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One approach to getting near-field ISAR measurement costs down is to dispense with track or turntable, and instead mount the SAR antenna on a vehicle and simply 'drive by' the target of interest, which might be a vehicle or aircraft standing on tarmac. In this situation the antenna path will depart from a perfect straight line or circular arc, and there will also be vibrational wobble at the antenna phase center. These effects can defocus the images obtained. One way to overcome the focus problem is to mount strategically placed corner reflector(s) in front of the target, each in a different range cell. These then act as phase references, used to refocus the image. However it is not strictly necessary to employ reflectors - a good focus can normally be obtained by suitably processing just the target return itself. This paper will describe autofocus procedures which have been sucessfully used in conjunction with chirp radar data, in the 'driveby' situation.
The measurement of the characteristic antenna data by means of conventional far-field ranges in frequencies up to 200 GHz requires measurement distances of some kilometers. The high atmospherical attenuation and the low available transmit power limit the dynamic range of the measurements considerably. The DASA Compensated Compact Range (CCR) /1/ is a high precision test facility; which avoids these disadvantages and allow measurements with considerably higher accuracy under controlled environmental conditions. The precision reflectors have an extremely high surface accuracy of 25 µm RMS, which allow their use even in the mm-wave range. For the frequency band of about 200 GHz, the relative roughness is in the order of N/60. This results in considerably lower degradation for the DASA CCR compared to the typical degradation on far-field ranges (N/16). For mm-wave application the test facility is equipped with broadband transmit and receive moduls, which covers the frequency range from 75 to 220 GHz. The basic transmit frequency is generated in a tunable Gunn oscillator, which is phaselocked to an externally supplied I 0 MHz reference signal. This optimized concept allows measurements with a dynamic range of more than 60 dB at 200 GHz. For a cost efficient solution the complete equipment for the transmit and receive moduls consists of commercial components. Keywords: MM-Wave Antenna Measurement, Compensated Compact Range, MM-Wave Transmit Module Tracking Converter
Gabor holography is an appropriate technique for near field measurements at THz frequencies when apertures of the order of thousands of wavelengths are involved. The method permits pattern prediction over a restricted angular range from intensity measurements, providing a direct method of recovering phase which overcomes cable, planarity and atmospheric effects; problematic to conventional near-field phase measurements. We demonstrate the feasibility and convenience of the method with an example planar near-field measurement at 94GHz for a 1.1m Cassegrain reflector and we determine the relationships governing dynamic range and the requirements for sampling. Finally, two-dimensional numerical simulations for a lm antenna at 0.5THz, with a 10m scan distance, will be presented to demonstrate the feasibility of the method for large terahertz antennas.
The DTU-ESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex tensive period of time been used for calibration of Standard Gain Horns (SGHs). A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent near-field to far-field (NF-FF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically. During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.
For frequencies above 30 GHz, RCS reference target method is, in general, more accurate than scanning the field by a probe. Application of mechanically calibrated targets with a surface accuracy of 0.01 mm means that the phase distribution can be reconstructed accurately within approximately 1.2 degrees across the entire test zone at 100 GHz. Furthermore, since the same result can be obtained for both azimuth and elevation patterns, all data is available for the characterization of the entire test zone. In fact, due to the fact that the reference target has a well known radar cross-section, important indication of errors in positioning can be obtained directly from angular data as well. In the first place the data can be used in order to recognize the first order effects (+/- 5 degrees in all directions). Applying this data, defocussing of the system reflector or transverse and longitudinal CATR feed alignment can be recognized directly. Furthermore, mutual coupling can be measured and all other unwanted stray radiation incident from larger angles can be recognized and localized directly (using timedomain transformation techniques). Inmost cases even a limited rotation of +/- 25 degrees in azimuth and +/- 10 degrees in elevation will provide sufficient data for analysis of the range characteristics. Finally, it will be shown that sufficient accuracy can be realized for frequencies above 100 GHz with this method.
Because the incident wave on an anisotropic material is likely to be depolarized, a complete characterization of such a media requires to measure its whole scattering matrix, which afterwards complicates the calibration process. A suitable technic is the Wiesbeck calibration method [1]. In this paper, we apply this method to two configurations, the reflection configuration and the transmission configuration, and obtain very good agreements between theoretical and experimental results.
