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Phase Responses of Harmonics Reflected from Radio-Frequency Electronics
Abstract
The phase responses of nonlinear-radar targets illuminated by stepped frequencies are studied. Data is presented for an experimental radar and two commercial electronic targets at short standoff ranges. The amplitudes and phases of harmonics generated by each target at each frequency are captured over a 100-MHz-wide transmit band. As in the authors’ prior work, target detection is demonstrated by receiving at least one harmonic of at least one transmit frequency. In the present work, experiments confirm that the phase of a harmonic reflected from a radio-frequency electronic target at a standoff distance is linear versus frequency. Similar to traditional wideband radar, the change of the reflected phase with respect to frequency indicates the range to the nonlinear target.
... iv List of Figures Fig. 1 General model of harmonic radar, 3 ...
... The original fundamental frequency ω is then stepped, or increased by an increment Δω, across a bandwidth until enough harmonic data are collected to verify detection and calculate distance to target. 3 A general diagram of harmonic radar is shown in Fig. 1. In the transmission chain, the fundamental ω is amplified to increase its incident power on the target, and then fed through a low pass filter to attenuate any artifacts created from amplification. ...
... In earlier studies of nonlinear radar, the phase of these reflected harmonics has largely been assumed to be constant versus the bandwidth of frequencies transmitted to the target. 3 This previously unconfirmed relationship is necessary to calculate the distance to target using an inverse Fourier transform. My experimental designs reexamine this assumption to affirm it and uphold that the harmonic stepped-frequency methods of radar are widely applicable for the detection and ranging nonlinear targets. ...
One of the latest challenges being investigated by the US Army Research Laboratory’s (ARL) Electronics and Radio Frequency (E&RF) Division is the development of a radar system that can accurately detect and range an electronically nonlinear target, such as a detonator of an improvised explosive device (IED). Previous nonlinear radar systems detect targets via transmission of a single frequency ω, stepping (incrementally increasing) this frequency through a wide bandwidth, and then listening for a response of the 2nd harmonic 2ω; however, the phase information that this harmonic contains and its relationship to target distance has been largely assumed and unconfirmed. Our most recent experimental tests, both wired and wireless, have confirmed that this harmonic phase response is constant versus frequency at the target. Using inverse Fourier transforms, the range of an electronic nonlinear target can be determined from that phase.
... when the doubler is operated in its square-law region where the memory-less power series approximation is applicable [14,18,27]. The full normalized measurement with a material or object under test can therefore be described by S I I,I 11,Norm. ...
This paper presents a new SIMO radar system based on a harmonic radar (HR) stepped frequency continuous wave (SFCW) architecture. Simple tags that can be electronically individually activated and deactivated via a DC control voltage were developed and combined to form an MO array field. This HR operates in the entire 2.45 GHz ISM band for transmitting the illumination signal and receives at twice the stimulus frequency and bandwidth centered around 4.9 GHz. This paper presents the development, the basic theory of a HR system for the characterization of objects placed into the propagation path in-between the radar and the reflectors (similar to a free-space measurement with a network analyzer) as well as first measurements performed by the system. Further detailed measurement series will be made available later on to other researchers to develop AI and machine learning based signal processing routines or synthetic aperture radar algorithms for imaging, object recognition, and feature extraction. For this purpose, the necessary information is published in this paper. It is explained in detail why this SIMO-HR can be an attractive solution augmenting or replacing existing systems for radar measurements in production technology for material under test measurements and as a simplified MIMO system. The novel HR transfer function, which is a basis for researchers and developers for material characterization or imaging algorithms, is introduced and metrologically verified in a well traceable coaxial setup.
... The initial phase of the transmit waveform is assumed to be zero without any loss of generality. For UHF-band transmission and L-band reception, the phase reflected from the target may be assumed constant with frequency, as recent experiments have demonstrated that ϕ-versusf 0 is flat over an ultrawide sweep within these bands [44]. ...
An extensive review of nonlinear radar systems is performed. Emphasis is placed on designs relevant to detecting RF electronics that were not intentionally manufactured as visible radar targets. The state of the art in nonlinear radar is conveyed by presenting high-level system architecture, explaining the rationale behind design decisions pertaining to that architecture, and listing the specifications that nonlinear radar designers have achieved. The authors’ recent advancements in nonlinear radar technology are summarized.
... The initial phase of the transmit waveform is assumed to be zero without any loss of generality. For UHF-band transmission and L-band reception, the phase reflected from the target may be assumed constant with frequency, as recent experiments have demonstrated that ϕ-versusf 0 is flat over an ultrawide sweep within these bands [44]. ...
An extensive review of nonlinear radar systems is performed. Emphasis is placed on designs relevant to detecting RF electronics that were not intentionally manufactured as visible radar targets. The state of the art in nonlinear radar is conveyed by presenting high-level system architecture, explaining the rationale behind design decisions pertaining to that architecture, and listing the specifications that nonlinear radar designers have achieved. The authors’ recent advancements in nonlinear radar technology are summarized.
