AMTA Paper Archive

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.

Antenna tracking systems are an important part of practical system designs. The goal of antenna tracking for communication applications is to provide sufficient accuracy to limit pointing loss, while for radar applications, to determine the target’s position as accurately as possible. Antenna tracking systems are reviewed describing both open and closed loop designs. Corresponding measurement techniques to quantify system performance are described.

This paper will discuss the design and performance of a small step-frequency homodyne monopulse radar. The radar is designed to sit on the ground and penetrate weedy foliage to observe moving vehicles. It operates with horizontal polarization near 3.3 GHz with approximately 500 MHz bandwidth. Only 8 dBm power is needed. We will show the results of tests done with a corner reflector and with a walking human. Tracking performance in both range and azimuth will be shown.

A system has been developed for acquiring an antenna’s complete (3D) radiation pattern and radar cross-section (RCS) measurements. The system consists of a motion controller, a network analyser and tower assembly. The tower assembly is in an anechoic chamber. The tower has a novel design. It uses three motors in a special configuration, thereby allowing 2 ½ degrees of freedom. This freedom gives the ability to run complete antenna or RCS measurements automatically. Another advantage stemming from the degrees of freedom is expansion of the range of measurements. This is enabled by a variety of possible positions inside the chamber. Tests have also been carried out on system performance. The data acquisition rate becomes crucial when dealing with 3D pattern measurements. The performance of an HP 8720 or 8753 network analyser series can be dramatically increased by using the power sweep mode for data acquisition. Together with the “external trigger-on-point” mode, this gives the best positioning accuracy. The six-month experience has demonstrated the flexibility and reliability of the set up and ideas.

In June 2001, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that the Helendale Measurement Facility (HMF) outdoor radar cross section (RCS) measurement Range Book met the ANSI-Z-540 documentation standards established by the DoD demonstration project. This paper describes how Lockheed Martin Aeronautics (LM Aero) applied the ANSI Z-540 [1,2,3] standard to obtain National Certification of the HMF RCS range. The dual calibration results for Pit #1 and Pit #3 are presented showing upper and lower uncertainty error bounds established by this process. Schedule, cost, range book format, and “lessons learned” from the LM Aero experience are also discussed.

This paper presents the results of NCTR research performed at the POSTECH compact range. The radar cross section data of five scaled aircraft models, such as F4, F14, F16, F117 and Mig29, have been measured over a frequency region of X-band and an angular sector of 29.6o. Afterwards, one-dimensional radar signatures at several aspects of each target are obtained by modern spectral estimation techniques, including MUSIC, Fast Root-MUSIC, TLS-Prony, matrix pencil, TLS-ESPRIT. The proposed features are based on the central moments of a given radar signature distribution, and they can provide scale and translation invariance, which are essential for the improvement of NCTR performance. After the appropriate post-processing, the proposed features are classified by the Bayes classifier. Results show that our proposed technique has a significant potential for use in NCTR or ATR areas.

An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.

In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets [1]. In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system [2]. In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.

Radar return data from various types of aircraft were collected and analyzed during varying flight profiles to determine the presence of consistent, dominant radar returns of point scatterers on the aircraft. These measurements were performed by integrating two separate X-band radars into one system with the ability to simultaneously track and image aircraft. Selected processed data from both radar systems were analyzed and are presented as a function of time, azimuth and elevation angle, and range. I/Q data, high-range resolution (HRR) profile data and inverse synthetic aperture range (ISAR) data are presented for selected flight profiles of helicopters, propeller aircraft, and jet aircraft.

The implementation of low observable (LO) materials and the fielding of aircraft with controlled signatures creates a new degree of difficulty for maintaining, executing prompt accurate inspections and achieving meaningful evaluations. To address this problem, Sensor Concepts, Inc (SCI) has prototyped a new radar system, (the SCI-Xe) to provide a test bed for a lighter, smaller RCS measurement and imaging system. The hardware consists of a suitcase containing RF hardware, computer and display and a hand-held or rail-mounted unit containing two X/Ku band antennas. In the rail-mounted application, imaging is followed by registration and image differencing, which allows an operator reproduce a baseline measurement geometry and evaluate RCS changes. The hand-held application forms a synthetic aperture by moving the antennas by hand. This can be used to quickly investigate an object under test.

This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery [1], where it is often referred to as the “Range Migration Algorithm (RMA)”[2]. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.

In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.

The growing need for a mobile radar system able to conduct measurements away from fixed radar ranges has prompted System Planning Corporation (SPC) to develop a mobile MkV radar system. Planned helicopter-based SAR measurements generated a requirement for a ground-based platform to verify functionality of X-band and VHF/UHF data collection and processing systems. Accordingly, SPC developed TruckSAR, a DGPS-equipped mobile testbed to collect side-looking and normal-incidence SAR data. Interleaved step chirp data were collected at 9.0-9.3 GHz (HH polarization) and 150-450 MHz (HH, VV, HV, and VH polarization). The system is self-contained and is proving useful for applications beyond ground and foliage penetration SAR investigations. This paper describes the TruckSAR hardware and data analysis systems. Results of measurements are presented, along with observations of challenges in data interpretation. Promising extensions of this mobile ground-based radar are also discussed.