Evaluation of Antenna Tracking Systems
Antenna tracking systems are an important part of practical system designs. The goal of antenna tracking for communication applications is to provide sufficient accuracy to limit pointing loss, while for radar applications, to determine the target’s position as accurately as possible. Antenna tracking systems are reviewed describing both open and closed loop designs. Corresponding measurement techniques to quantify system performance are described.
A New Antenna Laboratory for 3D Antenna and RCS Measurements
A system has been developed for acquiring an antenna’s complete (3D) radiation pattern and radar cross-section (RCS) measurements. The system consists of a motion controller, a network analyser and tower assembly. The tower assembly is in an anechoic chamber. The tower has a novel design. It uses three motors in a special configuration, thereby allowing 2 ½ degrees of freedom.
This freedom gives the ability to run complete antenna or RCS measurements automatically. Another advantage stemming from the degrees of freedom is expansion of the range of measurements. This is enabled by a variety of possible positions inside the chamber.
Tests have also been carried out on system performance.
The data acquisition rate becomes crucial when dealing with 3D pattern measurements. The performance of an HP 8720 or 8753 network analyser series can be dramatically increased by using the power sweep mode for data acquisition. Together with the “external trigger-on-point” mode, this gives the best positioning accuracy. The six-month experience has demonstrated the flexibility and reliability of the set up and ideas.
Helendale Measurement Facility Uncertainty Analysis
This paper reviews the Helendale Measurement Facility (HMF) ground plane range uncertainty analysis and associated data collection. Range uncertainty analysis is a requirement for ISO-25/ANSI-Z-540 range certification and is a priority one section in the Helendale Range Book. Targets used for the analysis were two sets of right circular “squat” calibration cylinders. These cylinders are the dual calibration cylinders for HMF. Calibration measurement uncertainties are established statistically from a large number of repeated measurements at S, C, X, and Ku bands. Each measurement was taken at two target support locations down range. The field data collected included monostatic scattering from two calibration cylinders, backgrounds with no target and support, and drift data for quality control. I and Q imbalance, frequency stability, range accuracy, linearity, and field uniformity at target locations were considered in the analysis. The uncertainty analysis is based on RSS addition of errors and assumes all errors are additive and that targets are not LO. The statistical approach used to perform the uncertainty analysis reported in this paper was developed cooperatively at AFRL and Mission Research Corporation.
Error Analysis of Circular-Polarization Components Synthesized From Linearly Polarized Measurements
A usual way of performing pattern-measurements on circularly polarized antennas is by measuring the linear components of the field and mathematically converting those to the left-hand and right-hand circular components.
These synthesized circular components are sensitive for a number of factors: The exact orthogonality of the measured linear components, the measurement-accuracy of both phase and amplitude of the measured linear components, the polarization-pureness (or the accuracy of the description of the polarization-characteristics) of the probe, etc. This paper analyzes these factors, using a computer-model. An indication on the requirements to be imposed on the measurement-equipment is provided.
Broadband Radar Cross Section Predictions and Measurements for a Canonical 3-Meter Ogive Body of Revolution
In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets . In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system . In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes.
In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.
Limitations of Near-Field Back Projection for Phased Array Tuning Applications
Simulated data is presented for a planar array to demonstrate the limitations of planar near-field back projections. It is well known that the result obtained in this way is of limited resolution and accuracy and these limitations are further illustrated through the data presented here. The impact of probe to AUT separation distance is shown as well as the correspondence between array excitation perturbations and that detected through the back projection technique. Results are shown for a simple iterative array excitation adjustment process. The purpose of this paper is to provide guidelines for the application of the planar near-field back projection technique.
Phased Array Calibration Method with Evaluating Phase Shifter Error
In this paper, the authors propose an improved Rotatingelement Electric-field Vector (REV) method taking into account amplitude and phase error of phase shifters in order to achieve more precise calibration. The conventional REV method has been used in order to determine and/or adjust amplitude and phase of electrical field radiated from each antenna element -element fieldin phased array antennas. However, amplitude and phase deviations due to phase shifter errors, and so on, reduce the measurement accuracy because the conventional REV method assumes no deviation.
On the other hand, the proposed REV method can evaluate element fields without error and error electrical fields -error fields- due to phase shifter errors in each bit, by measuring both amplitude and phase value of array composite electrical field. In a simulation for a 31- element array with 5-bit phase shifter, the evaluated element fields and error fields agree well with the expected values. This result shows that the proposed method allows the phased arrays to be calibrated more accurately as considering phase shifter errors.
A Simple Analysis of Near-Field Boresight Error Requirements
The need to measure the boresight pointing direction of radar antennas to a high degree of accuracy yields a requirement for excellent positioning accuracy on near-field antenna ranges. Evaluation of this requirement can be accomplished by a full and complete sensitivity analysis.
Alternatively, to gain an understanding of the effects of errors more simply, one can approach the question of accuracy required in the setup, by use of a physical model and straightforward physical reasoning. The approach starts with the assumptions of a collimated wave with planar phase fronts and the premise that the boresight direction of such a sum beam is along the normal to the phase fronts. A sensitivity analysis of the simple trigonometric boresight relationship between mechanical boresight and phase front normal, shows how accurate the receiver and the positioner must be to achieve a given boresight determination.
