AMTA Paper Archive

The requirement to calibrate and test large active pulsed planar array RADAR antennas, such as the one developed for the advanced synthetic aperture radar (ASAR), places certain requirements on the measurement facility and analysis software that are perhaps not encountered in other areas of application. This paper gives a brief overview of ASAR and an introduction to some of the difficulties encountered during the test and measurement campaign. Results are presented that compare measurement with theoretical prediction. Good agreement has been obtained for both far and near field data.

Near-field ground-to-ground imaging systems are widely used to discover damage that could degrade the radar signature of low observable vehicles. However, these systems cannot presently assess the impact of this damage on the far-field signature of these vehicles. We describe progress made on a method to accurately project the near-field data from these to the far­ field. Near-field data for the algorithm development is provided by the hybrid finite element/integral equation RCS computer code SWITCH. The near-field data is processed to extract the near-field scattering centers using imaging. The imaging algorithm used differs from the usual far-field imaging formulation in that it incorporates some near-field physics. The processing algorithm, which incorporates a modified version of the CLEAN technique, verifies that the scattering centers that were extracted reproduce the original data when illuminated in the near-field. These near-field scattering centers are then illuminated by a plane wave to produce far-field data. This procedure was tested using VHF band scattering data for a full size treated planform. The near field data was projected to the far-field and then compared to data from a far-field SWITCH computation.

Undesired antenna motion can significantly degrade SAR and ISAR image quality on an instrumentation radar operating in an outdoor or uncontrolled environment. Antenna vibration on the order of only a few hundredths of an inch at X-band frequencies can degrade performance to the point that one cannot reliably differentiate between the true and false peaks in the radar image. This paper describes a motion compensation technique that utilizes the measurements from an auxiliary antenna pointing at a stationary target. This "Phase Monitoring Subsystem" accurately records the linear antenna motion profile, which can then be used for compensation. Data collected at the US Naval Undersea Warfare Center (NUWC) Fisher Island Test Facility on a calibration target demonstrate that this compensation technique can reduce image artifacts by more than 20 dB.

Inverse synthetic aperture radar (ISAR) imaging on a turntable-tower test range permits convenient generation of high resolution two- and three­ dimensional of radar targets under controlled conditions, typically for characterization of the radar cross section of targets or to provide data for testing SAR image processing and automatic target recognition algorithms. However, turntable ISAR images suffer zero-Doppler clutter (ZDC) artifacts and near-field errors not found in the airborne SAR images they seek to emulate. In this paper, we begin by reviewing a technique to suppress ZDC while minimizing effects on the target signature. Next, turntable ISAR images of a vehicle formed at Georgia Tech's Electromagnetic Test Facility are used to demonstrate a computationally-efficient implementation of a backprojection (BP) image former. BP-formed ISAR images are free of all first­ order near-field errors. Finally, images generated using these techniques are compared to images obtained using electromagnetic prediction codes.

This paper presents the development and testing of a small low-cost (COTS) 3.5 GHz Ground Vehicle Detection Radar. In this frequency band, the signal can penetrate light brush and foliage. However, the detection and tracking of radar targets where both the radar and the target are close to the ground is particularly difficult because of ground wave attenuation and foliage dynamics. The radar uses all surface mount components for small size and low cost. A VCO is used to cover the frequency band from 3.1 to 3.6 GHz. A power splitter and a quadrature mixer follow the VCO. Thus the radar operates as a base-band system for direct down-conversion. We will show the design procedure for this radar as well as test results confirming the design. We will also show detection and tracking results for vehicle targets in this foliage/brush penetration close-to-the-ground environment.

The measurement of the frequency response of complex targets of interest for the purpose of radar cross section (RCS) analysis has become a common task for modern radar ranges. When carefully done to avoid transients, the stepped frequency continuous wave (CW) method directly measures the frequency response of the target. On the other hand, dechirp-on-receive processing utilized by linear frequency modulated (LFM) radars introduces certain distortions to the measurement that are rarely fully considered. In this paper, we derive the relationship between the true frequency response of a target and what is measured with an LFM radar utilizing dechirp-on-receive. One can use this relationship to analyze the effects of the LFM processing as a function of the target geometry or scattering mechanisms and radar parameters. Radar parameters may then be selected so as to minimize the differences between the LFM measured response and the true frequency response of the target.

In order to better understand the target-background interaction, we present new observations on the azimuthal and frequency dependences of the backgrounds, with the upper turntable (UTT) either kept stationary or in a constant rotation. In the stationary case, vector subtraction of backgrounds measured within seconds yields the lowest achievable residual levels between -50 and -60 dBsm. For the rotating UTT, the hot spots (regions of high background) exhibit a 4-fold symmetry in the azimuth, in frequency from 0.5to 4.0 GHz, and are positively identified as due to Bragg diffraction from the periodic 2-D structure pf absorbers with a 12"-square unit cell. Subtraction of backgrounds by azimuth yields a characteristic residual which mimic the structure of the hot spots. Aluminum rods (of small ka, supported by strings from the UTT in a horizontal position) provide an opportunity for studying the background interference with the echoes in HH, VH and VV, in order of decreasing signal. The results suggest that knowledge about the hot spots is essential for choosing the low background regions for measurements on low RCS objects.

