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Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. In the case of bistatic RCS measurements, the lack of information concerning calibration techniques is even greater. This paper will present the results of a preliminary investigation into calibration techniques and their suitability for use in the correction of cross-polarization errors when data is collected in a bistatic configuration. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. Results will be presented for data collected in the C and X bands on potential calibration targets. Recommendations for future efforts will also be presented.
The image-based near-field to far-field transformation is based on a reflectivity approximation that is commonly used in ISAR imaging. It is a limited but computationally efficient transform whose accuracy, for appropriate targets, rivals that of computationally more intense transforms. Previous results include applications of the transform to lOA. long wire and lOA. long conesphere numerical data. Here, 1-D and 2-D versions of the transform are applied to conesphere near-field measurements data and the results are compared to corresponding far-field measurements data. Transform errors obtained for these data are compared to corresponding results obtained using newly generated near-field and far-field numerical data. The image-based transform is believed to be especially applicable to the far-field correction of near-field measurements of complicated targets like aircraft or vehicles that are too large or too poorly defined to be simulated numerically.
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 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.
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.
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
Simulation of the antenna measurement errors caused by scattering of range clutters is presented in this paper. The Uniform Geometrical Theory of Diffraction (UTD) based NEC-Basic Scattering Code is used to simulate the measurement of antenna in a far-field range where structure scatterers present. It is known that these errors which come from various directions will impact the antenna under test differently dependent on the characteristics of the antenna under test. With the available computer codes, one can simulate and study various ranges in order to better understand the characteristics of the ranges and properly adjust, modify, and improve the facility such that better measurement results can be obtained.
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges and anechoic chambers contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The fabrication of a feasibility demonstration model Antimultipath System (AMPS) is complete. This AMPS uses a 10 MHz wide phase-shift-keyed spread spectrum modulated signal to illuminate the rotating AUT to tag each multipath by its delay. The novel receive section of the AMPS sorts out each multipath component by its delay and adaptively synthesizes a composite cancellation waveform (using delay, amplitude, and phase estimates of the scattered components) which is subtracted from the total signal received by the AUT. After subtraction the resultant is the desired direct path signal which produced the free space pattern of the AUT. Laboratory and antenna range test results are presented and show the promise of measuring sidelobe levels 60 dB below the main beam.
Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.
Planar near-field antenna measurements have developed into a mature science. Nonetheless, unique difficulties arise when measuring some modern antennas, such as high gain satellite antenna systems. In a typical planar near-field measurement, the antenna under test (AUT) has a collimated beam in the near-field which is perpendicular to the scan plane (i.e. the AUT boresight is parallel to the normal of the scan plane). On the other hand, the scan plane is positioned close to the AUT to maximize the valid angular range in the far-zone patterns. Unfortunately, it is not always possible to place the AUT very close to the scan plane and keep the near-field beam perpendicular to the scan plane. An investigation of the benefits and pitfalls of a planar near-field measurement where the AUT beam is not perpendicular to the scan plane is presented. The measurements of antennas tilted 45 degrees with respect to the scan plane normal are used as examples. With this atypical arrangement, some of the usual errors in a near-field measurement are emphasized. Procedures to identify and reduce these errors will be presented.
The standard planar near-field to far-field transformation method requires data points on a plane-rectangular lattice. In this paper we introduce a transformation algorithm in which measurements are neither required to lie on a regular grid nor are strictly confined to a plane. Computational complexivity is O (N log N), where N is the number of data points. (Actual calculation times depend on the numerical precision specified and on the condition number of the problem.) This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable for other measurement approaches, such as plane-polar scanning, where data are collected on a non-rectangular grid.
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.
Some precautions necessary for accurate RCS measurements using a short model range are discussed. Sources of error in these measurements such as non ideal range geometry, misalignment of the target and inappropriate time domain gating are discussed. A simple technique to estimate possible errors in RCS measurements due to factors such as bistatic angle due to finite separation of source and receive horns and finite length of the measurement range, is presented. The range of RCS values that can be measured within defined error bonds is identified.
A new approach for certification of RCS ranges is discussed. This new approach is based on evaluating the major expected sources of errors in a RCS range rather than evaluating each and every error source and then defining the error bar for a given RCS measurement. The new approach is, therefore, called a top-down approach. Based on our experience with many indoor RCS ranges, we can say that the main sources of errors in RCS measurements are range related. (stray signals, chamber drift, target/mount interactions etc.) One should, therefore, critically evaluate these errors such that the performance level of the range can be verified. A test approach is defined to characterize the range related errors. Various tests are based on the RCS measurement of specific targets, and thus, can be easily performed using standard RCS measurement procedure. This approach will provide range operators with the needed information to justify the use of their range to measure RCS of a given target. Also, one can spend more effort fixing the error sources which lead to large RCS measurement errors.
For the past seven years, ERIM has been studying RCS measurement error sources and processing methods by which these errors can be reduced. Image editing is an extension of range-gating where scattering measurements are improved by removing undesired scattering phenomena in the range-crossrange image domain. Conventional image editing methods rely on a user-supplied polygon to segment an image into desired and undesired scattering regions. However, the polygon method suffers from variability due to user and display characteristics, provides little hope for automation, and cannot be easily extended beyond two dimensions. An alterative approach based on peak region segmentation minimizes or eliminates these limitations and adds an element of optimally that can also improve the performance of image editing techniques. In this paper, we will discuss the application of peak region segmentation to the image editing problem and show examples that demonstrate some of the advantages of this approach.
In this paper we discuss proper RF system design for performing insertion-loss measurements using a microwave receiver and harmonic mixers. Specifically we will deal with problems caused by changing reflection coefficients of the devices which feed the mixer. When broadband mixers and coaxial isolators are used problems may be caused by the changing load seen by the local oscillator. This is due to local oscillator leakage through the mixer and isolator. We will elaborate on this problem, noting its impact on the measurement and suggest a procedure to properly minimize its effect.
To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.
A new 1-50 GHz Near-Field measurement system is now in operation at Matra Marconi Space, Portsmouth, UK. The system has the largest vertical planar scanner installed so far. The planar scanner is constructed of steel and has four moving axes: 22 meter horizontal X axis, 8 meter vertical Y axis, 25 cm Z axis for probe alignment and a 540o Roll axis for polarization. Precision bearings are used to ensure straightness over the full length of the X-Y travel. The vertical Y axis is exceptionally fast, 500 mm/sec, to minimize acquisition time. The scanner has extremely high positioning accuracy and planarity - ±0.2 mm over the entire 22m x 8m range – allowing uncorrected operation (without laser) up to 26.5 GHz. To achieve higher accuracy and a higher frequency range an advanced 3-axis (X, Y, Z) laser correction system automatically creates correction tables for use by the transformation routines. The scanner’s exceptional repeatability allows the use of correction tables created off-line, without need for an on-line laser correction system, considerably reducing measurement time. To create these correction tables, the scanner is fitted with laser interferometers for X and Y axes and with a spinning-diode laser to calibrate for planarity. Additional features include a shielded constant-radius cable carrier, giving minimal phase errors due to cable flexing.
The imaging of radar targets is typically accom plished by measuring the radar cross section (RCS) of the target as a function of frequency and az imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.