Derivation of TEC and GPS hardware delay based on dual-frequency GPS observations

The ground-based GPS receivers and hence networks enable studies of the ionosphere on a large regional or global scale by measuring the ionospheric TEC from dual-frequency GPS observations. However, originated in the hardware delay of the GPS satellite and receivers, a systematic bias affects the accuracy of the TEC derivation greatly. For accurate estimation of TEC, satellite and receiver instrumental biases should be removed from TEC measurements properly. This paper gives a comprehensive review of the algorithms on TEC and GPS hardware delays derivations. Starting from ionospheric radio wave propagation, the paper describes how the TEC is related with hardware delays. Then the TEC derivation methods are analyzed, evaluated and categorized into 3 main types. Finally, some suggestions are provided for further research on derivation of TEC and hardware delay.