Such an approach has been known for many years as it regards planar scanning; and, the results are known to be applicable.
In this paper this consideration is extended to spherical scanners to arrive at estimates of the mechanical positioner accuracies and electrical receiver accuracies needed to make boresight measurements of radar antennas with spherical near-field ranges.
Experimental Studies With Comparisons to Computational Model for Automobile Antennas
A series of experimental and theoretical tests designed to develop techniques for reliable computational modeling of automobile antenna performance is presented. The results from the experimental measurements are compared with the results of computational techniques to verify their accuracy and reliability.
The Electromagnetic Surface Patch (ESP5) code, a theoretical Method of Moment (MoM) general-purpose code developed at the Ohio State University, is used for computational modeling. We progress from the simple geometry of a single square plate and a monopole, to the more complex structure of a small copper-coated plastic model of an automobile. The computational simulation and measurements are configured with both a monopole antenna mounted at the center of the automobile roof and a backlite heater grid FM antenna. The input impedance, pattern, and polarization are all measured.
Comparisons between the results of the computational simulations are presented, as well as the procedures used to measure the antenna characteristics and compare the experimental data with the measured data.
Wide Band Compact Antenna Test Range
Ericsson Microwave Systems (EMW) in Sweden has several outdoor and indoor test ranges in operation , , . In line with future needs and requirements EMW has started building a new Compact Antenna Test Range to be used for a large range of projects and applications.
The Compact Antenna Test Range will cover the frequency range of 800 MHz to 75 GHz. The test range will have the possibility for both active and passive antenna measurements at both system and subsystem / unit levels.
The test zone will be 3 meters diameter. The maximum load the positioner can carry will be 700 Kg with very high position accuracy for special applications.
Due to the relatively low design frequency and the desired size of the test zone, special considerations have been taken in the conceptual design of the reflector system as well as the choice of absorbers.
Another important parameter in the design of the facility will be the access to the quiet zone and the time needed to change frequency bands and test objects. To accomplish this, preparations have to be made for easy alignment, very precise interfaces and a fast access to the test area.
Calibration and Verification Measurements in Compensated Compact Ranges Up to 500 GHz
Compensated Compact Ranges (CCR) represent a high standard of state-of-the-art test facilities with a fast and real time measurement capability up to the submm wave range. Future scientific and earth observation instruments of ESA/ESTEC such as MASTER, PLANCK and HERSCHEL are working within this frequency ranges and require a high measurement accuracy for large antenna apertures.
Within the ADMIRALS study for ESA/ESTEC, transmit and receive modules up to 500 GHz and an appropriate large offset reflector antenna with precise surface accuracy in form of a Representative Test Object (RTO) were applied. Related tests in the CCR 75/60 of Astrium were performed in order to qualify the test facility and verify the antenna measurements with theoretical pattern calculations.
The present paper shows measurement results with the highly accurate Plane Wave Scanner (PWS) of Astrium GmbH and the RTO. Through the measurements performed, the accuracy of the plane wave field as well as pattern accuracy in the quiet zone of the CCR 75/60 have been qualified up to 500 GHz.
Mitigation of Multipath and Ground Interactions in RCS Measurements Using a Single Target Translation
Translating pylon terminations are often used in narrowband RCS background measurements as means of separating the returns of the termination from those of the pylon itself. Typically, this is done by measuring the pylon while the fixture continuously translates in the range direction through a distance of at least half a wavelength. This paper describes a translated target processing (TTP) algorithmw hich is an extension of this technique to RCS measurements of rotating targets.
The technique is applicable to both narrowband and wideband measurements. The algorithm is applied to the problemof mitigating multipath and ground interaction contamination in indoor and outdoor RCS measurements, respectively. Its performance was evaluated as a function of signal-to-noise ratio, target-tocontamination ratio, and translation distance and accuracy using point target simulations. We conclude with a demonstration of the TTP algorithm using actual measurements from the Boeing 9-77 compact range.
Correcting for Range Measurement Errors Using Quiet Zone Synthesis
A method is presented for correcting for range measurement errors resulting from non-uniform quiet zone illumination in indoor tapered antenna chambers.
The interaction of the source antenna with the throat of the chamber causes undesirable amplitude and phase variations over the quiet zone, the region where the antenna under test (AUT) is located. These variations can impact the accuracy of the antenna pattern measurements, especially when the AUT has a significant aperture. These quiet-zone anomalies can be measured and removed from the antenna patterns by quiet-zone probing. The quiet zone can be probed planar, cylindrical, or spherical quiet zone probe configurations. A planar quiet-zone probe is used here. This process of calibrating the antenna pattern measurements for quiet-zone range errors is called quietzone synthesis (QZS) and is implemented here using MATLAB .
