Assessment of global ionosphere maps in view of ionospheric correction for coastal and inland altimetry: the case for average total electron content

Several low-Earth orbit (LEO) satellites are equipped with dual-frequency altimeters, theoretically scanning the entire ionosphere in the nadir direction. These two frequencies enable the determination of ionospheric delay and, thus, total electron content (TEC) below the satellite orbit. This information helps in altimetric range determination but is limited to sea and ocean areas. Therefore, global and local ionospheric models are needed for ionospheric corrections over coastal regions and lands. At the same time, altimetry-derived TEC is an important source of validation data for global navigation satellite system (GNSS)-TEC models over the oceans, where the number of GNSS stations is limited. This study compares the application of a high-resolution regional GNSS-TEC model determined from Precise Point Positioning and modeled by least-squares collocation (PPPLSC), and global ionosphere maps (GIMs), in the determination of ionospheric corrections along coastal altimetry tracks. The ionospheric delay values from 5 models are then compared with altimetry-derived TEC from 3 satellites, in the region of southeastern Asia, during a time of moderate TEC values and solar conditions. The reason for the choice of area is that altimetric observations from coastal zones meet difficulties related to atmospheric corrections, e.g., ionospheric correction, which can be affected by the land in the altimeter footprint. For this reason, along with the rapid progress of inland satellite hydrology, we are encouraged to study the consistency of ionospheric delays in coastal regions. The study shows overall discrepancies of 30% of the entire ionospheric delay, which is 2-3 cm even in the case of 35 TEC unit (TECU = 1016 el/m2) values. For this reason, in the case of increased solar activity, the GIMs can have even less TEC consistency with the altimetry-derived TEC, resulting from different orbital altitudes, data gaps, and modeling techniques. The GIMs, modeled by low-order spherical harmonics, have particularly low resolution and do not represent well the equatorial ionization anomaly (EIA).