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Troy, M.

Publications and source records attributed to Troy, M..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Adaptive Optics Communications Performance Analysis

The performance improvement obtained through the use of adaptive optics for deep-space communications in the presence of atmospheric turbulence is analyzed. Using simulated focal-plane signal-intensity distributions, uncoded pulse-position modulation (PPM) bit-error probabilities are calculated assuming the use of an adaptive focal-plane detector array as well as an adaptively sized single detector. It is demonstrated that current practical adaptive optics systems can yield performance gains over an uncompensated system ranging from approximately 1 dB to 6 dB depending upon the PPM order and background radiation level.

Srinivasan, M.↗
NASA NTRS2003

Daytime adaptive optics for deep space optical communications

The deep space optical communications subsystem offers a higher bandwidth communications link in smaller size, lower mass, and lower power consumption subsystem than does RF. To demonstrate the benefit of this technology to deep space communications NASA plans to launch an optical telecommunications package on the 2009 Mars Telecommunications orbiter spacecraft. Current performance goals are 30-Mbps from opposition, and 1-Mbps near conjunction (-3 degrees Sun-Earth-Probe angle). Yet, near conjunction the background noise from the day sky will degrade the performance of the optical link. Spectral and spatial filtering and higher modulation formats can mitigate the effects of background sky. Narrowband spectral filters can result in loss of link margin, and higher modulation formats require higher transmitted peak powers. In contrast, spatial filtering at the receiver has the potential of being lossless while providing the required sky background rejection. Adaptive optics techniques can correct wave front aberrations caused by atmospheric turbulence and enable near-diffraction-limited performance of the receiving telescope. Such performance facilitates spatial filtering, and allows the receiver field-of-view and hence the noise from the sky background to be reduced.

adaptive optics↗
NASA NTRS2002

Sparse matrix wavefront reconstruction: simulations and experiments

Adaptive optics systems with Shack-Hartmann wavefront sensors require reconstruction of the atmospheric phase error from slope measurements, with every sensor in the array being used in the computation of each actuator command. This fully populated reconstruction matrix can result in a significant computational burden for adaptive optics systems with large numbers of actuators. A method for generating sparse wavefront reconstruction matrices for adaptive optics isproposed. The method exploits the relevance of nearby slope measurements for control of an individual actuator, and relies upon the limited extent of the influence function for a zonal deformable mirror. Relying only on nearby sensor information can significantly reduce the calculation time for wavefront reconstruction. In addition, a hierarchic controller is proposed to recover some of the global wavefront information. The performance of these sparse wavefront reconstruction matrices was evaluated in simulation, and tested on the Palomar Adaptive Optics System. This paper will present some initial results from the simulations and experiments.

space↗
NASA NTRS1971

Physisorption of Ar, Kr, CH4, and N2 on 304 stainless steel at very low pressures.

Determination of physisorption isotherms of these gases on stainless steel by pressure change measurements in very low pressure cryogenic baths where a steel nipple was brought in contact with the test gas at 77 to 90 K in a sealed constant-volume system. The position of the nipple in the gas was changed in such a manner that gas adsorption on a 47.5 sq cm area of the steel surface could be measured. The Dubinin-Radushkevich (DR) equation (1947) was used for an empirical description of isotherms at different temperatures. The mean adsorption energies calculated from the DR plots were 1290, 1545, 1490 and 1903 cal/mol for Ar, Kr, CH4 and N2, respectively, being about 10% higher than the corresponding values on Pyrex.

Troy, M.↗