Russell Vela

Noise radar tomography

While traditionally employed in acoustics, tomographic imaging techniques have made a transition into the microwave regime and are finding application in areas such as ground penetrating radar (GPR). While excitation methods vary depending upon the application, little or no literature exists on the use of an arbitrary noise waveform as an excitation method in tomography. This paper complements the existing literature on tomography by presenting an ultra wideband (UWB) radar design, utilized for tomography, whose excitation source is of an arbitrary noise origin operating from 4 to 8 GHz at a full 4 GHz bandwidth. Evidence of this sources ability to be used in tomography is presented through the generation of two dimensional cross sectional images of three dimensional objects such as metallic and dielectric cylinders in various positions, under both unobscured and concealed conditions.

Development of a new software-defined S-band radar and its use in the test of wavelet-based waveforms

A new software-defined S-band radar (SDSR) has recently been developed at the Air Force Research Laboratory. The system is built upon individual off-the-shelf components, devices, and instruments, and the center of the architecture is an Arbitrary Waveform Generator (AWG). The AWG can be programmed to generate various kinds of radar waveforms, which are mixed with a carrier to create S-band radar signals. The AWG can be interfaced with a computer which runs on MatLab or LabView for software definition of radar waveforms. Furthermore, the computer can be connected to the Internet for receiving data from remote users. As a result, the new SDSR can be used to support studies of radar waveforms in a large area by remote users. The characteristics of the radar system are studied and the performance of the system is optimized by selecting the parameters of the building components. The newly developed SDSR is used to test the wavelet-based radar waveforms, which verifies the theoretical results produced earlier.

A calibration procedure for ground-based RF tomography

Ground-based RF Tomography requires accurate calibration to ensure that acceptable image quality is achieved. A calibration scheme that exploits the direct path between any transmitter and receiver pair was presented in [1], valid only when radiators are approximated as Hertzian dipoles. In this work, a generalized calibration procedure that is valid for most antennas is presented. The calibration is achieved by placing a known target slightly outside the area under observation. The known target can be of any shape, as long as numerical/analytical EM predictions are available. Preliminary experimental results are presented.

Range-Doppler-angle ambiguity function analysis in modern radar

Summary form only given. In this research, the impact of signal bandwidth on the ambiguity function is explored. While this effort focuses on monostatic radar, the concepts developed are fundamentally applicable to distributed sensor geometries. Consider an array of N antenna elements with non-uniform spacing, energized by a doublet, i.e., a single cycle of RF energy approximating the derivative of an impulse. Alternatively, an impulse may be considered, but no radiating element supports propagation of a DC current. In the simplest case, identical broadband signals, W(t), are radiated from each of the uniformly spaced elements. Broadside, these signals sum coherently, while they are dispersed off-broadside (angle θ), with the dispersion proportional to Nd sinθ / c, where d is the element spacing. For broadband signals W(t) of duration T, the far field signal is of duration T + Nd sinθ / c. In the computation of the ambiguity function, the effect of the cross correlation of the radiated waveform with a Doppler shifted replica of itself is dispersed by a factor of two. However, in the case of broad band radiation, the “pulse” duration is itself a function of angle. As such, the ambiguity function is severely distorted as a function of steering angle. In this paper, the impact of signal bandwidth on range-Doppler discrimination as a function of viewing angle will be discussed, as well as implications in MIMO radar.

Rediscovering monopulse radar with digital sum-difference beamforming

Classically, radar signals received from the monopulse quadrants are processed analogically to obtain sum (Σ) and difference (Δ) patterns. Exact Σ/Δ patterns are obtained only at the particular frequency in which the monopulse comparator is designed. In this paper, Σ/Δ patterns are computed via direct digizitazion of the signal coming from the quadrants, paving the way to the development of novel signal processing algorithms. Performance and limitations of digital monopulse processing will be discussed.

A technique for the generation of customizable ultra-wideband pseudo-noise waveforms

Noise excitation sources in radar systems have become increasingly useful in applications requiring wideband spectral responses and covertness. However, in applications requiring spectral controllability, traditional analog noise sources prove troublesome and require additional hardware such as sets of digital filters whose own spectral characteristics must also be accounted for. In an effort to reduce these issues and increase the applications of noise waveforms, a technique for generating a fully controllable pseudo-noise waveform is presented. This pseudo-noise waveform will be generated through the use of a multi-tone waveform. By randomizing the phase angles and setting the appropriate amplitudes to the individual tones, the result is a waveform whose temporal pattern resembles noise and frequency response is broadband. The capabilities of this digitally produced pseudo-noise multi-tone waveform is presented by optimization via a water-filling technique, thereby producing a flat spectral response for a user defined amplitude, effectively removing the spectral effects of the radar components. This optimized waveform is used to present methods for increasing signal to noise ratio (SNR) of cross-correlated responses of the waveform through the application of window functions to the waveform. As a whole, this paper showcases the ability to use this pseudo-noise multi-tone waveform for complete ultra-wideband (UWB) spectral control through water-filling and a method for increasing SNR of the cross correlated response of the transmitted and received radar waveform for a bandwidth of 2.5 GHz ranging from 2 to 4.5 GHz.

Target-adaptive radar pulse-train optimization

The possibility of optimizing pulse-train length in pulse-Doppler radar is investigated. Adapting pulse-train length in real-time changes the precision of range and velocity information present in radar returns and influences the eventual tracker performance.

Human thermal emissions and their exploitation in passive microwave radar

An investigation of the thermally generated microwave emissions of humans and their ability to be utilized in passive radar applications is to be presented. These investigations will be beneficial to the research areas of concealed human detection, identification and characterization, concealed object detection, through wall imaging and close-in sensing. While this paper conveys work performed in the 240 to 260 GHz regime, the presentation will also include research being performed in the X and W bands. It will be shown that sufficient spectral power is generated at these frequencies by humans not only to be receivable in sensitive passive radar systems but to aid in identification and classification as well.

RF Tomography: Self calibration of distributed RF sensors

Ground-based RF Tomography requires accurate calibration to ensure that acceptable image quality is achieved. A calibration scheme was presented in [1], valid only when radiators are approximated as Hertzian dipoles. In this work, a generalized calibration procedure that is valid for most antennas is presented. The calibration is achieved by placing a known target slightly outside the area under observation. Experimental results and discussions are presented.

A technique for the extraction of ultra-wideband (UWB) signals concealed in frequency band folded responses

Ultra-wideband (UWB) excitation sources in radar systems have allowed for enhancement in capabilities such as target spectral response, clutter suppression, and range resolution. While generation of generic UWB signals has become easily achievable, direct acquisition, or digitization, of these bandwidths (≥ 4 GHz) is not. To account for this, many UWB radar systems implement a single or multi-stage band folding technique in the receiver hardware chain which allows for the direct digitization of the UWB waveform at a smaller bandwidth (e.g., 4 GHz into 1 GHz). While the lower bandwidth allows for larger than narrowband capabilities, it reduces desired features such as range resolution (e.g., 3.75 cm to 15 cm). In an effort to address this problem, and allow for utilization of full bandwidth of an UWB waveform, this paper presents a signal processing technique which utilizes hardware band folding to wrap a spectrally unique UWB multi-tone waveform into a lower frequency, lower bandwidth signal allowing for both direct digitization and conservation of UWB features. The signal processing technique utilizes the multi-tone waveform to generate an UWB signal composed of sections whose separate spectral peaks fold into the inner ΔF regions of the previous band. It will be shown, that through reassignment of these peaks, as well as the phase, to the individual frequencies, the intended UWB capabilities can be restored.