This study investigates the use of a custom-built 10 GHz continuous wave micro-Doppler radar system to analyze external vibrations of idling vehicles under various conditions. Scenarios included different gear engagements with one occupant and parked gear with up to four occupants. Motivated by security concerns, such as the threat posed by idling vehicles with multiple occupants, the research explores how micro-Doppler signatures can indicate vehicle readiness to move. Experiments focused on a mid-size SUV, with similar trends seen in other vehicles. Radar data were compared to in situ accelerometer measurements, confirming that the radar system can detect subtle frequency changes, especially during gear shifts. The system’s sensitivity enables it to distinguish variations tied to gear state and passenger load. Extracted features like frequency and magnitude show strong potential for use in machine learning models, offering a non-invasive, remote sensing method for reliably identifying vehicle operational states and occupancy levels in security or monitoring contexts. Spectrogram and PSD analyses reveal consistent tonal vibrations around 30 Hz, tied to engine activity, with harmonics at 60 Hz and 90 Hz. Gear shifts produce impulse signatures primarily below 20 Hz, and transient data show distinct peaks at 50, 80, and 100 Hz. Key features at 23 Hz and 45 Hz effectively indicate engine and gear states. Radar and accelerometer data align well, supporting the potential for remote sensing and machine learning-based classification.

Micro-Doppler radar is a cutting-edge technology that has revolutionized the field of radar sensing to enable the detection and characterization of complex targets by leveraging their micro-motion dynamics. This paper discusses the design and construction of a 10-GHz continuous wave (CW) micro-Doppler radar, an explanation of how the system operates and extracts data, as well as a discussion of the device's possible applications for characterizing external vibrations of vehicles under different scenarios. The objective is to highlight the potential of micro-Doppler radar for remotely recognizing vehicle transmission shifts and occupancy.

In this work, we design a novel, integrated transmit (TX)/receive (RX) dual-band folded-probe-fed patch (DFPFP) antenna to improve nonlinear radar (NLR) system performance. Designed for down-looking, airborne NLR applications, the DFPFP antenna occupies an improved form factor ( $0.9\lambda \times 0.9\lambda \times 0.1\lambda $ , 1.8 kg) over existing commercial-off-the-shelf (COTS) designs and additionally offers minimal drag in airborne applications due to its low profile. The DFPFP antenna is simulated, fabricated, and tested, and excellent gain (9 dBi) and bandwidth (15%) performance are demonstrated. Iterations of the design from single, standalone TX and RX antennas to the final, integrated design are presented and discussed. The final DFPFP is compared to a pair of COTS antennas that are currently used for NLR system testing. Evaluated in the context of an NLR system, the custom DFPFP antenna significantly outperforms the COTS antennas, with signal-to-noise ratio (SNR) improvements as high as 17 dB, while offering significantly improved size, weight, and power (SWaP) performance.

Congestion in the frequency spectrum is an ever-growing issue for current and future radar systems due to the increasing number of commercial wireless devices and technologies. Both communications devices and radar continue to demand access to greater swaths of bandwidth, but do not coexist well. The cognitive radio and radar communities have investigated solutions to this problem through the use of dynamic spectrum access (DSA) and spectrum sharing (SS). For radar, the pulse-agility required to effectively share the spectrum with other rapidly changing signals complicates coherent integration due to the variation in the transmitted waveform. In particular, when non-identical pulses are processed with standard Fourier-based range-Doppler (RD) processing for moving target indication, a modulation / distortion effect is induced. The Richardson-Lucy deconvolution algorithm, an image processing technique, is implemented in a software-defined radar (SDRadar) system to remove the undesired modulation from the RD images. The approach is then verified via an over-the-air experiment where the SDRadar must share a radio frequency (RF) band with a communications device and detect a moving target simultaneously.

Ultra-wideband (UWB) ground-penetrating radar (GPR) technology has been widely employed for detecting targets that range from buried explosive devices such as landmines, improvised explosive devices (IEDs), to underground utilities and tunnels, etc. However, the backscatter signals from the ground surface pose a critical challenge for downward-looking GPR systems since (i) these ground return signals have significant power compared to the backscatter signal from subsurface targets, and (ii) the ground return and target signals completely overlap in both the time and frequency domains. Many techniques have been proposed to date; however, they all have limitations in mitigating the adverse effects of the very high power ground return interference (GRI) signals. This paper presents a novel technique for reconstructing and extracting the GRI signals from downward-looking UWB GPR signals. Our proposed technique performs an estimation of the return signal from the ground surface. This signal estimation, together with the estimated scatter center of the ground surface, is used to construct a dictionary that represents the ground return signal subspace. Finally, we employ a sparsity-driven optimization algorithm to reconstruct the GRI signals and then extract them from the received radar signals. All information used to construct the dictionary is completely derived from the data. Our technique performs this GRI extraction directly in the phase history data domain prior to synthetic aperture radar (SAR) image formation. Thus, it can be implemented as an additional step, completely independent from all other steps, in the pre-processing stage. Recovery results from simulated data set illustrate the robustness and effectiveness of our proposed technique.

