Search NASA⌕ Search

SEARCH · Search NASA

Results for “occultation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Diffraction calculation of occultation light curves in the presence of an isothermal atmosphere

From diffraction theory, light curves are calculated for stellar occultations by a planetary body with an isothermal atmosphere. The character of the resulting curves is determined by the scale height H, the Fresnel zone size l, the surface atmospheric refractivity, and the planetary radius. An exact general solution and two approximations are presented which are valid when H is much greater than l. The importance is assessed of accounting for diffraction effects of the limb when deducing atmospheric parameters from occultation light curves

French, R. G.↗
NASA NTRS1977

Verification by Viking landers of earlier radio occultation measurements of surface atmospheric pressure on Mars

The landing of Viking 1 in Chryse Planitia on July 20, 1976 provided the first opportunity to obtain measurements of atmospheric pressure directly from the surface of Mars. A computation was conducted to predict the atmospheric pressure at the landing site before the landing itself. The relative altitude between occultation points and the Viking 1 site was obtained with the aid of earth-based planetary radar data taken in 1967. The data cover Martian latitudes from 19 deg N to 24 deg N. The investigation indicates that the radio occultation results from Mariner 9 closely correspond to the actual surface pressure on Mars.

Kliore, A. J.↗
NASA NTRS1977

The occultation of Epsilon Geminorum by Mars - Analysis of McDonald data

An analysis of observations of the occultation of Epsilon Gem by Mars on April 8, 1976, is presented. The data were obtained by three neighboring telescopes at McDonald Observatory. Intensity fluctuations on time scales of the order of 100 ms were observed simultaneously at the three telescopes. As the observations compare well with predictions of turbulent scintillation theory, it is concluded that such fluctuations were probably largely the effect of stellar scintillations in the Martian atmosphere. The stellar diameter is included as a parameter in the theory but in a way which differs from previously published interpretations of occultations of extended sources by planetary atmospheres. Scintillations govern the experimental uncertainty in the deduction of the scale height of the high Martian atmosphere. A density scale height of 9.9 + or - 2.5 km is obtained at an altitude of 74 + or - 8 km above the mean surface. For CO 2 gas, this result corresponds to a temperature of 190 + or - 50 K.

Africano, J.↗
NASA NTRS1977

Recovery of refractivity profiles and pressure and temperature distributions in the lower atmosphere from satellite-to-satellite radio occultation data

The feasibility of recovering parameters from one-way range rate between two earth orbiting spacecraft during occultation of the tracking signal by the earth's lower atmosphere. The tracking data is inverted by an integral transformation (Abel transform) to obtain a vertical refractivity profile above the point of closest approach of the ray connecting the satellites. Pressure and temperature distributions can be obtained from values of dry refractivity using the hydrostatic equation and perfect gas law. Two methods are investigated for recovering pressure and temperature parameters. Results show that recovery is much more sensitive to satellite velocity errors than to satellite position errors. An error analysis is performed. An example is given demonstrating recovery of parameters from radio occultation data obtained during satellite-to-satellite tracking of Nimbus 6 by the ATS 6 satellite.

Murray, C. W., Jr.↗
NASA NTRS1977

Viking Radio Occultation Measurements of the Martian Atmosphere and Topography: Primary Mission Coverage

Radio occultation measurements were made at approximately 50 locations on Mars with the Viking Orbiter 1 S (2.3 GHz) and X (8.4 GHz) band tracking links during October 1976. The measurements have been used to study the topography and atmosphere of Mars at latitudes ranging from about 75 deg S to 70 deg N. By using the ingress and egress times obtained from the observed limb diffraction effects together with the best ephemerides available for the orbiter and the planet we have determined the surface elevations at the occultation points relative to the reference areoid. The observations agree with Mariner 9 and radar data to within 2 km. The mean atmospheric pressure at the areoid level was found to be 5.9 mbar during the northern midsummer season, a value which agrees quite well with data obtained at the landing sites. By comparing the new electron density measurements with earlier Mariner data we have determined that the temperature and the plasma scale height of the upper atmosphere appear to be functions of solar activity.