One of the key problems in SAR interferometry is the determination of the absolute phase of a scatterer by unwrapping the calculated phase difference of two SAR images. Since the phase difference is a function modulo 2p software algorithm are used to perform a phase unwrapping to obtain an unambiguous phase and thus the corresponding height information. This paper presents a procedure using two different transmit frequencies to enlarge the unambiguous range for height determination. Measurements performed in an anechoic chamber are used to test the processing. Using this procedure it is not only possible to resolve the height of a surface but also of single scatterers in space or in urban areas with steep slopes.
The recently introduced Fractional Fourier Transform (FRT) operation was shown to be useful for various spatial filtering, optical and signal processing applications. In contrast to the Fourier Transform, the real part of the FRT of a delta function can be "tailored" (by the position of the delta and the transform order) to have many zeros in specific areas of the spectrum and fewer zeros at other areas. This property was exploited to design new spectral windows, which pass through zero many times in the spectral points we define. These windows can be used in case of loss of information or partial information collected in a usual ISAR stepped-frequency data process. Another, more valuable property of the FRT, used here for radar imaging applications, is it's being a projection of a rotated Wigner transform of a signal. This property was used to filter resonant structures in the radar image, which cause the image to be smeared in the range.
This paper discusses the efforts of an on-going research program which has been exploring the use of expert systems (artificial intelligence) techniques to support automated analysis of wideband radar scattering data. The primary objective of this research is to explore and demonstrate the applicability of expert system techniques to the analysis of diagnostic radar images. There are two modes which are being explored. The first is an automated system that would allow lesser skilled (in radar imaging science) individuals to do the work of highly skilled engineers and analysts. The second mode would aid the highly skilled worker with the application and correct implementation of software tools, interpretation of phenomenology, and data quality assessment. In both cases, the expert system should allow for the increase through-put and accuracy of data being analyzed. A software prototype is being developed and tested with real data to demonstrate the feasibility and potential accuracy of such as system.
The Joint Strike Fighter (JSF) office sponsored a Navy directed limited technical demonstration of diagnostic Radar Cross Section (RCS) imaging on-board an aircraft carrier at sea. The overall objective was to obtain experience and data sufficient to assist the Navy in defining any future shipboard diagnostic imaging measurement system requirements. Measurements were conducted in the hangar bay to assess the challenges posed by the carrier environment. A technique for making diagnostic imaging measurements in spatially confined areas was developed.
A method is presented that gives a 3D ISAR image from a 2D measurement. An ordinary 2D image is created. An extra receive channel is used to give height information in every pixel. This channel gets its signal from two extra receive antennas with different elevation lobes. The antennas feed a hybrid which creates a difference signal that goes to the extra receive channel. Height information is derived like in an amplitude comparison monopulse radar. This way a height number is assigned to every pixel in the 2D image. Thus a 3D image is created. It is required that in a 2D resolution cell the reflexes come from only one height. If not, the height information given by the difference signal will be a weighted average of the heights of the reflexes. The method is applied to a ground plane RCS range. No measurements have yet been performed.
A waterline bistatic algorithm, based on the exact near field to far field transformation (NFFFT) and previously exercised on numerical data, is here applied to actual measured data taken at a traditional RCS range reconfigured for near field measurements. The resulting far field predictions for a lOA and 20A conesphere were initially worse than expected. Further examination of the data yielded two important observations. First, the data were found to have relative alignment errors from set to set, leading to a significant broadening of the predicted far field peaks. Second, a few data sets exhibited a constant phase offset inconsistent with the other measured data. This paper discusses the detection of the data misregistration issues highlighted above, along with their ad hoc correction. Predictions are give for the waterline bistatic NFFFT algorithm applied to the measured near field data, both before and after the corrections have been applied. The results are compared with analogous results for numerical input data.
New measurements on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz are presented. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, and combinations of these. A two dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd- or even-bounce nature. Use of the corner as a calibration target is discussed.
