SEARCH · Engineering Papers

Results for “direct detection”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 613 records · Page 34

All sources NASA NTRS DOE OSTI

NASA NTRS1972

Photodiodes for ten micrometer laser communication systems

The performance is discussed of 10-micron mercury-cadmiumtelluride and lead-tin-telluride photodiodes in laser heterodyne communication systems. The dependence of detector quantum efficiency, resistance, frequency response, and signal-to-noise ratio on temperature, bias, and local oscillator power are examined. Included in the discussion is an analysis of the feasibility of high temperature operation, and ability of the detector to dissipate power to a heat sink is explored. Some aspects of direct detection response are considered and figures showing flux levels from a blackbody presented.

Cohen, S. C.↗

NASA NTRS1973

Bragg crystal spectrometer for HEAO-B X-ray astronomy experiment

The X-ray astronomy experiment for the HEAO mission is described. The instrument to be flown on the satellite consists of two Bragg crystal spectrometers designed to make measurements with high spectral resolution in the energy range from 0.1 to 10 keV. No sharp spectroscopic features were detected directly in X-ray astronomy except emission lines in the sun; therefore, the objectives and design criteria of this instrument are based on the theoretical models of the different types of extra solar X-ray sources. From the point of view of instrumental techniques it is convenient to divide these into point or stellar sources, whose radiation is parallel, and diffuse sources, and to consider the possible spectral features in each type.

Angel, J. R. P.↗

NASA NTRS1972

Optical communication theory.

An introduction is provided to the important ideas involved in the design and the evaluation of optical communication systems which transmit digital or analog data. Field representations are discussed together with background noise, a detector statistical model, quantum receivers, and turbulent and scatter channels. Free-space channels are considered, taking into account heterodyne systems, preamplifier and homodyne systems, direct-detection systems, and optimum quantum systems.

Hoversten, E. V.↗

NASA NTRS1973

Applications of thermal remote sensing to detailed ground water studies

Three possible applications of thermal (8-14 microns) remote sensing to detailed hydrogeologic studies are discussed in this paper: (1) the direct detection of seeps and springs, (2) the indirect evaluation of shallow ground water flow through its thermal effects on the land surface, and (3) the indirect location of small volumes of ground water inflow into surface water bodies. An investigation carried out with this purpose in an area containing a complex shallow ground water flow system indicates that the interpretation of the thermal imageries is complicated by many factors, among which the most important are: (1) altitude, angle of view, and thermal-spatial resolution of the sensor; (2) vegetation type, density, and vigor; (3) topography; (4) climatological and micrometeorological effects; (5) variation in soil type and soil moisture; (6) variation in volume and temperature of ground water inflow; (7) the hydraulic characteristics of the receiving water body, and (8) the presence of decaying organic material.

Souto-Maior, J.↗

NASA NTRS1975

Application of LANDSAT to the Surveillance and Control of Eutrophication in Saginaw Bay

The author has identified the following significant results. LANDSAT digital data and ground truth measurements for Saginaw Bay (Lake Huron), Michigan, for 3 June 1974 can be correlated by stepwise linear regression technique and the resulting equations used to estimate invisible water quality parameters in nonsampled areas. Correlation of these parameters with each other indicates that the transport of Saginaw River water can now be traced by a number of water quality features, one or more of which are directly detected by LANDSAT. Five of the 12 water quality parameters are best correlated with LANDSAT band 6 measurements alone. One parameter (temperature) relates to band 5 alone and the remaining six may be predicted with varying degrees of accuracy from a combination of two bands (first band 6 and generally band 4 second).

Rogers, R. H.↗

NASA NTRS1975

Coupling characteristics of thin-film metal-oxide-metal diodes at 10.6 microns

Direct detection experiments have demonstrated the coherent coupling of 10.6 micrometer radiation into photolithographically fabricated metal-oxide-metal tunnel junctions. A CO2 laser beam mechanically chopped at 1 KHz was focused at a variable angle of incidence with a power density of about 10 W/sq cm at the diodes. Diodes in which the junction resistance was much greater than the lead resistance displayed angular characteristics dominated by coherent antenna coupling.

