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Long, D.

Publications and source records attributed to Long, D..

25 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Roundoff noise analysis for digital Doppler processors in radar scatterometers

The noise due to finite word length effects for digital Doppler processors (DPP) in radar scatterometers, is analyzed. The roundoff noise-to-signal ratio in the measurement of the radar return signal power is derived. Computer simulations which validate the analytical results are presented. The results can be used in tradeoff studies of hardware design such as number of bits required at each processing stage. The results are used in the design of a DPP for the NASA scatterometer planned to be launched in 1990.

Chi, Chong-Yung↗
NASA NTRS1986

Graphics Programs for the DEC VAX Computer

Variety of plots available in video or printed form. LONGLIB library of computer programs set of subroutines designed for vector plotting on cathode-ray tubes and dot-matrix printers. LONGLIB subroutines invoked by program calls similar to standard CALCOMP routines. In addition to basic plotting routines, LONGLIB contains extensive set of routines to allow viewport clipping, extended character sets, graphic input, gray-level plots, polar plots, and three-dimensional plotting with or without removal of hidden lines. LONGLIB written in FORTRAN 77 and C for batch execution.

Long, D.↗
NASA NTRS1984

NROSS scatterometer - An instrument for global oceanic wind observations

One of the instruments - the NROSS scatterometer or SCATT - to be launched on the Navy Remote Ocean Sensing System spacecraft in 1989 is discussed. To determine wind speed and direction that SCATT will measure the ocean normalized radar cross section (NRCS) by illuminating the ocean's surface with microwave pulses and measuring the return signal power. A wind retrieval algorithm is used to compute the wind speed and direction from the measured NRCS. Winds over at least 90 percent of the global, ice-free ocean will be observed at least once every two days. For NASA users, the system will retrieve winds with an accuracy of plus or minus 2 m/s or 10 percent whichever is greater, for wind speeds ranging from 3 to 30 m/s.

Li, F.↗
NASA NTRS1981

Evaluation of RCA thinned buried channel charge-coupled devices /CCDs/ for scientific applications

An experimental version of a thinned illuminated buried-channel 512 x 320 pixel CCD with reduced amplifier input capacitance has been produced which is characterized by lower readout noise. Changes made to the amplifier are discussed, and readout noise measurements obtained by several different techniques are presented. The single energetic electron response of the CCD in the electron-bombarded mode and the single 5.9 keV X-ray pulse height distribution are reported. Results are also given on the dark current versus temperature and the spatial frequency response as a function of signal level.

Zucchino, P.↗
NASA NTRS1981

Echelle spectroscopy with a charge-coupled device /CCD/

The recent availability of large format CCD's with high quantum efficiency makes it possible to achieve significant advances in high dispersion astronomical spectroscopy. An echelle CCD combination excels or equals other techniques presently available, and offers the advantage of complete spectral coverage of several thousand Angstroms in a single exposure. Attention is given to experiments which were conducted with a CCD camera head and an echelle spectrograph on a 4-meter telescope. It was found possible to achieve a signal-to-noise ratio of 150/1 on a 13th magnitude star at 6000 A in a two-hour exposure at 0.16 A/pixel, limited primarily by photon statistics. For fainter objects, readout noise is the limiting factor in precision. For 20 electron rms readout noise, an S/N = 15/1 at 18th magnitude is expected, all other things being equal.

York, D. G.↗
NASA NTRS1974

Simulation of low thrust guidance problems

The determination of optimal rocket control profiles and the guidance procedures used to approximate these profiles have received much attention. Simple guidance procedures have sufficed for the Apollo flights. A modified version of the linear tangent guidance law is developed which can be used for a low-thrust orbital maneuvering system such as the space shuttle. The modification allows the estimation of gravity over long thrusting arcs which is necessary for low thrust systems.

Dawkins, G. S.↗