Dynamic tracking method for radome characterization and measurement system
This paper describes an automated radome test and evaluation system, which quickly and accurately measures the electrical boresight shift and loss caused by the presence of a radome in front of a monopulse antenna. The system was required to measure the boresight deflection through all 60 spatial relative angles between the antenna and the radome. The conventional methods of radome characterization were useless for this range of relative angles (mechanically impossible).
To overcome this problem, a unique dynamic tracking method was developed. In this methos, the antenna is mounted on a dual-axis gimbal attached to the radome. The gimbal by itself is mounted on a second dual-axis positioner. The antenna gimbal scans the radome through all the required relative angles, while the monopulse error is continuously measured and used to control the radome positioner, in order to return the antenna to the boresight position. The readings of the angles and the values of the monopulse error establish the boresight deflection results, which are highly accurate because the apparent (deflected) source is accurately tracked, and the antenna is boresighted to it. The system measures all the 60 angles in 70 minutes time, at an accuracy of 0.3mRAD.
An Economical system for RF antenna measurements
This paper examines antenna pattern measurements of RF frequency antennas (300 kHz-3 GHs) using an integrated source/receiver and measurement control software. Current microwave measurement systems provide sufficient measurement capability but are often too expensive to be used on ranges which require test frequencies of less than 3 GHz such as aircraft communications, cellular radio, GPS, and satellite telemetry antenna. Several system block diagrams based on the HP 8753 network analyzer will be examined with respect to system performance, measurement accuracy, and cost. System considerations for outdoor RF ranges such as RFI susceptibility will also be addressed.
The Design and structural analysis of a large outdoor compact range reflector
A 75 foot diameter offset paraboloidal outdoor compact range reflector was designed for operation up to 95 GHz and installed at Ft. Huachuca, Arizona. The need for high frequency operation required that a highly accurate reflector surface be maintained in the desert’s harsh thermal and wind environment. The use of thermal modeling to predict the temperature distribution in the structure, along with extensive finite element analysis to determine the structure’s distortions from thermal, wind and gravity loads were integral to the reflector design. Using the above tools, thermal isolation techniques were developed to minimize the harmful effects of the thermal environment on surface accuracy.
A surface error budget based upon both calculations and measurements shows an overall rms error of 4.9 mils under optimal environmental conditions, degrading to only 6. Mils under the worst operating conditions.
An Overview of parameters determining productivity and sensitivity in RCS measurement facilities
A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes-one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.
The Effect of probe position errors on planar near-field measurements
Antenna engineers recognize that the planar near-field method for calibrating antennas provide accurate pattern and gain measurements. Bothe the pattern and gain measurements require some degree of probe position accuracy in order to achieve accurate results. This degree of accuracy increases for antennas that have structured near-field patterns. These are antennas in which the amplitude and phase change rapidly over a very small position change in the near-field scan plane.
The National Institute of Standards and Technology (NIST) has recently measured an antenna with a very structured near-field pattern. This measurement was performed using a new probe positioning system developed at NIST. This measurement will be discussed and results will be presented showing how slight probe position errors alter the antenna pattern and gain.
Effects of the mechanical deformation on the accuracy of a spherical near field testing facility
A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.
Amplitude accuracy of the PWS range probe
As the accuracy of antenna range instrumentation improves, multipath on the range is becoming the key limitation in antenna metrology. A fundamental requirement to improving range performance is the accurate and repeatable characterization of scattering on a range. A promising technique for range characterization is the planewave spectral (PWS) range probe.
Earlier papers have demonstrated the ability of the PWS probe to locate multiple scattering centers on a range. Of equal importance to the user is the ability to correctly assess the magnitude of the scattering centers. This paper presents the problem of spectral peak broadening due to phase curvature from localized scatterers. Methods for improving the accuracy of scattering center estimation are presented along with numerical studies of the performance of these methods.
Ramp sweep accuracy of RCS measurements using the HP 8530A
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.
