Parameters
s1 in, required structure	the data structure from the on or off source scan
s2 in, required structure	the data structure from the on or off source scan
tau in, optional float	tau at observation elevation
tsys in, optional float	tsys at observation elevation
ap_eff in, optional float	aperture efficiency
smthoff in, optional integer	smooth factor for reference spectrum
units in, optional string	takes the value 'Jy', 'Ta', or 'Ta*' same weight (1.0).
ret_tsys	.
ret_tau	.