A new method to determine the gravity field of small bodies from line-of-sight acceleration data

We present a new method to derive line-of-sight acceleration observables from spacecraft radio tracking data. The observables can be used to estimate the mass and gravity of a natural satellite as a spacecraft flyby. The corresponding observation model adapts to one-way and two/three-way tracking modes. As a test case for method validation and application, we estimated the mass and degree two gravity field for the Martian moon Phobos using simulated tracking data when the spacecraft Mars Express flew by Phobos on 2013 December 29. We have a few real tracking data during flyby and they will be used to confirm raw data simulation. The main purpose of this paper is to demonstrate the method of line-of-sight acceleration reduction from raw tracking data and the feasibility to estimate mass and gravity of a natural satellite using this type of observable. This novel method is potentially applicable to planet and asteroid gravity field studies combined with Doppler tracking data.

Ionospheric inversion of the Venus Express radio occultation data observed by Shanghai 25m and New Norcia 35m antennas

Electron density profiles of Venus＇ ionosphere are inverted from the Venus Express （VEX） one-way open-loop radio occultation experiments carried out by the Shanghai 25 m antenna from November 2011 to January 2012 at solar maximum conditions and by the New Norcia 35 m antenna from August 2006 to June 2008 at solar intermediate conditions. The electron density profile （from 110 km to 400 km）, retrieved from the X-band egress observation at the Shanghai station, shows a single peak near 147 km with a peak density of about 2 × 10^4 cm-3 at a solar zenith angle of 94° As a comparison, the VEX radio science （VeRa） observations at the New Norcia station were also examined, including S- and X-band and dual-frequency data in the ingress mode. The results show that the electron density profiles retrieved from the S-band data are more analogous to the dual-frequency data in terms of the profile shape, compared with the X-band data. Generally, the S-band results slightly underestimate the magnitude of the peak density, while the X-band results overestimate it. The discrepancy in the X-band profile is probably due to the relatively larger unmod- eled orbital errors. It is also expected that the ionopause height is sensitive to the solar wind dynamical pressure in high and intermediate solar activities, usually in the range of 200-1000 km on the dayside and much higher on the nightside. Structural variations （＂bulges＂ and fluctuations） can be found in the electron density profiles during intermediate solar activity, which may be caused by the interaction of the solar wind with the ionosphere. Considerable ionizations can be observed in Venus＇ nightside ionosphere, which are unexpected for the Martian nightside ionosphere in most cases.