Stepped-Frequency Radars (SFRs) have become increasingly popular with the advent of new technologies and increasingly congested RF spectrum. SFRs have inherently high dynamic range due to their small IF bandwidths, allowing for the detection of weak target returns in the presence of clutter. The Army Research Laboratory’s (ARL) Partnership in Research Transition program has developed a preliminary SFR for imaging buried landmines and improvised explosive devices. The preliminary system utilizes two transmit antennas and four receive antennas and is meant to act as a transitional system to verify the system’s design and imaging capabilities. The SFR operates between 300 MHz and 2000 MHz, and is capable of 1-MHz step-sizes. The SFR system will eventually utilize 16-receive channels and will be mounted on ARL’s existing Forward-Looking Ground Penetrating Radar platform, as a replacement for the existing Synchronous Impulse REconstruction (SIRE) radar. An analysis of the preliminary SFRs radio frequency interference mitigation, spectral purity dynamic range, and maximum detectable range is presented here.
This paper presents synthetic aperture radar (SAR) images of linear and nonlinear targets. Data are collected using a linear/nonlinear step frequency radar. We show that it is indeed possible to produce SAR images using a nonlinear radar. Furthermore, it is shown that the nonlinear radar is able to reduce linear clutter by at least 80 dB compared to a linear radar. The nonlinear SAR images also show the system’s ability to detect small electronic devices in the presence of large linear clutter. The system presented here has the ability to completely ignore a 20-inch trihedral corner reflector while detecting a RF mixer with a dipole antenna attached.
A new approach for detecting a particular class of moving targets is presented. This method exploits characteristics of specific non-linear targets to both eliminate moving objects that are not of interest and suppress stationary clutter. Details of the underlying physical phenomena are discussed, and the signal processing procedures leveraged by the non-linear radar system are outlined in detail.
In a harmonic radar system design, one of the most important components is the filter used to remove the self-generated harmonics by the high-power transmitter power amplifier, which is usually driven close to its 1-dB compression point. The obvious choice for this filter is a low-pass filter. The low-pass filter will be required to attenuate stop band frequencies with 100 dB attenuation or more. Due to the high degree of attenuation required, multiple low-pass filter will likely be required. Most commercially available low-pass filters are reflective devices, which operate by reflecting the unwanted high frequencies. Cascading these reflective filter causes issues in attenuating stop band frequencies. We show that frequency diplexers are more attractive in place of reflective low-pass filters as they are able to terminate the stop band frequencies as opposed to reflecting them.
Radio-frequency (RF) electronic targets, such as man-portable electronics, cannot be detected by traditional linear radar because the radar cross section of those targets is much smaller than that of nearby clutter. One technology that is capable of separating RF electronic targets from naturally-occurring clutter is nonlinear radar. Presented in this paper is the evolution of nonlinear radar at the United States Army Research Laboratory (ARL) and recent results of short-range over-the-air harmonic radar tests there. For the present implementation of ARL’s nonlinear radar, the transmit waveform is a chirp which sweeps one frequency at constant amplitude over an ultra-wide bandwidth (UWB). The receiver captures a single harmonic of this entire chirp. From the UWB received harmonic, a nonlinear frequency response of the radar environment is constructed. An inverse Fourier Transform of this nonlinear frequency response reveals the range to the nonlinear target within the environment. The chirped harmonic radar concept is validated experimentally using a wideband horn antenna and commercial off-the-shelf electronic targets.
RF electronic targets cannot be detected by traditional linear radar because their radar cross sections are much smaller than that of nearby clutter. One technology that is capable of separating RF electronic targets from clutter, however, is nonlinear radar. Presented in this paper is a combination of stepped-frequency ultra-wideband radar with nonlinear detection. By stepping the transmit frequency across an ultra-wide bandwidth and recording the amplitude and phase of the harmonic return signal, a nonlinear frequency response of the radar environment is constructed. An inverse Fourier transform of this response reveals the range to a nonlinear target.
In order to generate a detectable harmonic-radar response from an electronic device, the required power-on-target is comparable to that observed directly below a cellular base station. Also, the signal emitted from the target is often very weak. This weak signal must not be masked by harmonics generated by the radar itself. Thus, high transmit power must be provided with high linearity for detection of a nonlinear-radar target. In this paper, a technique is presented which achieves better than 135 dBc harmonic distortion at 7 W output power, at transmit frequencies between 800 MHz and 1 GHz.
Multitone harmonic radar is presented. The radar transmits multiple closely-spaced tones and receives nonlinear mixing products as well as harmonics. Harmonic and multitone responses are recorded from commercially-available RF devices. An original method for discriminating between electronic targets, by receiving at least two nonlinear mixing products near a harmonic, is presented. Target detection is demonstrated experimentally for a novel pulsed two-tone harmonic radar. Experimental results are extrapolated to estimate radar design parameters to achieve a realistic standoff range.
