Chapter 6 Multi-dimensional maps of the ionosphere

摘要

An autonomous Matlab-based software was developed for this study and further inte- grated. for computing twelve GIM per day with the same spatial and temporal resolution as IGS GIM, i.e. spatial resolution of 2.5° in latitude and 5° in longitude, and temporal reso- lution of 2h. To achieve this spatial resolution, VTEC is represented by spherical harmonic expansion of degree and order 15 (see Sect. 4.3.1). This leads to estimation of 256 coeffi- cients for every single two-hourly map. As already discussed in Sect. 4.3.1.3, IGS produces thirteen two-hourly maps par day with the reference epoch starting at 0 UT each day and ending at 0 UT next day. The IGS GIM corresponds to the results of the middle day of a 3-day combination analysis (Schaer, 1999). This solution requires processing observations over three consecutive days and solving for 37 times 256, or 9,472 VTEC parameters. Since our investigation is just experimental, to reduce the computational time, we shifted the ref- erence epoch from IGS reference epochs, i.e. {0,2,4,..., 24} to {1,3,5,..., 23}. Therefore, we could process the observations of only one day and to solve 12 times 256, or 3,072 VTEC parameters, to obtain the GIM for that day. In our solution, in addition to spherical harmonic coefficients, the daily values of the GNSS satellite and receiver DCB are also computed as a by-product. For this, a zero mean condition is imposed to the estimates of satellite bias, for defining the DCB datum. For more details please refer to Todorova (2008). Further- more, the corresponding RMS maps are developed as well. The final outputs are provided in the IONEX format (Schaer et al, 1998), in which the estimated VTEC values are presented globally in a grid in the Earth-fixed geomagnetic frame.