  Observations of ammonia in interstellar environments have revealed high
levels of deuteration, and all its D-containing variants, including ND$_3$,
have been detected in cold prestellar cores and around young protostars. The
observation of these deuterated isotopologues is very useful to elucidate the
chemical and physical processes taking place during the very early stages of
star formation, as the abundance of deuterated molecules is highly enhanced in
dense and cold gas. Nitrogen hydride radicals are key species lying at the very
beginning of the reaction pathway leading to the formation of NH$_3$ and
organic molecules of pre-biotic interest, but relatively little information is
known about their D-bearing isotopologues. To date, only ND has been detected
in the interstellar gas. To aid the identification of further deuterated
nitrogen radicals, we have thoroughly re-investigated the rotational spectrum
of NHD employing two different instruments: a frequency-modulation
submillimetre spectrometer operating in the THz region and a synchrotron-based
Fourier Transform infrared spectrometer operating in the 50-240 cm$^{-1}$
wavelength range. NHD was produced in a plasma of NH$_3$ and D$_2$. A wide
range of rotational energy levels has been probed thanks to the observation of
high $N$ (up to 15) and high $K_a$ (up to 9) transitions. A global analysis
including our new data and those already available in the literature has
provided a comprehensive set of very accurate spectroscopic parameters. A
highly reliable line catalogue has been generated to assist archival data
searches and future astronomical observations of NHD at submillimetre and THz
regimes.
