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Near field inverse synthetic aperture radar 3D is performed utilizing data for arbitrary, but known, positioning of the target. The imaging method was implemented and is described. This straightforward approach has many advantages. It geometrically correct in near field. Field corrections can be independently for each frequency, antenna position and point of interest in the target volume. The main disadvantage is that the processing using the algorithm is very time consuming. However, in many cases it is only necessary to perform the analysis on a few cuts through the object volume.
ERIM is investigating the use of modem spectral esti mation techniques for extracting (editing) desired or undesired contributions to RCS and ISAR measurements in two ways. The first approach involves using parametric spectral estimators to perform frequency sweep range compression and signal history editing, while the second involves using the associated stabilized linear prediction filters to extrapolate sweep data and perform "enhanced resolution" Fourier image editing. This paper summarizes our editing algorithms and illustrates RCS editing results using measurements of a conesphere target contaminated by a metal rod and foam support. The reconstructed "clean" conesphere measurements are compared quantitatively against numerically simulated ground truth. Editing was performed using three bandwidths at two center fre quencies to provide insight into the impacts of nominal resolution and scatterer amplitude variation with fre quency on editing efficacy, and to assess the degree to which superresolution algorithms can offset reduced nominal resolution.
Planar near-field measurements are the usual choice when testing phased array antennas. NSI recently delivered a large state-of-the-art near field measurement system for testing a multi beam, solid state phased-array antenna. The critical sidelobe and beam pointing accuracy specifications for the antenna required that special attention be paid to near-field system design. The RF path to the moving probe was implemented using a multiple rotary joint system to minimize phase errors. Additional techniques used to minimize system errors were an optical probe position correction system and a Motion Tracking Interferometer (MTI) for thermal drift correction.
Modem pulsed phased array radar systems bring new challenges to antenna measurement. These antennas generally consist of hundreds of Transmit-Receive (TR) modules controlled via a beam steering computer to fonn the antenna beam. Attempting to operate these modules with a CW wavefonn will not only quickly damage the mod ules but will not properly characterize the antenna. The Navel Surface Warfare Center, Crane Division, recog nized the need to add pulsed capability when specifying their latest antenna measurement system. Scientific Atlanta met these requirements by integrating their newly introduced Model l 795P Pulsed Microwave Receiver into their proven 2095 Microwave Measurement System to make the Model 2095P Pulsed Microwave Measurement System.
Deployable phased array antennas for satellite use have been of recent concern[ 11[2]. In a phased array antenna, the excitation phase is the most important factor. For the case when the observation point coincides with the point towards which the beam should be directed, the method for determining the excitation phase is presented in reference [1]. In this case, deflection of the main beam due to a change of the satellite attitude or distortion of the antenna surface can be corrected. However, in an antenna in which the shape of the main beam has to be maintained, the observation point should be placed apart from the main beam region. In this paper, we investigate the necessary number of observation points to correct main beam deflection and present a method that makes it possible not only to correct the deflection of the main beam but also to measure displacement of relative element positions.
We investigated two methods of probe-position error correction to determine how well the corrected results compare to the uncorrupted far field: the k-correction method and the Taylor-series method. For this investigation, we measured a 1.2 m dish at 4 GHz and a 1.2m by 0.9m phased array at 2.2 GHz. Measurements were made first without position errors and then with deliberate z-position errors. We perfonned probe position error correction using both methods and compared the results to the error-free far field. For errors up to A/4, the fifth-order implementation of the Taylor series correction was slightly better than the k-correction. For errors of ')..J2, the k-correction was better than the Taylor-series correction.
Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.
Evaluation of serrated edge and blended rolled edge compact range reflectors is presented in this paper. An interactive approach is used to design the serrated reflectors. Several issues associated with the serrated reflectors are also discussed in this paper. Quiet zone fields for various serrated edge with an optimally designed blended rolled reflector are presented for comparison. In addition, simulations of a low sidelobe phased array measurement using serrated and blended rolled edge reflectors are shown to investigate their impact on the measurement accuracy.
The radiation characteristics for an active phased array receive antenna operating at K Band were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured for several scan angles. In this paper we'll discuss the general design of the antenna and the instrumentation ensemble used to perform the far field and near field characterization of this antenna. Measurements taken on a 2600 foot far field range vs. a near field planer scanner are compared.