Advancement in radar component technology has led to a reduction in the size, weight, and power consumption of radar systems.Experimental radar systems can now be integrated onto smaller, maneuverable platforms, such as small unmanned aerial vehicles (sUAVs).Integration onto rotorbased sUAVs enables data collection over novel synthetic apertures which can be optimized for different scenarios.The design, simulation, and experimentation of a light-weight, ultra-wideband synthetic aperture radar (SAR) is presented here that will be used for the detection of obscured surface targets.The approach outlined herein uses 3-dimensional (3-D) imagery to vertically resolve clutter from the target.A vertical-grid aperture is presented which yields vertical resolution.Point spread functions are derived for both linear and vertical-grid apertures.The analytical expressions are verified using simulations.Finally, experimental data is used to form 3-D imagery and demonstrate the importance of vertical resolution in the discrimination between scatterers above the ground, as well as clutter mitigation.

The US Army Combat Capabilities Development Command Army Research Laboratory is developing a dualband, full-polarization, side-looking synthetic aperture radar using an RF system-on-a-chip for the detection of landmines. The system employs two separate front-ends to operate in the bands from 0.5 to 1.8 GHz and from 2.1 to 3.8 GHz. An antenna array is set up with two transmitters (one vertical and one horizontal) and two receivers (one vertical and one horizontal) to enable fully-polarimetric operation. A continuous wave stepped-frequency waveform is employed, and each combination of polarizations is simultaneously transmitted and received. This system was tested at a desert site. The targets that were tested were remote anti-armor mine system landmines, M20 metal landmines, and VS2.2 plastic landmines. The targets are imaged under a number of emplacement scenarios so that imaging results address targets made of various materials at different orientations and ranges. Furthermore, obscured targets and buried targets are also investigated. The effect of antenna coupling and techniques for reducing this effect are discussed. Then, the imaging results for each target scenario is shown and analyzed. Imaging results between data from the two frequency bands are compared and the success of detection for different emplacements is analyzed.

When an electromagnetically-nonlinear radar target is illuminated by a high-power stepped-frequency probe, a sequence of harmonics is unintentionally emitted by that target. Detection of the target is accomplished by receiving stimulated emissions somewhere in the sequence, while ranging is accomplished by processing amplitude and phase recorded at multiple harmonics across the sequence. The strength of the harmonics reflected from an electronic target depends greatly upon the orientation of that target (or equivalently, the orientation of the radar antennas). Data collected on handheld wireless devices reveals the harmonic angular-dependence of commercially-available electronics. Data collected on nonlinearly-terminated printed circuit boards implies the origin of this dependency. The results of this work suggest that electronic targets may be classified and ultimately identified by their unique harmonic-response-vs.-angle patterns.

Intermodulation radar is an established technique for locating electromagnetically-nonlinear junctions. For this type of radar, the probe consists of multiple simultaneous frequencies, usually two tones of equal amplitude. The multiple frequencies illuminate the target, mix with each other, and generate integer sums and differences of the original transmitted tones. This work studies a variation on the intermodulation-radar technique. Some targets, such as AM/FM transmitters, emit radio frequencies without being actively probed; thus, some collections of (powered) nonlinear junctions generate at least one internal tone which might be mixed with an externally-applied probe tone. This internal-external mixing is referred to as "carrier modulation," where the carrier is associated with the target and its modulation is induced by the transmit probe. This paper documents an experiment conducted using a transverse electromagnetic cell: contactless excitation of carrier modulation from active nonlinear junctions. Data recorded from two radio transmitters indicate that, for this internal-external mixing technique, a reduction in transmit power results in less of a reduction in received power compared to traditional intermodulation radar.

In this work, a set of custom dual-arm folded-probe-fed patch (DFPFP) antennas are designed to improve the performance of a non-linear radar (NLR) system. To improve the system performance, the custom antennas are designed to have high gain, relatively narrow bandwidth, good linearity, and good size, weight, and power (SWaP) performance. Two DFPFP antennas are built and tested, one each for transmit and receive. The VSWR and gain of the fabricated antennas are measured. The measurements show good agreement with simulated results. The performance of the fabricated antennas is then compared to a pair of commercial off-the-shelf (COTS) antennas that are currently used for NLR system testing. The VSWR and gain measurement results of the DFPFP antennas compare favorably with those of the COTS antennas, and suggest improved NLR performance. Lastly, the performance of a NLR system using the two sets of antennas is evaluated. By replacing the COTS antennas with the custom DFPFP antennas, signal-to-noise ratio (SNR) improvements as high as 20 dB are demonstrated.

