GNSS Radio Occultation Technique and Space Weather Monitoring

X. Yue, W.S. Schreiner, Y-H. Kuo, D.C. Hunt, C. Rocken

Abstract:	Since the pioneer GPS/MET mission, low Earth orbit (LEO) based GNSS Radio Occultation (RO) technique has been a powerful technique in ionosphere monitoring. After that, many LEO satellites were launched with RO payload, include: CHAMP, GRACE, SAC-C, COSMIC, C/NOFS, Metop-A, TerraSAR-X, and etc. COSMIC was the first constellation of satellites dedicated primarily to RO and delivering RO data in near-real-time. Currently in University Corporation for Atmospheric Research (UCAR) COSMIC data analysis and archive center (CDAAC), we process all these missions’ RO data for the community. As for space weather, we provide the following products: slant total electron content (TEC) along GNSS ray, Abel retrieved electron density profile (EDP) of each occultation, and amplitude scintillation index (S4) along the GNSS ray. We will talk the following 2 aspects in the paper: 1: GNSS RO space weather data process and quality: (1) Overview of CDAAC data process for ionosphere: How the slant TEC, EDP, and S4 are processed; available data types and time interval for each satellite mission; data access; data volume. (2) Quantity evaluation of slant TEC quality: 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. (3) Quantity evaluation of Abel retrieved EDP quality: Comparison between CHAMP PLP observed and RO estimated orbit electron density on board CHAMP shows that RO estimation tends to overestimate the true orbit electron density by 10% averagely. The average relative deviation is ?20% and decreases slightly with the increase of the ionospheric peak height and the satellite orbit. It is larger at nighttime than daytime and peaks around sunrise time. 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. Abel inversion can reproduce the aurora region night time E region electron density enhancement well. It tends to underestimate electron density in the aurora region and overestimate it in both sides. 2: GNSS RO space weather monitoring examples: (1) Solar radio burst (SRB) effect on RO signals: An extreme SRB case during December 6 2006 were studied. The LEO RO signals show frequent loss of lock (LOL), simultaneous decrease and oscillations on L1 and L2 signal-to-noise ratio (SNR) globally during daytime, small scale oscillations of SNR, decreased successful retrieval percentage (SRP) for both ionospheric and atmospheric occultations during SRB occurrence. Either decreased data volume or data quality will influence weather prediction, climate study, and space weather monitoring by using RO data during SRB time. (2) Ionospheric response by COSMIC satellites during 2009 January Stratospheric Sudden Warming (SSW) event: 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 high-latitude 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.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2508 - 2522
Cite this article:	Yue, X., Schreiner, W.S., Kuo, Y-H., Hunt, D.C., Rocken, C., "GNSS Radio Occultation Technique and Space Weather Monitoring," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2508-2522.
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