Kenneth I. Schultz

Ground-based laser radar measurements of satellite vibrations

Vibration signatures from the low-power atmospheric compensation (LACE) satellite are obtained by using the MIT Lincoln Laboratory Firepond coherent CO(2) laser radar facility located in Westford, Mass. The LACE satellite is equipped with IR germanium retroreflectors on deployable/retractable booms to enhance ground-based IR laser radar measurements of on-orbit boom vibrations. Analysis of pulsed cw laser radar measurements of the satellite during and subsequent to boom retraction indicates the presence of a complex time-varying model structure. The observed vibration spectra include vibration modes not previously predicted. These data represent the first observations of satellite vibration modes from a ground-based laser radar.

Design approaches for digitally dominated active pixel sensors: leveraging Moore's Law scaling in focal plane readout design

Although CMOS technology scaling has provided tremendous power and circuit density benefits for innumerable applications, focal plane array (FPA) readouts have largely been left behind due to dynamic range and signal-to-noise considerations. However, if an appropriate pixel front end can be constructed to interface with a mostly digital pixel, it is possible to develop sensor architectures for which performance scales favorably with advancing technology nodes. Although the front-end design must be optimized to interface with a particular detector, the dominant back end architecture provides considerable potential for design reuse. In this work, digitally dominated long wave infrared (LWIR) active pixel sensors with cutoff wavelengths between 9 and 14.5 μm are demonstrated. Two ROIC designs are discussed, each fabricated in a 90-nm digital CMOS process and implementing a 256 x 256 pixel array on a 30-μm pitch. In one of the implemented designs, the feasibility of implementing a 15-μm pixel pitch FPA with a 500 million electron effective well depth, less than 0.5% non-linearity in the target range and a measured NEdT of less than 50 mK at f/4 and 60 K is demonstrated. Simple on-FPA signal processing allows for a much reduced readout bandwidth requirement with these architectures. To demonstrate the potential for commonality that is offered by a digitally dominated architecture, this LWIR sensor design is compared and contrasted with other digital focal plane architectures. Opportunities and challenges for application of this approach to various detector technologies, optical wavelength ranges and systems are discussed.

Satellite vibration measurements with an autodyne CO 2 laser radar

Vibration signatures of the Low Power Atmospheric Compensation Experiment satellite were obtained with a ground-based CO(2) laser radar. The laser radar operated in a cw mode and used autodyne receivers to extract relative target velocity information between a germanium retroreflector located at the base of the satellite and a retroreflector array located at the tip of an extended forward boom. Time-frequency analysis algorithms were applied to the vibration data to investigate the correlation between excitations and modal structure. The resultant analysis suggests that vibration modes of an on-orbit spacecraft can be suppressed with simple open-loop techniques.

Acquisition algorithm for direct-detection ladars with Geiger-mode avalanche photodiodes

An optimal algorithm for detecting a target using a ladar system employing Geiger-mode avalanche photodiodes (GAPDs) is presented. The algorithm applies to any scenario where a ranging direct detection ladar is used to determine the presence of a target against a sky background within a specified range window. A complete statistical model of the detection process for GAPDs is presented, including GAPDs that are inactive for a fixed period of time each time they fire. The model is used to develop a constant false alarm rate detection algorithm that minimizes acquisition time. Numerical performance predictions, simulation results, and experimental results are presented.

Spectrogram processing of laser vibration data

Laser vibration sensing has traditionally relied on the use of limiters and frequency modulation (FM) discriminators to process frequency modulated laser radar returns. The performance of the traditional FM discriminator approach can be limited by laser radar target characteristics and motion (speckle noise) and laser temporal coherence. In this paper we examine a novel laser vibration signal processor, a spectrogram processor, and compare its performance with the traditional limiter/FM discriminator signal processor used to process laser radar vibration measurements. The two processes are also compared using some laser radar measurement data.

The Standoff Aerosol Active Signature Testbed (SAAST) at MIT Lincoln Laboratory

The Standoff Aerosol Active Signature Testbed (SAAST) is the aerosol range within the MIT Lincoln Laboratorys Optical System Test Facility (OSTF). Ladar and Lidar are promising tools for precise target acquisition, identification, and ranging. Solid rocket effluent has a strong Lidar signature. Currently, calculations of the Lidar signature from effluent are in disagreement from measurements. This discrepancy can be addressed through relatively inexpensive laboratory measurements. The SAAST is specifically designed for measuring the polarization-dependent optical scattering cross sections of laboratory-generated particulate samples at multiple wavelengths and angles. Measurements made at oblique angles are highly sensitive to particle morphology, including complex index of refraction and sample shape distribution. With existing hardware it is possible to re-aerosolize previously collected effluent samples and, with online and offline diagnostics, ensure that these samples closely represent those found in situ. Through comparison of calculations and measurements at multiple angles it is possible to create a realistic model of solid rocket effluent that can be used to extrapolate to a variety of conditions. The SAAST has recently undergone a dramatic upgrade, improving sensitivity, flexibility, sample generation, sample verification, and level of automation. Several measurements have been made of terrestrial dust and other samples.

An Airborne Passive/Active Electro‐Optic Sensor System for Theater Ballistic Missile Defense

An airborne passive/active surveillance aircraft-based, electro-optic sensor system, named Gatekeeper, is being developed for the US Navy Theater Ballistic Missile Defense Program. The sensor is designed to detect theater ballistic missiles (TBMs), either in boost or post-boost phase, and make precision three-dimensional measurements of the TBMs post-boost, ballistic trajectory with sufficient state-vector accuracy for handover to naval, air and land based missile defense systems. The sensor includes a dual-band IRST for acquisition, a precision angle tracker (MWIR focal-plane array and high-bandwidth mirror), and a short-pulse, direct detection laser radar. The sensor subsystems are coupled and controlled by a sophisticated multiprocessor computer control system. The system has a highly compressed engagement timeline resulting in a substantial target handling capacity. This paper describes the sensor, its specifications, and performance in terms of the accuracy of the state-vector, and its target handling capability in a realistic engagement scenario.

Ladar measurements of satellite vibrations with post-heterodyne detection autodyne receivers

Vibration signatures of the Low Power Atmospheric Compensation Experiment (LACE) satellite were obtained using a ground based CO2 laser radar. The laser radar operated in a cw mode and utilized autodyne receivers to extract relative target velocity information between a germanium retroreflector located at the base of the satellite and a retroreflector array located at the tip of an extended forward boom. Time-frequency analysis algorithms were applied to the vibration data to investigate the correlation between excitations and modal structure. The resultant analysis suggests that vibration modes of an on-orbit space craft can be suppressed using simple open-loop techniques.

Ladar measurements of satellite vibrations

Vibration signatures from the Low Power Atmospheric Compensation (LACE) satellite were obtained using a ground-based coherent CO2 laser radar facility. Analysis of pulsed CW laser radar measurements of the satellite indicates the presence of complex time-varying vibration modes. These data represent the first observations of satellite vibration modes from a ground-based laser radar.

Analytic approach to centroid performance analysis

A hybrid analytic computer simulation approach is used to estimate centroid performance of laser radar images. This approach includes the statistical effects of shot-noise and speckle and provides the flexibility to model complex target geometry and surface scattering, image point spread response characteristics, and target glints. Results suggest that image edge effects have a pronounced impact on centroid accuracy. A bi-level threshold is shown to provide improved centroid stability over intensity and binary images.