  For a ground-based radio interferometer observing at low frequencies, the
ionosphere causes propagation delays and refraction of cosmic radio waves which
result in phase errors in the received signal. These phase errors can be
corrected using a calibration method that assumes a two-dimensional phase
screen at a fixed altitude above the surface of the Earth, known as the
thin-layer model. Here we investigate the validity of the thin-layer model and
provide a simple equation with which users can check when this approximation
can be applied to observations for varying time of day, zenith angle,
interferometer latitude, baseline length, ionospheric electron content and
observing frequency.
