable cost.
This system uses the NIST planar near-field scanner's existing DC motors, computer and laser. The programmable power supplies are connected to the motors, with a separate power supply for each motor'a armature and a common power supply for each of the motor's field windings. This allows for concurrent movement in each axis and eliminates delays in switching between axes. Directional control, motor protection, and special software features are implemented by logic control.
There is often a need for a laboratory to make quick, inexpensive, and accurate measurements on individual absorber samples. Different types and sizes of absorber need to be quickly analyzed at several frequencies to determine which type best maintains or improves the facility's RF characteristics. The National Institute of Standards and Technology has devised an improved version of the Doppler shift method to measure the scattering levels of different sizes and types of microwave absorber. This technique is useful as an inexpensive and simple method for measuring individual absorber pieces with good accuracy and sensitivity. The system does not require a large anechoic facility nor a sophisticated measurement system for gating out background scattering. Reflectivity levels on the order of -80 dB can be measured and relative changes of 1 dB can be detected. Sample results for absorber with and without fire retardant salts and different sizes are presented.
Characterizing the bistatic performance of anechoic absorbers
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited.
This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence.
Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
A Synthetic aperture imaging method for evaluating anechoic chamber performance
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance.
This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed.
The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
Circularly polarized RCS measurements
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
Next generation Harris compact range, The
After having delivered a model 1630 and a model 1640 compact range plus a number of smaller 1606 and 1603 ranges, Harris has improved their product to meet the demanding needs for operating frequencies of 35 GHz and higher. In characterizing the two large ranges, it was discovered that the surface accuracy as originally optimized would not support the highest operating frequency. Achieving the required surface accuracy required additional surface measurement data in combination with RF contour plots and was very time consuming. From those lessons learned, several features have been incorporated into the next generation of compact ranges that make more accurate reflector surfaces easily achievable. The features include optimally located adjustment mechanisms, additional targets on each panel, software for best fitting the panel surface to minimize steps, techniques for eliminating panel steps in place, and gravity bias setting of panels.
VHF/UHF RCS measurements in indoor microwave facility
Radar cross section (RCS) measurements were performed in the 0.1-1 GHz band in an anechoic chamber optimized for microwave frequencies.
Selection of proper instrumentation, antennas, measurement techniques and processing software are discussed.
Experimental results, showing the accuracy and sensibility of the system are presented.
Antenna phase measurements at 105-190 GHz
A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.
Laser tracker for radar calibration sphere position measurement
A laser tracker using a computer controlled feedback loop has been designed and tested. The tracker follows a small retroreflector embedded in a radar calibration sphere. Angle encoders coupled to two orthogonal scanning mirrors give azimuth and elevation pointing angles to the target. Phase measurements of an intensity modulated laser beam give change in distance to the target, while absolute range is determined by knowing the initial 2p ambiguity interval of the target position. The crossrange accuracy of the system is limited by the scanning mirror encoders to =.063 inches rms at 105 feet (50 microradians). The downrange accuracy of the system is ˜.015 inches rms. This versatile system can be used for: a) contour measurements of models with the aid of a retroreflector moving over the surface, b) accurate determination of the coordinates of a single moving target, and c) determination of the orientation of a large extended target. Anticipated modifications of the system, with their potential precision measurement capabilities and applications, are discussed.
High performance hardware gate improves compact range performance
Comparative measurements have been made in a compact range to determine the performance improvements that can be achieved when adding a hardware gate to a CW-based measurement system. Starting with conventional stepped frequency CW measurements made in the time domain mode, high resolution downrange data was collected to determine the background levels of the compact range. This was followed by comparative measurements under the same conditions adding a narrow pulsed hardware gate to reject inter-horn coupling and high returns from the compact reflector. A second mode of comparison was examined by collecting aspect data with a specific range gate fixed about the target. Software gated measurements required more points to insure alias free operation, while the hardware gated measurements allowed fewer points which reduced measurement time without sacrificing any accuracy. Finally, imaging measurements were made with both software and hardware gating to compare the measurement time and accuracy
Short term stability performance of pulsed instrumentation radars using TWTAS
Pulse-to-pulse amplitude and phase noise can affect the overall measurement accuracy of RCS instrumentation radars. Depending upon the measurement requirements, such noise can limit the overall performance whenever pulse-to-pulse repeatability is required in the signal processing. Radar systems using pulsed TWTAs are subject to high noise due to limitations in the performance of the TWTA modulators and power supplies. A characterization of this additive noise is important to understand the limitations in system performance. Measurements have been made on kilowatt power TWTAs at L and X band as well as 20 watt pulsed TWTAs at S, C, and X/Ku band at various duty cycles and PRFs.
Determining measurement accuracy in antenna tests
The task of making accurate antenna measurements is complicated by the numerous sources of measurement error in the antenna test range. In addition to the test system performance, the overall measurement uncertainty depends strongly upon the range configuration and user-selected operating conditions. A correct understanding of these systematic and random error sources can help optimize the test range, instrument configuration, and measurement technique to achieve the highest levels of measurement accuracy.
This paper describes dominant error sources present on an antenna test range and gives methods for quantifying their effects on measurement accuracy.