GPS radio occultation (GPS-RO) is a method of probing the atmosphere using GPS signals. Such measurements can be assimilated into operational numerical weather prediction (NWP) systems and can reduce stratospheric temperature biases. This indicates that they could have an important role in climate monitoring and climate reanalyses. However, the measurements must be corrected to account for the effects of the ionosphere and this paper proposes an enhanced correction that removes residual biases.