Statistics of momentum flux estimation using the dual coplanar beam technique

We examine the statistical estimation error for the dual coplanar beam mome ntum flux estimator in the presence of geophysical noise. Included in our d erivation are the effects of measurement noise and finite spatial correlati on of the wind fluctuations. We show that there is an optimal beam separati on that minimizes the statistical estimation error at a value nearly equal to the irreducible error observed by an "ideal" anemometer measuring u and w at a point. For typical mesopause parameters this beam separation is appr oximately +/-13 degrees. The optimal angle is dependent in part on the rati o of vertical to horizontal wind variances and is therefore variable. For o ptimal momentum flux measurements a system, radar or lidar, with steerable beams becomes necessary.