GPS Solut (2005) 9: 63–66 DOI 10.1007/s10291-004-0129-z
Zhizhao Liu Susan Skone Yang Gao Attila Komjathy
GPS ON THE WEB
Ionospheric modeling using GPS data
Received: 16 December 2004 Accepted: 20 December 2004 Published online: 10 February 2005 Ó Springer-Verlag 2005 This column provides the web-based global positioning system (GPS) resources and their technical background information. Its purpose is to inform the reader about the data, software and electronic documents that are available on-line. This column is coordinated by Dr. Jinling Wang, The University of New South Wales, Sydney. Comments and suggestions are appreciated (
[email protected]). In this issue, the GPS researchers, from the University of Calgary and JPL, introduce the resources on ionospheric modeling from several websites. Z. Liu (&) Æ S. Skone Æ Y. Gao Department of Geomatics Engineering, The University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada, T2N 1N4 E-mail:
[email protected] Tel.: +1-403-2204916 Fax: +1-403-2841980 A. Komjathy Jet Propulsion Laboratory, NASA, 4800 Oak Grove Drive, M/S 238-634A, Pasadena, CA 91109, USA
the GPS positioning accuracy. The abundance of GPS measurements from worldwide-distributed GPS reference networks, which provide 24-h uninterrupted operational services to record dual-frequency GPS measurements, provides an ideal data source for ionospheric modeling research. The measurements from dual-frequency GPS receivers allow the users to precisely determine the magnitude of ionospheric delay at their location. Due to high spatial variability of the ionosphere, the ionospheric delay determined at one location cannot directly be used for ionospheric correction at another location, where a single frequency receiver is deployed. However, the goal of providing correction to single frequency users can be achieved by employing a given mathematical model to describe the ionosphere using GPS data as model observations; this data can be derived from a number of dual-frequency GPS receivers within a GPS network. In the past decade, a large number of GPS reference networks have been deployed worldwide. Therefore, the GPS network facilities provide ionospheric model researchers an excellent data source, allowing the researchers to test, analyze and validate their ionospheric models with extensive GPS data sets. Many efforts of ionospheric model studies have been invested in developing innovative mathematical approaches, to produce better modeling performance and to generate near real-time ionospheric updates.
Ionospheric modeling overview JPL ionospheric and atmospheric remote sensing Ionospheric modeling using Global Positioning System (GPS) data is the focus of extensive efforts within the GPS community. The range error caused by ionospheric delay in GPS signals is currently the largest component that affects the accuracy of positioning and navigation determination using single frequency GPS measurements. Ionospheric modeling is an effective approach for correcting the ionospheric range error and improving
http://iono.jpl.nasa.gov/ The ionospheric and atmospheric remote sensing group at Jet Propulsion Laboratory (JPL) uses GPS measurements from globally distributed ground stations, to generate in real-time global maps of ionospheric total electron content (TEC). Ionospheric modeling is based
64
on an assumption of a thin single ionospheric shell assumed to be located 350-km above the Earth’s surface. These global maps are updated at a 5-min interval and they can be used to provide accurate ionospheric calibrations to single frequency GPS users. Moreover, these real-time maps can be employed to monitor ionospheric space weather, particularly ionospheric storms. Since 1999 another global ionospheric model, the global assimilative ionospheric model (GAIM) has been under development at JPL and University of Southern California as well. This three-dimensional, time-dependent model is constructed on the basis of combining first-principle ionospheric physics and optimization techniques, which allows the assimilation of various types of ionospheric measurements. Ionospheric data ingested into GAIM include TEC data from both ground-based and space-borne GPS receivers, satellite UV limb scans and ionosonde, etc. Recently, a new GAIM development has been undertaken assimilating GPS data available in real-time. The real-time and postprocessed GAIM model is routinely validated using TOPEX and Jason TEC measurements. Most recently, a new algorithm has been developed at JPL, to routinely estimate vertical total electron content (VTEC) on a daily basis using all available GPS receivers worldwide. Every day, JPL has access to more than 1,000 GPS receivers supplied by global and regional GPS networks, to estimate the spatial and temporal variability of the global ionosphere. The JPL group is now processing all 1,000+ GPS receivers on a daily basis. Current products include global or regional vertical electron content maps, TEC movies and satellite and receiver differential biases that are made available to the public upon request. The daily results of processing the 1,000+ stations are stored at ftp://sideshow.jpl.nasa.gov/pub/axk/allsites/.
Global ionospheric maps at CODE http://www.cx.unibe.ch/aiub/ionosphere.html The Center for Orbit Determination in Europe (CODE) employs a spherical harmonics functions to model the global vertical total electron content (VTEC), at an interval of 2 h, using about 200 GPS/GLONASS worldwide stations. CODE provides two types of ionospheric products. One product is Klobuchar-style ionospheric coefficients, where eight coefficients are estimated daily and used to model ionospheric delay in latitude and local time for global single frequency users. Another product is global ionospheric maps (GIM), which are generated at 2-h intervals and 13 snapshots are available to users each day. The GIM are usually provided in two formats: IONosphere Map EXchange
format (IONEX) (Schaer et al. 1998) and Bernese ION format. The map file in IONEX format can be directly employed at user locations to estimate TEC values for a given satellite-receiver line-of-sight via an interpolation method, while the latter format is specifically for users of Bernese software. The final versions of GIM data files and Klobuchar-style coefficients usually have a latency of 3 days; rapid versions with only 12-h delay are also available to users. In addition, predicted versions of the IONEX files are also produced at CODE. The prediction is performed at two intervals: 1 day and 2 day. All the ionospheric files can be downloaded from http:// www.aiub.unibe.ch/download/CODE/YYYY, where YYYY denotes the year of the product.
