The US Total Electron Content (US-TEC) product, which evolved through a collaboration between the Space Weather Prediction Center (SWPC), the National Geodetic Survey (NGS), the National Geophysical Data Center ( NGDC), and the Global Systems Division (GSD), is designed to specify Vertical and Slant TEC over the Continental US (CONUS) in near real-time. The product uses a Kalman Filter data assimilation model, described in the Technical Documentation. This technique is driven by data from ground-based Global Positioning System (GPS) dual frequency receivers. The primary data stream comes from the Maritime and Nationwide Differential GPS (M/NDGPS) real time network of stations operated by the US Coast Guard (USCG). Secondary data streams are provided by the GPS/Met network (meteorological application of GPS data) and the IGS (International GNSS Service) network. Currently, there are about 80 CORS, 30 GPS/Met, and 15 IGS stations ingested into the model. This number has been gradually increasing and will be augmented by Federal Aviation Administration/Wide Area Augmentation System (FAA/WAAS) data, etc.
In order to deliver a reliable product, SWPC has designed and implemented dual parallel processing for US-TEC. This scheme includes independent data ingest, CORS-A for one branch, CORS-B for the other. RTIGS and GPS/MET data are common for both. These data are controlled by independent, in-house FTP machines, and are pulled by the two independent processing machines. The final product is selected, after a quality-control evaluation, from the two outputs.
The US-TEC products cover the Continental US (CONUS), and include:
The main display is the map of Vertical TEC over the CONUS for a given 15 minute interval in TEC units (1 TECU=1016 electrons m-2). New maps are normally available about 13 minutes after a given interval, and are updated every 15 minutes. The contour interval is variable, but is typically about 1 TEC unit up to 20 TEC, and gradually increases for values above 20 TEC units. The map also displays the GPS sites used in the current assimilation cycle, using different symbols for each network. The total number of sites is displayed below the map.
The other displays are the TEC uncertainty and the Recent trend. The TEC uncertainty map is based on a combination of the Kalman filter state uncertainty, estimates of accuracy from the Validation analysis, and 5% of the trend. The latter contribution to the uncertainty estimate is designed to warn users during storm events, when the TEC can rise many times over its normal quiet time value, steep gradients can exist, and the uncertainty is likely to grow. As more GPS sites are included in the assimilation scheme the uncertainty is likely to decrease. Part of the uncertainty estimation is fully described in the Validation Documentation. The contour interval for the uncertainty map is 0.1 TEC units. The Recent trend displays the difference between the current map and an average over the previous ten days, for the particular Universal Time (UT) interval. The contour interval for the trend display is 3 TEC units. For comparison, all three displays can be seen together from the All Plots tag. Links to animations of the current day and the full 24-hour of the previous day can be found below each figure.
The display and data files cover regions outside the CONUS where no data are available. The uncertainty in the TEC is expected to increase in these areas. Users are advised to only use the values within the CONUS, and only when sufficient CORS data are available to provide a reliable estimate of TEC. Several warning flags have been built into the display. If data are unavailable for up to an hour, a banner will warn users that no data have been used in the assimilation cycle. If no data have been available for greater than one hour, a statement with the time the input data have been missing and the latest TEC map are displayed.
The product is designed for single and dual frequency GPS applications. The TEC and slant path maps can be used to estimate the GPS signal delay due to the ionospheric electron content between a receiver and a GPS satellite. Of particular interest to users is the link to Data files which provides not only tabular values of the total electron content, but also values of line-of-sight (LOS) or slant path electron content to each GPS satellite in view over the CONUS. For each GPS satellite in view the LOS maps provide 1°x1° two-dimensional grids over the CONUS.
The full description of the Kalman filter process is provided in the Technical Documentation. The model calculates receiver biases, but uses satellite biases from the University of Bern (http://www.aiub.unibe.ch/download/). Some statistics on the reliability of the GPS data feed and on the accuracy of the TEC estimates can be found on the Validation link, together with the referenced papers on it. Note that the model has not performed in real-time through many extreme storm events so the estimates of uncertainty should be treated with caution during large events. The link " Storm Animations " illustrates the performance of the model during several storms, including the Halloween storm of Oct 29-30, 2003. The animations illustrate the extreme TEC values and large gradients that are possible during geomagnetic storms.
The basic data ingested by USTEC are ground-based GPS receiver data from three different sources, CORS (Continuously Operating Reference Stations, http://www.ngs.noaa.gov/CORS/cors-data.html), CORS/MET (Ground-Based GPS Meteorology, http://gpsmet.noaa.gov/jsp/index.jsp), and RTIGS (Real Time International GNSS Service, http://igscb.jpl.nasa.gov/). The system also requires other data for the processing; the satellite coordinates are obtained from SP3 files from JPL (http://igscb.jpl.nasa.gov/igscb/product/), the satellite biases are downloaded from the University of Bern (http://www.aiub.unibe.ch/download/), and the RZ12 solar index is obtained from the NASA IRIís site (ftp://nssdcftp.gsfc.nasa.gov/models/ionospheric/iri/iri2001/fortran_code/).
The link to Data files provides the Daily Empirical Orthonormal Functions (EOFs), the Coefficients for the EOFs, the tabular values of vertical TEC and slant path TEC, TEC Uncertainty, Recent Trend ( expressed as the difference between the current TEC and the previous 10-day average), and the list of stations used for the assimilation for each time interval. The detailed instructions to use these data and the description of each data format is at Data Files Format.
US-TEC web page general information
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