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Abstract— Electromagnetic properties of aircraft and missile skins have a large effect on radar cross sections and determine the level of stealth that is achieved over the various RF bands currently in use. RF absorption, reflection, and propagation along the skin surface all serve as important measures of the electromagnetic performance of the coated surfaces. Non-invasive probing of the electromagnetic field just above the propagating wave at multiple spots along the propagation direction can be used to determine and measure wave propagation parameters, including effective RF index, loss per length, wave impedance, and frequency dependent material properties of the coatings. Wide-band photonic electric field sensors have been demonstrated for probing of dielectric layers by measuring the traveling waves along the coated aircraft surface. The photonic E-field sensors are extremely linear and produce an exact real time analog RF representation of the electric field, including phase information. These ultra-wideband (UWB) photonic RF sensors are very small and contain negligible metal content, allowing them to be placed at close proximity without perturbing the RF surface waves. This is very important in accurately characterizing highly damped surface waves on absorber layers. This paper discusses the linearity, bandwidth, polarization, and sensitivity of the unique UWB photonic E-field sensor design. Experimental results are presented on surface-wave characterization measurements using these sensors.
Radar sensor working at 76-77GHz band, because of its long detection range, high resolution and excellent performance in different weather and illumination conditions, has been used to develop on-road pedestrian collision avoidance system. Therefore, studying the pedestrian radar scattering features is important to develop reliable on-road pedestrian detection algorithm. In this paper, we first discuss the measurement setup requirement at 76-77GHz to obtain reliable radar cross section (RCS) data of human subjects. Then the RCS pattern of human subjects with different postures and different body features are measured and studied. The observed radar features could be further developed into stable radar signatures to improve the pedestrian identification algorithm.
Antenna test engineers are faced with testing increasingly complex antenna systems, one of these being the AESA (Active Electronically Steered Array) antennas used for cell communications, jammers, and radars. Often these antennas have integrated electronics and RF components that are an intricate part of the antenna, and as a result must be tested with the waveforms generated by the antenna itself. One cannot simply inject an unmodulated continuous wave signal. These antennas require new measurement techniques which are compatible with their broadband waveforms. The reference channel of a measurement receiver can be used to collapse the spectrum of the modulated signal into a single CW measurement. Done properly all the energy in the signal is captured with noise and interference being dispersed, resulting in no loss of DR (dynamic range) over a CW measurement. A receiver employing this technique can capture all the energy in modulated and pulsed signals wielding wide dynamic range measurements. Phased locked loops (PLL) are not used as they can preclude such measurements. A measurement receiver that uses a digital correlator to collapse the spectrum of modulated and pulsed signals will be presented. This paper will describe the technique used to do this and show measured results on example broadband signals.
Compact range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signature. This paper investigates strategies for improving compact range measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational finite-difference time-domain (FDTD) model at VHF/UHF frequencies to determine baseline performance and develop trade rules for more advanced designs. Physical optics (PO) models were used to analyze the final design at the frequencies of interest.
Reconfigurable radar antennas with rapid, real-time control of the radiation pattern beamwidth provide expanded performance for many instrumentation radar applications, including RCS signature measurement and dynamic Time Space Position Information (TSPI) radar tracking applications. Adaptive adjustment of antenna radiation patterns was traditionally accomplished by electro-mechanically selecting predefined aperture dimensions that corresponded to desired beamwidths (e.g., ? ?/D). For an array antenna consisting of as few as 200 elements, beam shaping can be accomplished by adjusting the relative phase of individual array elements, a technique defined as beam spoiling or decollimation. This paper analyzes an operational radar antenna array incorporating reconfigurable beamwidth and beam shape through independent phase control of each subaperture. By adjusting the relative phase of radiating elements, the system can illuminate a programmable field of regard with full transmit power. For this array, the phase distributions across the elements map to a smaller "virtual aperture" displaced behind the physical array. Theoretical and measured results are presented to validate the reconfigurable array pattern control technique.
There are many ways to acquire the current location, global positioning system (GPS), triangulation, radar, and dead reckoning. Today GPS is the most reliable and accurate navigation technique when there is a clear, unobstructed view of the satellite constellation. However, when GPS is not available, another means of reliable navigation must be accomplished. The Air Force Institute of Technology (AFIT) random noise radar (RNR) is a possible solution to the indoor navigation problem. However, the current implementation of the RNR requires a large amount of data to be transfered between radar pulses. This research determined if using a template replay strategy has the same RNR performance as using an analog noise source. Using the template replay approach, each RNR node has a priori knowledge about the transmitted waveforms of other nodes and does not require the large data transfer between radar pulses. The analysis here revealed that modifications do not significantly alter RNR functionality. The analysis revealed that even at signalto- noise plus interference ratio (SNIR) equal to 0 dB, there are no parameters that can be reliably extracted other than transmitted signal bandwidth and transmitted template length; the transmitted message length was able to be extracted because the message was repeated over and over. If the message was not replayed the analysis showed that there would be no ability to extract parameters. Finally, by using the RNR to transmit digitally generated templates, digital communication is possible and the symbol error rate (SER) is traceable to simulated SER.
