  Inferences of sub-surface flow velocities using local domain ring-diagram
helioseismology depend on measuring the frequency splittings of oscillation
modes seen in acoustic power spectra. Current methods for making these
measurements utilize maximum-likelihood fitting techniques to match a model of
modal power to the spectra. The model typically describes a single oscillation
mode, and each mode in a given power spectrum is fit independently. We present
a new method that produces measurements with greater reliability and accuracy
by fitting multiple modes simultaneously. We demonstrate how this method
permits measurements of sub-surface flows deeper into the Sun while providing
higher uniformity in data coverage and velocity response closer to the limb of
the solar disk. While the previous fitting method performs better for some
measurements of low-phase-speed modes, we find this new method to be
particularly useful for high phase-speed modes and small spatial areas.