Cross-modulation produced by nonlinear junctions is described mathematically and captured experimentally. The effect is modeled by the same memory-less power series which predicts the generation of harmonics by electromagnetic nonlinearities. Nominal specifications for a single-transmitter harmonic detector are presented; that architecture is compared to one which splits the original broadcast-probe power between two transmitters and receives cross modulation. Assuming equal total transmit power, the system-generated distortion of the cross-modulation detector is lower than that of the harmonic detector, while the peak strength of the received target response is the same. Data collected on a variety of radio-frequency mixers, amplifiers, and antennas indicate that assemblies of such components may act as calibration targets for harmonic radars or similar nonlinear-junction detectors.

A unique apparatus is constructed for generating and capturing acoustic-radar responses from electronic targets. The concept of acoustic radar is reviewed, assembly of the apparatus is described in detail, and pictures of the apparatus are provided. The assembly is used to measure the acoustic-radar responses of two handheld radios to demonstrate detection as well as distinction between particular electronic devices.

Dynamic spectrum allocation will require cognitive radar transmitters to change operating frequency and bandwidth in real time. This will require high-power reconfigurable circuitry to improve radar performance by simultaneously increasing 1) the output power of the transmit waveform and 2) the power-added efficiency of the power amplifier. This circuitry is also used to mitigate cochannel interference by maintaining sufficiently linear performance so that the output waveform conforms to a given spectral mask. In this approach, a 90-W evanescent-mode cavity tuner is reconfigured using a specially designed gradient search to find the best combination of resonant cavity position numbers. This approach will be much more flexible for field use than typical Smith-chart-based load-pull searches, which require a characterization that is susceptible to drift. Experimental results are presented showing that the efficiency, output power, spectral performance, and estimated maximum radar detection range are improved significantly by retuning the matching network at each operating frequency. Additionally, this article discusses innovations to reduce or eliminate time bottlenecks in a cognitive radar system for impedance tuning, reducing the time needed for complete impedance tuning searches from minutes to seconds. This significant timing reduction makes tunable power amplifiers a feasible option for future use in spectrum sharing by cognitive radar systems.

A broad review of publications relevant to nonlinear radar is conducted. The principle-of-operation of nonlinear radar is summarized and applications for this technology are listed. Targets addressed by this type of radar follow a power-series model, and from this model a nonlinear radar range equation is derived. An extensive survey of publicly-available literature, including specifications for systems already tested, guides the design of harmonic radar for finding electronics. The authors have combined a stepped-frequency architecture with harmonic radar to create a system which is capable of imaging and tracking nonlinear targets with very high clutter rejection.

The use of software defined radios (SDRs) for radio frequency (RF) applications has spread to research labs, commercial industry and hobbies in recent years. This is because SDRs are low cost, readily available and software-tunable over a wide range of RF. Many SDRs are capable of full duplex on multiple channels and contain all the RF hardware needed for a wide variety of applications. Unfortunately, this high flexibility and low price point come at a cost of RF performance. This paper illustrates the limitations of SDR RF hardware and the impact of these limitations on radar performance. It then presents a technique for improving radar performance on a SDR.

Common approaches for radar and communication system spectrum sharing consider protection zones with power allocation for in-band operation, dynamic spectrum access (DSA) with spectrum sensing for in-band operation, and sense-and-avoid, frequency-agile approaches for out-of-band operation. In this paper we introduce a cooperative spectrum sharing model that combines multiple aspects of the previously mentioned approaches for in-band and out-of-band coexistence. This model jointly optimizes multiple radar and communication system parameters for improved frequency agility and performance while mitigating mutual interference between secondary radiofrequency (RF) users. Spectrum sensing is implemented to form a power spectral estimate of the electromagnetic environment (EME) to identify the secondary users. Multi-objective optimization then adjusts the output power, center frequency, and bandwidth parameters of the radar and communication system to maximize range resolution, radar signal to interference plus noise ratio (SINR), and channel capacity. Simulations are used to evaluate the model for different RF spectra. The results indicate that spectrum sharing is achieved for all systems.

Congestion in the RF spectrum is rapidly increasing, which has motivated the need for efficient spectrum sharing techniques. A cognitive radar system has been developed to implement a perception action cycle, for spectrum sharing, in which the RF spectrum is sensed, other RF signals are identified, and the radar frequency band of operation is adapted to avoid interfering signals in the spectrum. The system operates in real time and is capable of coexisting with common communications signals. A system with this capability requires efficient programming that pushes the limits of the technology available. In order to properly test the performance of a radar system designed for this kind of reactive spectrum sharing, a rigorous set of synthetic interference signals is generated and several informative evaluation metrics are defined. Additionally, the system's performance is evaluated with common communications signals such as LTE and GSM. The performance of the system is found to be adequate for avoiding signals that are either varying in frequency or turning on and off at rates on the order of 10 ms.