In 2008 and 2011 we proposed at AMTA a method of radar imagery based on Fast Fourier Transform. We demonstrated then that this method was faced with interpolation problematic (which could be almost solved) and that the obtained radar image had a poor resolution imposed by the low bandwidth used during the measurement. We saw then that because of this resolution issue, it was impossible to distinguish two scatterers when they were too close. This article presents methods allowing to improve the resolution of radar image without increasing the bandwidth of measurement but using 1D super-resolution techniques adapted to 2D or 3D measurements. More precisely, this article proposes first to explain the principle of three 1D super-resolution methods: the Capon method, the MUSIC (Multiple Signal Classification) method and the ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) method. This principle understood, these methods will be then adapted to 2D or 3D measurements in order to be used to calculate high resolution radar images. Eventually, a true target will be used in order to compare the different radar images obtained from the different super-resolution techniques.
Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.
The RF electromagnetic spectrum, extending from 2 MHz to 94 GHz, can be considered an evolving, but finite resource. This span of spectrum is used for a multitude of purposes including communications, radio and television broadcasting, radio navigation, sensing and radar. The portion of the spectrum from 2-4 GHz has become particularly problematical due to the influx of wireless systems such as WiMAX into a region which has traditionally been associated with radar systems. This paper will discuss the different types of measurements made on radars and other non-radar users of this spectrum. First, the propagation physics of why different types of radars reside in selected frequency bands and why other non-radar systems seek and have gained entrance to these bands is reviewed. The unique spectral characteristics of radars, not generally found in communications systems will be addressed. The trade-offs in using conventional superheterodyne and FFT based spectrum analyzers versus the newer real time spectrum analyzers will be discussed in terms of their capability in evaluating radar spectrum. The spectral characteristics of a variety of communications waveforms will be explored, such as OFDM, and how they can complicate the process of measuring radar type signals in a radiated test. The types of antennas that are used in these radar systems will be discussed and how these antennas influence the reliability of these measurements. Finally, some of the issues associated with separating out antenna effects from waveform effects in legacy & future radars will be discussed. The need for improved measurement techniques and systems will also be addressed.
Ground to air imaging poses numerous technical challenges, a number of which relate to target motion throughout its inverse synthetic aperture. A well-tracked target benefits not only its illumination, but provides an accurate description of the target’s position as a function of time. Tracking may be accomplished using a monopulse tracking radar or precision GPS/telemetry techniques; either of which are sufficiently accurate for coarse translational motion compensation. However, without target attitude telemetry, the inverse synthetic aperture may still be inferred from the target’s spherical coordinates and their first derivatives, and coarse rotational motion compensation may be performed. Further refinement is available from the in-phase/quadrature data. Residual translational motion may be characterized and corrected by considering both intra-chirp (i.e., within stepped chirps) and inter-chirp phase migration, accommodating fine translational motion compensation. The data become reasonably bandlimited, allowing rotational motion compensation to be performed via bandlimited resampling, yielding a focused ISAR image.
All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.
As a novel concept for field probes, RCS measurements on long rigid objects rotated within a small angular range about the broadside condition are reported. The rotation was maintained either in a horizontal (H) plane or in a vertical (V) plane containing the center of the quiet-zone (QZ). Processing the RCS data by DFT yields a spectrum which is recognized as the field distribution along that object. This spectrum compares extremely well to traditional field-probes taken earlier by translating a sphere across the QZ in H- or V-direction. Preliminary results at several S-band frequencies are presented and discussed.
This paper describes the development of a method based on measurements of the radar cross section (RCS) in the forward direction to determine the extinction cross section for the 2.5-38GHz frequency range using the optical theorem. Forward RCS measurements are technically complicated due to that the direct signal has to be subtracted from the total signal at the receiving antenna in order to extract the forward RCS. The efficiency of this subtraction as a function of time is evaluated. A traditional calibration method using a calibration target and a second method that does not require a calibration target are investigated and compared. The accuracy of the forward RCS measurements is determined using small spheres of different sizes. The spheres have a forward RCS that is straightforward to calculate with good accuracy. The method is also extended to polarimetric measurements on a small helix that are compared to theoretical calculations.
The goal of this project was to design and test a UWB S21 measurement system for less than $2,000. We use a synthesized source and a coherent demodulator. The bandwidth extends from 0.5 to 4.5 GHz with frequency steps = 100 MHz. The selected synthesized source is a Windfreak SynthNV module based on the Analog Devices wideband fractional-N synthesizer chip. This chip can output signals in the 137-4,400 MHz range. It has enabled a number of very low cost modules to be developed, and the selected module is a USB controlled and powered synthesizer. The I/Q mixer is a Polyphase Microwave quadrature demodulator with a bandwidth from 0.5 to 4.0 GHz with built in LO amplifier and I/Q low pass filters. We will show the design, performance parameters and cost of this radar and show results of the use of this radar to detect and characterize the human heartbeat.
