Engineering topics

Johnson, P. E.

Publications and source records attributed to Johnson, P. E..

16 records

NASA NTRS1986

Spectral mixture modeling - A new analysis of rock and soil types at the Viking Lander 1 site

A Viking Lander 1 image was modeled as mixtures of reflectance spectra of palagonite dust, gray andesitelike rock, and a coarse rocklike soil. The rocks are covered to varying degrees by dust but otherwise appear unweathered. Rocklike soil occurs as lag deposits in deflation zones around stones and on top of a drift and as a layer in a trench dug by the lander. This soil probably is derived from the rocks by wind abrasion and/or spallation. Dust is the major component of the soil and covers most of the surface. The dust is unrelated spectrally to the rock but is equivalent to the global-scale dust observed telescopically. A new method was developed to model a multispectral image as mixtures of end-member spectra and to compare image spectra directly with laboratory reference spectra. The method for the first time uses shade and secondary illumination effects as spectral end-members; thus the effects of topography and illumination on all scales can be isolated or removed. The image was calibrated absolutely from the laboratory spectra, in close agreement with direct calibrations. The method has broad applications to interpreting multispectral images, including satellite images.

Adams, J. B.↗

NASA NTRS1985

Quantitative Analyses of Planetary Reflectance Spectra

The development of a set of quantitative models to analyze planetary reflectance spectra as a function of microscopic and macroscopic mineral mixtures, particle size, and illumination geometry is considered. The approach has been to simplify more sophisticated algorithms to include the smallest number of parameters possible, consistent with being able to use them to produce useful results. This means that they should be able to model the data to within the accuracy obtainable by laboratory, telescopic, and space instrumentation (roughly 1%). The algorithms are ideally given in terms of parameters that are directly measureable (such as spectral reflectance or particle size).

Johnson, P. E.↗

NASA NTRS1985

Quantitative determination of mineral types and abundances from reflectance spectra using principal components analysis

A procedure was developed for analyzing remote reflectance spectra, including multispectral images, that quantifies parameters such as types of mineral mixtures, the abundances of mixed minerals, and particle sizes. Principal components analysis reduced the spectral dimensionality and allowed testing the uniqueness and validity of spectral mixing models. By analyzing variations in the overall spectral reflectance curves, the type of spectral mixture was identified, mineral abundances quantified and the effects of particle size identified. The results demonstrate an advantage in classification accuracy over classical forms of analysis that ignore effects of particle-size or mineral-mixture systematics on spectra. The approach is applicable to remote sensing data of planetary surfaces for quantitative determinations of mineral abundances.

Smith, M. O.↗

NASA NTRS1985

Quantitative analysis of planetary reflectance spectra with principal components analysis

A technique is presented for quantitative analysis of planetary reflectance spectra as mixtures of particles on microscopic and macroscopic scales using principal components analysis. This technique allows for determination of the endmembers being mixed, their abundance, and the scale of mixing, as well as other physical parameters. Eighteen lunar telescopic reflectance spectra of the Copernicus crater region, from 600 nm to 1800 nm in wavelength, are modeled in terms of five likely endmembers: mare basalt, mature mare soil, anorthosite, mature highland soil, and clinopyroxene. These endmembers were chosen from a similar analysis of 92 lunar soil and rock samples. The models fit the data to within 2 percent rms. It is found that the goodness of fit is marginally better for intimate mixing over macroscopic mixing.

Johnson, P. E.↗

NASA NTRS1983

Design considerations for high-power VHF radar transceivers: The Poker Flat MST radar phase control system

Sixty-four separate 50-kW peak-power transmitters are distributed throughout the 200 x 200 meter Poker Flat MST radar antenna array. The relative phase of each transmitter is automatically controlled by a 64-channel unit located in the main building at the edge of the antenna. The phase control unit is described. In operation the RF pulse from a transmitter coupler is power divided and compared with the phase reference in a mixer. The mixer output is low-pass filtered and sampled near the center of the resulting video pulse by an amplifying sample-and-hold integrated circuit. Phase control is effected by maintaining the mixer output pulse near zero volts by amplifying the sample-and-hold output which then drives the voltage-controlled phase shifter in the direction to null the mixer output. The voltage-controlled shifter achieves over 360 deg phase shift in the range from 0.7 to 24 volts. When the voltage into the shifter tracks to either voltage limit the wrap-around control resets the voltage so that the shifter is always operating within its control range.

