Atmospheric soundings derived from Global Positioning System radio occultations (GPSRO) acquired in low-Earth orbit have the potential to be global climate benchmark observations of significant value to the Global Climate Observing System (GCOS). Geophysical observables such as atmospheric pressure and temperature are derived by measuring propagation delay induced by the atmosphere, a measurement whose fundamental unit-the second-is absolutely determined by calibration against atomic clocks. In this paper, we analyze the sources of systematic and random error for GPSRO soundings to determine the steps needed to establish GPSRO as a climate benchmark observation. Benchmarks require specific processing strategies and specific forms of documentation so that confidence in the accuracy and precision of the measurements is assured. Establishing calibration traceability to absolute standards (SI-traceability) is an essential strategy. We discuss a wide range of error sources in a geophysical retrieval, such as orbit determination error, signal delay in the Earth's ionosphere, and quality control strategies. Uncalibrated ionospheric delay is identified as the error source deserving the most attention in establishing SI-traceability of the retrievals, to meet stringent climate observation requirements of 0.5 K accuracy and 0.04 K stability. Profile comparisons from the recently launched COSMIC constellation establish strong upper limits on systematic error arising from the individual instruments. These encouraging results suggest that GPSRO should become a permanent resource for the GCOS. These highly precise and accurate instruments can be deployed on future Earth Observation satellites at a low per-sensor cost and minimal interference to existing and planned observational programs.
Transmissions of the Global Positioning System (GPS) satellites can be used to measure the total electron content (TEC) between a receiver and several GPS satellites in view. This simple observable is yielding a wealth of new scientific information about ionosphere and plasmasphere dynamics. Data available from thousands of ground-based GPS receivers are used to image the large-scale and mesoscale ionospheric response to geospace forcings at high-precision covering all local times and latitudes. Complementary measurements from space-borne GPS receivers in low-Earth orbit provide information on both vertical and horizontal structure of the ionosphere/plasmasphere system. New flight hardware designs are being developed that permit simultaneous measurement of integrated electron content along new raypath orientations, including zenith, cross-track and nadir antenna orientations (the latter via bistatic reflection of the GPS signal off ocean surfaces). We will discuss a new data assimilation model of ionosphere, the Global Assimilative Ionosphere Model (GAIM), capable of integrating measurements from GPS and other sensors with a physics-based ionospheric model, to provide detailed global nowcasts of ionospheric structure, useful for science and applications. Finally, we discuss efforts underway to combine GPS space-based observations of plasmaspheric TEC, with ground-based magnetometer measurements, and satellite-based images from NASA's IMAGE satellite, to produce new dynamic models of the plasmasphere.
Atmospheric soundings using signals received in low Earth orbit from Global Positioning System (GPS) satellite transmissions are widely recognized as important data for establishing a precise climate record of upper-air temperatures, due to their self-calibrating nature and all-weather acquisition. More recently, advances in retrieval methods using the same GPS data have opened the possibility of new scientific studies related to atmospheric processes and climate change. We will present recent innovations in extracting scientifically useful information from the phase and amplitude of received GPS transmissions, and discuss the technical challenges that need to be overcome to achieve new scientific results. Promising areas being pursued include: remote sensing of the planetary boundary layer from space, important for understanding ocean-atmosphere coupling; retrieving tropopause temperature structure at high vertical resolution, important for understanding troposphere-stratosphere exchange mechanisms and the role of convection; high accuracy and precision of upper altitude (25+ km) retrievals in the stratosphere. Using an end-to-end simulator recently developed at JPL, we will investigate in realistic detail the relationship between the atmospheric state and retrieved scientific parameters, and discuss retrieval research needed to address new scientific applications.