  The rotational spectra of the two stable conformers syn- and gauche-propanal
(CH$_3$CH$_2$CHO) were studied in the millimeter and submillimeter wave regions
from 75 to 500 GHz with the Cologne (Sub-)Millimeter wave Spectrometer.
Furthermore, the first excited states associated with the aldehyde torsion and
with the methyl torsion, respectively, of the $syn$-conformer were analyzed.
The newly obtained spectroscopic parameters yield better predictions, thus
fulfill sensitivity and resolution requirements in new astronomical
observations in order to unambiguously assign pure rotational transitions of
propanal. This is demonstrated on a radio astronomical spectrum from the
Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line
Survey (ALMA-PILS). In particular, an accurate description of observed
splittings, caused by internal rotation of the methyl group in the
$syn$-conformer and by tunneling rotation interaction from two stable
degenerate $gauche$-conformers, is reported. The rotational spectrum of
propanal is of additional interest because of its two large amplitude motions
pertaining to the methyl and the aldehyde group, respectively.
