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At least 415 records · Page 23

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NASA NTRS1984

Pre-launch Absolute Radiometric Calibration of LANDSAT-4 Protoflight Thematic Mapper

From both scientific and applications perspectives, the usefulness of TM digital imagery is significantly determined by its radiometric characteristics. This includes both the accuracy to which the dynamic range is known and its radiometric reproducibility or precision. Results from several pre-launch tests with a 122-cm Integrating Sphere (IS) used as part of the absolute radiometric calibration experiments for the protoflight TM sensor carried on the LANDSAT-4 satellite are summarized and analyzed. Topics covered include TM radiometric calibration and sensitivity; the TM internal calibrator; and the dynamic range after calibration.

Barker, J. L.↗

NASA NTRS2003

Hydroxide-catalyzed bonding

A method of bonding substrates by hydroxide-catalyzed hydration/dehydration involves applying a bonding material to at least one surface to be bonded, and placing the at least one surface sufficiently close to another surface such that a bonding interface is formed between them. A bonding material of the invention comprises a source of hydroxide ions, and may optionally include a silicate component, a particulate filling material, and a property-modifying component. Bonding methods of the invention reliably and reproducibly provide bonds which are strong and precise, and which may be tailored according to a wide range of possible applications. Possible applications for bonding materials of the invention include: forming composite materials, coating substrates, forming laminate structures, assembly of precision optical components, and preparing objects of defined geometry and composition. Bonding materials and methods of preparing the same are also disclosed.

Gwo, Dz-Hung↗

NASA NTRS1967

Precision capacitor has improved temperature and operational stability

Vacuum dielectric capacitor is fabricated from materials with very low temperature coefficients of expansion. This precision capacitor in the 1000-2000 picofarad range has a near-zero temperature coefficient of capacitance, eliminates ion chamber action caused by air ionization in the dielectric, and minimizes electromagnetic field charging effects.

Brookshier, W. K.↗

NASA NTRS1972

Time synchronization via lunar radar.

The advent of round-trip radar measurements has permitted the determination of the ranges to the nearby planets with greater precision than was previously possible. When the distances to the planets are known with high precision, the propagation delay for electromagnetic waves reflected by the planets may be calculated and used to synchronize remotely located clocks. Details basic to the operation of a lunar radar indicate a capability for clock synchronization to plus or minus 20 microsec. One of the design goals for this system was to achieve a simple semiautomatic receiver for remotely located tracking stations. The lunar radar system is in operational use for deep space tracking at Jet Propulsion Laboratory and synchronizes five world-wide tracking stations with a master clock at Goldstone, Calif. Computers are programmed to correct the Goldstone transmissions for transit time delay and Doppler shifts so as to be received on time at the tracking stations; this dictates that only one station can be synchronized at a given time period and that the moon must be simultaneously visible to both the transmitter and receiver for a minimum time of 10 min.-

Higa, W. H.↗

NASA NTRS1973

3-D multilateration for measurement of earth crustal deformation and network densification

Discussion of how range and range-difference data types can make possible precise three-dimensional measurement of ground station positions and the position of an artificial signal source, without explicit dependence upon the signal source trajectory. An effective strategy for such measurement is to combine the multilateration approach with a VLBI system using natural radio sources. The VLBI methods would provide a coarse grid of three-dimensional benchmark locations on a regional and global scale. Multilateration stations would then occupy these coarse grid locations and provide a means for highly portable, relatively low-cost units to then densify networks on a regional and local scale. Because a multilateration approach can make use of strong artificial radio sources, it makes possible the use of relatively low-cost, highly mobile stations. Such mobile stations are virtually essential for three dimensional surveying in heavily urbanized areas or in rugged terrain.

Ong, K. M.↗

NASA NTRS1975

The X-band low noise antenna measurement cone

An X-band low-noise receiving cone with 17-MHz instantaneous bandwidth was built to evaluate the 26-m antenna at the Venus Station (DSS 13), Goldstone, California, at X-band frequencies, and also to provide a means for precise radio source calibrations in this frequency range. These measurments are necessary to provide more accurate antenna gain performance calibrations for the 64-m antenna subnet. The zenith system temperatures with the cone on the ground and on the antenna are 17.5 and 20.8 K, respectively.

