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Antenna pattern measurements are dominantly influenced by the presence of extraneous fields in the test zone. A fast and simple way to recognize problems in pattern measurements provides the Antenna Pattern Comparison-technique (APC). This method usually consists of recording azimuthal patterns on different positions across the test zone. Differences in the amplitude data give a rough indication for the magnitude of the interfering signal. The "Novel APC-method" (NAPC) employs both amplitude- and phase-data so that it becomes possible to separate the direct and the extraneous signals from each other. It will be shown that this method is eminently suited to correct radiation patterns of high-gain and low-sidelobe antennas. For verification purposes corrected patterns are compared with time-dated ones and the resemblance is excellent. It is concluded that the NAPC-method is promising and powerful technique for accurate antenna pattern determination, mainly because it can be easily implemented for most applications.
Unique instrumentation is required for dynamic (in-flight) measurements of aircraft radar cross section (RCS), jammer-to-signal (J/S), or chaff signature. The resulting scintillation of the radar echo of a dynamic target requires special data collection and processing techniques to ensure the integrity of RCS measurements. Sufficient data in each resolution aspect cell is required for an accurate representation of the target's signature. Dynamic RCS instrumentation location, flight profiles, data sampling rates, and number of simultaneous measurements at different frequencies are important factors in determining flight time. The Chesapeake Test Range (CTR), NAVAIRWARCENACDIV, Patuxent River, Maryland, is a leader in quality dynamic in-flight RCS, J/S ratio, and chaff measurements of air vehicles. The facility is comprised of several integrated range facilities including range control, radar tracking, telemetry, data acquisition, and real-time data processing and display.
RCS measurement accuracy is degraded by reflections occurring between the feed antenna, the range, and the radar subsystem. These reflections produce errors which appear in the image domain (both 1-D and 2-D). The errors are proportional to the RCS magnitude of the target under test and they are present in each of the typical range calibration measurements. Current 2-term error models do not predict or account for the above errors. An improved 8-term error model is developed to do so. The model is based on measurable reflections and losses within the range, the feed antenna, and the radar. By combining the improved error model with the commonly used 2-term RCS range calibration equation, we are able to quantify the residual RCS errors. The improved error model is validated with measured results on a direct illumination range and is used to develop specific techniques which can improve RCS measurement accuracy.
This paper describes a calibration technique for reducing the errors due to mismatch between the measurement receiver and the antenna in microwave antenna relative gain measurements. In addition, this technique provides an accurate method for measuring the input return loss of the antenna under test. In this technique, a microwave reflectometer is mounted between the measurement receiver and the antenna test port. The reflectometer is calibrated and used to measure the return loss of both the test and calibration antennas. Using this information in conjunction with the HP 8530A antenna gain calibration, the corrected gain of the antenna under test is computed. Compact range antenna measurements verifying the calibration model and error analysis are presented. Practical implementation considerations are discussed.
A method to estimate the rms error in the compact range reflector surface is presented. The method uses the target zone field of the reflector and is based on the fact that the random errors in the reflector surface cause energy to subtract from the main beam resulting in reduction of the axial gain. The reduction in the axial gain can be used to estimate the rms error. It is shown that if the target zone fields of the reflector are probed at high frequencies such that the irregularities in the reflector surface are the main source of error in the target zone fields, then the proposed technique gives a good estimate of the rms error in the reflector surface.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
The National Institute of Standards and Technology (NIST) has written a certification plan to ensure that a proposed planar near-field facility is capable of measuring high-performance phased arrays. Generally for a complete plan, one must evaluate many aspects including scanner alignment, near-field probe alignment, alignment of the antenna under test, RF crosstalk, probe position errors RF path variations, the receiver's dynamic range and linearity, leakage, probe-antenna multiple reflections, truncation effects, aliasing, system drift, room multipath, insertion loss measurements, noise, and software verification. In this paper, we discuss some of the important aspects of the certification plan specifically written for the measurement of high-performance phased-array antennas. Further, we show how the requirements of each aspect depend on the measurement accuracies needed to verify the performance array under test.
A Planar Near Field to Far Field (PNF/FF) Transformation Program has been developed. This PNF/FF package includes probe correction, spectral filtering, position errors correction and sampled data expansion. In order to evaluate how measurement system errors affect PNF/FF transformation results, a whole set of simulation routines have also been implemented. In this paper, main modules of the PNF/FF package are discussed and error simulation models together with correction routines are described.
The remote capability of the ORBIT AL-8000-5 Microwave Receiver is described. The use of a high speed fiber optic link between the remote receiver and the control room unit allows range distances of up to 19,000 feet. With repeaters, the range distance limitation is removed. This eliminates many of the distance cable and EMI problems associated with receivers which use a remote LO. The small size and weight of the remote unit, allow the system to be mounted on the probe carriage of near-field scanner systems. This eliminates the high frequency phase errors as well as the phase error due to cable bending and temperature variation during the measurement. The result is a lower cost and more accurate measurement system. The advantages of this type of remote system are discussed for both near-field and far-field applications. Measurement data which show the performance of the fiber-optic system are presented. A description and pictures of recent installations are to be provided.
