Benefits of a SmallSat Radio Occultation Constellation for Mars Science and Exploration

Radio Occultation (RO) is a remote sensing technique with long standing heritage on planetary missions. In typical planetary RO, the communications system on board the spacecraft doubles as the scientific instrument by measuring the Doppler shift of radio signals through a planets atmosphere to infer physical parameters such as temperature, pressure and number density of the neutral atmosphere and ionosphere. With the development of smallsat technologies along with the success of deep space smallsat missions such as MarCO, we present a smallsat mission concept for a dedicated RO Mars mission. The primary mission goal addresses existing knowledge gaps identified by the planetary science community to assist in safely landing spacecraft on the surface in the Entry Descent and Landing (EDL) and Ascent from the Surface (AST) phases of a mission. EDL and AST benefit from precise knowledge of wind speeds close to the surface that can be derived from RO data. A dedicated RO mission consisting of multiple orbiters offer much higher spatiotemporal coverage than possible from traditional orbiter-to-Earth communication links, giving insight into seasonal and diurnal cycles. The mission targets preliminary requirements of 100 RO global profiles per day with a temperature uncertainty of 1K between 6-12 km above the surface. The baseline concept consists of a fleet of 6 smallsats in high inclination orbits to maximize global coverage. The quantity derived directly from RO data is the bending angle, which is converted to physical parameters pertaining to EDL and AST requirements. Expected noise is added into a Monte Carlo code and then converted to the physical parameters. Current configurations allow for ~200 RO profiles per day. Using a dual frequency one-way approach of X-Band and UHF Band frequencies, the ionosphere and neutral atmosphere contributions can be effectively separated. A trade study of 3 different oscillators has been conducted and findings show that clocks with an Allan Deviation (AD) of 10-13 s/s meet the requirement of 1K temperature uncertainty between 6-12 km above the surface. In addition, we investigate the use of RO profiles from such a constellation in deriving 3-D winds based on the geostrophic wind approximation, as has been done from GPS radio occultation for the Earths atmosphere.