GPS Radio Occultation as Part of the Global Earth Observing System

Radio occultation measurements of the atmosphere using transmissions of the Global Positioning System (GPS) are discussed in the Decadal Survey for Earth Science released in 2007. Several successful examples of RO missions are currently in orbit: CHAMP, SAC-C, COSMIC and GRACE. RO retrievals have the fortunate characteristic of being based on time delay measurements, whose fundamental unitiquestthe secondiquestis absolutely calibrated using atomic clocks. Due to the absence of bias or long-term drift, multi-decadal time series of GPS RO retrievals are natural to develop for climate monitoring of atmospheric properties from the troposphere to the stratosphere. Highly accurate temperature profiles with high vertical resolution (50 m-200 m) are retrieved from the stratosphere to the mid-troposphere. Water vapor profiles are available from approximately 5 km altitude to the surface. The technique has sufficient vertical resolution to resolve the planetary boundary layer over much of the globe. Understanding the spatial sampling properties of GPS RO is important when bringing these data into the broader Earth observing context. In this paper, we discuss the unique relationship between vertical and horizontal resolution and describe the technology development needed to achieve maximum benefit for climate and weather applications.