  In coming years, gravitational wave detectors should find black hole-neutron
star binaries, potentially coincident with astronomical phenomena like short
GRBs. These binaries are expected to precess. Gravitational wave science
requires a tractable model for precessing binaries, to disentangle precession
physics from other phenomena like modified strong field gravity, tidal
deformability, or Hubble flow; and to measure compact object masses, spins, and
alignments. Moreover, current searches for gravitational waves from compact
binaries use templates where the binary does not precess and are ill-suited for
detection of generic precessing sources. In this paper we provide a closed-form
representation of the single-spin precessing waveform in the frequency domain
by reorganizing the signal as a sum over harmonics, each of which resembles a
nonprecessing waveform. This form enables simple analytic calculations (e.g., a
Fisher matrix) with easily-interpreted results. We have verified that for
generic BH-NS binaries, our model agress with the time-domain waveform to 2\%.
Straightforward extensions of the derivations outlined here [and provided in
full online] allow higher accuracy and error estimates.
