Simulation Studies of Radio Occultation Errors Using MGS-Type Orbits and Mars GCM Results

Physics – Optics

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Scientific paper

The temperature-pressure profiles obtained from Abelian inversion of radio-occultation data may be in error when non-spherically-symmetric refractivity structures exist in the atmosphere. GCM results for the Martian atmosphere suggest that the height of constant refractivity surfaces drops significantly going from mid-latitudes towards the pole in the winter hemisphere [Pollack et al. 1990, J. Geophys. Res. 95, 1447-1473]. This creates a ``tilt'' in the atmosphere as seen along the refractivity contours on a latitude versus height map. Recently we simulated the transmission of radio waves through the Martian atmosphere, represented by the results of NASA Ames GCM, using geometrical optics approximation. Subsequent inversion of Doppler data using the Abel transform confirms our previous estimates of errors in the retrieved profiles [Bilal and Tyler 1993, Bull. Amer. Astron. Soc. 25, 1062-1063]. For example, for an approximately 3.8 mrad tilt the temperature profiles are biased by 1.5, 2.5, and 5.2 Kelvins for MGS type orbit openings of 30deg , 45deg , and 60deg respectively. The constant refractivity contours, however, tend to flatten out at lower altitudes. In this example the tilt of 3.8 mrad gradually disappears between 15 km and 10 km altitude. The bias in the recovered temperature also starts decreasing at about 15 km and is down to 0.5 and 1.4 Kelvins at the surface for 45deg and 60deg orbit openings, respectively. This behavior will be explained in light of the image spread function associated with the inversion of radio occultation Doppler data on the assumption of spherical symmetry [Ahmad and Tyler 1995, Bull. Amer. Astron. Soc. 27, 1140]. Knowledge of the horizontal structure is needed to invert radio occultation data accurately and to estimate the uncertainty in retrieved profiles. This information may be available from other experiments. Also, after several occultation events have been observed in a region of interest, it may be possible to construct a refractivity contour map of the region which can then be used to estimate and possibly to correct for systematic errors if the distribution of occultation points is favorable.

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