Ionospheric total electron content (TEC) studies with GPS in the equatorial region

This paper essentially deals with the effects of equatorial ionization anomaly gradient on space-based navigation systems like GPS. The equatorial region of the ionosphere, which extends about ±30dip about the magnetic equator, is characterized by a steep latitudinal gradient, not only in the maximum ionization but also in the total electron content (TEC), through a major part of the day. This region also accounts for about one-third of the global electron content. The high ambient TEC results in large range errors for a major part of the day, affecting navigation and position-fixing using GPS. The gradient of the equatorial ionization anomaly between the trough and the crest is very sharp, which results in large temporal and spatial variation of the ionospheric electron content. A prediction of the range error introduced by the ionosphere in the equatorial zone is very difficult. Identification of a suitable ionospheric model for prediction of these errors in the geophysically sensitive equatorial region is necessary prior to the introduction of Indian SBAS network, GAGAN (GPS And Geo Augmented Navigation). For this purpose, ionospheric TEC measured from Calcutta, situated underneath the northern crest of the equatorial anomaly, has been compared with values generated by models like PIM1.6 and IRI-95 during 1977-1990. The equatorial anomaly gradient not only extends in the horizontal direction but with altitude also. Problems related to conversion of vertical to slant TEC and vice versa, as required for ionospheric range error corrections in satellite-based navigation with GPS, have been indicated and diagnostics suggested. It has been observed that sharp latitudinal gradient of TEC during the afternoon hours of equinoctial months of high sunspot number years is usually followed by generation of irregularities over the magnetic equator in the form of ‘bubbles’ or depletions. These depletions have sharp edges resulting in large range error rates on GPS links. Characteristics of bubbles, namely, amplitude and leading and trailing edge slopes, have been studied using GPS TEC data recorded at the Giant Meterwave Radio Telescope (GMRT) site during the vernal equinox of 2004. Use of GPS TEC measurements as a tool for studying ionospheric response to earthquakes has also been indicated.