Wang, S. Y.↗

NASA NTRS1978

Infrared heterodyne interferometry

The possibilities for infrared heterodyne interferometry are examined. First the characteristics of atmospheric seeing in the infrared are considered, in order to determine the sizes of collecting apertures and baselines which can be used for relatively simple interferometers. The advantages of heterodyne over direct detection are pointed out. For example, when long baselines are used, the narrow bandwidth of heterodyne detection makes matching the path lengths easier. Experimental results on the spatial distribution of warm dust around stars and on astrometric precision obtained with prototype systems are given. Telescope design for a heterodyne interferometer is discussed.

Townes, C. H.↗

NASA NTRS1979

Application of a microprocessor controlled lidar to tropospheric ozone measurements

A microprocessor controlled lidar system under construction at the NASA Goddard Space Flight Center is described and the problems in making space-based measurements of tropospheric ozone are considered. The differential absorption lidar using a dual wavelength, pulsed CO2 laser and direct detection receiver can significantly improve the existing global data base on tropospheric ozone burden. Sensitivity to tropospheric ozone can be obtained in the spaceborne version of lidar by selecting laser lines located in the wings of the target zone lines. Simulation studies using various laser line pairs in the P-branch of the CO2 9.4 micron band show that the ozone burden retrieval may be weighted to particular altitude regions. These simulation studies are based on numerical integration of differences in absorption coefficient at the two selected laser wavelengths using the AFGL absorption line parameter compilations, U.S. Standard Atmosphere ozone profile, and laser software. Simulation, data collection and reduction are performed by a microprocessor subsystem of the CO2 lidar.

Stewart, R. W.↗

NASA NTRS1979

Remote measurement of tropospheric ozone

It is shown that a differential absorption lidar employing a pulsed CO2 laser and a direct detection receiver is capable of significantly improving the existing data base on the tropospheric ozone burden. As a ground-based system, the lidar could obtain urban to regional scale O3 measurements with a vertical or horizontal resolution of at least 1 km in the troposphere. As a space-based system, it could obtain global scale coverage of the O3 burden below the stratospheric maximum of O3.

Bufton, J. L.↗

NASA NTRS1980

IUE observations of the hot components in two symbiotic stars

Recent IUE observations reveal striking differences in the UV spectra of two symbiotic stars, R Aqr and RW Hya. RW Hya is found to be an unexpectedly intense source of UV radiation. The measurements reported demonstrate the presence of a hot component in each star, supporting the view that each is a binary system with a luminous red primary and a hot, subluminous companion. In one case, the hot companion manifests itself by exciting a compact nebulosity; in the other case the continuous spectrum of the hot star is directly detected, while the continuum of nebulosity excited by the hot star is detected at longer wavelengths.

Michalitsianos, A. G.↗

NASA NTRS1981

Temporal correlation measurements of pulsed dual CO2 lidar returns

A pulsed dual-laser direct-detection differential-absorption lidar DIAL system, operating near 10.6 microns, is used to measure the temporal correlation and statistical properties of backscattered returns from specular and diffuse topographic targets. Results show that atmospheric-turbulence fluctuations can effectively be frozen for pulse separation times on the order of 1-3 msec or less. The diffuse target returns, however, yielded a much lower correlation than that obtained with the specular targets; this being due to uncorrelated system noise effects and different statistics for the two types of target returns.

Menyuk, N.↗

NASA NTRS1981

Remote sensing of sea state by laser altimeters

The reflection of short laser pulses from the ocean surface was analyzed based on the specular point theory of scattering. The expressions for the averaged received signal, shot noise and speckle induced noise were derived for a direct detection system. It is found that the reflected laser pulses have an average shape closely related to the probability density function associated with the surface profile. This result is applied to estimate the mean sea level and significant wave height from the receiver output of the laser altimeter.

