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At least 343 records · Page 19

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NASA NTRS1985

A water-vapor radiometer error model

The water-vapor radiometer (WVR) is used to calibrate unpredictable delays in the wet component of the troposphere in geodetic microwave techniques such as very-long-baseline interferometry (VLBI) and Global Positioning System (GPS) tracking. Based on experience with Jet Propulsion Laboratory (JPL) instruments, the current level of accuracy in wet-troposphere calibration limits the accuracy of local vertical measurements to 5-10 cm. The goal for the near future is 1-3 cm. Although the WVR is currently the best calibration method, many instruments are prone to systematic error. In this paper, a treatment of WVR data is proposed and evaluated. This treatment reduces the effect of WVR systematic errors by estimating parameters that specify an assumed functional form for the error. The assumed form of the treatment is evaluated by comparing the results of two similar WVR's operating near each other. Finally, the observability of the error parameters is estimated by covariance analysis.

Beckman, B.↗

NASA NTRS1986

A full CI treatment of Ne atom - A benchmark calculation performed on the NAS CRAY 2

Full CI calculations are performed for Ne atom using Gaussian basis sets of up to triple-zeta plus double polarization quality. The total valence correlation energy through double, triple, quadruple and octuple excitations is compared for eight different basis sets. These results are expected to be an important benchmark for calibrating methods for estimating the importance of higher excitations.

Bauschlicher, C. W., Jr.↗

NASA NTRS1987

Calibration of the EGRET high-energy gamma-ray telescope in the range 20-10,000 MeV with a tunable beam of quasi-monoenergetic gamma rays at SLAC

The EGRET telescope, one of the instruments to be carried on NASA's Gamma Ray Observatory, is designed to explore the high-energy portion (20-24,000 MeV) of the electromagnetic spectrum. A calibration source of gamma-rays over this wide energy range has been used together with an appropriate set of intensity-monitoring instruments, to conduct a calibration of the EGRET telescope over this energy range for the full telescope aperture. The calibration method and an overview of the results are described.

Thompson, D. J.↗

NASA NTRS1987

Full CI benchmark calculations for several states of the same symmetry

Full CI (FCI) wave functions are used to compute energies for several electronic states of the same symmetry for SiH2, CH2, and CH2(+). It is found that CASSCF/multireference CI wave functions yield results very similar to FCI, irrespective of whether the CASSCF MOs are optimized independently for each state or using an average of the CASSCF energies for all desired states. The ionization potentials and excitation energies obtained from the FCI calculations should help calibrate methods (such as Green's function approaches, equations of motion and propagator methods, and cluster expansions) in which energy differences are computed directly.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1986

Dead-time effects in pulse-counting Digicon detectors

The Hubble Space Telescope's High Resolution Spectrograph employs two pulse-counting Digicon detectors to record UV spectra. At higher measured count rates, the intervals between photon arrivals become comparable to the response time of the electronics, so that not every pulse will be counted; this 'paired pulse' effect causes departures from linearity and must be corrected for during raw data reduction. Attention is given to two analytic equations that quantify the dead-time losses. The less attenuated spectra are noted to be severely distorted by these losses; a comparison with the undistorted low count rate data allows a detailed analysis to be made of the pulse-counting characteristics over five decades of input event rates. The present equations and calibration methods should be applicable to all kinds of multichannel pulse-counting detectors.

Ebbets, Dennis C.↗

NASA NTRS1988

Fast optical pyrometry

Design and operation of accurate millisecond and microsecond resolution optical pyrometers developed at the National Bureau of Standards during the last two decades are described. Results of tests are presented and estimates of uncertainties in temperature measurements are given. Calibration methods are discussed and examples of applications of fast pyrometry are given. Ongoing research in developing fast multiwavelength and spatial scanning pyrometers are summarized.

Cezairliyan, Ared↗

NASA NTRS1988

Radiometric calibrations of portable sources in the vacuum ultraviolet

The primary UV radiometric calibration standards of the Atomic and Plasma Radiation Division of the NBS are the hydrogen arc and the blackbody line arc. The secondary standards are the argon mini- and maxiarcs, and the deuterium arc lamp. Calibration methods involving both spectral radiance and irradiance, as well as the evaluation of their uncertainties, are discussed. Attention is given to the establishment of an irradiance scale, and the absolute spectral irradiance, measured as a function of wavelength, at a 50-cm distance from the field aperture for five different continuum sources.

