The correction for the satellite-receiver longitude difference in ionospheric tomography

The technique of ionospheric tomography involves the inversion of a number of total electron content measurements to provide a two-dimensional image of electron density. In an experimental situation the initial data are oriented in an approximate plane between a longitudinal chain of ground-based receivers and the trajectory of a polar-orbiting satellite. The effect of the longitude difference between the polar-orbiting satellite and the receiver chain on the tomographic image is investigated in both an ionospheric-model and the experimental environment. Tomographic measurements recorded during an experimental campaign held in May 1995, together with independent electron density information from the EISCAT incoherent scatter radar, are used in the studies. Electron-density comparisons between a number of tomographic images and corresponding incoherent-scatter measurements allow an optimal longitudinal correction to be verified in an experimental situation. The results indicate that the longitudinal corrections can be made to high accuracy, provided that a reliable estimate of the mean ionospheric height is obtained. A preliminary result from the experimental campaign, showing the early-evening east-west electron-density gradient, demonstrates that ionospheric tomography has the potential to produce three-dimensional images from a number of receiver chains displaced in longitude.