Ionospheric Delay Forecast Using GNSS Data

M. Cueto, E. Sardon, A. Cezon, F. Azpilicueta and C. Brunini

Abstract:	The ionosphere affects signals broadcast by Global Navigation Satellite Systems (GNSS) by delaying the propagation of the code carried by the signal. In general ionospheric effects in mid-latitude regions are not severe, causing only gradual variations in ionospheric delays (except in the presence of magnetic storms) whose magnitude can be roughly predicted. However in the equatorial region ionospheric effects are more severe: added to the higher ionospheric delays in this area, the presence of the equatorial anomaly and other equatorial features complicate the ionospheric modeling and make more difficult the prediction of its main features and its effects on GNSS and other applications that depends on the ionospheric status. Nowadays ionospheric algorithms are mainly focused on post-processing and now cast estimation. However, an ionospheric predicting tool could be very useful for several applications, above all for unstable conditions and for the equatorial region: an algorithm capable of predicting the ionospheric behavior in advance could be used to set up early warnings for different uses, among others civil aviation based on Ground Based Augmentation Systems (GBAS) or Satellite Based Augmentation System (SBAS), the protection of valuable communication satellites from space weather adverse conditions, as well as other technologies affected by space weather, including geophysical exploration and protection of long distance pipelines, HF radio systems, communication and surveillance systems, spacecraft operations, defense needs and alarm systems for safety applications. Taking into account the aforementioned need, a new ionospheric forecasting tool based on the use of GNSS measurements has been developed. This tool provides predicted ionospheric delays for a set of Ionospheric Grid Point (IGP) located in the service area defined by the user. The ionospheric forecast algorithm is based on magicSBAS tool, which is an innovative tool developed by GMV which computes SBAS corrections and additional information required by a SBAS system in real-time to be broadcast to SBAS users. magicSBASimplements multi-constellation (GPS, GLONASS) state-of-the-art algorithms for precise orbit determination and time synchronization, ionospheric delay estimation, SBAS wide area correction computation and SBAS integrity determination. The tool uses as input GNSS raw data in several formats (such as Networked Transport of RTCM via Internet Protocol, NTRIP, Receiver INdependent Exchange, RINEX and European Geostationary Navigation Overlay Service, EGNOS) and provides as output SBAS information compliant with SBAS international standards (International Civil Aviation Organization Standards and Recommended Practices and Radio Technical Commission for Aeronautics DO-229D Minimum Operational Performance Standards) in two ways, as SBAS binary messages for GEO (Geostationary Earth Orbit) broadcast and as SBAS signal-in-space through Internet (SISNET). The forecast algorithm used for ionospheric delays prediction is based on the ionospheric delay estimation from previous epochs using GNSS data and the main dependence of ionospheric delays on solar and magnetic conditions. On account of the fact that the ionospheric behavior is highly dependent on the region of the Earth, different algorithmic modifications have been implemented in GMV´s magicSBAS ionospheric algorithms to be able to estimate and forecast ionospheric delays worldwide, adapting the ionospheric algorithms to the ionospheric characteristics at different latitudinal regions. Simulated data provided by a GNSS End-to-end simulator developed by GMV have been used to check the performances of the ionospheric delay prediction tool. The predicted ionospheric delays for different forecast periods have been compared with the ionospheric delay values estimated by magicSBAS for those predicted epochs. This paper shows how the new ionospheric delay forecasting tool is able to provide very good forecasting results for middle latitudes, and even for those equatorial latitudes where the ionosphere is much more complicated the results obtained are quite encouraging. Forecast periods from 1/2 hour to few hours are provided, showing that the prediction periods for which adequate forecast ionospheric results are achieved at middle latitudes are longer than those for equatorial latitudes, due to the higher (and less predictable) spatial and temporal variability in this region. Several examples of comparisons between predicted and estimated ionospheric delays for different locations using Global Navigation Satellite System data for different latitudinal regions and space weather conditions are also provided in this paper.
Published in:	Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011) September 20 - 23, 2011 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	634 - 642
Cite this article:	Cueto, M., Sardon, E., Cezon, A., Azpilicueta, F., Brunini, C., "Ionospheric Delay Forecast Using GNSS Data," Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 634-642.
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