Klose, Jules Z.↗

NASA NTRS1988

Full configuration-interaction benchmark calculations for AlH

Full CI and CASSCF/SOCI theoretical computations are presented for the X 1Sigma(+), A 1Pi, and C 1Sigma(+) states of AlH. The aim is to establish benchmark data to calibrate methods which describe valence-Rydberg transitions in other molecules. The results are presented in extensive tables and graphs and compared with published theoretical and experimental values, and the conditions under which CASSCF/SOCI calculations accurately reproduce the FCI results are defined.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1989

Improved capabilities of the Multispectral Atmospheric Mapping Sensor (MAMS)

The Multispectral Atmospheric Mapping Sensor (MAMS) is an airborne instrument being investigated as part of NASA's high altitude research program. Findings from work on this and other instruments have been important as the scientific justification of new instrumentation for the Earth Observing System (EOS). This report discusses changes to the instrument which have led to new capabilities, improved data quality, and more accurate calibration methods. In order to provide a summary of the data collected with MAMS, a complete list of flight dates and locations is provided. For many applications, registration of MAMS imagery with landmarks is required. The navigation of this data on the Man-computer Interactive Data Access System (McIDAS) is discussed. Finally, research applications of the data are discussed and specific examples are presented to show the applicability of these measurements to NASA's Earth System Science (ESS) objectives.

Jedlovec, Gary J.↗

NASA NTRS1988

Calibration of single particle sizing velocimeters using photomask reticles

The development of photomask reticle calibration standards for single particle instruments is discussed. The calibration method studied involves the use of photomask reticles where the particle artifacts are actually disks of chrome thin film in the clear field reticles produced by photolithography and etching processes. Consideration is given to various aspects of theory, design, and performance.

Hirleman, E. D.↗

NASA NTRS1990

SR90, strontium shaped-charge critical ionization velocity experiment

In May 1986 an experiment was performed to test Alfven's critical ionization velocity (CIV) effect in free space, using the first high explosive shaped charge with a conical liner of strontium metal. The release, made at 540 km altitude at dawn twilight, was aimed at 48 deg to B. The background electron density was 1.5 x 10(exp 4) cu cm. A faint field-aligned Sr(+) ion streak with tip velocity of 2.6 km/s was observed from two optical sites. Using two calibration methods, it was calculated that between 4.5 x 10(exp 20) and 2 x 10(exp 21) ions were visible. An ionization time constant of 1920 s was calculated for Sr from the solar UV spectrum and ionization cross section which combined with a computer simulation of the injection predicts 1.7 x 10(exp 21) solar UV ions in the low-velocity part of the ion streak. Thus all the observed ions are from solar UV ionization of the slow (less than critical) velocity portion of the neutral jet. The observed neutral Sr velocity distribution and computer simulations indicate that 2 x 10(exp 21) solar UV ions would have been created from the fast (greater than critical) part of the jet. They would have been more diffuse, and were not observed. Using this fact it was estimated that any CIV ions created were less than 10(exp 21). It was concluded that future Sr CIV free space experiments should be conducted below the UV shadow height and in much larger background plasma density.

Wescott, Eugene M.↗

NASA NTRS1990

The limits of direct satellite tracking with GPS

The expected accuracy of the direct user orbit solution and the magnitude of the principal error sources are evaluated using the techniques of covariance analysis. Utilization of advanced GPS receivers, measurement calibration methods, and geophysical models developed for high-precision GPS-based geodesy and differential satellite tracking are assumed in order to explore the limiting accuracy of the above technique. In addition, the dependence of user orbit accuracy on such factors as data arc length, the time interval between the end of the ground data arc used for GPS orbit solutions and the beginning of the user data arc, and data types used are studied. For comparison, results from the analysis of a full differential orbit solution are also presented. It is shown that submeter real-time accuracy can be readily achieved for a user above 700 km altitude, even when the user solution is based on a GPS ephemeris that is more than 12 h old.

