Radio-frequency navigation and timing signals can be excellent sources of remotely-sensed science data, revealing structural details of the ionosphere and neutral atmosphere. Perhaps the most promising technique is GPS-based radio occultation (GPSRO), which yields electron density and precipitable water vapor or temperature profiles useful for numerical weather prediction (including space weather). Together with colleagues at UT and Cornell University, we have developed the first software-defined GPSRO sensor suitable for deployment on a cubesat.  We have also developed an instrument for ionospheric scintillation monitoring, which leverages our work in software-defined radio and applies techniques we developed for robust GPS signal tracking during scintillation.