  We describe progress toward a precise measurement of the recoil energy of an
atom measured using a perturbative grating-echo atom interferometer (AI) that
involves three standing-wave (sw) pulses. With this technique, a perturbing sw
pulse is used to shift the phase of excited momentum states---producing a
modulation in the contrast of the interference pattern. The signal exhibits
narrow fringes that revive periodically at twice the two-photon recoil
frequency, $2\omega_q$, as a function of the onset time of the pulse.
Experiments are performed using samples of laser-cooled rubidium atoms with
temperatures $\lesssim 5$ $\mu$K in a non-magnetic apparatus. We demonstrate a
measurement of $\omega_q$ with a statistical uncertainty of 37 parts per $10^9$
(ppb) on a time scale of $\sim 45$ ms in 14 hours. Further statistical
improvements are anticipated by extending this time scale and narrowing the
signal fringe width. However, the total systematic uncertainty is estimated to
be $\sim 6$ parts per $10^6$ (ppm). We describe methods of reducing these
systematic errors.
