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A large compact range facility required a foam column for RCS testing where the center of the quiet zone was six meters above the floor level. The RCS measurement after vector background subtraction, had to be accurate down to a –50 dBsm level from 1.5 GHz to 40 GHz. A foam column was constructed from a single billet of material. The foam column was evaluated as to its RCS level in both whole body and ISAR imaging modes. This paper describes the specification, construction and RCS evaluation of this column in the compact range facility. The column was evaluated at single frequencies and with RCS images from 2 GHz to 36 GHz using a gated CW radar. Data is presented that shows the effects of the column on the response of a calibration sphere and the response of the column itself. A study of the foam column imaging response used as the background for vector background subtraction is also described. Targets in the –60 dBsm range were successfully imaged with vector background subtraction of the foam column.
ABSTRACT Conventional radar cross-section measurement ranges have limitations. Indoor anechoic chamber ranges have limitations with respect to the size of the objects that can be measured. Outdoor RCS ranges cannot be used in bad weather conditions and also pose a security problem when the designs are classified or proprietary. Limitations in availability are also common for both outdoor and indoor ranges. An alternative is to use a conventional lab area. The key to successful measurements of LO-targets in such high clutter environments is efficient coherent background subtraction. Coherent background subtraction was performed for ISAR and SAR and compared to the zero-Doppler subtraction method for ISAR in this study. The results from the measurements are compared with calculated results. We find that the ISAR and SAR techniques are comparable in performance but that it is advantageous to use ISAR for small objects due to practical reasons. We conclude that both SAR and ISAR can be utilized for LO targets.
This paper presents a method for measuring signal backscattering from an RFID tag and calculating tag radar cross-section (RCS), which depends on the chip input impedance. We present a derivation of a theoretical formula for RFID tag radar cross-section and an experimental RCS measurement method using a network analyzer connected to an antenna in an anechoic chamber where the tag is also located. The return loss of the antenna measured with and without the tag present in the chamber allows one to calculate the power backscattered from the tag and find tag RCS. Measurements were performed in anechoic chamber using RFID tag operating the base station (called “RFID reader”). RFID tag antenna is loaded with the chip whose impedance switches between two impedance states, usually high and low. At each impedance state, RFID tag presents a certain radar cross section (RCS). The tag sends the information back by varying its input impedance and thus modulating the back-scattered signal.
As NASA prepared the Space Shuttle for its first return to flight mission (STS-114) in July of 2005, a number of new visual and radar sensors were used during the critical ascent phase of the flight to assess if unintentional debris was liberated from the Shuttle as it raced into orbit. New high-resolution C-Band and X-Band radars were used to help ascertain the location and speed of released debris. We also used both radars to monitor debris generated by routine flight events such as Solid Rocket Booster (SRB) separation. To assure these new radars did not interfere with flight-critical engine subsystems, an Electromagnetic Interference (EMI) measurement was performed on the Shuttle Orbiter "Discovery" in January 2005, using the Air Force Research Laboratory's Mobile Diagnostic Laboratory (MDL). This portable EM Measurement system performed a large number of attenuation measurements the night of January 17-18, 2005. This paper describes how the attenuation data was acquired, and the methodology used to reduce the data to predict average attenuation of the radar energy from the outside world to the inside of the aft engine bay of the Orbiter. This data was combined with a separate NASA-performed avionics EMI analysis to demonstrate that the new C and X-Band Debris Radars could be operated without adversely interfering with the Orbiter aft bay Avionics systems.
This paper presents measurements of the CloudSat Collimating Antenna (CA) as fabricated for the 94.05 GHz CloudSat radar, which is to be used to measure moisture profiles in the atmosphere. The CloudSat CA is a 3 reflector system consisting of the 3 "final" (relative to the transmitted energy) reflecting surfaces of the CloudSat instrument. This assembly was fed by a horn designed to approximate the illumination from a Quasi-Optical Transmission Line (QOTL). This same horn was employed as a "standard" for measurement of the CA gain via substitution, and its patterns were also measured (this substitution represents a departure from the standard insertion loss technique in the near field range). The CloudSat CA presented a substantial measurement challenge because of the frequency and the electrical size of the aperture is approximately 600 wavelengths in diameter, with a nominal beamwidth of 0.11 degrees. In addition, very high accuracy was needed to characterize the lower sidelobe levels of this antenna. The CA measurements were performed on a 3122-ft outdoor range (this distance was 41% of the far field requirement), which were immediately followed by measurements in an indoor cylindrical Near Field (NF) range. The instrumentation challenges, electrical, mechanical, and environmental are described. Comparison of the outdoor vs. indoor pattern data is presented, as well as the effect of the application of tie-scans to the near field data.