IGS ionosphere working group http://gage152.upc.es/ionex3/igs_iono/ The IGS (International GPS Service) ionosphere working group (Iono-WG) was established in May 1998, to produce ionospheric VTEC maps as one of the IGS products for the GPS community. The objective of this working group is to employ the IGS GPS reference network and its infrastructure, to derive global ionosphere maps and an IGS ionosphere model. Currently five IGS Ionosphere Associate Analysis Centers (IAACs) operated at different agencies contribute their ionosphere products, which are computed using different approaches, to the Iono-WG, to generate the final IGS combination product. Both the VTEC maps computed at five IAACs and the final combined IGS VTEC map products have a temporal resolution of 2 h and a spatial resolution of 5° in longitude and 2.5° in latitude. Since first of April 2003, official IGS Global TEC maps (IGTEC) in IONEX format have become an official IGS product, which are uploaded with a delay of about 11 days to ftp://cddisa.gsfc.nasa.gov/gps/products/ionex/ for public downloading. Meanwhile, a rapid version of IGTEC is also available to the public since December 2003 with a delay of less than 24 h. In order to evaluate the performance of the IGS ionosphere TEC map, a thorough examination and comparison was performed using 3 months of data. The IGTEC data were compared to TEC data inferred from the JASON mission and an agreement of about 5.0 TECU was found. The details of the comparison result can be found in the report http://maite152.upc.es/ionex3/doc/IGS_IONO_report_April2003_7.pdf. The future goals of IGS working group include further improving the accuracies of IGTEC products, enhancing product temporal and spatial resolution, as well as validating ionospheric maps with more external data.
65
NOAA real-time US total electron content http://www.ngdc.noaa.gov/stp/IONO/USTEC/ home.html In November 2004 new ionospheric products, which include real-time slant and vertical TEC for the continental US (CONUS), were developed at the US National Oceanic and Atmospheric Administration (NOAA) Space Environment Center. This effort is conducted in collaboration with the National Geodetic Survey, National Geophysical Data Center, and the University of Colorado. The ionospheric products include maps of vertical TEC over CONUS, estimate uncertainty, recent trends based on the past 10 days of TEC information, and ASCII data files for both vertical and slant TEC. The ionospheric products are computed by a Kalman filter-based data assimilation algorithm called ‘‘MAGIC’’ (Spencer et al. 2004). The current ionospheric products are generated using approximately 60 real-time GPS sites from continuously operating reference stations (CORS) and the products are calculated near real-time with a sample interval of 15 min. At present, the project products are experimental and under review through March 2005. The real-time TEC products can be accessed via http://www.sec.noaa.gov/ustec/ and historical data can be accessed via http://www.ngdc.noaa.gov/stp/ IONO/USTEC/products/2004/.
Fusion Numerics’ IonoNumerics http://63.231.68.133/ionosphere/DesktopDefault.aspx Funded by the US Air Force Research Laboratory, a global numerical ionospheric forecasting system, IonoNumerics, has been developed by Fusion Numerics Inc. (Boulder, CO, USA). This system is built on the basis of integrating two components. One is a first principles physics-based model of the ionosphere that is constructed in a magnetic coordinate system; the other is a module assimilating real-time ionospheric observations. The ultimate objective is to use such a system to nowcast and forecast electron densities in the Earth’s ionosphere, which may be used for GPS navigation applications, among others. Currently, the IonoNumerics model computes ion and electron densities at a grid of more
than 1-million points and the grid point resolution is expected to increase in the future. Such a large number of grid points ensure that the model-specified information of ion and electron densities can be sufficiently accurate. The model uses real-time solar activity data from the NOAA’s Space Environment Center to solve energy, momentum, and mass conservation equations governing distribution of the ionospheric plasma. The other data source used by IonoNumerics is a network of GPS reference ground stations. The GPS observations are real-time assimilated into the model to adjust the ionospheric model calculation. The result from the overall system is global three-dimensional distribution of electron densities, which allows the direct calculation of slant total electron content from any location on the ground or space to all visible GPS satellites. The IonoNumerics also has the capability to produce short-term forecasts of global electron densities and the likelihood of occurrence of scintillation.
Concluding remarks Due to its highly temporal and spatial variability of the ionosphere, ionospheric modeling will continue to be a research focus within the GPS and space science communities. Generally, several characteristics in the near future development trend can be summarized. First, more ionospheric models pay attention to assimilate different types of ionospheric data, such as radio occultation measurements, into their systems for modeling and validation, in addition to the popularly used TEC data derived from ground-based GPS stations. Second, real-time data acquisition and modeling is another distinct development compared to the past postprocessing mode. This allows the users to monitor the ionospheric variations and employ the modeling results in a timely fashion. Thirdly, more modeling systems are developing forecast capability to support the researches and applications in other disciplines such as space science. With the availability of additional signals from the future GPS modernization and European Galileo system, the ionospheric modeling research will be further augmented and model performance is expected to be significantly enhanced subsequently.
66
References Schaer S, Gurtner W, Feltens J (1998) IONEX: The IONosphere Map EXchange Format Version 1, Proceedings of the 1998 IGS Analysis Centers Workshop, ESOC, Darmstadt, Germany, February 9–11, 1998
Spencer PSJ, Robertson DS, Mader GL (2004) Ionospheric Data Assimilation Methods for Geodetic Applications, Proceedings of IEEE PLANS 2004, Monterey, California, April 26–29, 2004, 510–517