Fjeldbo, G.↗
NASA NTRS1977

Pioneer 10 and 11 radio occultations by Jupiter

Results on the temperature structure of the Jovian atmosphere are reviewed which were obtained by applying an integral inversion technique combined with a model for the planet's shape based on gravity data to Pioneer 10 and 11 radio-occultation data. The technique applied to obtain temperature profiles from the Pioneer data consisted of defining a center of refraction based on a computation of the radius of curvature in the plane of refraction and the normal direction to the equipotential surface at the closest approach point of a ray. Observations performed during the Pioneer 10 entry and exit and the Pioneer 11 exit are analyzed, sources of uncertainty are identified, and representative pressure-temperature profiles are presented which clearly show a temperature inversion between 10 and 100 mb. Effects of zonal winds on the reliability of radio-occultation temperature profiles are briefly discussed.

Kliore, A. J.↗
NASA NTRS1978

Independent radio-occultation studies of Venus' atmospheres

Closed-loop frequency data from Mariner 10 have provided S and X-band temperature-pressure profiles for Venus' neutral atmosphere, as well as an S-band profile for the nightside ionosphere. The dual-frequency radio-occultation experiment was independently monitored by two radio telescopes: Goldstone and Owens Valley (X-band only). Comparisons are also made with data obtained from Mariner 5 and in-situ measurements from Veneras 8, 9, and 10. Attention is given to anomalies in the amplitude and frequency of spacecraft signals during their exit from the Venusian atmosphere, as evidenced by radio occultation.

Nicholson, P. D.↗
NASA NTRS1978

Analysis of stellar occultation data - Effects of photon noise and initial conditions

An occultation light curve can be analyzed to provide information about a planetary atmosphere. Temperature, pressure, and number density profiles for the atmosphere of Mars are derived from a series of boundary layer equations, which invert equal increments of altitude (as opposed to time) in order to predict the noise quality of the occultation. Numerical results are given for a noisy isothermal light curve, with special attention to error analysis.

French, R. G.↗
NASA NTRS1978

Lunar occultation of Saturn. III - How big is Iapetus

The orbital light curve of Iapetus is analyzed, and the bright material on the trailing face of the satellite is found to have an effective limb-darkening parameter of 0.62 (+0.10, -0.12). This result is combined with observations of a lunar occultation of Iapetus to derive a radius of 718 (+87, -78) km for the satellite. The specific patchy albedo model of Morrison et al. (1975) is shown to fit the observed orbital light curve precisely and to yield an occultation radius of 724 + or - 60 km. It is noted that the values determined for the limb-darkening parameter and for the normal reflectance (in V) of the material at the center of the bright face are consistent with a partial water-frost cover on the bright side of Iapetus. It is concluded that if the albedo model of Morrison et al. reliably represents the surface of Iapetus, the radius of that satellite is 724 + or - 60 km, which is consistent with the radiometric radius of 835 (+50, -75) km derived by Morrison et al. (1975).

Veverka, J.↗
NASA NTRS1978

Radiative equilibrium temperatures in the stratosphere and mesosphere - A comparison for the stellar occultation and BUV ozone data

Recently published ozone densities in the equatorial stratosphere and mesosphere from satellite stellar occultation measurements are appreciably larger than previous independent midlatitude measurements and ozone densities predicted by photochemical theory. Radiative equilibrium temperature profiles with a stratosphere-mesosphere optimized model have been calculated for each of these ozone profiles; it is found that the stellar occultation studies cannot be discounted on energy balance grounds.

Kuhn, W. R.↗
NASA NTRS1978

Direct imaging of extra-solar planets with stationary occultations viewed by a space telescope

The use of a telescope in space to detect planets outside the solar system by means of imaging at optical wavelengths is discussed. If the 'black' limb of the moon is utilized as an occulting edge, a hypothetical Jupiter-Sun system could be detected at a distance as great as 10 pc, and a signal-to-noise ratio of 9 could be achieved in less than 20 min with a 2.4 m telescope in space. An orbit for the telescope is proposed; this orbit could achieve a stationary lunar occultation of any star for a period of nearly two hours.

Elliot, J. L.↗
NASA NTRS1978

Lunar occultation of Saturn. II - The normal reflectance of Rhea, Titan, and Iapetus

An inversion procedure to obtain the reflectance of the central region of a satellite's disk from lunar occultation data is presented. The scheme assumes that the limb darkening of the satellite depends only on the radial distance from the center of the disk. Given this assumption, normal reflectances can be derived that are essentially independent of the limb darkening and the diameter of the satellite. The procedure has been applied to our observations of the March 1974 lunar occultation of Tethys, Dione, Rhea, Titan, and Iapetus. In the V passband we derive the following normal reflectances: Rhea (0.97 plus or minus 0.20), Titan (0.24 plus or minus 0.03), Iapetus, bright face (0.60 plus or minus 0.14). For Tethys and Dione the values derived have large uncertainties, but are consistent with our result for Rhea.

