There is a clear need for observing systems that characterize decadal-scale changes of the climate system. A National Research Council (NRC) report, “Earth science and applications from space” (2007); recommends the radio occultation (RO) technique to monitor climate change. Radio occultation establishes a global climate benchmark by measuring the effect of atmospheric temperature, pressure, and water vapor on radio signals propagating above the outer edge of Earth's surface, known as the limb. The atmospherically induced delays are tied to atomic clock standards via the Global Positioning System (GPS). Measurements obtained decades apart can be intercompared without concern for intersatellite bias or calibration drift.