This paper deals with High Resolution (HR) Filtering. Extracting the frequency dependence of radar scatterers is a common task in Radar Cross Section Analysis (RCS). This is usually achieved with signal processing tools like finite impulse response filters allowing filtering in the range domain. However, when range resolution is poor, it becomes impossible to extract the exact feature since it is not deconvolved in the range domain. We thus propose to use HR methods to overcome these difficulties. These methods are applied to estimate the frequency response of the creeping wave of a small sphere. The results show good agreement with the theoretical response.
NASA Langley Research Center (LaRC) is participating in a technology program element in Synthetic Aperture Microwave Radiometry. This includes deployable antenna technology for "sparse" arrays to provide improved spatial resolution with lower mass and less packaging volume. One instrument under consideration includes a deployable L-Band antenna made up of 16 slotted waveguide array elements. Mutual coupling between elements is known to be critical to the calibration and performance of these systems. Currently, waveguide element portions of a 37 GHz "minimum redundancy" array, on loan from the US Naval Research Laboratory, were characterized in an effort to develop a computer model of such a system. Coupling measurements were performed on the WR-28, slotted array elements at spacings out to 50 wavelengths. Measurement results will be used for radiometric modeling and validation of a new coupling prediction code developed at NASA LaRC.
A sampling strategy for array diagnosis is discussed. The proposed strategy is able to obtain a minimum (i.e. equal to the number of array elements), and optimum (i.e. working as well as a uniform l/2 sampling rate) number of measurements in a given region of a plane in front of the array.
Measurements of antennas with integrated electronics is an upcoming topic. In many cases the antennas can only work in pulsed mode which requires synchronisation between radar and measurement equipment. Up and down mixing by internal LO's causes additional problems, especially with Near Field measurements where amplitude and phase data is required. Based upon hands-on experience, this paper treats some of the problems and pitfalls related to the Near Field measurements of an active antenna and alignment of the elements by means of backtransformation of the data.
When a phased array antenna consists of a number of complex subarrays, efficient and accurate testing of the subarrays is essential for overall project success. This paper presents a flexible system for testing various parameters of a subarray of an active aperture phased array antenna including Sparameters, noise figure, spurs, oscillations, and peak and average power. Testing is done for both CW and a variety of pulsed signals. A system block diagram is presented and system architecture explained. Timing diagrams are included for testing multiple states (which correspond to antenna beams), channels, and frequencies. Measured verification results are presented.
The Antenna Repair Facility at McClellan AFB, which has been responsible for the repair and maintenance of Air Force antennas and radars over several decades, now faces a new challenge: transferring their many years of skill in antenna maintenance from their closing base to acquiring bases or contractors. This must be done while maintaining their in-house expertise and production levels, while manpower decreases. A possible solution to these problems is the implementation of expert systems. This is where human knowledge and expertise is transferred to computerized systems. Currently, the repair shop is testing such a system for analyzing diagnostic data on an electronically steered, phased-array satellite ground station antenna. This paper will examine the design options considered, such as using a commercial software package versus building in a higher order language. It will also discuss the design process for such an expert system, and cover the key issues of knowledge acquisition and selection. The paper will include details of the current design, such as structure and control, and discuss plans for future enhancements.
Instrumentation Radar systems evolution includes improved stability. Metrologists know frequency within Hertz. Amplitude and Phase variations are low. Ranges check drift with reference systems. Still, with increased capability, expectations of accuracy have increased. Todays instrumentation makes analysis of stability factors practical. This study analyzes Radar Cross Section (RCS) return of a stable target under controlled conditions. Methodology will be an analysis of a constant RCS target return. The target is a stable object at a typical measurement site. Data points are at several discrete frequencies in bands between S and Ku. This study sample is a set of data taken over a 87 hour span with several duty factors. Duty factors will range from minimal 0.1% to 1.5%, near the 2% maximum for the output amplifiers. Acquisition times for data sets are chosen for outdoor temperatures ranging from hot -- desert afternoon -- through cool in the early morning. This data will be analyzed statistically. If statistical correlations exist, analysis will quantify factor contributions with multiple linear regression. Hypothesis: Drift does not correlate to variables such as duty factor, & temperature.