The use of “Spectral Analysis” algorithm in RADAR imaging is mainly motivated by the fact that RADAR signals are supposed to follow a very convenient model which says that the target echo (its scattering coefficient) can be decomposed into elementary scatterers, and that it can be modelled with a sum of complex exponential functions. This “high frequency” approach leads to the extensive use of Fourier imaging. When resolution becomes poor, due to the fact that the extent of the set of data is too small, one invokes High Resolution Algorithms like MUSIC or ESPRIT. This becomes particularly the case at low frequencies, where such a model is not valid anymore. The aim of the paper is to show that the use of the MUSIC algorithm can be related theoretically to the “characteristic fields” decomposition of the scattered electromagnetic field. The Harrington-Mautz “characteristic currents” theory leads to a decomposition of the bistatic scattering matrix of the target allowing naturally the use of the MUSIC algorithm to reconstruct the target. We show an application of this on a F117 calculated dataset. 3D bistatic images are obtained.
ABSTRACT Conventional radar imaging techniques combine information in angle and frequency to obtain the location of the scatterers which contribute to the radar cross section (RCS) of a target. From these information, supposing that the scatterers have a white and isotropic behavior, a high resolution 2D image can be built. However, in certain circumstances (for example low frequency), the narrowness of the available frequency band and/or the frequency dependence of the scatterers may limit the resolution of the produced images. To circumvent this difficulty, an imaging technique based on multistatic data at fixed frequency is proposed. The use of monochromatic data to image a target was already studied in monostatic configuration. In this case, even if the resolution is very fine, the presence of high sidelobe which decrease slowly limits this technique to target’s reflectivity produced by a limited number of reflectors. In multistatic configuration, the situation is more favorable because weighting functions can be applied to control the level of the sidelobes. To illustrate the performances of this imaging technique, images obtained from the response of various targets measured at low frequencies are presented. Keywords: multistatic RCS, monochromatic radar imaging,
A portable, handheld imaging verification radar (HIVeR) system is designed to verify the RCS integrity of a low observable (LO) platform. The HIVeR is the latest generation to a previously designed and field-tested system (SARBAR) that produced radar images of targets in real-time. For applications with LO aircraft, an objective of the present technology is to extend the first-generation SARBAR system performance to easier use, higher sensitivity, and effective pass/fail decisions for selected regions on the aircraft outer mold line (OML). A novelty of the HIVeR design is an automatic registration scheme incorporated into the radar set. The location and orientation of the HIVeR unit is continually recorded using a precision position and orientation monitoring system. This registration process locates the handheld radar antenna position and orientation with respect to a fixed coordinate system. Similarly, the region-of-interest (ROI) on the aircraft surface is registered in this fixed coordinate system. An important feature of the new HIVeR is its capability to form calibrated radar images along a surface defined by the OML of the LO aircraft. This enables the radar to produce images that can be related to the RCS integrity of the ROI. The image along the OML can be used for pass/fail decision-making by comparing the image with a “gold standard” image for the same region.
We examine how accurately the transmit and receive parameters of a radar cross section measurement system can be determined by use of a rotating dihedral as the polarimetric calibration device. We derive expressions for the errors due to misalignment in the angle of rotation. We obtain expressions for the angles a0,hv and a0,vh for which the measured cross-polarization ratios of a target vanish. Since the theoretical cross-polarization of a cylinder is 0, we can .nd the calibration bias-correction angles. We use simulated and real data to demonstrate the robustness of this bias-angle correction technique. We derive expressions for the uncertainty in the polarimetric system parameters.
wall or ceiling. The resulting motion changes the angle of the The incident field in the test zone can be measured using one-way string with respect to the incident field direction, during which probe antennas or passive two-way reflectors. In most cases time the coherent radar echo is recorded as a function of the string neither the probe antenna nor the passive reflector is very large, angle. The coherent signal-vs-angle data are then transformed to usually only a few wavelengths at best. It must be rolled across the cross-range domain using the fast Fourier transform (FFT), the range on carriages or raised up and down on towers, and the whence we obtain a chart of the incident field amplitude as a distance moved must somehow be measured. If a passive function of cross-range distance. Numerical examples are reflector is used as a probe, the carriage system must be shielded presented that show how variations in the incident field influence with absorbers and the reflector must be mounted on low-the string echo. A sample of experimental data shows that the reflectivity support structures. In addition to the hazard of processed data are readily interpreted. contaminating reflections, the mere assembling of all this equipment can be a significant task.
