Development of Radio Science Techniques and a Planetary Mission Concept

Abstract

Radio Occultation (RO) is a remote sensing observation which takes advantage of radio signals from the communications system of a spacecraft to measure the Doppler shift of the signals as they pass through the atmosphere. Only if the atmospheric composition is known, pressure, temperature and number density can be derived using Abelian transforms. Such observations at Mars are crucial to enhancing modelling software which engineering teams have to use to predict atmospheric conditions during Entry Descent and Landing (EDL), the most high-risk phase of any surface mission. Most RO experiments at Mars have been performed with an Earth based Deep Space Network (DSN) station and with an orbiting spacecraft at Mars. These observations have to be conducted when the viewing geometry between Earth and Mars is optimal, and they take time away from normal DSN communications tasks for spacecraft operations. As such, the observations lack in the frequency and coverage necessary to properly initialize models used by engineering teams. RO observations rely on highly precise clocks on board the spacecraft to reduce error in Doppler shift measurements that propagate into the retrieval of physical atmospheric parameters. The Ultra Stable Oscillators (USOs) of these precise clocks are expensive, and require a large volume and power. The use of Commercial off the Shelf (COTS) parts for such a USO is a low-cost solution that provides a less accurate clock. However, we present an analysis that shows the use a Dual One-Way (DOW) frequency method, commonly used in gravity ranging experiments, can achieve the same accuracy as highly stable clocks using a lower stability COTS USO. Given recent interest in SmallSats for interplanetary missions such as MarCO, a dedicated fleet of SmallSats at Mars is a low-cost solution that could yield unprecedented RO global, diurnal and seasonal coverage. We present a detailed mission concept to use SmallSats over the course of 1 Mars year developed in a collaboration between the University of Michigan and the Jet Propulsion Laboratory (JPL). The concept has been developed in preparation for submission to a NASA SIMPLEx (Small Innovative Missions for Planetary Exploration) call for proposals. A secondary mission science objective is to measure Electrostatic Discharge (ESD) events, that have been observed during global dust storms (e.g., Ruf et al., 2009; Renno and Ruf, 2012) . Such ESD events present an unknown risk to humans on the surface of Mars. Past Earth based measurements of ESD are lacking in frequency, and the viewing geometry cannot pinpoint the originating source of ESD. However, orbiting spacecraft will have the ability to identify areas which ESD can originate from.