Globally Distributed and High-Resolution Mars Atmospheric Profiles from Small Spacecraft Constellations

Radio occultation experiments have been carried out on numerous planetary missions since 1965, when Mariner IV flew by Mars. The observations of radio links from spacecraft to ground stations have yielded valuable information on the structure of planetary ionospheres and neutral atmospheres. By utilizing links from a single orbiting spacecraft, global coverage of the atmospheric measurements can be achieved, but only after extended periods of time, often many months or more. However, following work by the Earth atmospheric community that advanced crosslinks between spacecraft in Earth orbit, the transmitter side being a Global Positioning System GPS spacecraft, crosslink occultations can be envisoned at Mars and other planets. Crosslink occulations between two or three spacecraft can produce much more rapid global coverage that enables examination of the variability of vertical profiles of temperature-pressure and ionospheric electron density profiles on daily, seasonal, and annual time scales. Such small-spacecraft planetary constellations are enabled by recent advances in small and less costly spacecraft, such as CubeSats. Such smallsat occultation missions offer a means to dramatically improve knowledge of atmospheric structure and dynamics for several planets and the moon Titan.We report on our studies for a small constellation of Mars satellites that perform atmospheric/ionospheric occultation studies. We developed a science traceability matrix that yielded technical design specifications linked to key science goals such as the atmospheric temperature accuracy and dense geographical/ temporal measurement distribution that would allow global determination of atmospheric variations from diurnal to seasonal scales over the entire planet. Radio link systems scenarios were examined that met the measurement accuracy requirements at maximum spacecraft ranges. We also explored various mission design concepts and an initial Cubesat design that included considerations of cost, size, power, mass, longevity, and radiation tolerance. The occultation data obtained by each smallsat would be relayed to a large orbiter which then sends the data on to the Earth, so the CubeSats themselves are not required to have Earth communication capability. Precision knowledge of the CubeSat orbits is also required to properly interpret the occultation data. This is achieved by Doppler measurements on radio links between the CubeSats before or after each occulation and by occasional radio contact between each CubeSat with the large orbiter whose position and velocity are well known from Earth-based tracking by the Deep Space Network.