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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.
A recently developed analytic technique that can correct for probe position errors in planar near-field measurements to arbitrary accuracy [1] 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 [1], 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.
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.
Measurements in the compact range system are susceptible to errors. Some of these errors are caused by chamber stray signals illuminating the target such as sidewall, backwall, ceiling and floor scattering. One of the major source of these stray signals is the feed spillover or the feed/subreflector spillover in a dual reflector system where the feed/subreflector is not isolated from the main chamber. Due to the limited chamber size, some of these errors cannot be eliminated by either hardware gating or software processing. An alternative approach to reduce these errors is by use of a feed cover such that the spillover field is highly attenuated before it can reach the target or chamber clutter sources. The feasibility of using feed cover in a compact range system to reduce the feed spillover has been studied in this paper. The effectiveness and problems associated with using a feed cover have been investigated in terms of numerical simulation and experimental measurements.
The Antenna Metrology Group of the National Institute of Standards and Technology (NIST) has recently developed and implemented measurement procedures to diagnose faults on a flat phased-array antenna. First, the antenna was measured on the NISTplanar near-field (PNF) range, taking measurements on a plane where the multiple reflections between the probe and the antenna under test are minimized. This is important since the PNF method does not directly allow for these reflections. Then, the NIST PNF software which incorporates the fast Fourier transform (FFT) was used to determine the antenna’s gain and pattern and to evaluate the antenna’s performance. Next, the inverse FFT was used to calculate the fields at the aperture lane. By using this technique, errors in the aperture fields due to multiple reflections can be avoided. By analyzing this aperture plane data through the use of detailed amplitude and phase contour plots, faults in the antenna were located and corrected. The PNF theory and utilization of the inverse FFT will briefly be discussed and results shown.
This paper describes the nature of the errors introduced by amplitude and phase corruptions in the detection process of a classical quadrature mixer and a means for on-line hardware correction of those errors. The discussion defines the nature of the signal corruption produced by first order circularity errors, describes the hardware correction technique, and presents test results that demonstrate the effectiveness of the technique. The method had to meet the requirements for high precision and high speed sampling. The configuration described provides the correction with direct digital processing at the output of the in-phase and quadrature analog-to-digital converters on a pulse-by-pulse basis. The processor operates to pulse rates greater than 3 MHz and has demonstrated corrections with residual errors of approximately 0.01 dB.
In the present work, different configurations of reflector systems for indoor antenna and RCS measurements have been studied and compared. These include the Single Offset reflector, Dual Parabolic Cylinder configuration, Shaped Cassegrain, Front-fed Cassegrain and Dual Chamber Gregorian. The above comparison between the different systems is made in terms of: Configuration efficiency; Cross Polar level introduced by the reflector configuration; Scanning capability; ratio of the configuration equivalent focal length to main reflector aperture diameter and ratio of subreflector area to main reflector area; RCS background levels; phase errors due to reflectors surface roughness as a function of the frequency. In order to illustrate the above discussion, several examples of commercially available compact ranges (S.A., March, Harris) are examined, as well as some recently developed European facilities (MBB, ESTEC, RYMSA). As it will be shown, each configuration is best suited to satisfy different user requirements. For example Shaped Cassegrain/Gregorian configurations seem to be the most efficient for RCS measurements whereas the Front-fed Cassegrain quiet zone can be scanned with low degradation.
Accurate scattering and antenna measurements require excellent plane wave purity in the target zone; however all measurement systems are contaminated by various stray signals which result in measurement errors. In this paper, a technique of evaluating the stray signal sources in a compact range using a diagonal plat plate as a test target is presented. The scattering cross section of the diagonal flat plate as a function of frequency and angle of rotation is first measured. Then the time domain response for each projection angle is processed to obtain a two dimensional ISAR image of the plate as well as the stray signals. From the stray signal images, the location and relative strength of the stray signals can be determined. Experimental results from the OSU/ESL Compact Range Facility are presented to demonstrate this stray signal imaging technique.
Problems exist with the measurement of large aperture antennas due to the far field requirement. This paper discussed a new method to measure a phased array at about 1/10 the normal far field. The basic idea involves focusing the test array at probe antenna a distance R away from the aperture. In the described measurement technique the probe antenna is placed on an arm that rotates 100º on the focal arc given by Rcos(?). This arc minimizes defocusing due to phase aberrations. To minimize the amplitude errors, the pattern of the probe antenna is carefully matched in order to compensate for the 1/R variation induced amplitude error. The application of this technique will enable arrays to be measured in anechoic chambers, allowing convenient classified testing, while avoiding the effects of weather, and will reduce the risks inherent in the high power testing on transmit. The results of a computer simulation is presented that characterizes the validity and limitations of the technique.
In the search for an ideal test-object support for simulate free-space radar-cross-section (RCS) measurements, low-density polystyrene foam has achieved considerable popularity. However, significant error can be introduced into a measurement by the use of an inappropriately designed support. Although low back-scatter radar cross section (RCS) can be obtained with this material, interactions can occur between the test object and the mount which will cause measurement errors in excess of several dB. We present results of measurements performed on a simple test object supported on a low-density foam column which demonstrate this effect. As we discuss, this error can be incorrectly interpreted to be caused by poor alignment of the test object with the radar-range coordinate system. Finally, we show that the errors can be explained by differential propagation effects. In addition, this simple theory provides the insight necessary to devise appropriate measures to minimize the errors cause by the presence of the support.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.
A novel low-cost automatic system is described to measure both the complex permittivity and permeability of solid materials at 2 to 18 GHz. It is particularly useful for evaluating the frequency dependence of radar absorbing materials (RAM). The RF and the mechanical setups are described, including the computer algorithm and the measurement procedure. The results and the experimental errors of three materials are presented, which agree with results that were obtained by other methods, while the cost of putting up the system is considerably lower than any comparable alternative.
The scanning-probe spherical near-field system at GE Aerospace uses a roll over azimuth positioner with probe horn on a cantilevered arm to scan spherical sector centered over a stationary antenna. The main sources of measurement errors in this system are: 1. Signal drift, 2. Deviations of recorded angles from commanded values, 3. Differences of actual sample positions from ideal ones. Unless corrected, these arrors may alter the transformed field.