2012.B.2.1 Interplanetary Radio Occultation CubeSat Constellation

Author(s)

Kerri Cahoy (1), Ingrid Beerer (1), Anne Marinan (1), Sami Asmar (2), Paul Withers (3) and Luke Moore (3)

MIT
NASA JPL
Boston University

Session

B.2 – Technologies and Missions to Enhance Interplanetary CubeSat Science

Keywords

constellation, radio, radio occultation, atmosphere, ionosphere, interplanetary

Abstract

A constellation of CubeSats can use small and simple spacecraft radio transmitters and receivers to globally and frequently measure temperature, pressure, and electron density profiles of a planet’s atmosphere and ionosphere with a technique called radio occultation. During a radio occultation experiment, a stable radio signal is transmitted to or from a spacecraft as it drops behind the limb of a planet. The electromagnetic signal interacts with the molecules in the planet’s atmosphere and the charged particles in its ionosphere. The vertical distribution of the molecules and charged particles creates a refractivity gradient. In geometrical optics terms, the electromagnetic ray travels straight through “empty” space but is symmetrically “bent” as it encounters the refractivity gradients of the atmosphere and ionosphere. The received frequency of the signal is thus slightly but detectably shifted from the initial frequency. These measured frequency residuals or amount of “bending” can be inverted to calculate atmospheric neutral densities or ionospheric electron densities of the volume of atmosphere through which the ray passed, from which high vertical resolution profiles can be derived.

The Interplanetary Radio Occultation CubeSat Constellation (IROCC) concept consists of six 3U CubeSats, each contained by a Poly-Picosatellite Orbital Deployers (P-PODs) as a secondary payload on a larger interplanetary spacecraft. We discuss system design trades and requirements for this constellation, from deployment of the CubeSats after orbit insertion around a planet or satellite to orbital decay of the constellation. Trades analyzed include the timing of P-POD deployments into the desired orbit planes that also minimize any risk to a primary mission, the radio occultation experiment configuration itself (multiple frequencies, intersatellite links, signal to noise ratios, timing, and tracking), the different architectures for transmission of the collected radio occultation data back to Earth, the opportunity to study atmospheric drag as the CubeSat orbits decay, and planetary protection requirements on Cubesats. Additional target-specific parameters are also considered, such as the reduced amount of available solar power for the already-tiny CubeSats at larger distances from the Sun, the radiation environment, the presence or absence of a magnetic field, orbital decay and mission duration, and the selection of radio occultation frequencies to correspond to the spectral features and compositions of the targets.