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Ellis, J.

Publications and source records attributed to Ellis, J..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Structural Evolution of a Cyclooctatetraene Adlayer on Cu(111) during Isothermal Desorption

Here, the use of helium diffraction patterns to study desorption processes is explored as a novel extension to traditional methods based on helium specular reflection. The sample, cyclooctatetraene, adsorbed on Cu(111) provides a rich but complex structure. The modulation of cyclooctatetraene by Cu(111) is manifested as a convolution in the diffraction pattern, displaying an averaged super cell symmetry of (7$\sqrt{3}$ x 7$\sqrt{3}$)R30°. The adlayer expands during isothermal desorption, and the change in lattice constant provides a direct measure of the coverage as a function of time. We find a desorption energy of 0.96 +/- 0.01 at saturation of the first layer and an upper limit of 1.62 +/- 0.07 for isolated molecules. These values and details of the assigned structure indicate chemisorbed molecules with a planar conformation.

36 MATERIALS SCIENCE↗
NASA NTRS2017

Solar System Ephemerides, Pulsar Timing, Gravitational Waves, & Navigation

In-spiraling supermassive black holes should emit gravitational waves, which would produce characteristic distortions in the pulse time of arrival residuals from millisecond pulsars. There are multiple national and regional consortia that have constructed pulsar timing arrays by precise timing of different sets of millisecond pulsars. An essential aspect of precision timing is the transfer of the pulsar times of arrival to a (quasi-)inertial reference frame, conventionally taken to be the solar system barycenter. The solar system barycenter is determined from the knowledge of the planetary masses and orbits, which has been re ned over the course of the past 50 years by multiple spacecraft missions. Within the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), uncertainties on the solar system barycenter are emerging as an important element of the overall NANOGrav noise budget. We describe what is known about the solar system barycenter, touch upon how uncertainties in it affect gravitational wave studies with pulsar timing arrays, and consider future trends in spacecraft navigation.

Vallisneri, M.↗
NASA NTRS1998

Navigation of the Space VLBI Mission - HALCA

This paper describes the spacecraft dynamics model and orbit determination strategies developed to meet the stringent trajectory accuracy requirements for generating predicts for the transfer of a stable uplink frequency to the spacecraft and for determining reconstructed orbits for delivery to the NRAO VLBI correlator and the international VLBI science community.

navigation↗
NASA NTRS1991

Orbit determination support for Hiten's aerobraking in the Earth's atmosphere

Two passes of the ISAS (Japan's Institute of Space and Astronautical Science) Hiten spacecraft through the Earth's atmosphere, at perigee altitudes of 125 km and 120 km, during Mar. 1991 marked the first aerobraking technology demonstrations for an object in cis-lunar orbit traveling at near Earth escape velocity. Prediction and control of perigee altitude to better than 1 km was desired to assure spacecraft survival. Covariance analysis provided confidence that prediction accuracy better than 200 m for support of final trim maneuver design was achievable with NASA DSN (Deep Space Network) tracking support. ISAS used orbit determination results in their decisions to cancel final trim maneuvers. Post flight reconstructions, marking the first combined use of DSN and ISAS tracking data, yielded perigee altitude solutions which agree with the near real time mission operations support predictions to better than 50 m.

Efron, L.↗
NASA NTRS1991

Navigation for Muses-A (HITEN) aerobraking in the earth's atmosphere - Preliminary report

Aerobraking phase covariance study results and preliminary joint JPL/ISAS orbit determination findings are presented. Possible explanations for differences in the JPL and ISAS perigee altitude results are discussed. Agreement between predicted and reconstructed values for the altitude and time of perigee passage attests to the accuracy of the JPL orbit solutions.

Kawaguchi, J.↗