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Lee, T. H.

Publications and source records attributed to Lee, T. H..

13 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Hydrogen permeability and stability of palladium films in a high pressure reactor

Palladium (Pd) thin films were fabricated by colloidal spray deposition (CSD) on substrates of porous yttrium stabilized zirconium oxide (YSZ). The films were sintered at 1140–1160 °C for 2–3 h in argon and had thickness of 5–11 μm. The hydrogen (H 2 ) permeation flux for the Pd film was measured with simulated feed gases (90% H 2 /He, syn-gas/steam, or syn-gas/steam with H 2 S) flowing on the film side and sweep gas (Ar) flowing on the substrate side. Here, the hydrogen flux with the Pd film was obtained by adjusting the flow rate and pressure of the gas. Hydrogen flux values of 121–173 cm 3 min -1 cm -2 were measured at 388–596 °C (feed: 90% H 2 /He at 200 mL min -1 ; sweep: Ar at 1400 mL min -1 ; and ΔP = ~170 psi), and the permeability was calculated and compared with literature values. Because Pd is known to be sensitive to some gases, the stability of Pd films was evaluated by examining the microstructures of samples after their exposure to gas mixtures containing CO, CO 2 , H 2 O, and H 2 S. From the results of stability tests in H 2 S-containing atmospheres, the Pd/Pd 4 S phase boundary was determined, and this measured boundary was compared to the Pd/Pd 4 S boundary calculated from the thermodynamic data in the literature.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Planar doped barrier subharmonic mixers

The Planar Doped Barrier (PDB) diode is a device consisting of a p(+) doping spike between two intrinsic layers and n(+) ohmic contacts. This device has the advantages of controllable barrier height, diode capacitance and forward to reverse current ratio. A symmetrically designed PDB has an anti-symmetric current vs. voltage characteristic and is ideal for use as millimeter wave subharmonic mixers. We have fabricated such devices with barrier heights of 0.3, 0.5 and 0.7 volts from GaAs and InGaAs using a multijunction honeycomb structure with junction diameters between one and ten microns. Initial RF measurements are encouraging. The 0.7 volt barrier height 4 micron GaAs devices were tested as subharmonic mixers at 202 GHz with an IF frequency of 1 GHz and had 18 dB of conversion loss. The estimated mismatch loss was 7 dB and was due to higher diode capacitance. The LO frequency was 100.5 GHz and the pump power was 8 mW.

Lee, T. H.↗
NASA NTRS1991

Planar doped barrier devices for subharmonic mixers

An overview is given of planar doped barrier (PDB) devices for subharmonic mixer applications. A simplified description is given of PDB characteristics along with a more complete numerical analysis of the current versus voltage characteristics of typical structures. The analysis points out the tradeoffs between the device structure and the resulting characteristics that are important for mixer performance. Preliminary low-frequency characterization results are given for the device structures, and a computer analysis of subharmonic mixer parameters and performance is presented.

Lee, T. H.↗
NASA NTRS1989

Target signature modeling and bistatic scattering measurement studies

An annual report that summarizes the work done on Grant No. NAG2-542 is presented. There are four areas of study to be summarized: (1) bistatic scattering measurement studies for a compact range; (2) target signature modeling for test and evaluation hardware in the loop situations; (3) aircraft code modification study; and (4) SATCOM antenna studies on aircraft.

Burnside, W. D.↗
NASA NTRS1988

A diagnostic technique used to obtain cross range radiation centers from antenna patterns

A diagnostic technique to obtain cross range radiation centers based on antenna radiation patterns is presented. This method is similar to the synthetic aperture processing of scattered fields in the radar application. Coherent processing of the radiated fields is used to determine the various radiation centers associated with the far-zone pattern of an antenna for a given radiation direction. This technique can be used to identify an unexpected radiation center that creates an undesired effect in a pattern; on the other hand, it can improve a numerical simulation of the pattern by identifying other significant mechanisms. Cross range results for two 8' reflector antennas are presented to illustrate as well as validate that technique.

