  Astronomical observations in the molecule rich 3 mm window using large
reflector antennas provide a unique view of the Universe. To efficiently carry
out these observations gravitational and thermal deformations have to be
corrected. Terrestrial laser scanners have been used to measure the
deformations in large reflector antennas due to gravity, but have not yet been
used for measuring thermal deformations. In this work we investigate the use of
a terrestrial laser scanner to measure thermal deformations on the primary
reflector of the Green Bank Telescope (GBT). Our method involves the use of
differential measurements to reduce the systematic effects of the terrestrial
laser scanner. We use the active surface of the primary reflector of the GBT to
validate our method and explore its limitations. We find that when using
differential measurements it is possible to accurately measure deformations
corresponding to different Zernike polynomials down to an amplitude of 60
$\mu$m. The difference between the amplitudes of known deformations and those
measured are $<140~\mu$m when the wind speed is $\lesssim2$ m s$^{-1}$. From
these differences we estimate that it should be possible to bring the surface
error of the GBT down to $240\pm6~\mu$m. This suggests that using a commercial
off-the-shelf terrestrial laser scanner it is possible to measure deformations
induced by thermal gradients on a large parabolic reflector.