Method of determining the phase center location and stability of wide beam GPS antennas
Some proposed attitude control systems will require sub-millimeter position accuracy and GPS signals. Toward this end, two antenna parameters must be determined and optimized. These two parameters are phase center location and phase center stability. The phase center location is defined as the point whose ideal spherical phase front has the minimum RMS difference between itself and the measured phase data. Phase center stability is the effective movement of a GPS antenna’s phase center caused by the deviation of the radiated phase front from an ideal spherical phase front. The RMS difference between the ideal phase and the measured phase is a good measure of phase center stability.
This paper describes a method of determining the phase center location of a wide-beam GPS antenna. Once the phase center is determined, the phase center stability throughout the coverage region (usually a hemisphere) is characterized. Finally, some sources of error are identified. Methods of minimizing the effects of these error sources are addressed.
Compact range bistatic scattering measurements
This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented.
Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.
Some differences between gated CW and pulse radars in RCS and imaging measurements
This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases.
For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system.
This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
Radar cross section measurements for computer code validation
Computer codes for the computation of scattering are based on physical, mathematical, and numerical assumptions and approximations that impact the accuracy of the results in ways that are not obvious or quantifiable analytically. This paper stresses the usefulness of a concurrent measurement program to provide reliable RCS data for targets of special interest in establishing the range of validity of the various assumptions upon which a specific computer code is based. This in turn assists in developing “modelling guidelines” restricting the design of computer models for input to the code such that reasonable accurate results are likely to be obtained.
Range instrumentation performance verification and traceability
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.
Range field compensation
The accuracy of antenna measurements can be improved by compensating for the effects of extraneous fields present in an antenna range using analytical compensation techniques. Range field compensation is a new technique to provide increased measurement accuracy by compensating for extraneous fields created by refection and scattering of the range antenna field from fixed objects in the range and by leakage of the range antenna RF system from a fixed location in the range. The range antenna field must be the dominant field in the range, and the range field cannot change for different AUTs. Existing compensation techniques are limited in the amount of compensation they can provide. The range field is measured over a spherical surface encompassing the test zone using a low gain probe. The measured range field is used in subsequent antenna measurements to compensate for the effects of extraneous fields. This technique is demonstrated using measurements simulated for an anechoic chamber far-field range.
Measurement receiver error analysis for rapidly varying input signals
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system.
This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas.
Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
Applications of portable near-field antenna measurement systems
Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flight-line testing, anechoic chamber quiet zone imaging, and EMI testing.
Many of NSI’s near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for use of a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.
General analytic correction of probe-position errors in spherical near-field measurments
A recently developed analytic technique that can correct for probe position errors in planar near-field measurements to arbitrary accuracy  is shown to be also applicable to spherical near-field data after appropriate modifications. The method has been used to successfully remove errors in the near-field, hence leading to more accurate far-field patterns, even if the maximum error in the probe’s position is as large as 0.2?. Only the error-contaminated near-field measurements and an accurate probe position error function are needed to be able to implement the correction technique. It is assumed that the probe position error function is a characteristic of the near-field range, and that it has been obtained using state-of-the-art laser positioning and precision optical systems. The method also requires the ability to obtain derivatives of the error contaminated near-field defined on an error-free regular grid with respect to the coordinates. In planar geometry the derivatives are obtained using FFTs , and, in spherical geometry, one needs to compute derivatives of Hankel functions for radical errors, and derivatives of the spherical electric and magnetic vector basis functions for errors in the ? and Ø coordinates. The error-correction technique has been shown to work well for errors in and of the spherical coordinates r, ? or Ø. Efficient computer codes have been developed to demonstrate the technique using computer simulations.
The Effect of range errors on phase measurements of a spiral antenna
Phase relationships between the three dominant modes on a four armed spiral can be used to perform broad band, direction of arrival estimates, but this requires accurate estimates of the phase behavior of the antenna both in the design stage and for calibration purposes. Unfortunately, imperfections in range design make the measurement and interpretation of phase information extremely difficult. This paper describes an approach where the imperfections of the range and the behavior of the antenna are modelled, and range effects removed from antenna data through antenna motion, and frequency change. This technique obtained tremendous accuracy at the cost of large amounts of data processing.