Bertiger, Willy I.↗

NASA NTRS1991

Kinematics of the six-degree-of-freedom force-reflecting Kraft Master

Presented here are kinematic equations for a six degree of freedom force-reflecting hand controller. The forward kinematics solution is developed and shown in simplified form. The Jacobian matrix, which uses terms from the forward kinematics solution, is derived. Both of these kinematic solutions require joint angle inputs. A calibration method is presented to determine the hand controller joint angles given the respective potentiometer readings. The kinematic relationship describing the mechanical coupling between the hand and controller shoulder and elbow joints is given. These kinematic equations may be used in an algorithm to control the hand controller as a telerobotic system component. The purpose of the hand controller is two-fold: operator commands to the telerobotic system are entered using the hand controller, and contact forces and moments from the task are reflected to the operator via the hand controller.

Williams, Robert L., II↗

NASA NTRS1991

Precise orbit determination of high-earth elliptical orbiters using differenced Doppler and range measurements

Recent advances in Deep Space Network station calibration methods have led to renewed interest in the use of differenced Doppler and range data types for interplanetary navigation. Described here is an orbit determination error analysis of the performance of these differenced data types when used with conventional two-way Doppler for precise navigation of High-Earth Orbiters. Three highly elliptical Earth orbits are investigated, with apogee heights on the order of 20,000 km, 70,000 km, and 156,000 km. Results indicate that the most significant navigational accuracy improvements, relative to the performance obtained from two way Doppler alone, are achieved for the lowest altitude orbit by using differenced Doppler measurements with two way Doppler (assuming that spacecraft onboard downlink antennas have no ground footprint limitation in the near-apogee regime). In the case of the two higher altitude orbits, accuracy improvements over Doppler-only performance, although less dramatic, are also achieved when differenced range measurements are combined with two-way Doppler.

Estefan, J. A.↗

NASA NTRS1992

Accounting For Gains And Orientations In Polarimetric SAR

Calibration method accounts for characteristics of real radar equipment invalidating standard 2 X 2 complex-amplitude R (receiving) and T (transmitting) matrices. Overall gain in each combination of transmitting and receiving channels assumed different even when only one transmitter and one receiver used. One characterizes departure of polarimetric Synthetic Aperture Radar (SAR) system from simple 2 X 2 model in terms of single parameter used to transform measurements into format compatible with simple 2 X 2 model. Data processed by applicable one of several prior methods based on simple model.

Freeman, Anthony↗

NASA NTRS1991

Observations of the Earth in polarized light from the US Space Shuttle

During the four American Space Shuttle missions of year 1985, the crewmembers took pictures of the Earth in polarized light. Different problems were encountered in the quantitative use of the data: induced polarization by the shuttle window, lack of calibration correction of the window polarization and enveloped in flight calibration methods. The analysis of the selected data first confirmed the previous observation over snow and sand. A low polarization on these surfaces was observed. On the other hand, the measurements show the potentiability of the polarization for agricultural inventory. Contamination of the atmosphere is well characterized.

Roger, Jean-Claude↗

NASA NTRS1990

Hubble Space Telescope: Wide field and planetary camera instrument handbook. Version 2.1

An overview is presented of the development and construction of the Wide Field and Planetary Camera (WF/PC). The WF/PC is a duel two dimensional spectrophotometer with rudimentary polarimetric and transmission grating capabilities. The instrument operates from 1150 to 11000 A with a resolution of 0.1 arcsec per pixel or 0.043 arcsec per pixel. Data products and standard calibration methods are briefly summarized.

Griffiths, Richard↗

NASA NTRS1992

Quantitative photoabsorption and fluorescence spectroscopy of benzene, naphthalene, and some derivatives at 106-295 nm

Photoabsorption and fluorescence cross sections of benzene, (o-, m-, p-) xylenes, naphthalene, 1-methylnaphthalene, and 2-ethylnaphthalene in the gas phase are measured at 106-295 nm using synchrotron radiation as a light source. Fluorescences are observed from the photoexcitation of benzene and xylenes at 230-280 nm and from naphthalene and its derivatives at 190-295 nm. The absolute fluorescence cross section is determined by calibration with respect to the emission intensity of the NO(A-X) system, for which the fluorescence quantum yield is equal to 1. To cross-check the current calibration method, the quantum yield of the SO2(C-X) system at 220-230 nm was measured since it is about equal to 1. The current quantum-yield data are compared with previously published values measured by different methods.

Suto, Masako↗