IDS has developed a RCS measurement solution capable to operate both in indoor and outdoor test ranges. The solution is based on the Agilent PNA series of network analyzers, whose performance are enhanced by a dedicated RF front end (named "Pulser"), resulting in a low-cost, compact and flexible system covering the frequency band from 2GHz to 18GHz. At first, the capability of the measurement solution was verified in a near field test range, demonstrating sensitivity compliant with low observable platform requirements (typical values of Noise Equivalent RCS can be in the order of -50 dBsm indoor at 30 m). Recently the RF front end has been upgraded to be usable for outdoor dynamic RCS measurements as well, being the upgraded solution named "Pulser_EV". This paper describes the performance of the Pulser_EV, its application field and possible developments.
A low cost transportable short range Synthetic Aperture Radar (SAR) measurement system is constructed by placing a linear positioner on the bed of a truck. The radar system is based on a network analyzer and a pair of standard horn antennas. The SAR system gives a resolution of 0.3 m at X-band frequencies and 100 m measurement distance. By using the terrain the objects can be measured at depression angles up to at least 10. . The near field SAR images are processed using back projection algorithms. The system can be used to estimate camouflage efficacy in realistic environments. Flexible software based entirely on open source components is developed for the data processing, presentation and analysis of the SAR images. Results from the evaluation of two generic camouflage nets using reference targets in different backgrounds and SAR images of vehicles are presented.
From measurements of RCS of a target as a function of the frequencies and the bearings, it is possible to make RADAR imagery. A common way is to use a bi-dimensional Fast Fourier Transform (FFT2) while this algorithm being very fast. Yet this algorithm demands that the grid on which the RCS is known fulfils some particular conditions. Now such conditions are not respected by the grid of measurement. Consequently an interpolation of this grid is necessary in order to be able to apply the FFT2 algorithm. The choice of the method of interpolation will directly impact the quality of the calculated RADAR image. In this article we propose to study this impact while giving the analytical expression of the interpolation then while giving the analytical expression of the RADAR image calculated from the interpolated RCS and while specifying eventually the method interpolation which limits the degradation of quality of the calculated RADAR image.
The installation of power-generating wind turbines near outdoor radar cross-section measurement ranges for low-observable targets presents a number of problems for accurate measurements on those ranges. The turbines, with towers up to 100 meters tall and blades 45 meters long, are enormous scatterers that raise clutter levels well above returns from LO targets. The movement of the rotors, rotating about both a horizontal and vertical axis, generates a dynamic and unpredictable Doppler smear that cannot be mitigated with phase coding techniques or Doppler filtering. Lastly, the large wind farms over which the turbines are installed lowers the unambiguous-return PRF to extremely low levels, raising test times and dropping rotation speeds below acceptable levels. A method of varying pulse spacing into a burst mode is presented that maintains data throughput in RCS collection while avoiding the clutter returns of downrange wind turbines. This burst mode has the radar transmit and receive a string of closely-spaced pulses, and then idle while that string of pulses propagates through the wind farm. By careful selection of timing parameters, clean-range clutter levels can be maintained with little to no degradation in test time.
We are developing a new-generation weather radar to observe and predict short-term weather phenomena like severe storms, gust and so on. Therefore, an active phased array antenna (APAA) with digital beam-forming (DBF) receivers could be used for the new-generation weather radar to reduce the observation time. In Toshiba Corporation, 33m x 16m vertical near-field antenna range including the digital instrumentation receivers have been working for multi-beam DBF antenna measurements. This near-field antenna range is used to evaluate the performance of an APAA. In this paper, we describe the characteristics of this new-generation weather radar and the APAA. And we demonstrate the antenna measurement set-up using the near-field antenna range and the measurement results of this antenna.
The use of computational electromagnetics (CEM) prediction codes in the analysis and interpretation of RCS measurements has become increasingly prevalent. This is in large part due to rapid advances in computing capability over the last several years, particularly for rigorous techniques such as the method of moments (MoM). In many instances, however, these codes are still limited to plane waves and/or elementary dipoles as the sources of target illumination. Modeling of the illumination from an arbitrary antenna therefore requires meshing and solution of the combined antenna-target geometry for each frequency and aspect angle, with an associated increase in the computational complexity of the problem, even if the interactions between the antenna and the target are negligible. In this paper, we describe a method by which measurements or predictions of the antenna pattern are used to develop an equivalent representation of the antenna in terms of an array of non-interacting elementary dipole current sources in a MoM code that uses RWG basis functions. The representation can then be used to efficiently derive the antenna’s illumination on the target as a function of frequency and aspect angle with only a minor increase in the computational burden relative to plane wave illumination. Results are presented using antenna pattern predictions for an ETS-Lindgren 3164-01quad-ridged VHF antenna which illustrate the accuracy and efficiency of the technique.