Near real-time monitoring of the ionosphere using dual frequency GPS data in a Kalman filter approach

The ionosphere is an important source of errors for the GPS signals that travel through the ionosphere on their way to the ground-based receivers by introducing a frequency dependent path delay proportional to the total electron content (TEC) along the signal path. For dual-frequency GPS receivers, the ionospheric effects can be accounted for by taking advantage of the dispersive nature of the ionosphere in the microwave region of the electromagnetic spectrum, while for the single frequency GPS receivers the ionospheric effects can be minimized by modeling them using, for example, empirical or physics-based ionospheric models. On the other hand, the errors imposed by the ionosphere on the GPS signals can provide important temporal and spatial information about the electron density distribution in the ionosphere. Besides the ionospheric errors, there are some other sources of errors that can affect the GPS signals, such as the satellite and receiver instrumental biases, carrier phase ambiguities, multipath effects, clock errors, orbital errors, tropospheric errors, but which can be compensated for, estimated, or neglected depending on the particular application. In this paper, we are only concerned with the ionospheric effects on the GPS signals, and describe a Kalman filter-based algorithm for near real-time estimation of the line-of-sight and vertical ionospheric TEC and of the combined satellite and receiver instrumental biases, using data from dual-frequency GPS receivers.