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This paper presents the design and performance characteristics of a novel active stability control capability that Aeroflex Incorporated has developed and implemented in the élan-2000 pulsed-IF instrumentation radar. The real-time technique incorporates an internal power reference loop that continuously monitors and compensates for phase and amplitude drifts within the radar RF analog circuitry through high-speed processing of the streaming data collections. Vector corrections are applied to each recorded data point, using internal loop samples of the transmit pulse from a common RF channel and digitizer, without degrading other overall system performance capabilities. Demonstrated stability levels exceed –50 dB over the full operational RF bandwidth, for periods of several hours, with environmental temperature variations of several degrees. This measurement mode provides ~30 dB of improvement over conventional instrumentation radar systems under similar test conditions, which consequently enables significant improvements in measurement applications incorporating background subtraction or where extremely stable system parameters are required.
Sensor Concepts Inc. has prototyped a fast, lightweight, dechirp-on-receive radar called the SCI-Lr to provide the capability of a range instrumentation radar in a highly portable package. The small weight, size and power requirements of the SCI-Lr allow a variety of new deployment options for the user including in a small general aviation aircraft or on a mountaintop that is accessible only by four wheel drive. Pulse rates up to 20 KHz enables investigation of high Doppler bandwidth phenomenon such as ground vehicle microdoppler features. The dual integration from dechirp-on-receive matched filtering in fast time and Doppler processing in slow time provides high sensitivity with low output power. Planned enhancements of waveform bandwidth up to 2 GHz , frequency operation between .2 and 18 GHz and pulseto- pulse polarization switching will provide high information content for target discrimination. The flexibility provided by the hardware is augmented by software tools to examine data in near real time to monitor data quality and sufficiency. A variety of applications are being investigated including RCS measurement, SAR and ISAR imaging, Ground Moving Target Indication, and signature collection for ATC.
Traditional data collection strategy for antenna measurement is to perform a step and scan operation. This method moves a particular axis while holding all other source and AUT axes in a fixed location. Modern radome measurements require the coordinated motion of two or more axes due to the desired measurements, the radome testing geometries or a combination of both. An example would be transmission efficiency testing of a radome associated with a tracking antenna. In this measurement scenario, the antenna azimuth and elevation axes must maintain an orientation along the range axis while the radome is moved in front of the antenna. The axis coordination could be linear or non-linear in nature. This paper describes the concept of coordinated motion and the needs for coordinated motion in radome measurements that have been identified. Additional potential applications for coordinated motion in radome measurements are described. Two methods of coordinated motion that have been implemented in instrumentation are described. They are geared motion, which is a linear master/slave relationship between two axes and generalized coordinated motion where the relationship of axes motion is described via linear or non-linear equations.
The true RF spectral response represents In- phase and Quadrature (I and Q) data in the frequency domain, and is identical to that mea- sured in many anechoic chambers including the one at Mission Research Corporation. Given a Linear FM (LFM) response, a method is derived that extracts the true RF spectral response. In the process some basic features of LFM systems are explained. The derivation depends on the assumption that the received signal is zero outside a de¯ned interval. Validation consists of applying the extraction process to both sim- ulated and measured LFM data from the ERIM DCS radar system.
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.
In a conventional manner, a majority of compact ranges are currently used between 2 GHz and 200 GHz. Mechanical stiffness limits compact ranges at high frequency and diffraction effects are dominant at low frequency. However, CNES has installed a single reflector with dedicated serrations to perform accurate measurements between 800 MHz and 2 GHz. These serrations are 2 meters long and minimize the ripple in both amplitude and phase within the quiet zone. In order to further improve its measurement capabilities at lower frequencies, CNES has installed, in co-operation with SATIMO, a spherical near field measurement system directly inside of its compact range building. The goal is to measure antennas within the frequency range 80 MHz – 400 MHz with a relatively good accuracy. The spherical near field measurement facility has been tested and validated with four antennas that had been previously measured in the compact range of CNES and other external ranges. This paper focuses in this smart approach, which allows to extend the lower frequency domain of compact ranges. This paper describes in details the measurement facility, the test and the validation of the system.
We at MI Technologies have employed the Hansen error analysis [1] developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design. I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges [2]. The result is an appropriate extension of the TUD uncertainty analysis. Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field [5]. In this paper, I offer such an extended tabulation for spherical measurements.
