  Because of lunar librations, the retroreflectors left on the moon do not, in
general, face directly at the Earth. Usually this is regarded as a
disadvantage. It results in a spread of arrival times, because each cube that
comprises the retroreflector is at a slightly different distance from the
earth. However, we can turn this same effect into an advantage. Using pulses
and detectors somewhat faster than those currently used for lunar ranging, we
can resolve at least some of the structure of a retroreflector, at least when
the libration angles are large. This additional structure in the transfer
function means that a unique mm level fit can be obtained with many fewer
photons. Fitting to the expected reflector transfer function in general
requires fewer photons than straight averaging, and smoothly reduces to
averaging in the cases where no structure can be resolved. The gains from
resolving the reflectors are largest at large libration angles, exactly the
case where averaging is most inefficient. In these cases the number of photons
needed can be reduced by an order of magnitude or more. Analysis shows that
angles for which the gain is very high happen several times each month, with
the details depending on the exact librations. Experimental validation of this
technique should be possible with existing SLR stations and mockups of the
lunar reflectors.