Under support from the Army Research Laboratory's Partnerships in Research Transition program, a stepped-frequency radar (SFR) is currently under development, which allows for manipulation of the radiated spectrum while still maintaining an effective ultra-wide bandwidth. The SFR is a vehicle-mounted forward-looking ground-penetrating radar designed for high-resolution detection of buried landmines and improvised explosive devices. The SFR can be configured to precisely excise prohibited or interfering frequency bands and also possesses frequency-hopping capabilities. This paper discusses the expected performance features of the SFR as derived from laboratory testing and characterization. Ghosts and artifacts appearing in the range profile arise from gaps in the operating band when the system is configured to omit specific frequencies. An analysis of these effects is discussed and our current solution is presented. Future prospects for the SFR are also discussed, including data collection campaigns at the Army's Adelphi Laboratory Center and the Countermine Test Site.
The essence of radar, radio and wireless sensor engineering is extracting small information-bearing signals. This is notoriously difficult and engineers compensate by transmitting high power signals, reducing range, and spacing wireless systems in frequency and time. New understandings of passive intermodulation distortion, thermal effects, time-frequency effects, and noise are presented. It is seen that the familiar frequency-domain-based abstractions have missed important underlying physics. Through greater understanding, RF engineers can develop microwave systems with far lower levels of distortion and noise.
Interest in harmonic and multitone radar has motivated new design efforts based on the standard approach found in the radar literature, i.e., the classical radar equation. In this paper we show that such approaches can be problematic in the presence of an active or nonlinear target, and indicate how a proper link budget should be constructed based on anticipated target behaviors at high signal levels.
Recent research has examined several novel radio frequency (RF) remote probing techniques. These new results are joined with the prior art leading to a unified understanding of RF remote probing that articulates the fundamental principles involved and allows extension to the analysis of additional “new” RF probing techniques. Two significant aspects in RF probing are emphasized: 1) information, coding, and signal theory considerations are identified as these establish bounds on probe performance against a specific information task and 2) building upon the well-known radar equation, the object cross section is generalized to be an interaction term describing a much broader range of interactions than simply reflection. These results are significant in enabling the rapid analysis and evaluation of “new” RF sensing techniques proposed to accomplish a variety of functions in the detection and identification of both natural and manmade objects.
Reference, which appears in the Proceedings of the RADAR'09 Conference at Bordeaux in October 2009, describes nonlinear currents generated by an electromagnetic wave incident on a metal surface. This paper supplements the results of with a full calculation of the RCS per unit area for second harmonic generation. These results verify that the level of such emission from a metal surface is anomalously low.
A crucial issue in the exploitation of harmonic and multitone radar is the nature of target signatures at the various emission frequencies. In this paper we describe some preliminary results on how the nonlinear response of a target depends on its composition for the specific example of a target with a bare metal surface. Our results show that, in contrast to linear radar signatures, those with metal surfaces are the most difficult targets to detect.
Short-range harmonic radar: Chirp waveform, electronic targets Stepped-frequency nonlinear radar simulation Nonlinear synthetic aperture radar imaging using a harmonic radar Moving target indication with non-linear radar
- G J Mazzaro
- K A Gallagher
- A F Martone
- K D Sherbondy
- R M Narayanan
- G J Mazzaro
- K A Gallagher
- A F Martone
- R M Narayanan
- K A Gallagher
- G J Mazzaro
- K I Ranney
- H Lam
- K D Nguyen
- R M Sherbondy
- Narayanan
G. J. Mazzaro, K. A. Gallagher, A. F. Martone, K. D. Sherbondy, and R. M. Narayanan, " Short-range harmonic
radar: Chirp waveform, electronic targets, " Proc. SPIE, vol. 9461, pp. 946108(1–12), Apr. 2015.
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G. J. Mazzaro, K. A. Gallagher, A. F. Martone, and R. M. Narayanan, " Stepped-frequency nonlinear radar
simulation, " Proc. SPIE, vol. 9077, pp. 90770U(1–10), May 2014.
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K. A. Gallagher, G. J. Mazzaro, K. I. Ranney, Lam H. Nguyen, K. D. Sherbondy, and R. M. Narayanan,
" Nonlinear synthetic aperture radar imaging using a harmonic radar, " Proc. SPIE, vol. 9461, pp. 946109(1-11),
Apr. 2015.
[9]
K. A. Gallagher, R. M. Narayanan, G. J. Mazzaro, K. I. Ranney, A. F. Martone, and K. D. Sherbondy, " Moving
target indication with non-linear radar, " Proc. IEEE Radar Conf., pp. 1428–1433, May 2015.