Lee, T. H.↗
NASA NTRS1986

Simulation of an aircraft flying through a ground station to satellite link

The effect of an aircraft which flies through a ground station-to-satellite link is determined. There are two aspects to the work reported here: (1) an aperture blockage theoretical solution developed by Rudduck and Lee was used to calculate the effect of a large aircraft (C5) for various satellite ground station antenna diameters, and (2) the compact range facility at the Ohio State University was used to measure various targets, including a 737 aircraft, and to validate the theoretical solution in item 1.

Rudduck, R. C.↗
NASA NTRS1985

Infrared horizon sensor modeling for attitude determination and control: Analysis and mission experience

The work performed by the Attitude Determination and Control Section at the National Aeronautics and Space Administration/Goddard Space Flight Center in analyzing and evaluating the performance of infrared horizon sensors is presented. The results of studies performed during the 1960s are reviewed; several models for generating the Earth's infrared radiance profiles are presented; and the Horizon Radiance Modeling Utility, the software used to model the horizon sensor optics and electronics processing to computer radiance-dependent attitude errors, is briefly discussed. Also provided is mission experience from 12 spaceflight missions spanning the period from 1973 to 1984 and using a variety of horizon sensing hardware. Recommendations are presented for future directions for the infrared horizon sensing technology.

Phenneger, M. C.↗
NASA NTRS1984

The accuracy of mesoscale temperature and dewpoint fields retrieved from VAS satellite and conventional surface data

The Visible Infrared Spin-Scan Radiometer (VISSR) Atmospheric Sounder (VAS) is a twelve channel imaging radiometer on the Geostationary Operational Environmental Satellites (GOES). It is pointed out that the instrument is designed to monitor the evolution of tropospheric temperature and moisture fields at mesoscale resolution. Since the numerical qualities of VAS soundings affect their usefulness, a quantitative verification of the absolute and relative accuracy of VAS soundings appears highly desirable. The present investigation is concerned with an objective verification for two VAS case studies. Absolute accuray is measured by comparing the VAS retrievals to the radiosonde values at the network sites. Absolute temperature accuracy is found to be + or - 2 C below 500 mb, while absolute dewpoint accuracy is + or - 2 to + or - 3 C below 850 mb.

Chesters, D.↗
NASA NTRS1983

The impact of conventional surface data upon VAS regression retrievals in the lower troposphere

Surface temperature and dewpoint reports are added to the infrared radiances from the VISSR Atmospheric Sounder (VAS) in order to improve the retrieval of temperature and moisture profiles in the lower troposphere. The conventional (airways) surface data are combined with the twelve VAS channels as additional predictors in a ridge regression retrieval scheme, with the aim of using all available data to make high resolution space-time interpolations of the radiosonde network. For one day of VAS observations, retrievals using only VAS radiances are compared with retrievals using VAS radiances plus surface data. Temperature retrieval accuracy evaluated at coincident radiosonde sites shows a significant impact within the boundary layer. Dewpoint retrieval accuracy shows a broader improvement within the lowest tropospheric layers. The most dramatic impact of surface data is observed in the improved relative spatial and temporal continuity of low-level fields retrieved over the Midwestern United States.

Lee, T. H.↗
NASA NTRS1983

Use of the VAS for analysis of a prethunderstorm environment

The information content of the VAS radiances can be converted to meteorological parameters useful for analyzing a severe weather environment. The method by which the VAS variances are converted to vertical profiles of temperature, dewpoints, and equivalent potential temperature involves a basic regression technique using the most local radiosonde data available for establishing a correlation matrix. The results indicate that mesoscale features apparent within images of the radiances can be converted to usable temperature and moisture fields using regression when surface temperature and dewpoint observations are included within the total data base. In addition, results indicate that surface data are very important for better defining lower tropospheric structure that the VAS radiances alone cannot properly resolve. Analyses of these retrievals distinctly show mesoscale structure in the temperature and moisture fields derived with VAS radiances collected every 3 hours, and 0000 GMT. The retrievals capture the moisture structure. More important, convective instability is clearly detected immediately before the onset of convection. The results indicate that the VAS is capable of providing valuable mesoscale information suitable for analyzing a preconvective environment that is generally clear.

Uccellini, L. W.↗