  Faraday rotation measurements using the current and next generation of
low-frequency radio telescopes will provide a powerful probe of astronomical
magnetic fields. However, achieving the full potential of these measurements
requires accurate removal of the time-variable ionospheric Faraday rotation
contribution. We present ionFR, a code that calculates the amount of
ionospheric Faraday rotation for a specific epoch, geographic location, and
line-of-sight. ionFR uses a number of publicly available, GPS-derived total
electron content maps and the most recent release of the International
Geomagnetic Reference Field. We describe applications of this code for the
calibration of radio polarimetric observations, and demonstrate the high
accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar
observations. These show that we can accurately determine some of the
highest-precision pulsar rotation measures ever achieved. Precision rotation
measures can be used to monitor rotation measure variations - either intrinsic
or due to the changing line-of-sight through the interstellar medium. This
calibration is particularly important for nearby sources, where the ionosphere
can contribute a significant fraction of the observed rotation measure. We also
discuss planned improvements to ionFR, as well as the importance of ionospheric
Faraday rotation calibration for the emerging generation of low-frequency radio
telescopes, such as the SKA and its pathfinders.