This paper describes the experience of using a tracking laser interferometer to align and characterize the planarity of a planar near-field scanner. Last year, The Aerospace Corporation moved their planar near-field antenna range into a new larger room with improved environmental controls. After this move, the near-field scanner required careful alignment and characterization. The quality of the scanner is judged by how accurately the probe scans over a planar surface. The initial effort to align the scanner used a large granite block as a planarity reference surface and cumbersome mechanical probe measurements. However, a tracking laser interferometer was used for the final alignment and characterization. The laser interferometer was included as part of an alignment service purchased from MI Technologies. The tracking laser interferometer emits a laser beam to a mirrored target called an SMR (Spherically Mounted Retroreflector). Encoders in the tracker measure the horizontal and vertical angles while the laser interferometer measures the distance. From these measurements, the three-dimensional SMR location is determined. The laser has the ability to very accurately (within about 0.001 inch) measure the location of the scanning near-field probe. This paper includes a description of the mechanical alignment of the scanner, the tracking laser interferometer measurements, and the final planarity characterization.
Calibrated probe complex pattern data is used in planar NFR (near field range) data processing to remove the effects of the probe on the measurement. In a prior paper [1] we proposed a procedure to estimate the measurement error (uncertainty) introduced into a near field antenna radiation pattern measurement due to test frequencies that do not coincide with available calibration frequencies of the range probe. Our prior paper resulted in a “19th term” which was added to the well known NIST NFR 18 Term Error Table used to evaluate the unavoidable uncertainty of far-field radiation patterns derived from a near field scan of a given AUT (antenna under test). A limitation of this procedure, pointed out in our prior paper, is that it was most accurate for a test frequency falling midway between two nearest neighbor probe calibration frequencies. The estimated uncertainty became overly pessimistic as the test frequency of interest moved closer to one of the neighboring calibrated frequencies. The procedure is improved in the present paper by the inclusion of a new term that is a function of the test frequency and the two nearest neighbor probe calibration frequencies. Examples are shown of the use of the new procedure to obtain an improved estimate of this measurement uncertainty and to create the 19th term for use with the standard 18 Term Error Table.
In previous AMTA presentations, we developed and evaluated an image-based near field-to-far field transformation (IB NFFFT) algorithm for monostatic RCS measurements. We showed that the algorithm’s far field RCS pattern prediction performance was quite good for a variety of frequencies, near field measurement distances, and target geometries. In this paper, we quantify the statistical RCS prediction performance of the IB NFFFT using simulated data from a generalized point scatterer model and method of moments (MoM) code, both of which allow modeling of targets with single and multiple interactions. It is shown that the predicted RCS statistics remain quite accurate under conditions where the predicted far field patterns have significantly degraded due to multiple interactions and other effects.
This paper describes a unique three axis gimbal assembly used to test the performance of small radomes. The gimbal assembly supports thin, low profile antennas within the radome, and achieves the correct orientation of the antenna relative to the radome as the entire radome/antenna assembly is rotated for measurement of parameters such as transmission loss and boresight error. The gimbal provides roll, elevation, and azimuth compensation for the antenna within the radome using a small package, with the gimbal point located very close to the rear of the antenna. The axes are equipped with high resolution encoders to provide the very accurate antenna positioning required to demonstrate compliance with tight boresight error tolerances for the radome under test. The entire assembly is removable from the master positioning system for the purpose of switching the test range configuration from radome testing to standard antenna pattern testing.
This paper describes the use of simultaneous axis motion for various antenna, RCS, and radome applications, and the use of off the shelf hardware to support the corresponding measurement requirements. This is particularly relevant to polarization, low reflectivity target characterization, and radome measurements. Specific motion profiles required to accomplish various classes of tests are discussed, along with the implications on the mode of operation of the measurement system in order to achieve the most efficient collection of the required data. These simultaneous axis motion requirements may typically be user defined from the available set of axes composing the positioning system. Evaluation of the speed and real time tracking capability of the multiple axes are examined as they relate to the accuracy of the measurements that are required.