Radar cross section (RCS) is a primary determinant of ship susceptibility to attack by antiship cruise missiles. RCS management benefits from the clear association of individual scatterers on a ship with measured ship RCS data, which is the scatterer identification problem. It is an. inverse scattering problem in which the scattering object is extremely complex, and environmental effects such as multipath and ducting corrupt the measurement channel. This paper describes a new method of solution to this important problem. The approach uses high­ fidelity models of ship RCS, of the radar signal processing, and of the environment in a constrained optimization framework. In so doing, advances are made in the areas of scatterer identification and predictive RCS model validation. Promising experimental results are presented that directly relate scatterers in a predictive RCS model of a ship to measurements of the ship taken in a maritime environment.

The NRTF and MRC have recently completed the first bistatic RCS test utilizing the Bistatic Coherent Measurement System (BICOMS). BICOMS is the first true far-field, phase coherent, bistatic RCS measurement system in the world and is installed at the NRTF Mainsite facility. The test objects include a 10 foot long ogive and a 1/3 scale C-29 aircraft model. Full pol rimetric, 2-18 GHz monostatic and bistatic RCS measurements were performed on both targets at 17 degree and 90 degree bistatic angles. BICOMS data demonstrates excellent agreement to method-of­ moments RCS predictions (ogive) and indoor RCS chamber measurements (monostatic, ogive). This paper describes the BICOMS system and the test process, highlights some process improvements discovered during testing, assesses the quality of the collected data set, and analyzes the accuracy of the bistatic equivalence theorem.

The Radar Signature Management Group of Racal Defence Electronics Limited specializes in the measurement, prediction and analysis of radar signatures. Types of measurement ranges used by the Group fall into three categories: • Indoor instrumented ranges • Outdoor measurement ranges • Full-scale trials, in which dynamic measurements are made of the target in its normal operational environment This paper describes a methodology used for characterizing the uncertainties within data from one of the outdoor RCS measurement ranges, at frequencies from 8 to 12 GHz. The results are summarized and uncertainties arising from the following sources are quantified: • Linearity • Absolute Accuracy • Stability and Repeatability • Polar Diagram The effects of background and target-to-pylon support interface are also discussed. The individual uncertainties are combined in a simple manner in order to obtain an overall uncertainty bound for the range, and recom mendations are made for reducing uncertainties against the difficulty and cost of implementation.

Radar Cross Section measurements require the target to be in the far field of the illuminating and receiving antennas. Such requirements are met in a compact range in the SHF band, but problems arise when trying to measure at lower frequencies. Typically, below 500 MHz, compact ranges are no more efficient, and one should only rely upon direct illumination. In this case, the wavefront is spherical and the field in the quiet zone is not homogeneous. Furthermore, unwanted reflections from the walls are strong due to the poor efficiency of absorbing materials at these frequencies, so the measurement that can be made have no longer something to see with RCS, especially with large targets. We first propose a specific array antenna to minimize errors caused by wall reflections in the V-UHF band for small and medium size targets. Then an original method based upon the same array technology is proposed that allows to precisely measure the RCS of large targets. The basic idea is to generate an electromagnetic field such that the response of the target illuminated with this field is the actual RCS of the target. This is achieved by combining data collected when selecting successively each element of the array as a transmitter, and successively each other element of the array as a receiver. Simulations with a MoM code and measurements proving the validity of the method are presented.

A radar transceiver operating at 1.56 THz has recently been developed to obtain coherent, fully polarimetric W-band (98 GHz) RCS images of 1:16 scale model targets. The associated optical system operates by a scanning a small focused beam of swept­ frequency radiation across a scale model to resolve individual scattering centers and obtain the scaled RCS values for the centers. Output from a tunable microwave source (10 - 17 GHz) is mixed with narrow band submillimeter-wave radiation in a Schottky diode mixer to produce the chirped transmit signal. Two high-frequency Schottky diode mixers are used for reception of the V-pol and H-pol receive states, with a fourth mixer providing a system phase reference. The full 2x2 polarization scattering matrix (PSM) for each resolved center is obtained following off-line data processing. Measurement examples of five simple calibration objects and a tank are presented.

The purpose of millimeter wave RCS measurements is often to evaluate the performance of scale model aircraft. To representative ISAR it is important that also the resolution cell size is scaled in proportion to the frequency. A typical bandwidth used for full scale aircraft measurements at 10 GHz is 2 GHz. This means that for at a 1:10 scale model measured at 100 a bandwidth of 20 GHz should be used. By modifications of a HP83558A W-Band antenna measu rement equipment, a powerful RCS measurement equipment covering 75 - 103 GHz with high receiver have been achieved. The hardware modifications and the radar and turntable performance are presented. This paper also shows the W-Band requirements for the SAAB indoor RCS measu rement facility in Linkoping, Sweden, and how these requirements are fulfilled. RCS measurements have been performed on 1:50 and 1:10 model aircraft. These measurements are discussed and ISAR images with resolution cell sizes down to 10 mm x 10 mm are presented.