For many years now, GDAIS has described the development, characterization, and performance of an image-based circular near field-to-far field transformation (CNFFFT) for predicting far-field radar cross-section (RCS) from near-field measurements collected on a circular path around the target. In this paper, we present an improved version of the algorithm that avoids a stationary phase approximation inherent in earlier versions of the technique. The improvement is realized by modifying the range-domain weighting used to implement the frequency derivative in the existing method. A similar modification was presented in the context of linear near-field measurements in an earlier AMTA paper. Numerical simulations are presented that demonstrate the improvement afforded by the technique in predicting far-field RCS patterns from near-field data collected using typical bandwidths and standoff distances. An additional benefit of the revised algorithm is that it readily admits a formulation that includes antenna pattern compensation, as described in a companion paper.
In previous work [1], we presented an antenna pattern compensation technique for linearly-scanned near field measurements. In this paper, we present a similar technique to mitigate the errors from uncompensated azimuthal antenna pattern effects in circular near-field monostatic radar measurements. The antenna pattern co mpensation is implemented as part of an improved algorithm for transforming the near-field measurements to the far-field RCS. A description of this improved circular near field-to-far field transformation CNFFFT technique for isotropic antennas is presented in a companion paper [2]. In this paper, we formulate the near-field signal model in the presence of an azimuthal antenna pattern under the same scattering approximation used in the isotropic CNFFFT. Using this model, we derive a modified version of the CNFFFT that includes antenna pattern compensation. Numerical simulations are presented that demonstrate the ability of the technique to remove antenna pattern errors and improve the accuracy of the far field RCS patterns and sector statistics.
A low-profile, high efficiency sixteen-element stacked patch microstrip array operating in the L-band frequencies of 1.26GHz and 1.413GHz was designed, fabricated and tested for use in applications to airborne sensors operating on small aircrafts. The array was optimized for element spacing, excitation amplitude taper, low cross-polarization and high beam-efficiency using Particle-Swarm Optimization (PSO) and Finite-Difference Time Domain (FDTD) methods. The design and measurement of sixteen-element array topology, stacked patch elements, and power-divider beam forming network are presented in detail. The study highlights the repeatability measurements and characterization of array with the effect of dielectric radomes in a spherical near-field test facility at UCLA. The results met the requirements of center-frequencies and frequencybands(1.26GHz ± 10MHz, 1.413GHz ± 15MHz), side-lobes, very good beam-efficiency (>90%) and low-cross polarization (<-40dB) in main-beam region of array. The measured results compared well with simulations for the two frequencies. Based on measurement results, the microstrip array design has a potential to be used as a feed for deployable mesh antennas for future spaceborne L-band passive and active sensing systems that can operate at integrated active radar (1.26GHz) and passive radiometer (1.413GHz) frequencies with dual polarization capabilities to study soil-moisture and sea-surface salinity.
A generally practiced way to improve the sidelobe accuracy in antenna measurements is by repeating and averaging the measurements in different positions in the quiet zone (also referred to as APC or AAPC, depending on the application). An alternative new way for improving the accuracy of compact range measurements is by moving the compact range feed in different locations. This can easily be achieved for both horizontal and vertical directions. Although feed scanning causes a boresight shift, this can be easily compensated if the feed positions are selected intelligently. A significant measurement speed improvement can be realized by using multiple feeds in the relevant locations, instead of moving a single feed sequentially into these locations. Feed scanning APC has been successfully tested in the Ericsson Microwave Systems Compact Range, where it is now practiced in high accuracy radar antenna measurements.
