Aseel Anabtawi

Ionosphere of Callisto from Galileo radio occultation observations

heights of 29.6 and 49.0 km. Four different methods, based on both photoionization and electron impact ionization, were used to obtain estimates of the corresponding neutral densities at the surface. The various assumptions inherent in these methods required using a variety of parameters, (cross sections, rate constants, etc.) all with their associated uncertainties. It was rather surprising and reassuring to find that all of the methods used to estimate the surface neutral density gave very similar results in each of the eight cases. The estimated values fall between 1 and 3 � 10 10 cm � 3 , leading to an estimate for the column density of from 3 to 4 � 10 16 cm � 2 . INDEX TERMS: 6218 Planetology: Solar System Objects: Jovian satellites; 6028 Planetology: Comets and Small Bodies: Ionospheres—structure and dynamics; 6026 Planetology: Comets and Small Bodies: Ionospheres—composition and chemistry; 6025 Planetology: Comets and Small Bodies: Interactions with solar wind plasma and fields; KEYWORDS: Callisto, ionosphere, atmosphere, radio, occultation

The ionosphere of Saturn as observed by the Cassini Radio Science System

Fifty-nine ionsopheric radio occultation observations of the vertical electron density profile in the Saturn ionosphere have been made since the Cassini spacecraft was inserted in orbit around Saturn in 2004. Significant orbit to orbit variations were observed, but the general trend noted in earlier orbits, namely, increasing electron densities with increasing latitude was reconfirmed and bolstered with this extended data base. This trend is likely to be due to some combination of increasing ionization rates and decreasing water influx with latitude.

A numerical technique for two‐way radio occultations by oblate axisymmetric atmospheres with zonal winds

The Ultra Stable Oscillator aboard the Cassini spacecraft failed in late 2011, which means that all radio occultations after that date have to be done in two-way mode, using a ground-based signal transmitted to the spacecraft as the frequency reference. Here we present the numerical technique we use to analyze the data from the two-way atmospheric radio occultations of both Saturn and Titan that have occurred since the Ultra Stable Oscillator (USO) failure, along with the theoretical reasons behind this technique. Since our two-way technique is based upon our earlier one-way technique which used the USO as the frequency reference, we also present our one-way technique which we used for Saturn occultations prior to the loss of the USO.

Experience from Ground Tracking of Huygens Radio Signal

The Huygens probe was released by the Cassini spacecraft and landed onto the surface of Saturn’s moon Titan. During the descent on 14 January 2005, radio signals were transmitted from the probe to the Cassini orbiter. One radio link, referenced to an ultra-stable oscillator was the prime link to conduct a Doppler experiment for the purpose of deriving the wind speeds in the atmosphere of Titan. The link to the Cassini orbiter failed but the signal was received and recorded at ground-based radio telescopes. This paper will discuss the experience and lessons learned from this activity, considered outside the usual tracking support of a deep space probe and performed in collaboration and coordination with Very Long Baseline Interferometry activity. Discussion will include the options and decisions on the recorded frequencies, polarizations, types of receivers transported to the Green Bank Telescope, Parkes radio telescopes and several VLBA stations for this purpose. It will also show the resulting science product of the ground-based Doppler Wind Experiment.

Limitations on the subcarrier frequency-to-symbol rate ratio

The residual-carrier tracking performance for PCM/PSK/PM/NRZ for square-wave and sine-wave subcarriers is evaluated as a function of the symbol SNR degradation. Explicitly, the RF residual carrier tracking performance is defined in terms of the subcarrier-to-symbol rate ratio, n. The upper and lower bound values for n are derived, and used to determine the minimum required bandwidth for maximum residual-carrier tracking performance. The analytical results obtained from the mathematical models are validated by computer simulation.<<ETX>>