This is a report on work in progress to understand the wide variations in sphere calibration data observed on dynamic radar cross section measurement ranges. The magnitude of these fluctuations indicate an uncertainty of greater than 2 dB in some cases. The range of fluctuations in the received power (which is well beyond fluctuation due to received noise) underlines the need for a thorough understanding of sources of uncertainties in dynamic radar cross section measurements. In addition to the fluctuations, we observe a systematic error with respect to the mean of the data segments, possibly due to drift, pointing errors and I or target-background interactions. Understanding the error mechanisms in these measurements allows us to reduce the overall uncertainty and to improve data quality.

(U) Precise radar cross-section (RCS) calibration are needed for all RCS measurement facilities. In 1996, AFRL began to advocate the use of a series of precision, short cylinder RCS calibration standards, demonstrating consistently greater accuracy than traditional sphere targets. Previous AMTA publications [1,2,3,4] demonstrated the overall measurement fidelity of these targets. However, questions regarding the accuracy and stability of the numerical RCS solutions to these cylinders continue to be raised. This paper will strictly and thoroughly examine the accuracy of several numerical techniques used to predict the AFRL calibration cylinder RCS, and will examine such "real world" issues as gridding sensitivity, conductivity vanat1ons, frequency bandwidth, and practical manufacturing tolerances.

In order to have definitive measurement traceability according to, ANSI-Z-540, a radar cross section measurement facility must have solid traceability to a known and accepted measurement standard. The Air Force Research Laboratory choose short right circular cylinders as calibration standards for their facilities. We describe a general RCS uncertainty analysis technique, and apply the method to our calibration standards to establish absolute traceability to a known standard. Though applied to cylinders in the current paper, the uncertainty method is general enough for any arbitrary target

This paper describes how ANSI standard Z-540 [l,2,3] was applied in a DoD demonstration project to organize radar cross section (RCS) range documentation for the Air Force Research Laboratory Advanced Compact Range (ACR) and Patu:xent River Atlantic Test Range (ATR) Dynamic RCS measurement facility. Both parts of this paper represent a follow-up report on the DoD demonstration program introduced at AMTA 97 [4]. In June 2000, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that these two facilities met the ANSI-Z-540 documentation standards established by the DoD demonstration project. Since AFRL plans to require mandatory ANSI-Z-540 compliance for DoD contractors performing RCS measurements with AFRL after January 1, 2004, the review process described in this paper will be the likely model for industrial compliance. After a brief review of the ANSI-Z-540 standard, Part 1 of this paper will outline the certification review process and discuss the outcomes, results, and lessons learned from the DoD demonstration program from the perspective of the AFRL range and volunteer range reviewers.

This paper describes how ANSI/NCSL standard Z- 540 [1, 2] was applied in a DoD demonstration project to organize radar cross section (RCS) range documentation for the Air Force Research Laboratory (AFRL) Advanced Compact Range (ACR) and the Naval Ai:r Warfare Center - Aircraft Division (NAWC-AD) Atlantic Test Range (ATR) Dynamic RCS measurement facility. Both parts of this paper represent a follow-up report on the DoD demonstration program introduced at AMTA 97 [3]. In June 2000, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that these two facilities met the ANSI/NCSL Z-540 documentation standards established by the DoD demonstration project. Since AFRL plans to require mandatory ANSI/NCSL Z- 540 compliance for DoD contractors performing RCS measurements with AFRL after January 1, 2004, the review process described in this paper will be the likely model for industrial compliance. Part I of this paper contained a brief summary of the ANSI/NCSL Z-540 standard, outlined the certification review process and discussed the outcomes, results, and lessons learned from the DoD demonstration program from the perspective of the AFRL range and volunteer range reviewers. Part II will discuss the review process as it applied to ATR, as well as the outcomes, results, and lessons learned.

An efficient algorithm for the RCS evaluation of the Monostatic Radar Cross Section (RCS) from a reduced set of bistatic near-field data is proposed. The algorithm allows to evaluate the monostatic RCS from near field data collected in an angular region centered on the direction of interest, whose amplitude depends on the size of the scatter and the distance of the measurement zone. Numerical examples on two dimensional elliptical cylinders show the effectiveness of the proposed technique.

The U.S. Navy has considerable experience in the radar cross section (RCS) measurement of dynamic targets. An understanding of the possible error sources and their relative magnitudes is critical to obtaining accurate and repeatable results. In addition to the usual potential sources of error in RCS measurements of stationary items, considerations with dynamic targets include target range and angle tracking, calibration, and various environmental effects. The primary considerations are identified and discussed, and an error budget is developed for a particular test scenario.