  We present a new global study of the millimeter (mm) wave, submillimeter
(sub-mm) wave, and terahertz (THz) spectra of the lowest three torsional states
of methyl mercaptan (CH$_3$SH). New measurements have been carried out between
50 and 510 GHz using the Kharkiv mm wave and the Cologne sub-mm wave
spectrometers whereas THz spectra records were used from our previous study.
The new data, involving torsion-rotation transitions with $J$ up to 61 and
$K_a$ up to 18, were combined with previously published measurements and fit
using the rho-axis-method torsion-rotation Hamiltonian. The final fit used 124
parameters to give an overall weighted root-mean-square deviation of 0.72 for
the dataset consisting of 6965 microwave (MW) and 16345 far-infrared line
frequencies sampling transitions within and between the ground, first, and
second excited torsional states. This investigation presents a two-fold
expansion in the $J$ quantum numbers and a significant improvement in the fit
quality, especially for the MW part of the data, thus allowing us to provide
more reliable predictions to support astronomical observations.
