William Buller

Distributed RADAR waveform design based on compressive sensing considerations

In this paper we describe a joint waveform design methodology for distributed imaging RADARs using the concepts of compressive sensing. Compressive sensing is an active area of research that offers the promise of good object reconstruction with a sparse measurement set. The measurement set of the scene is based on a set of dasiaprobes,psila the radar waveforms. The set of measurements must satisfy the restricted isometry property and the scene being interrogated must be dasiacompressiblepsila meaning that it can be sparsely represented in some basis. We examine waveform and position considerations for a distributed radar system to satisfy these constraints and show their impact on waveform design and image reconstruction.

An analysis of coded aperture acquisition and reconstruction using multi-frame code sequences for relaxed optical design constraints

We present an investigation of the performance of coded aperture optical systems where the elements of a set of binary coded aperture masks are applied over a sequence of acquired images. In particular, we are interested in investigating code sequences and image reconstruction algorithms that reduce the optical fidelity and hardware requirements for the system. Performance is jointly tied to the mask design, the image estimation algorithm, and the inherent optical response of the system. As such, we adopt a simplified reconstruction model and consider generalized optical system aberrations in designing masks used for multi-frame reconstruction of the imagery. We also consider the case of non-Nyquist sampled (aliased) imagery. These investigations have focused on using a regularized least-squares reconstruction model and mean squared error as a performance metric. Masks are found by attempting to minimize a closed form objective that predicts the mean squared error for the reconstruction algorithm. We find that even with suboptimal solutions that binary masks can be used to improve imagery over the case of an uncoded aperture with the same aberration.

Statistical modelling of measured automotive radar reflections

A statistical analysis was performed on measured radar reflections from a broad range of personal vehicle classes. The outcome of this study is two-fold: 1.) An improved understanding of the radar scattering components of automobiles, which informs the design of surrogate test targets for evaluating automotive pre-collision system (PCS) radars, and 2.) statistical models for evaluating surrogate targets and characterizing target models for PCS radar system designs. We examined the validity of two-parameter distribution models applied to measurements of subject vehicles radar cross-section (RCS) and found the Weibull distribution to be the best fit. In evaluating the goodness-of-fit of the Weibull distribution model, using the Kolmogorov-Smirnov test, we deem an acceptable fit between the model and the measured RCS data for our intended project outcome.

Radar Scattering Design Elements for a Pre-Collision System Surrogate Target

The aim of our research is to design surrogate test targets for evaluation of automotive Pre-Collision Systems (PCS). To design these surrogate test targets, we used derived scattering sources from measured radar returns of selected subject vehicles. Twenty-five vehicles were selected from a broad range of vehicle classes. This group includes several vehicles that are frequently struck in rear-end collisions as determined by the U.S. National Crash Database [1]. We used a wideband, millimeter wave, instrumentation radar to measure radar returns of these subject vehicles. Range profiles and real-beam images were used to isolate scattering sources, which were then used to design surrogate targets that mimic the radar response of real cars. To verify the design of such surrogate test targets, we developed and applied a statistical model [2] of radar signatures from the group of selected vehicles at a fixed set of viewing angles. The collected signatures for each vehicle and test-surrogates were fit to a Weibull- distribution using a maximum-likelihood estimator. We then applied results of the statistical studies to evaluate the surrogate test targets.

Radar characterization of automobiles and surrogate test-targets for evaluating automotive pre-collision systems

We measured and characterized the radar response of twenty-five vehicles to evaluate the quality of surrogate test targets. The vehicles are selected form a broad range of vehicle classes and include several that are frequently struck in rear-end collisions as determined by the U.S. National Crash Database [1]. Each vehicles response is measured with a wideband, millimeter wave, instrumentation radar at a set of identical angles near tail-aspect. The collected signatures for each vehicle and test-surrogates are fit to a Γ-distribution using a maximum-likelihood estimator. The wideband waveform provides spatial separation of scattering sources for additional analysis. The main result of this research is a process for characterizing automobile responses at W-band, how we use these results to identify good design practices for developing radio-frequency (RF) test surrogates and the impacts of these designs on test procedures for evaluating RF pre-collision systems (PCS).

Methods for modeling multimode waveguides with abrupt changes in propagation axis

Characterization and performance estimates for polymer, step-index, multi-mode, channel waveguides requires simulation of light propagation in models requiring large numbers of spatial samples using the method of finite-difference time-domain methods (FDTD). As noted in [1], full 3-D solutions for multi-mode waveguides with a cross-section of tens of microns are nontrivial. In many instances, designers are forced to use a ray-tracing model to characterize the channel. However, ray-tracing is not intended to characterize spectral and modal properties. Approximation of Helmholtz equation for slowly varying field in the propagation plane, termed beam propagation methods (BPM), has proved efficient for modeling optical waveguides [2–4]; however, these are primarily intended for modeling waveguides with slowly varying axes of propagation.

Effects of lithographic roughness and sidewall slope on the optical performance of polymer rectangular waveguides: Modeling

Fabrication of polymer, multimode, channel waveguides have gained much attention recently because of usefulness in replacing fiber in backplane interconnects. Waveguides are more desirable because they are able to achieve greater bandwidth, they are immune to electromagnetic interference and they consume less power compared to traditional electrical communication links. They are however subject to insertion losses due to fabrication defects such as the waveguide sidewall roughness and sidewall slope. This requires fabrication tolerances that are on the micron level. Determining the acceptable minimum tolerances required in the fabrication process in terms of the overall channel link budget is critical for determining how precise of fabrication methods must be utilized.

Potential application of micro-bolometer coupled antenna-pairs in beam synthesis

The IR antenna-pair coupled micro-bolometers has demonstrated its unique power response features compared to the single antenna coupled micro-bolometers. The response pattern is determined by that of the single antenna and an interference oscillation term of the antenna-pair with respective to the angle of incidence of the radiation field, and can be steered by shifting the location of the bolometer. This paper explores the potential application of antenna-pair coupled detector in beam synthesis. It describes an array configuration based upon these micro-bolometers, and discusses the corresponding coherent data processing method for the purpose of obtaining response pattern narrowing effects from such an array. This directional gain enhancement, together with the beam steering control, could potentially lead to an array capable of providing a novel IR lensless imaging technique.