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 .

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.

[1] Collocated global atmospheric temperature, humidity, and refractivity profiles from radiosondes and from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation data for April 2008 to October 2009 are compared for two purposes. The first is to quantify the error characteristics of 12 radiosonde types flown in the global operational network, as a function of height and for both day and nighttime observations, for each of the three variables. The second is to determine the effects of imperfect temporal and spatial collocation on the radiosonde-COSMIC differences, for application to the general problem of satellite calibration and validation using in situ sounding data. Statistical analyses of the comparisons reveal differences among radiosonde types in refractivity, relative humidity, and radiation-corrected temperature data. Most of the radiosonde types show a dry bias, particularly in the upper troposphere, with the bias in daytime drier than in nighttime. Weather-scale variability, introduced by collocation time and distance mismatch, affects the comparison of radiosonde and COSMIC data by increasing the standard deviation errors, which are generally proportional to the size of the time and distance mismatch within the collocation window of 6 h and 250 km considered. Globally, in the troposphere (850-200 hPa), the collocation mismatch impacts on the comparison standard deviation errors for temperature are 0.35 K per 3 h and 0.42 K per 100 km and, for relative humidity, are 3.3% per 3 h and 3.1% per 100 km, indicating an approximate equivalence of 3 h to 100 km in terms of mismatch impact.

[1] This paper investigates the global ionospheric response during the January 2009 stratospheric sudden warming (SSW) event by using electron density profiles derived from GPS radio occultation measurements of the COSMIC satellites. The peak density (NmF2), peak height (hmF2), and ionospheric total electron content (ITEC) increase in the morning hours and decrease in the afternoon globally for 75% of the cases, in which electron density profiles during SSW and non-SSW days are available around the same location and local time bins. NmF2, hmF2, and ITEC during SSW days, on average, increase 19%, 12 km, and 17% in the morning and decrease 23%, 19 km, and 25% in the afternoon, respectively, in comparison with those during non-SSW days from global COSMIC observations. These results agree well with previous results from total electron content observations in low-latitude and equatorial regions. Interestingly, the unique COSMIC observations also revealed that during this SSW event the ionosphere responds globally, not only in the equatorial regions but also at the high and middle latitudes. The highlatitude ionosphere shows increased NmF2 and ITEC and decreased hmF2 in either the morning or afternoon sector. Thus, these results indicate that the ionospheric response in low-middle latitude and equatorial regions during SSW can be explained by either the modulated vertical drift resulting from the interaction between the planetary waves and tides through E region dynamo or the possible direct propagation of tides from the lower atmosphere, whereas the ionospheric variations at the middle and high latitude during the SSW might be attributed to the neutral background changes due to the direct propagation of tides from the lower atmosphere to the ionospheric F2 region. The competitive effects of different physical processes, such as the electric field, neutral wind, and composition, might cause the complex features of ionospheric variations during this SSW event.

With the increased number of low Earth orbit (LEO) satellites equipped with GPS receivers, LEO based GPS observations play a more important role in space weather research because of better global coverage and higher vertical resolution. GPS slant total electron content (TEC) is one of the most important space weather products. In this paper, the LEO based slant TEC derivation method and the main error sources, including the multipath calibration, the leveling of phase to the pseudorange TEC, and the differential code bias (DCB) estimation, are described systematically. It is found that the DCB estimation method based on the spherical symmetry ionosphere assumption can obtain reasonable results by analyzing data from multiple LEO missions. The accuracy of the slant TEC might be enhanced if the temperature dependency of DCB estimation is considered. The calculated slant TEC is validated through comparison with empirical models and analyzing the TEC difference of COSMIC colocated clustered observations during the initial stage. Quantitatively, the accuracy of the LEO slant TEC can be estimated at 1–3 tecu, depending on the mission. Possible use of the LEO GPS data in ionosphere and plasmasphere is discussed.

Global Positioning System (GPS) radio occultation (RO) has provided continuous observations of the Earth's atmosphere since 2001 with global coverage, all-weather capability, and high accuracy and vertical resolution in the upper troposphere and lower stratosphere (UTLS). Precise time measurements enable long-term stability but careful processing is needed. Here we provide climate-oriented atmospheric scientists with multicenter-based results on the long-term stability of RO climatological fields for trend studies. We quantify the structural uncertainty of atmospheric trends estimated from the RO record, which arises from current processing schemes of six international RO processing centers, DMI Copenhagen, EUM Darmstadt, GFZ Potsdam, JPL Pasadena, UCAR Boulder, and WEGC Graz. Monthly-mean zonal-mean fields of bending angle, refractivity, dry pressure, dry geopotential height, and dry temperature from the CHAMP mission are compared for September 2001 to September 2008. We find that structural uncertainty is lowest in the tropics and mid-latitudes (50° S to 50° N) from 8 km to 25 km for all inspected RO variables. In this region, the structural uncertainty in trends over 7 yr is <0.03% for bending angle, refractivity, and pressure, <3 m for geopotential height of pressure levels, and <0.06 K for temperature; low enough for detecting a climate change signal within about a decade. Larger structural uncertainty above about 25 km and at high latitudes is attributable to differences in the processing schemes, which undergo continuous improvements. Though current use of RO for reliable climate trend assessment is bound to 50° S to 50° N, our results show that quality, consistency, and reproducibility are favorable in the UTLS for the establishment of a climate benchmark record

A new type of radio occultation (RO) data, recorded in open‐loop (OL) mode from the SAC‐C satellite, has been tested for monitoring refractivity in the Atmospheric Boundary Layer (ABL). Previously available RO signals, recorded in phase‐locked loop mode were often unusable for sensing the lower troposphere (LT) or resulted in significant inversion errors, especially in the tropics. The OL RO signals allow sensing of the LT and accurate monitoring of the ABL and, especially, its depth. Comparison of RO‐inverted refractivity profiles to ECMWF analysis and available radiosondes generally shows good agreement in the depth of the ABL. However, in a number of cases, ECMWF fails to reproduce the top of ABL. Future OL RO signals will provide information about the ABL depth which is an important parameter for weather prediction and climate monitoring.