The Range Commanders Council Signature Measurement Standards Group (RCC/SMSG) Performed a Demonstration program with three DOD Radar Cross Section Ranges to evaluate and improve their documentation and evaluation process and criteria documented in what is known as a "Range Book". After a successful Demonstration Program, The RCC/SMSG has embarked on the evaluation of Industry RCS Range Calibration and measurement processes and procedures and compliance with the RCC/SMSG ANSI-Z540 (ISO-25) evaluation criteria. The Lockheed Martin Helendale RCS Range was evaluated by a committee of industry volunteers appointed by the RCC/SMSG after a review of their experience and credentials. The Lockheed Martin Orlando RCS Range requested an evaluation of their "Range Book" shortly after the completion of the Helendale evaluation. Each review committee is made up of three RCC/SMSG approved reviewers, at least one of which has participated in a previous review either as a review requester or a review committee member. This paper will put forth the process used by this review committee and the lessons learned from this and previous reviews. This paper will also discuss the RCC/SMSG process for obtaining an RCC/SMSG review.
This paper will deal with basic rectangular chamber design and the choices that most affect the performance characteristics of a typical Rectangular Anechoic Chamber. The first and foremost criterion that needs to be addressed is “What is the chamber for”. The answer to this question is the primary driving factor regulating the overall chamber design. Is the chamber to be used to evaluate low gain, low frequency antennas? Is the chamber going to be used for RCS measurements of unique test bodies? Is the chamber going to be used to test high gain high frequency antennas? Is the chamber going to be used for far field measurements? Is the chamber going to be used for near field measurements? On and on. The answers to these very basic questions have a dramatic effect on the overall design of the anechoic chamber. Since there are so many preliminary criteria that have to be decided before we can even attempt a design I will make the following assumptions: 1) The chamber is to be a far field antenna measurement facility 2) The chamber is to operate from 2.0 Ghz to 18.0 Ghz 3) The chamber is to be of a rectangular design 4) The quiet zone is to be a 4’ diameter sphere 5) The range length is to be 20’ 6) The desired Quiet Zone performance is a. –30 dB @ 2.0 Ghz b. –40 dB @ 4.0 Ghz c. –50 dB @ 10.0 Ghz d. –50 dB @ 18.0 Ghz With these parameters we will first look at the effect that source antenna selection has on the chamber deign. The first design example will be with a low gain broadband antenna chosen as the source and the second case will be with a high gain antenna chosen as the source. This paper will detail the different design approaches that this choice has on the overall size and absorber placement in the chamber. These will have a dramatic effect on overall chamber size and cost.
NPL has recently commissioned a new indoor test range. This test range has been designed to offer Extrapolation Gain Measurements, Far-Field Probe Calibrations, and eventually, a Spherical Near-Field Test Capability. This paper describes this new range and the results of the initial validation measurements. It also compares the gains of a standard gain horn calibrated in NPL’s old Extrapolation Range with those from the new one.
For the past twenty years, the NASA Glenn Research Center (formerly Lewis Research Center) in Cleveland OH, has developed and maintained facilities for the evaluation of antennas. This effort has been in support of the work being done at the center in the research and development of space communication systems. The wide variety of antennas that have been considered for these systems resulted in a need for several types of antenna ranges at the Glenn Research Center (GRC). Four ranges, which are part of the Microwave Systems Laboratory, are the responsibility of the staff of the Applied RF Technology Branch. A general description of these ranges is provided in this paper.
This paper refers to a special type of antenna, called frequency independent antenna, used in Stepped Frequency Continuous Wave (SFCW) radar employed for humanitarian demining. The radar transmits 128 frequencies within the frequency range from 400 MHz to 4845 GHz, in groups of 8 simultaneously transmitted frequencies. It has been built at the International Research Center for Telecommunications transmission and Radar (IRCTR), Delft University of Technology. Two Archimedean spiral antennas with opposite sense of rotation, in order to decrease coupling signal below –55dB, have been chosen. Precise antenna behavior characterization is needed because SFCW radar is phase sensitive. The paper is focused on antenna footprint measurements, translating data from frequency domain to time domain and gating in order to remove any unwanted signals. Some phase and amplitude pattern using gating measurements are presented.
Two of NASA’s Deep Space Network (DSN) 70-meter reflectors are measured using a Leica TDM-5000 theodolite. The main reflector surface was measured at five elevation angles so that a gravity deformation model could be derived that described the main reflector distortions over the entire range of elevation angles. The report describes the measurement equipment and accuracy and the results derived from the data.
Although the theory is straightforward, practical implementation of spherical near-field scanning for evaluating test chambers presents some significant challenges. Among these are the requirement for accurate probe positioning and the difficulty in minimizing support-structure blockage. We report on recent NIST efforts to mitigate these difficulties and present our most recent results.
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.