  The ionosphere contributes time-varying Faraday Rotation (FR) to radio
signals passing through it. Correction for the effect of the ionosphere is
important for deriving magnetic field information from FR observations of
polarized cosmic radio sources, as well as providing valuable diagnostics of
the structure of the ionosphere. In this paper, we evaluate the accuracy of
models commonly used to correct for its effects using new observations of
pulsars at low frequencies, which provide total rotation measures (RM) at
better precision than previously available. We evaluate models of the
ionosphere derived from modern digital ionosondes that provide electron density
information as a function of height, as well as GPS-derived Total Electron
Content (TEC) measurements. We combine these density models with reference
global magnetic field models to derive ionospheric RM contributions. We find
that the models disagree substantially with each other and seek corrections
that may explain the differences in RM prediction. Additionally we compare
these models to global TEC models and find that local high-cadence TEC
measurements are superior to global models for ionospheric RM correction.
