Computer Science
Scientific paper
May 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992tdar.nasar..22l&link_type=abstract
In its The Telecommunications and Data Acquisition Report p 22-39 (SEE N93-26105 09-32)
Computer Science
Coordinates, Ecliptic, Frequencies, Occultation, Planetary Ionospheres, Troposphere, Venus (Planet), Venus Atmosphere, Diffraction Patterns, Ionopause, Ionospheric Electron Density, Pressure Measurement, Solar Flux, Solar Wind, Wind Pressure
Scientific paper
An occultation of the compact radio source P 0507+17 by Venus on 19 July 1988, was observed in Tidbinbilla, Australia at a frequency of 2.3 GHz. The purpose of this observation was to measure the position of Venus in the radio reference frame. When data from both ingress (Venus day side) and egress (Venus nightside) were used to solve for the position of Venus in ecliptic longitude and latitude, the results were consistent with zero offsets from the nominal values, with an uncertainty of approximately 0.2 arcsec in both coordinates. By using the nightside data alone, a value of -0.026+/-0.04 arcsec was obtained for the linear combination delta(lamda) + 0.51delta(beta) where delta(lambda) and delta(beta) were the offsets from their nominal values of the ecliptic longitude and latitude of Venus. Distortion of a vacuum Fresnel fringe pattern by the Venus troposphere, and especially by the Venus ionosphere, was observed. The day side ionosphere of Venus caused very large distortions; the amplitude of the first Fresnel fringe in the ingress data was eight times larger than had been expected for an airless planet. The observed fringe patterns were modeled by using plausible ionospheres (i.e., consistent with spacecraft measurements of the Venus ionosphere and with solar extreme ultraviolet flux and solar wind pressure measurements at the occultation epoch). However, the range of Venus ionospheric profiles (electron density as a function of altitude) allowed by a priori constraints and by the occultation data was large (e.g., the ionopause height on the day side was uncertain by a factor of two). This ionospheric uncertainty (particularly on the day side) translated into a large position uncertainty (0.2 arcsec for the day side and 0.04 arcsec for the nightside). If it had been possible to calibrate the Venus ionosphere by some external means, the accuracy in delta(lambda) and delta(beta) would have been 0.01 arcsec or better.
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