Reid, M. S.↗

NASA NTRS1977

Portable aerosol-particle counter

Device makes real-time, precise measurements of aerosols of small particle size. Measurement combines laser velocimetry and light scattering. Technique offers advantages over currently practiced techniques: by measuring aerosol size in real-time, measuring size precisely, and having high sensitivity in size range of greatest biological concern.

Weber, F. N., Jr.↗

NASA NTRS1977

Design of an airborne lidar for stratospheric aerosol measurements

A modular, multiple-telescope receiving concept is developed to gain a relatively large receiver collection aperture without requiring extensive modifications to the aircraft. This concept, together with the choice of a specific photodetector, signal processing, and data recording system capable of maintaining approximately 1% precision over the required large signal amplitude range, is found to be common to all of the options. It is recommended that development of the lidar begin by more detailed definition of solutions to these important common signal detection and recording problems.

Evans, W. E.↗

NASA NTRS1978

Nanosecond time transfer via shuttle laser ranging experiment

A method is described to use a proposed shuttle laser ranging experiment to transfer time with nanosecond precision. All that need be added to the original experiment are low cost ground stations and an atomic clock on the shuttle. It is shown that global time transfer can be accomplished with 1 ns precision and transfer up to distances of 2000 km can be accomplished with better than 100 ps precision.

Reinhardt, V. S.↗

NASA NTRS1978

HEAO-B and -C engineering design and scientific objectives

The HEAO-B observatory will be launched in late 1978 for a one-year mission to analyze individual X-ray objects and extended sources in detail using a high precision X-ray telescope. The energy range of the telescope is 0.1 keV to 4 keV. Four interchangeable instruments are at the focal plane; a high resolution imaging instrument, an imaging proportional counter, a focal plane crystal spectrometer, and a solid state spectrometer. A monitor proportional counter will collect data independently of the telescope. The HEAO-C observatory is a scanning mission and will be launched in 1979 for six months operation. It will contain two cosmic ray experiments and a gamma ray spectrometer.

Dailey, C. C.↗

NASA NTRS1981

Fourier spectroscopy on planetary missions including Voyager

For more than a decade, spaceborne Fourier transform spectrometers have been employed in investigations concerning the earth and other objects in the solar system. The wide spectral range at moderately high spectral resolution, the precise wavenumber and radiometric calibrations, and the reliability achieved by these instruments, have permitted scientific investigations which would otherwise have been impossible. Weight limitations and the long flight durations prohibited the use of highly sensitive, cryogenically cooled detectors on these missions. A brief review is presented of the evolution of the instruments known as IRIS (InfraRed Interferometric Spectrometer). The concept of remote sensing by infrared emission spectroscopy is discussed, and examples of results obtained with IRIS are presented. Attention is given to the most recent results from Voyager with emphasis on those from Titan.

Hanel, R. A.↗

NASA NTRS1984

Solidification and crystal growth of solid solution semiconducting alloys

Problems associated with the solidification and crytal growth of solid-solution semiconducting alloy crystals in a terrestrial environment are described. A detailed description is given of the results for the growth of mercury cadmium telluride (HgCdTe) alloy crystals by directional solidification, because of their considerable technological importance. A series of HgCdTe alloy crystals are grown from pseudobinary melts by a vertical Bridgman method using a wide range of growth rates and thermal conditions. Precision measurements are performed to establish compositional profiles for the crystals. The compositional variations are related to compositional variations in the melts that can result from two-dimensional diffusion or density gradient driven flow effects ahead of the growth interface. These effects are discussed in terms of the alloy phase equilibrium properties, the recent high temperature thermophysical data for the alloys and the highly unusual heat transfer characteristics of the alloy/ampule/furnace system that may readily lead to double diffusive convective flows in a gravitational environment.

Lehoczky, S. L.↗

NASA NTRS1985

Satellite laser ranging and its applications

Satellite laser ranging (SLR) provides an important capability for precise orbit determination and for geophysical parameter estimation to support a number of contemporary geodynamic and oceanographic investigations. The precision of the SLR measurement has improved from the early meter-level systems to the current capabilities of a few centimeters for the best systems. The accuracy of the orbits and geophysical parameter recovery have shown an associated improvement. Polar motion with accuracies of 2 mas, station coordinates better than 10 cm, and interstation baseline rates indicative of tectonic motion are determined routinely with the current set of global SLR data. This discussion reviews the SLR measurement, analysis approach, and some of the recent results derived from the current SLR data set.

