Publications
Volume 3, Number 2
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A History of Vocoder Research at Lincoln Laboratory During the 1950s, Lincoln Laboratory conducted a study of the detection of pitch in speech. This work led to the design of voice coders (vocoders)—devices that reduce the bandwidth needed to convey speech. The bandwidth reduction has two benefits: it lowers the cost of transmission and reception of speech, and it increases the potential privacy. This article reviews the past 30 years of vocoder research with an emphasis on the work done at Lincoln Laboratory. The Airborne Seeker Test Bed The Airborne Seeker Test Bed is a recently operational instrumentation system containing a closed-loop tracking, semi-active seeker with the capability to record high-fidelity signals pertaining to radar seeker phenomenology, target scattering characteristics, electronic countermeasures, and acquisition and tracking performance. The unique capabilities of the test bed will be used to collect data and develop computer models for evaluating and predicting missile performance. Test bed data will be used to evaluate the susceptibility of U.S. aircraft to missile attack, and to explore new directions for future systems. The test bed is also designed to support the development of advanced seekers and new electronic counter-countermeasure techniques, and to demonstrate their capabilities in flight. Beam Path Conditioning for High-Power Laser Systems Heating of mirrors and windows by high-power radiation from a laser transmitter produces turbulent density gradients in the gas near the optical surfaces. If the gradients are left uncontrolled, the resulting phase errors reduce the intensity on the target and degrade the signal returned to a receiver. Beam path conditioning maximizes the efficiency of the optical system by alleviating thermal turbulence within the beam path. A Space-Qualified Transmitter System for Heterodyne Optical Communications A space-based optical communications system requires the development of high-precision yet rugged electro-optical hardware. As part of a program to develop this technology, Lincoln Laboratory has designed and constructed a laser transmitter and a companion diagnostics module that have passed a rigorous space-qualification test program. The transmitter and diagnostics module are critical components of a satellite-to-satellite, 220 Mb/sec heterodyne communications experiment. The transmitter includes four redundant 30-mW diode lasers in a compact, lightweight package. The diagnostics module enables precise and autonomous setting of the transmitter laser power, wavelength, and modulation characteristics. The successful qualification of these components is a first, and a major milestone in the development of spaceborne optical communications systems. Optimal Processing of Polarimetric Synthetic-Aperture Radar Imagery The Advanced Detection Technology Sensor can detect, discriminate, and classify stationary ground targets-during the day or night—even through cloud cover, fog, smoke, dust, or rain. The sensor is a coherent, fully polarimetric, 35-GHz synthetic-aperture radar (SAR) with a resolution of 1 ft x 1 ft. And, to minimize SAR speckle while preserving image resolution, it uses the polarimetric whitening filter, our recently developed method for processing fully polarimetric data into SAR imagery. An Ultralow-Sidelobe Adaptive Array Antenna One of the most important functions of a radar antenna is to provide spatial filtering that maximizes the radar's sensitivity in the desired surveillance direction while suppressing interference signals that enter the radar from other directions. This article describes a UHF radar antenna system with exceptional interference-rejection capabilities that are achieved through a combination of precision passive beamforming in the azimuth plane, and active, digital adaptive nulling in elevation. The antenna is composed of 14 stacked rows; each row contains a stripline, low-sidelobe, passive corporate-feed network. For signal reception, a separate receiver and analog-to-digital converter are used at each row output. The system adaptively combines the digitized row signals to form an elevation pattern with nulls at the elevation angles of interference sources. The antenna system is part of an advanced air surveillance radar that Lincoln Laboratory is developing for the Navy. This article describes the design of the antenna system and presents performance analyses and measured results. The Best Approximation of Radar Signal Amplitude and Delay The estimation of receiver signal amplitude and delay, which can be converted to target cross section and range, is one of the fundamental functions of signal processing algorithms in a narrowband radar. The problem of tracking high-earth-orbit (HEO) satellites with ground-based radars requires a generalization of simple filter-bank signal processing architectures traditionally used to estimate signal amplitude and delay. A solution of the mathematical best-approximation problem leads to a new signal processing architecture that efficiently estimates signal amplitude and delay in all of the generality necessary to address the BEO satellite-tracking problem. |
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