Dale F. Dubbert

A high-resolution, four-band SAR Testbed with real-time image formation

This paper describes the Twin-Otter SAR Testbed developed at Sandia National Laboratories. This SAR is a flexible, adaptable testbed capable of operation on four frequency bands: Ka, Ku, X, and VHF/UHF bands. The SAR features real-time image formation at fine resolution in spotlight and stripmap modes. High-quality images are formed in real time using the overlapped subaperture (OSA) image-formation and phase gradient autofocus (PGA) algorithms.

Affordable, miniaturized SAR for tactical UAV applications

Sandia’s fielded and experimental SAR systems are well known for their real time, high resolution imagery. Previous designs, such as the Lynx radar, have been successfully demonstrated on medium-payload UAVs, including Predator and Fire Scout. However, fielding a high performance SAR sensor on even smaller (sub-50 pound payload) UAVs will require at least a 5x reduction in size, weight, and cost. This paper gives an overview of Sandia’s system concept and roadmap for near-term SAR miniaturization. Specifically, the “miniSAR” program, which plans to demonstrate a 25 pound system with 4 inch resolution in early 2005, is detailed. Accordingly, the conceptual approach, current status, design tradeoffs, and key facilitating technologies are reviewed. Lastly, future enhancements and directions are described, such as the follow-on demonstration of a sub-20 pound version with multi-mode (SAR/GMTI) capability.

Bistatic SAR: Proof of Concept.

Typical synthetic aperture RADAR (SAR) imaging employs a co-located RADAR transmitter and receiver. Bistatic SAR imaging separates the transmitter and receiver locations. A bistatic SAR configuration allows for the transmitter and receiver(s) to be in a variety of geometric alignments. Sandia National Laboratories (SNL) / New Mexico proposed the deployment of a ground-based RADAR receiver. This RADAR receiver was coupled with the capability of digitizing and recording the signal collected. SNL proposed the possibility of creating an image of targets the illuminating SAR observes. This document describes the developed hardware, software, bistatic SAR configuration, and its deployment to test the concept of a ground-based bistatic SAR. In the proof-of-concept experiments herein, the RADAR transmitter will be a commercial SAR satellite and the RADAR receiver will be deployed at ground level, observing and capturing RADAR ground/targets illuminated by the satellite system.

Radar cross section statistics of cultural clutter at Ku-band

Knowing the statistical characteristics of the radar cross-section (RCS) of man-made, or cultural clutter, is crucial to the success of clutter mitigation, radar target detection algorithms, and radar system requirements in urban environments. Open literature studies regarding the statistical nature of cultural clutter focus primarily on radar probability models or limited experimental data analysis of specific locations and frequencies. This paper seeks to expand the existing body of work on cultural clutter RCS statistics at Ku-band for ground moving target indication (GMTI) and synthetic aperture radar (SAR) applications. We examine the normalized RCS probability distributions of cultural clutter in several urban scenes, across aspect and elevation angle, for vertical transmit/receive (VV) polarizations, and at diverse resolutions, using experimental data collected at Ku-band. We further describe frequency and RCS strength statistics of clutter discretes per unit area to understand system demands on radars operating in urban environments in this band.

Spurious effects of analog-to-digital conversion nonlinearities on radar range-Doppler maps

High-performance radar operation, particularly Ground Moving Target Indicator (GMTI) radar modes, are very sensitive to anomalous effects of system nonlinearities. System nonlinearities generate harmonic spurs that at best degrade, and at worst generate false target detections. One significant source of nonlinear behavior is the Analog to Digital Converter (ADC). One measure of its undesired nonlinearity is its Integral Nonlinearity (INL) specification. We examine in this paper the relationship of INL to radar performance; in particular its manifestation in a range-Doppler map or image.

Radome effects on coherent change detection radar systems

A radome, or radar dome, protects a radar system from exposure to the elements. Unfortunately, radomes can affect the radiation pattern of the enclosed antenna. The co-design of a platform’s radome and radar is ideal to mitigate any deleterious effects of the radome. However, maintaining structural integrity and other platform flight requirements, particularly when integrating a new radar onto an existing platform, often limits radome electrical design choices. Radars that rely heavily on phase measurements such as monopulse, interferometric, or coherent change detection (CCD) systems require particular attention be paid to components, such as the radome, that might introduce loss and phase variations as a function of the antenna scan angle. Material properties, radome wall construction, overall dimensions, and shape characteristics of a radome can impact insertion loss and phase delay, antenna beamwidth and sidelobe level, polarization, and ultimately the impulse response of the radar, among other things, over the desired radar operating parameters. The precision-guided munitions literature has analyzed radome effects on monopulse systems for well over half a century. However, to the best of our knowledge, radome-induced errors on CCD performance have not been described. The impact of radome material and wall construction, shape, dimensions, and antenna characteristics on CCD is examined herein for select radar and radome examples using electromagnetic simulations.

A new approach for in-situ scan impedance characterization of scanned antenna arrays

Scanned phased array antennas require active scan impedance determination and mitigation. This paper addresses the former by introducing a novel in-situ measurement architecture and associated mathematics for efficiently determining the real-time active scan impedance of arbitrary sized scanned arrays in the field. The in-situ nature of the proposed architecture reduces the need for large numerical simulation and/or estimation of scan impedance variations due to possible diverse antenna array placement in the field. Direct experimental characterization also enables direct validation of numerical simulation. The mathematics developed are for an M by N antenna array utilizing direct in-situ mutual coupling characterization. The mathematical model was implemented in MATLAB and verified through simulation using CST Microwave Studio (MWS) for a 2×2 monopole planar antenna array. The models robustness is tested by varying the inter-element spacing.

Results of the sub-thirty-pound high-resolution miniSAR demonstration

Sandia-developed SAR systems are well known for their real-time, high quality, high resolution imagery. One such system, the General Atomics Lynx radar, has been successfully demonstrated on medium-payload UAVs, including the Predator and Fire Scout. Previously, Sandia reported on its system concept and roadmap for SAR miniaturization, including details of the miniSAR program. This paper and its companions provide an update for miniSAR and discuss the results of the successful May 2005 demonstration of the 26 pound, 4-inch resolution system. Accordingly, the miniSAR system and software implementation and performance are reviewed. Additionally, future plans for miniSAR and the Sandia SAR/GMTI miniaturization efforts are discussed, such as the currently planned miniSAR demonstration onboard a small-payload UAV.