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Note: These comparisons serve as working tools for all participants. Software used in each case is in continuous development and is subject to various tests for the ultimate purpose of achieving more accurate results. Do not be surprised if sometimes you see such tests underway.
Global Ionospheric Maps (GIM) are generated hourly and daily at JPL using data from up to 100 GPS sites of the IGS and other institutions. The vertical TEC is modeled in a solar-geomagnetic reference frame using bi-cubic splines on a spherical grid. A Kalman filter is used to solve simultaneously for instrumental biases and vertical TEC on the grid (as stochastic parameters). Contact address: email@example.com.
The Dynasonde estimate of TEC used for this and other comparisons is obtained by integration of NeXtYZ's vertical N(h) profile up to the F region peak, supplemented by the TEC of a "Chapman Topside" which is defined by the observed peak height, maximum electron density, and scale height. Physics-based justification for the Chapman form is given in references [1,2] of the Bibliography. Use of a constant scale height above the peak surely underestimates the topside TEC. Also, the estimate does not include the plasmaspheric component of TEC (approximately 5 TECU).
These results are from an operational system "IonoNumerics", being developed by the Fusion Numerics Inc and sponsored by the US Air Force, for generating and distributing near real-time three-dimensional ionospheric electron densities and corresponding GPS propagation delays. The system adapts technologies developed and routinely used for operational weather forecasting in order to nowcast and forecast ionospheric conditions. It consists of two parts: a first-principles numerical model of the ionosphere and a data assimilation component. The core ionospheric model solves plasma dynamics and composition equations governing evolution of density, velocity and temperature for 7 ion species on a fixed global three-dimensional grid in magnetic coordinates. It uses a realistic model of the Earth's magnetic field and solar indeces obtained in real time from the NOAA Space Environment Center. While the core model is capable of delivering realistic results, its accuracy can be significantly improved by employing a special set of numerical techniques known as data assimilation. These techniques originated in and are currently used for numerical weather forecasting. The core ionospheric model is continuously fed data from a network of reference GPS ground stations. This improves both the nowcast and the forecast. Web-based access to the system is provided at: fusionnumerics.com/ionosphere.
The International Reference Ionosphere (IRI) is an international project sponsored by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). These organizations formed a Working Group in the late sixties to produce an empirical standard model of the ionosphere, based on all available data sources. For given location, time and date, IRI describes the electron density, electron temperature, ion temperature, and ion composition in the altitude range from about 50 km to about 2000 km; and also the electron content. The major data sources are the worldwide network of ionosondes, powerful incoherent scatter radars (Jicamarca, Arecibo, Millstone Hill, Malvern, St. Santin), the ISIS and Alouette topside sounders, and in situ instruments on several satellites and rockets. More information can be found at IRI's official Web site. Results presented here are obtained using the software package distributed by the National Space Science Data Center (NSSDC). Details of its implementation may be found here.