Ecklund, W. L.↗

NASA NTRS1983

Design considerations for high-power VHF radar transceivers: Phase matching long coaxial cables using a cable radar

The Poker Flat 49.92-MHz MST radar uses 64 phase-controlled transmitters in individual shelters distributed throughout the antenna array. Phase control is accomplished by sampling the transmitted pulse at the directional coupler of each transmitter and sending the sample pulse back to a phase-control unit. This method requires phase matching 64 long (256 meter) coaxial cables (RG-213) to within several electrical degrees. Tests with a time domain reflectometer showed that attenuation of high frequency components in the long RG-213 cable rounded the leading edge of the reflected pulse so that the cables could only be measured to within 50 cm (about 45 deg at 49.92 MHz). Another measurement technique using a vector voltmeter to compare forward and reflected phase required a directional coupler with unattainable directivity. Several other techniques were also found lacking, primarily because of loss in the long RG-213 cables. At this point it was realized that what was needed was a simple version of the phase-coherent clear-air radar, i.e., a cable radar. The design and operation of this cable are described.

Johnson, P. E.↗

NASA NTRS1983

10 micron polarimetry of Ceres

Linear polarimetry of Ceres at 10 micron is presented. These data represent the first published polarization measurements of an asteroid in the thermal infrared. It is found that Ceres is polarized at the 0.2-0.6 percent level. This data set is compared with theoretical models of the linear polarization of emitted radiation from a spherical plane. These models are used to derive the pole position and thermal inertia of Ceres. Ceres is best fit with a thermal inertia of 0.0010 + or 0.0003 cal (sq cm K) per square root of second and a pole orientation of beta(p) = 36 deg + or - 5 deg + or -5, lambda(p) = 270 deg + or - 3. It is concluded that 10 micron polarimetry is a potentially powerful technique for remotely sensing the pole orientation and thermal inertia of asteroids.

Johnson, P. E.↗

NASA NTRS1983

A semiempirical method for analysis of the reflectance spectra of binary mineral mixtures

A simple semiempirical method is presented for determination of the spectral reflectance of a powdered binary mineral mixture. This technique uses a two-stream radiative transfer model (a modified Kubelka-Munk model) on a particulate medium of isotroic scatterers The particles are assumed to be much larger than the wavelength of light under consideration. This same method can be used to determine the relative proportion of components in a mixture for which the spectral reflectance is known. Binary mixtures of olivine, two pyroxenes, and magnetite are used to test this model. The theoretical and empirical results agree approximately within experimental errors.

Johnson, P. E.↗

NASA NTRS1981

Measurement of stratospheric aerosol on Saturn using an eclipse of Titan

Measurements of the aerosol content in the Saturn atmosphere made during an eclipse of Titan by Saturn are presented. Observations of the ingress of Titan into the shadow of Saturn on December 20.08, 1979 were made by the 74-in telescope at Kottamia Station in Egypt at an effective wavelength of 6728 A with FWHM of 1000 A. The deviation of the observed light curve after a satellite darkening of 3 mag from that predicted for a clear Saturn atmosphere is interpreted in terms of an aerosol layer decreasing in extinction coefficient from a base altitude 5 km below the planetary tropopause to an altitude of up to 75 km above the tropopause. For an aerosal layer extending to 75 km above the tropopause calculations reveal possible optical depths ranging from 0.020 to 0.050. The range of the optical depth is comparable to that of the Jupiter atmosphere, and suggests that similar processes may be responsible for the stratospheric haze on both planets.

Smith, D. W.↗

NASA NTRS1981

Spectrophotometry of J8, J9, and four Trojan asteroids from 0.32 to 1.05 microns

New 30-channel narrowband photometry from 0.32 to 1.05 microns of the retrograde Jovian satellites J9 (to 0.7 micron) and J8 and the trailing Trojan asteroids 617, 884, 1172, and 1173 is presented. The data confirm previous measurements of J8, 617, 884, and 1172 at wavelengths less than 0.8 micron, but the extension into the infrared shows that the normalized spectral reflectance of these objects rises steadily from approximately 0.8 at 0.4 micron to approximately 1.4 at 1.05 microns, suggesting they are too bright in the near infrared to be C-type asteroids. The C classification of 1173 is confirmed. J9 is markedly redder than J8 at visible wavelengths. The results indicate a greater taxonomic contrast between these distant objects and main-belt asteroids than previously thought.