This paper describes a software based receiver post processor that corrects circularity and gain errors in coherent receivers. The receiver post processor additionally provides range gating capabilities, signal quality estimation, mixer non-linearity detection and various display functions. This paper will concentrate primarily on the identification of circularity errors by the receiver post processor.
The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.
As part of an effort to provide improved measurement services at frequencies above 30 GHz, scientists at the National Institute of Standards and Technology (NIST) have completed development of a swept frequency gain measurement service for the 33-50 GHz band. This service gives gain values with an accuracy of ± 0.3 dB. In this paper we discuss an example measurement and the associated errors.
Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.
A bistatic calibration technique for wide-band, full-polarimetric instrumentation radars is presented in this paper. First general bistatic measurement problems are discussed, as there are the coordinate systems, the definition of polarization and the bistatic scattering behavior of convenient calibration targets. In chapter two the new calibration approach is presented. The general mathematical and physical description of errors introduced in the bistatic system is based on the radiation transfer matrix. The calibration procedure is discussed for the application with a vector network analyzer based instrumentation radar. For verification purposes measurements were performed on several targets.
Edge and corner diffraction and non-planewave illumination both cause measured free space relativity data to deviate from the infinite sample/planewave result which is predicted when using the Transmission Line Methos (TLM) for planar surfaces. The amount by which each of the two factors perturbs the measured data depends on the measurement system used; compact ranges, near field focused antennas and far field antennas on an NRL arch are all susceptible to the effects of non-planewave illumination and perimeter diffraction. Perimeter diffraction is virtually eliminated in the case of a near field focused system or where the sample is semi-infinite; however, the truncated illumination inevitable yields additional angular planewave components. In a far field system, the quadratic phase variation at the sample surface is shown to cause significant errors in the depth of resonant nulls. A uniform illumination is required to accurately map the depth of resonant nulls, but the consequent perimeter diffraction causes errors in null position. Perimeter diffraction does not cause errors in the null depth providing the illumination in uniform.
An alpha-beta-gamma (a-ß-?) tracking algorithm has been devised to improve the performance of a laser-references planarity control servo. GTRI is developing a field probe for the USAEPG Compact Range at Ft. Huachuca, Arizona. The probe scans a surface whose planarity is controlled by a servo. A reference plane is generated by sweeping a laser beam with a pentaprism. The beam is detected by a photodiode mounted with the probe. The servo nulls any error detected. The servo must correct dynamic errors in the presence of high frequency electronic noise and low frequency atmospheric scintillation. A control algorithm based on the alpha-beta-gamma tracker has been developed and tested by simulation. The algorithm and simulation results are presented.
This paper will discuss the need for performance verification, or calibration, of the transmitter and receiver systems used in an antenna or RCS range. Errors introduced by the range and positioning system means the instrumentation’s performance must be measured independently of the range and positioner. The performance verification should insure that the measurement system exceeds the manufactures’ specifications by a reasonable margin. The verification must be performed with the equipment installed on the range to insure adequate performance on the range. The system must als be verified as a system, rather than individual instruments. This guarantees that measurement errors in each instrument will not add together to exceed the system’s specifications. Testing of the system should be easy and repeatable to insure accuracy of the verification by the test technician. The tests should also be documented for later reference. The measurements should be traceable to a local standard such as NIST to certify the accuracy and stability of the measurement. The verification should be repeated on a regular basis to insure continued accuracy of the measurement system.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
Compact range systems have been widely used for antenna measurements. However, the amplitude taper can lead to significant measurement errors especially as the dimension of antenna is larger than quiet zone area. An amplitude taper removing technique by software implement is presented for compact range system. A 12 feet by 1.0 feet S-band rectangular slot array antenna is measured in SA5751 compact range system, which provides a quiet zone area with a 4 feet diameter. Results of corrected far-field patterns from compact range are compared with that taken by planar near-field range.
A performance simulation for analyzing the measurements of target RCS in a compact radar range has been applied to a small indoor range which will be installed at RRI. A dual reflector collimator has been examined with respect to both quiet-zone quality and the amount of stray energy in the chamber which eventually end up as clutter or multipath interference. The complicated ray geometries, beyond the reach of hand calculation, are discovered by complete tracing of all the rays from the feed source. The ray pats which interfere with target measurements are shown convincingly by graphical display. Vector clutter subtraction is widely used in compact ranges in order to reduce the background clutter to an acceptable level. Some of the effects which limit the effectiveness of clutter subtraction are also addressed in the paper. The sources of measurement errors which are obtained by this simulation are used in the measurement-error budget analysis, which is the subject of the follow-on paper.