Elliot, J. L.↗
NASA NTRS1979

The geometry of stellar occultation measurements on long-duration atmospheric monitoring missions

Geometrical considerations are presented for analyzing use of the stellar occultation technique on long duration earth atmosphere monitoring missions. The simulated mission data are presented for three representative orbits. Bright near infrared stars are used as examples of how extensive global longitude-latitude coverage is obtained by performing occultation measurements on several stars during the course of a one year mission. A comparison is made with similar missions using the sun as a light source.

Brooks, D. R.↗
NASA NTRS1978

The rings of Uranus - Results of the 10 April 1978 occultation

Observations of the April 10, 1978, stellar occultation by the rings of Uranus are presented. Nine rings were observed, and their radii and widths are calculated. Rings eta, gamma, and delta are found to be most likely circular and coplanar, in agreement with previous analyses; the remaining rings are either noncircular or slightly inclined. The width of the epsilon ring is a linear function of its radius from the center of Uranus, projected onto the satellites' orbital plane; this suggests that it forms one continuous noncircular ring. The optical-depth profile of the epsilon ring has not changed significantly since March 1977. A model of this ring which fits all available observations adequately is that of a uniformly precessing Keplerian ellipse coplanar with the satellites' orbits. This model permits predictions of the radius and width of the epsilon ring for future occultations. The precession rate is used to determine J2 for Uranus, on the assumption that precession is caused solely by the planetary oblateness and not by satellite-ring interactions.

Nicholson, P. D.↗
NASA NTRS1979

The diameter of Pallas from its occultation of SAO 85009

The May 29, 1978, occultation of SAO 85009 by Pallas was observed photoelectrically at seven widely spaced sites. The observations are well represented by an elliptical apparent limb profile having semimajor and semiminor axes of 279.5 + or - 2.9 and 262.7 + or - 4.5 km, respectively. Combining these results with published information on the light curve and rotational pole position, Pallas's mean diameter is found to be 538 + or - 12 km, which yields a mean density for Pallas of 2.8 + or - 0.5 g/cu cm and a visual geometric albedo of 0.103 + or - 0.005. The diameter of Pallas as determined from this occultation is significantly smaller than the values derived by radiometric, polarimetric, and double-image techniques.

Wasserman, L. H.↗
NASA NTRS1979

Deep radio occultations and 'evolute flashes' - Their characteristics and utility for planetary studies

Deep radio occultation signals from spacecraft passing behind planets may provide data on atmospheric absorption, turbulence, and structure, as well as information on the effects of planetary gravitational moments, rotation and zonal winds on the atmospheric shape. The strength of radio signals from a spacecraft passing behind a planet will at first decrease because of defocusing in the atmosphere, but then increase as the evolute of the planetary limb is neared, due to focusing caused by limb curvature within the evolute. Within the evolute, the availability of four simultaneous signal paths over four limb positions may render focused signals instantaneously great. The passage of Voyager 1 behind Jupiter and Voyager 2 behind Saturn will provide a test of deep radio occultation studies.

Eshleman, V. R.↗
NASA NTRS1979

Occultation of Epsilon Geminorum by Mars. III - Temperature structure of the Martian upper atmosphere

High-quality photoelectric observations of the April 8, 1976, occultation of Epsilon Gem by Mars are analyzed which were obtained at five different observatories. Temperature profiles are derived by numerical inversion of the star's light curves and found to be in mutual agreement. This is taken as evidence for the presence of strong atmospheric waves in the Martial upper atmosphere between number densities of 10 to the 13th and 10 to the 15th per cu cm. It is estimated that the vertical wavelength is about 20 km, the horizontal wavelength is greater than 600 km, and the peak-to-peak wave amplitude is approximately 40 K. The results are shown to be qualitatively consistent with Zurek's (1976) model of thermally driven tidal modes. No compelling evidence for turbulence effects is found in the occultation data.

French, R. G.↗
NASA NTRS1979

The geometry of stellar occultation measurements on long-duration atmospheric monitoring missions

Geometrical considerations are presented for analyzing use of the stellar occultation technique on long-duration earth atmosphere monitoring missions. Simulated mission data are presented for three representative orbits. Bright near-IR stars are used as examples of how extensive global longitude-latitude coverage can be obtained by performing occultation measurements on several stars during the course of a 1-year mission. A brief comparison is made with similar missions using the sun as a light source.

Brooks, D. R.↗