Tapley, B. D.↗

NASA NTRS1986

Cryogenic star-tracking telescope for Gravity Probe B

This paper describes the design, development and preliminary testing of the cryogenic star-tracking telescope used as an optical reference for the gyroscopes in the Gravity Probe B Relativity Gyroscope experiment. The telescope is operated at 1.8 K; it is fabricated entirely from fused quartz components held together by optical contacting; it has a physical length of 14 in., a focal length of 150 in. and an aperture of 5.6 in. Readout is by two photomultiplier chopper-detector assemblies at ambient satellite temperature. When fully operational, the telescope may be expected to have a precision approaching 0.1 milliarcsec over a linear range of 70 + or 70 milliarcsec. Its projected noise performance corresponds to an angular resolution of 1 milliarcsec in 1 Hz bandwidth. The paper includes a theoretical analysis, a description of the design and fabrication of a laboratory version of the telescope, a discussion of techniques of optical contacting, an account of vibration tests on a separate mass model of the telescope, a description of the artificial star developed for optical tests, and an account of preliminary experimental results.

Everitt, C. W. F.↗

NASA NTRS1989

Miniature Flow-Direction/Pitot-Static Pressure Probes

Precision flow-direction/pitot-static pressure probes, ranging from 0.035 to 0.090 inch (0.89 to 2.29 mm) in outside diameter, successfully fabricated and calibrated for use in Langley 20-inch Mach 6 Tunnel. Probes simultaneously measure flow direction and static and pitot pressures in flow fields about configurations in hypersonic flow at temperatures up to 500 degree F (260 degree C).

Ashby, George C., Jr.↗

NASA NTRS1989

PAH formation in carbon-rich circumstellar envelopes

While there is growing observational evidence that some fraction of interstellar carbon is in polycyclic aromatic hydrocarbons (PAH's), the mechanisms by which these molecules might be formed have not been extensively studied. A detailed investigation of PAH production in the outflowing molecular envelopes of carbon-rich red giant star is presented. The gasphase kinetics of a chemical reaction mechanism developed to study soot production in hydrocarbon flames is modified to apply in circumstellar environments. It was found that astrophysically significant quantities of PAH's can be formed in carbon star envelopes provided the gas is sufficiently dense and resides for a long time in the temperature range of 900 to 1100 k. The precise yield of PAH's is very sensitive to astronomical parameters of the envelope (e.g., mass loss rate, outflow velocity, and acetylene abundance) and certain poorly determined chemical reaction rates.

Feigelson, Eric D.↗

NASA NTRS1993

Multi-beam range imager for autonomous operations

For space operations from the Space Station Freedom the real time range imager will be very valuable in terms of refuelling, docking as well as space exploration operations. For these applications as well as many other robotics and remote ranging applications, a small potable, power efficient, robust range imager capable of a few tens of km ranging with 10 cm accuracy is needed. The system developed is based on a well known pseudo-random modulation technique applied to a laser transmitter combined with a novel range resolution enhancement technique. In this technique, the transmitter is modulated by a relatively low frequency of an order of a few MHz to enhance the signal to noise ratio and to ease the stringent systems engineering requirements while accomplishing a very high resolution. The desired resolution cannot easily be attained by other conventional approaches. The engineering model of the system is being designed to obtain better than 10 cm range accuracy simply by implementing a high precision clock circuit. In this paper we present the principle of the pseudo-random noise (PN) lidar system and the results of the proof of experiment.

Marzwell, Neville I.↗

NASA NTRS1993

A ground track control algorithm for the Topographic Mapping Laser Altimeter (TMLA)

The results of an analysis of an algorithm that will provide autonomous onboard orbit control using orbits determined with Global Positioning System (GPS) data. The algorithm uses the GPS data to (1) compute the ground track error relative to a fixed longitude grid, and (2) determine the altitude adjustment required to correct the longitude error. A program was written on a personal computer (PC) to test the concept for numerous altitudes and values of solar flux using a simplified orbit model including only the J sub 2 zonal harmonic and simple orbit decay computations. The algorithm was then implemented in a precision orbit propagation program having a full range of perturbations. The analysis showed that, even with all perturbations (including actual time histories of solar flux variation), the algorithm could effectively control the spacecraft ground track and yield more than 99 percent Earth coverage in the time required to complete one coverage cycle on the fixed grid (220 to 230 days depending on altitude and overlap allowance).

Blaes, V.↗