A Large Aperture 650 GHz Near-Field Measurement System for the Earth Observing System Microwave Limb Sounder
This paper describes a large aperture, 650 GHz, planar near-field measurement system developed for field of view characterization of the Earth Observing System Microwave Limb Sounder (EOS MLS). Scheduled for launch in 2003 on the NASA EOS Aura spacecraft, EOS MLS is being developed by the Jet Propulsion Laboratory to study stratospheric chemistry using radiometers from 118 to 2500 GHz. The combination of a very high operating frequency and a 1.6-meter aperture, coupled with significant cost and weight restrictions, required a new look at near-field scanner design approaches. Nearfield Systems Inc. (NSI) developed a planar scanner that provides a planar accuracy of 4 microns RMS over the entire 2.4 x 2.4 meter scan area. This paper presents an overview of this system including the sub-millimeter wave RF subsystem and the ultrahigh precision scanner.
Representative measurement results will be shown.
Positioning System Upgrade of an Existing Measurement System
An accurate and reliable target positioning system is mandatory for a good antenna and/or radar cross section (RCS) measurement facility. Most measurements involve characterizing the radiation or scattering of the unit under test as a function of angle and frequency. Accuracy and repeatability become increasingly important in RCS measurements where background subtraction is utilized. Any error in target position will reduce the subtraction effectiveness.
Wear and tear of existing equipment coupled with improvements in motion control technology may compel some measurement facilities to upgrade their positioning system. Doing so, while keeping the rest of the measurement system intact, poses integration challenges that cannot be over emphasized. Problems will inevitably be encountered. Their source could be the new positioning system, the old measurement system, or the communication between the two.
Subtleties of how the motion control system works can be overlooked during the requirements definition phase of the project. Further idiosyncrasies can be missed during acceptance testing of the system. The Air Force Research Lab compact range has recently upgraded their target positioning system and will share the lessons learned as a result.
High Accuracy Wide Band Compact Antenna Test Range
ORBIT/FR-Europe is in the process of finishing a new Compact Antenna Test Range for Ericsson Microwave Systems (EMW) in Sweden.
The design was presented at AMTA 2001. Here the progress in the project is presented. Most subsystems are now installed, and system level acceptance will follow in the near future.
Algorithms and Mechanics Employed for Successful Portable Imaging Via the SCI-Xe Microwave Imaging System
Sensor Concepts, Inc. has developed the SCI-Xe Portable Microwave Imaging System prototype for use in the assessment of the low observable (LO) characteristics of fielded military platforms in their native environments.
The SCI-Xe is a single man deployable suitcase-size system that employs a small linear rail in order to acquire Linear Synthetic Aperture Radar (LSAR) data in the 8-18 GHz frequency range. Data collections are performed via a single button push and the data is stored on a removable harddrive for comparison to an existing database for analysis.
Recent deployment of the SCI-Xe prototype has provided excellent feedback on the viability of performing repeatable field measurements using alignment techniques that do not significantly affect the overall system size and weight. The SCI-Xe employs a video camera and uses video image algorithms such as edge detection, thresholding, and overlay masks to provide a simple coarse alignment to a stored baseline position. Once positioned, a single LSAR collection is performed to provide the radar data necessary for analysis, which includes a robust image registration algorithm to first, perform a quantitative assessment of the positioning accuracy and second, align the data for further image filtering and statistical processing.
Development of Highly Accurate Measurement Techniques for State-of-the-Art Antenna Test Facilities
Contoured multi-beams achieved by multi-feed reflector antennas, realized in modern communication satellites, like Intelsat VIII and IX generations satellite, require an economic measurement of their antenna characteristic. Further, highly accurate, but also fast and therefore real-time measurements are assumed to be applied for the testing of the antenna performance.
For that aim, the Compensated Compact Range CCR 75/60, applied at e.g. Space Systems Loral (SSL) in Palo Alto (USA), at ALCATEL in Cannes (France), at the MISTRAL facility in Toulouse (France) and at Astrium GmbH (Germany) was developed and installed by Astrium GmbH. In order to optimize the measurement accuracy of the CCR, detailed error analyses and investigations for improvement measures were performed.
Within this paper, the accuracy analyses and improvement steps will be presented in order to establish accuracy values, which can be realized in state-of-the-art compact range test facilities.
An Evaluation of Errors Encountered Using the NUWC/NPT Overwater Arch Antenna Measurement Range
The NUWC/NPT Overwater Arch Antenna Range consists of a 70 ft radius measurement arch located over an elevated 90 ft x 65 ft salt water pool. This facility, located outdoors, presents mechanical and electrical challenges. Measurement accuracy and precision are a function of environmental parameters (including unwanted signals), physical plant and instrumentation characteristics. Measured data variation will be presented along with techniques which could be employed to improve range performance.
Improved Antenna Radiation Pattern Measurements Using an Equalization Technique
This paper describes a novel system that overcomes the inaccuracies in antenna radiation pattern measurements caused by multipath propagation. The system operates by specifically compensating for the effects of unwanted signals rather than by attempting to remove, or minimize, their effects through the use of screens or baffles or an anechoic chamber.
Compensation is achieved through the use of an equalization technique, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration.
The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy.
Most importantly, however, the basic idea leads to the design of a completely new type of real-time 3-D range in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.