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The Ohio State University ElectroScience Laboratory (OSU/ESL) has built a series of radars that transmit UWB random noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. When the response of the system is taken as a function of the delay time, the result is proportional to the impulse response of the system. After background subtraction and calibration, the impulse response of the target results. We will present a description of the variable delay line system and show an example ISAR image made from measurements taken in the OSU compact range.
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.
New results on the complete scattering matrix measurements during the interference between a sphere and a second object are presented. The objects involved are strings of two sized, a rod, and a dihedral. In cases for the strings or the rod, in-phase oscillations in HH and VV are observed. For the dihedral, the HH and VV responses are exactly out-of-phase. We find that the results are in excellent agreement with the characteristics of the scatterer types. Use of targets other than the sphere for cross-polarized calibrations is discussed.
It is possible to build a very inexpensive radar which transmits wide band radio noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. In this paper we will discuss the design and operation of a UWB noise radar which was installed in the OSU compact RCS measurement range. Scattering measurements were made for a number of targets over 360 degrees of aspect angle. Calibration was performed, and then the data converted to ISAR images. Example ISAR images will be shown.
In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.
A fully-polarimetric compact range operating at 160 GHz has been developed for obtaining X-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, CW, X-band synthesizer driving dual X16 transmit multiplier chains and dual X16 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and a rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 40 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 20” quiet zone. A description of this 160 GHz compact range along with measurement examples are presented in this paper.
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.
The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.
Compact Payload Test Ranges (CPTR) for test zones of 5 meters or larger can be used for both payload and advanced antenna testing. In both cases accurate calibration, including amplitude and phase characteristics across the test zone, is required. Accurate data analysis is needed in order to establish corresponding error budgets. In addition, boresight determination will be required in both measurement types for most applications. Since it may be difficult or even impossible to scan the test zone field using a (planar) scanner, application of a large reference target (a rectangular or circular flat plate) can be seen as in interesting alternative. RCS measurements are then performed and test-zone field characteristics are determined in both amplitude and phase. Time- and spectral domain techniques can provide valuable information as to the location of possible disturbances. The evaluations is complemented with the measurement of a VAlidation STandard (VAST) antenna in combinations with an advanced APC technique. These techniques have been demonstrated at the CPTR at ESTEC, Noordwijk, the Netherlands. Results and practical considerations are presented in this paper.
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.
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
A method to generate time and direction of arrival (TADOA) spectra of the quiet zone fields of a RCS/ antenna range is presented. The TADOA spectra is useful for locating the stray signal sources in the RCS/ antenna range. To generate the TADOA spectra, quiet zone fields along a linear scan over the desired frequency band are probed. The probed data are calibrated to remove the magnitude and non-linear phase variation versus frequency. A calibration technique is also proposed in the paper. The TADOA spectra for simulated probed data as well as experimental probed data are shown.
A portable system for measuring the RF environment at remote sites is described. A frequency range between 500 MHz and 18 GHz is covered by this system. The design, calibration and use of this system are discussed.
Photogrammetry, as its name implies, is the science of obtaining precise coordinate measurements from photographs. Until recently, photo-grammetry used film photographs taken with specially designed, high-accuracy film cameras. With the development of h igh resolution solid-state imaging sensors, a new era in photogrammetry has arrived. Video grammetry, as it is often called, provides far faster results and greater capability than film based photogrammetry, and therefore eliminates the major impediments to more widespread use of photogrammetry in the antenna manufacturing industry. Video-grammetry is a powerful enabling technology that not only performs many current measurement tasks faster and more efficiently th an existing technologies, but also, now makes feasible many types of measurements, that pre viously were not practical or possible. The capability for quick, accurate, reliable, in place measurements of static or moving objects in vibrating or unstable environments is a powerful combination of features all in one package. There are many applications for this emerging new technology in the antenna manufacturing industry. This paper will describe some of the successfu l implementation of video-grammetry into the MSA T program at Hughes Space and Communications Company located in Los Angeles, California.
The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.
We discuss how RCS target depolariza tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro tated, there is no need to mount and align multiple sepa rate standards, and clutter and noise may be rejected by averaging over rotation angle.
Boresight and gain determination play an important role in antenna measurements. Traditionally, on outdoor ranges, optical methods are used to determine the boresight. Accuracy requirements better than 0.001 degrees are difficult if not impossible to obtain on outdoor ranges using these method since the effect of incident electromagnetic fields are not taken into account. On indoor ranges no technique is available at present that achieves the desired accuracy demands. In this paper, an improved method for boresighting will be presented. It will be shown that using this technique, desired accuracy demands on both outdoor and indoor can be obtained. Furthermore, the method can also be combined with accurate gain calibration. Advantages and disadvantages of this technique will be discussed.
Calibration of monostatic radar cross section (RCS) measurements is a well-defined process that has been optimized through many years of theoretical investigation and experimental trial and error. On the other hand, calibration of bistatic RCS measurements is potentially a very difficult problem; the range of bistatic angles over which calibration must be achieved is essentially unlimited and devising a calibration target that will provide a calculable scattering solution over the required range of bistatic angles is difficult, particularly for cross-polarized measurements. GTRI has developed a solution for amplitude calibration of both co-polarized and cross-polarized bistatic RCS, as well as a bistatic phase-calibration procedure for coherent systems.
Several approaches are known for the identification of noncooperative air-borne targets with radar. Assuming that the tar get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.