Convergence of Technologies Enabling Radio Crosslink Missions for Planetary Atmospheric Structures

The atmospheric radio occultation (RO) technique was developed six decades ago for planetary missions and has since resulted in numerous scientific discoveries throughout the Solar System. The traditional experimental configuration utililizes spacecraft communication links with phase-stable radio signals transmitted from a spacecraft orbiting or flying past a planet and received at a ground station after propagating through the atmosphere. Alterations in the phase and amplitude are used to infer properties of that atmosphere. A reverse configuration with a ground-to-spacecraft one-way link has been used with the appropriately instrumented mission to Pluto to overcome severe signal-to-noise ratio limitations. A more recently investigated spacecraft-to-spacecraft occultation technique utilizes UHF communication links between Mars landers and orbiters to measure ionospheric properties.In 1995, the Earth science community demonstrated a variation of this technique that enabled remote sensing of Earth’s atmosphere and ionosphere. By tracking spacecraft-to-spacecraft crosslinks utilizing the global navigation satellite constellations as the transmitting terminals and specially instrumented satellites as the receiving terminals, high resolution vertical profiles are routinely retrieved with valuable utility in numerical weather prediction and atmospheric dynamics. Applying crosslinks more generally to planetary missions would allow large increases in spatial and temporal coverage of atmospheric structures and composition; however, this has been prohibitively costly until recent technological advances and breakthroughs.The advances of the required technologies for crosslink RO experiments currently enable realistic planning of their use in future planetary missions. These crosslinks could use orbiting small spacecraft for one or multiple links, and possibly probes, aerobots (e.g., balloons), and landers equipped for relay communications. These technologies and techniques include:1.Small software-defined radios2.Multi-frequency communication links3.Smaller ultra-stable oscillators4.Advancedsignal processing and holographic methods5.Interplanetary flight, orbit insertion, and formation-flyingThis paper presents the state of the art of these technologies and introduces lower cost mission concepts to Venus, the giant planets, and other planetary targets in the context of recent Decadal Surveys. These multi-spacecraft missions provide natural opportunities for international collaboration. Design requirements, recent simulations, radio-holographic analysis methods, and lessons from an opportunistic UHF crosslink demonstration at Mars will be discussed.