We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground‐based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time‐varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground‐based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.

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Abstract. Using temperature profiles obtained by the GPS/MET (GPS

Abstract. The Global Positioning System/Meteorology (GPS/MET) Program was established in 1993 by the University Corporation for Atmospheric Research (UCAR) to demonstrate active limb sounding of the Earth's atmosphere using the radio occultation technique. The demonstration system observes occulted GPS satellite signals received by a low Earth orbiting (LEO) satellite, MicroLab-1, launched April 3, 1995. The system can profile ionospheric electron density and neutral atmospheric properties. Neutral atmospheric refractivity, density, pressure, and temperature are derived at altitudes where the amount of water vapor is low. At lower altitudes, vertical profiles of density, pressure, and water vapor pressure can be derived from the GPS/MET refractivity profiles if temperature data from an independent source are available. This paper describes the GPS/MET data analysis procedures and validates GPS/MET data with statistics and illustrative case studies. We compare more than 1200 GPS/MET neutral atmosphere soundings to correlative data from operational global weather analyses, radiosondes, and the GOES, TOVS, UARS/MLS and HALOE orbiting atmospheric sensors. Even though many GPS/MET soundings currently fail to penetrate the lowest 5 km of the troposphere in the presence of significant water vapor, our results demonstrate iøC mean temperature agreement with the best correlative data sets between 1 and 40 km. This and the fact that GPS/MET observations are all-weather and self-calibrating suggests that radio occultation technology has the potential to make a strong contribution to a global observing system supporting weather prediction and weather and climate research.

Abstract. Global Positioning System (GPS) radio occultation signals received by a low Earth orbit (LEO) satellite provide information about the global distribution of electron density in the ionosphere. We examine two radio occultation inversion algorithms. The first algorithm utilizes the Abel integral transform, which assumes spherical symmetry of the electron density field. We test this algorithm with two approaches: through the computation of bending angles and through the computation of total electron content (TEC) assuming straight line propagation. We demonstrate that for GPS frequencies and for observations in LEO, the assumption of straight-line propagation (neglecting bending) introduces small errors when monitoring the F2 layer. The second algorithm, which also assumes straight-line propagation,

[1] The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC)/Formosa Satellite 3 (FORMOSAT-3) is a six-satellite radio occultation mission that was launched in mid-April, 2006. The close proximity of the COSMIC satellites provides a unique opportunity to estimate the precision of the radio occultation remote sensing technique from closely collocated occultations (<10 km separation of tangent points). The RMS difference of refractivity between 10 and 20 km altitude is less than 0.2%, which is approximately twice better than previous estimates obtained from CHAMP and SAC-C collocated occultations, apparently, due to smaller separation of the occultation pairs and due to parallel occultation planes. In the lower troposphere, the maximal RMS is $0.8% at 2 km altitude and decreases abruptly to $0.2% between 6 and 8 km altitude. The RMS difference of electron density in the ionosphere between 150 and 500 km altitude for collocated occultations is about 10 3 cm À3 .

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Abstract. This letter reports for the first time the simulated error distribution of radio occultation (RO) electron density profiles (EDPs) from the Abel inversion in a systematic way. Occultation events observed by the COSMIC satellites are simulated during the spring equinox of 2008 by calculating the integrated total electron content (TEC) along the COS-MIC occultation paths with the "true" electron density from an empirical model. The retrieval errors are computed by comparing the retrieved EDPs with the "true" EDPs. The results show that the retrieved NmF2 and hmF2 are generally in good agreement with the true values, but the reliability of the retrieved electron density degrades in low latitude regions and at low altitudes. Specifically, the Abel retrieval method overestimates electron density to the north and south of the crests of the equatorial ionization anomaly (EIA), and introduces artificial plasma caves underneath the EIA crests. At lower altitudes (E-and F1-regions), it results in three pseudo peaks in daytime electron densities along the magnetic latitude and a pseudo trough in nighttime equatorial electron densities.