  Spin waves in the the rare earth orthorferrite YFeO$_3$ have been studied by
inelastic neutron scattering and analyzed with a full four-sublattice model
including contributions from both the weak ferromagnetic and hidden
antiferromagnetic orders. Antiferromagnetic (AFM) exchange interactions of $J_1
= -4.23 \pm 0.08$ (nearest-neighbors only) or $J_1 = -4.77 \pm 0.08$ meV and
$J_2 = -0.21 \pm 0.04$ meV lead to excellent fits for most branches at both low
and high energies. An additional branch associated with the hidden
antiferromagnetic order was observed. This work paves the way for studies of
other materials in this class containing spin reorientation transitions and
magnetic rare earth ions.