A uniquely inexpensive solution to Frequency-Modulated Continuous-Wave (FMCW) radar was developed, using low cost Gunn oscillator based microwave transceiver modules. However these transceiver modules have stability problems causing them to be unsuitable for use in precise FMCW radar applications, when just one module is used. In order to overcome this problem, a unique radar solution was developed which uses a combination of 2 transceiver modules to create a precise FMCW radar system. This FMCW radar system was then used in a small Synthetic Aperture Radar (SAR) imaging system. The SAR imaging system was composed of a 12 foot long linear track to which the FMCW radar system was mounted. The FMCW radar system would traverse the linear track, acquiring data to be used for producing SAR imagery. The combination of the small aperture length, narrow bandwidth transmit chirp, and overall frequency instability of the FMCW radar system created a number of SAR imaging problems which were unique in this application. However, it was found that when these issues were properly addressed it was possible to create SAR imagery on a low budget.
The reflection coefficient of an antenna impacts the power transmitted by the antenna. Accurate characterization of this parameter is important in a communication or radar system. This paper discusses an implementation whereby a reflectometer is located near the antenna under test in an antenna range albeit far from the receiver. By placing the reflectometer near the antenna, the measurement uncertainty intrinsic to long cable runs can be minimized.
Antennas that are used aboard next generation airborne, maritime and ground vehicles are increasingly required to satisfy both conventional radiation pattern and gain requirements as well as new radar cross section (RCS) requirements. In response to these requirements, Nurad and ORBIT/FR recently completed design, installation, and verification of a high performance, multi-purpose antenna and RCS measurement facility at the Nurad site in Baltimore, Maryland. This compact range facility features a 60x36x26 foot shielded anechoic chamber and a precision machined, serrated edge, offset-fed reflector system that produces a 5.3’H x 8’W x 8’L quiet zone over the 2-50 GHz frequency range. The facility includes a unique feed room structure that positions the primary radar components close to the feed mount for RCS measurements, and allows for easy change of compact range feed antennas. A removable pylon assembly is used for test body support during RCS testing, and a unique add on section to the pylon rotator allows for inclusion of a roll axis that enables measurement of small and medium size antenna assemblies without removing the pylon. Measurements performed on low RCS standard targets and antennas made in the chamber demonstrate that the chamber provides a high performance measurement environment while providing ease of use and rapid configuration and target changeover.
In this paper, we present an analysis of the impact of positioning errors on the performance of the GDAIS circular image-based near field-to-far field RCS transformation (CNFFFT). The analysis is part of our continuing investigation into the application of near fieldto-far field transformations to ground-based signature diagnostics. In particular, the analysis focuses on the errors associated with ground-to-ground, near-field, whole-body measurements where the radar moves on a nominally circular path around the target. Two types of positioning errors are considered: slowly-varying, long term drift and rapidly-varying, random perturbations about the nominal circular path. The analyses are conducted using simulated data from a target comprised of an array of generalized point scatterers which model both single and multiple interactions on the target. The performance of the CNFFFT was evaluated in terms of the angle sector cumulative RCS statistics. The analyses were performed as a function of frequency for varying amounts of position error, both with and without (approximate) motion compensation. As expected, the results show that the CNFFFT is significantly more sensitive to rapidly-varying position errors, but that acceptable performance can be achieved with motion compensation provided an accurate estimate of the errors is available.
The time-dependent spectrum of rotating structures presents many significant challenges to radar cross section (RCS) test design, instrumentation parameter selection, signal processing methodology, data analysis, and data interpretation. This paper presents a multi-dimensional signal processing tool and a suite of associated data products, based on an efficiently scripted test design and execution strategy, that are responsive to the high throughput, high data volume requirements and real time data analysis demands associated with rotorcraft testing. We specifically address the NRTF’s realization of a suite of spectral, cepstral and statistical signal processing tools supported by animation that facilitate near-real time parametric data analysis and interpretation.
This paper presents a Radar Crossed Section (RCS) time-domain near-field measurement and its Inverse Synthetic Aperture Radar (ISAR) imaging. The target includes a pyramidal horn and a metallic aircraft scale model. A pulse generator excites the transmit antenna and a digital sampling unit collects the data at the receiving side. A time gating window is subsequently applied to reject the multiple reflections. An efficient 3-D algorithm for ISAR based on time-domain near-field data is presented. The test results for six cases demonstrate excellent ISAR images. In particular the geometry of 3-D reconstructed target can be displayed in perspective manner. The advantage of using time-domain near-field measurements is three-fold. First, it reduces measurement time in the order of one-tenth compared to frequency-domain measurements. Second, it mitigates the multiple reflection effects via time gating. Third, near-field measurements require relatively little real estate which reduces the cost tremendously since a compact range is not needed.