Smith, D. W.↗

NASA NTRS1981

An outburst of comet Tempel-2 observed spectrophotometrically

Periodic comet Tempel-2, first discovered in 1873, is a possible target of a spacecraft flyby in the 1980s. Accurate physical measurements of this comet have only recently been attempted. Few spectrophotometric data exist for any faint comet, but such data are presented for Tempel-2 at a heliocentric distance of 2.97 AU. The data were obtained during an outburst in which the comet exhibited heightened CN and C3 emission over a quiescent phase. Comet flares of such magnitude are exceptional and are particularly surprising for a periodic comet at such a large heliocentric distance.

Johnson, P. E.↗

NASA NTRS1980

Impact of changing fuel characteristics on supersonic cruise airplane

The question of an advanced supersonic cruise research airplane is related to future oil supplies and prices. Technical data on the impact of changing fuel characteristics on the SCR airplane were developed. Projections of crude oil characteristics typical of the 1985 to 2000 time period were made with the help of consultants to the oil industry. Refineries for the future were modeled to establish jet fuel of engine and aircraft systems for future airplanes, with emphasis on supersonic cruise airplanes. Study results do not show a need for broadening the fuel specification. Hypothetical study fuels with broader specifications were defined, however, as was the impact of their properties on the SCR airplane and systems.

Hadaller, O. J.↗

NASA NTRS1980

New results from optical polarimetry of Saturn's rings

Linear polarimetry of Saturn's rings, obtained through the period of the 1979 opposition, is presented. The polarisation clearly correlates in direction with the plane containing the Sun, planet and Earth, but not the ring plane. The results are consistent with local scattering on the surface of individual ring bodies, covered with frost

Johnson, P. E.↗

NASA NTRS1979

Eclipse spectrophotometry of the Galilean satellites

Narrowband spectrophotometry of satellite eclipses is presented for each of the Galilean satellites. Comparing the partially eclipsed full-phase satellite disk to the uneclipsed disk can reveal colorimetric inhomogeneities on the surface. The trailing half-disk of Ganymede is slightly blue compared to the leading half-disk. The trailing and leading half-disks of Callisto are similar to each other, as are the northern and southern half-disks. The northern half-disk shows evidence of blue and red portions. The trailing half-disk of Europa is redder than the leading half-disk.

Smith, D. W.↗

NASA NTRS1978

Narrow-band spectrophotometry of Ariel, Umbriel, Titania, Oberon, and Triton

The spectral reflectances of Ariel, Umbriel, Titania, Oberon, and Triton were measured in 28 bandpasses between lambda 326 and lambda 976 nm on the night of 28/29 June 1974. These observations were made with the 200-in. Hale telescope and multichannel spectrometer. Bandpasses of 8 nm from lambda 326 to lambda 566 nm and 16 nm from lambda 592 to lambda 976 nm were employed. The spectral reflectances of Ariel, Oberon, and Titania increase from lambda 342 to lambda 534 nm and are relatively flat from lambda 550 to lambda 976 nm. Umbriel's reflectance decreases monotonically with increasing wavelength through the entire range of measured wavelengths. Triton is found to have a constant spectral reflectance.

Johnson, P. E.↗

NASA NTRS1975

Design definition study of a NASA/Navy lift/cruise fan technology V/STOL airplane: Risk assessment addendum to the final report

An assessment of risk, in terms of delivery delays, cost overrun, and performance achievement, associated with the V/STOL technology airplane is presented. The risk is discussed in terms of weight, structure, aerodynamics, propulsion, mechanical drive, and flight controls. The analysis ensures that risks associated with the design and development of the airplane will be eliminated in the course of the program and a useful technology airplane that meets the predicted cost, schedule, and performance can be produced.

Zabinsky, J. M.↗