Tsai, B.↗

NASA NTRS1981

ISIS observations of auroral particles and large-scale Birkeland currents

Simultaneous electron and positive ion observations made with single-component magnetic perturbations on the ISIS-2 satellite are used to compare and contrast the relationships between primary and secondary auroral particle distributions at 5 eV-15 keV, and the large-scale Birkeland currents, in the pre- and post-midnight local time sectors. No unique relation is found between the regions of the Birkeland current system and regions of auroral particle distribution, though repeatable systematics in the region of upward-directed current are observed, and little evidence exists in either local time sector for the direct detection of the downward current-associated current carriers.

Klumpar, D. M.↗

NASA NTRS1982

Model studies of laser absorption computed tomography for remote air pollution measurement

Model studies of the potential of laser absorption-computed tomography are presented which demonstrate the possibility of sensitive remote atmospheric pollutant measurements, over kilometer-sized areas, with two-dimensional resolution, at modest laser source powers. An analysis of this tomographic reconstruction process as a function of measurement SNR, laser power, range, and system geometry, shows that the system is able to yield two-dimensional maps of pollutant concentrations at ranges and resolutions superior to those attainable with existing, direct-detection laser radars.

Wolfe, D. C., Jr.↗

NASA NTRS1982

Limitations of signal averaging due to temporal correlation in laser remote-sensing measurements

Laser remote sensing involves the measurement of laser-beam transmission through the atmosphere and is subject to uncertainties caused by strong fluctuations due primarily to speckle, glint, and atmospheric-turbulence effects. These uncertainties are generally reduced by taking average values of increasing numbers of measurements. An experiment was carried out to directly measure the effect of signal averaging on back-scattered laser return signals from a diffusely reflecting target using a direct-detection differential-absorption lidar (DIAL) system. The improvement in accuracy obtained by averaging over increasing numbers of data points was found to be smaller than that predicted for independent measurements. The experimental results are shown to be in excellent agreement with a theoretical analysis which considers the effect of temporal correlation. The analysis indicates that small but long-term temporal correlation severely limits the improvement available through signal averaging.

Menyuk, N.↗

NASA NTRS1983

Ice in Comet Bowell

Results of observations of the reflected light in the 1-5 micron region of the spectrum from the Comet Bowell are presented as part of the attempt to directly detect the presence of frozen volatiles in the nucleus of a comet. A deep absorption at 3.25 microns is detected in Comet Bowell which is due to the presence of H2O ice. While other volatile ices have absorption features near 3 microns, it is shown that only H2O ice is a plausible identification at the heliocentric distance of Comet Bowell during the observations (3.4 AU) since only H2O grains are stable enough to have produced the observed absorption. The implications of these findings for models of cometary nuclei are examined.

Campins, H.↗

NASA NTRS1984

Optical communication with laser diode arrays

The performance of a direct-detection optical communication system in which the laser transmitter sends short optical pulses of selected nonoverlapping center frequencies is analysed. This modulation format, in which a single light pulse is sent in one of M time slots at one of N optical center frequencies, is referred to as color coded optical pulse position modulation (CCPPM). The optimum energy-efficiency of this system, as measured by the channel capacity in nats per photon, exceeds that of ordinary optical pulse position modulation which uses a pulsed laser of fixed optical frequency. Reliable communication at optimal energy efficiency is easily achieved through the use of modest block length Reed-Solomon codes with the code words represented as CCPPM symbols.

Davidson, F.↗

NASA NTRS1984

Evaluation of LANDSAT-D Thematic Mapper performance as applied to hydrocarbon exploration

Work with digital data of Oklahoma, Colorado, Wyoming, Utah and California demonstrate that the increased spectral refinement and spatial resolution of TM over MSS data greatly increase the value of the data to petroleum exploration in roles ranging from logistic planning to direct detection of phenomena related to microseepage of hydrocarbons. The value of the spatial content versus the spectral content of the data increases as soil and vegetation cover increase. The structural detail visible in the imagery can contribute to exploration at the prospect level. Examination of the variance/covariance matrix suggests that a combination of bands 1, 4, and 5 displays the most information for most areas.

Dykstra, J. D.↗