Norden Inc. has developed and instrumented its JSTARS 1000' Outdoor Antenna Range with a multi-port antenna measurement system designed to acquire antenna data (patterns and other related signals) at the antenna's respective radar system's intermediate frequency (IF). The measurement system utilizes the JSTARS RF microwave receivers attached to the multiple channels of the JSTARS antenna. These receivers obtain the RF signal from these multiple channels and provide the IF signals to the measurement system.
As mobile and satellite phased array antennas move from to concept production the demands on test station throughput increases dramatically. Completely characterizing a Transmit/Receive (TIR) module may require thousands of S-parameter measurements under CW and high-power pulsed conditions, as well as, harmonics, spurious, and noise figure measurements. The measurement throughput of instrumentation used in characterizing the prototype TIR modules simply may not be capable of handling the added volume of a production environment. The volume of measurements, the multiport nature of the device, and the integrated TIR module control make it necessary to reexamine the traditional approaches of separate network analyzers, noise figure meters, and spectrum analyzers. The result is a high-speed modular test ystem that completely characterizes the device in a single connection. The system contains a single receiver and a dedicated controller that utilizes the instrumentation in the most efficient method while maintaining or increasing the accuracy of traditional approaches. This paper describes the high-speed test stations that have been designed and built and are currently in use in several production facilities. Test system architecture is discussed and measurement throughput numbers are given and compared to conventional approaches.
Applications that require tight tolerances on dielectric thickness control need accurate sensors. A technique has been developed that will allow for the measurement of thickness without requiring surface contact. High resolution radar imaging, commonly used in RCS measurements , is now being used to measure thickness. Electromagnetic fields reflected from the front and rear surface are detected and the time response delta is converted into thickness. A major advantage of this method is that it is not affected by varying sensor offset height.
Characterizing antennas under pulsed RF conditions has focused attention on a class of measurement challenges not normally encountered in CW measurements. The primary problems often include high transmit power, thermal management of the AUT, and a close interaction between the antenna and its transmitting circuitry. This paper presents instrumentation techniques for pulsed RF antenna measurements using the Scientific-Atlanta 1795P Pulsed Microwave Receiver as an example of a commercially available solution applicable to both active and passive apertures. Emphasis is given to measurement speed, dynamic range, linearity, single pulse versus multiple pulse measurements, pulse width, pulse repetition frequency (PRF), frequency coverage, system integration and automation, and suitability of equipment for antenna range applications.
This paper describes the significant upgrades to METRATEK's Model 100 AIRSAR Dynamic Imaging System since the earlier version was discussed at last year's conference. This system consists of three wideband radars mounted on a A-3 aircraft. It can generate diagnostic images airborne targets up to 200 feet in length and width. We will present examples and discussions of the solutions found to the many difficulties involved in generating high quality, high resolution, fully-calibrated SAR images of aircraft in flight from aircraft in flight. Data collection and processing hardware and software, as well as lessons learned from over 6 months of flight tests will also be described.
The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
The LX/LH organization of McClellan AFB has been using near-field (NF) technology for the past two years to test an Air Force phased array receiving antenna. McClellan uses both a close range surface RF scanner and a larger offset, fain and back-transform near-field scanner. NF testing is done for both trouble-shooting purposes to support repair efforts, and for acceptance-testing to certify the antenna. The purpose of this paper is to show how McClellan interprets its planar near-field data for diagnosing antenna faults. First the various near-field techniques used by the LX/LH organization will be discussed. Following, will be an examination of the antenna defects pointed out by the NF test date. Failures will be traced to the component level where possible. Techniques other than near-field, such as electronic test, will be used to verify these problems. Additionally, the repair methods will be discussed.
Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.
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.
A phased array antenna has been considered the favorite candidate and been developed for land mobile satellite communications. However, in communication experiments, a noise level in a receiving frequency band is found to be increased when a signal is transmitted. The amount of noise increase is found to depend on a scanning angle in azimuth directions to track the satellite, and the value is up to 20 dB in maximum and 5 dB in minimum. The noise increase was found to be caused by an nonlinear (sic) effect of a PIN diode, which are essentially used in phased array antennas.
Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.