The use of spatial filters, or weighting functions, on transmitting and receiving apertures used on line-of-sight propagation paths is a powerful technique for probing the velocity and refractivity structure of the transmission medium. Provided that the weak-scattering approximation is appropriate, simple and inexpensive techniques offer on the order of 100 resolution elements along the path, with little or no data processing required. A simple optical experiment has been conducted over a 61 m path, with a resolution-cell size of about 0.7 m long, 0.1 m<sup>3</sup>. volume, with results that compare well with anemometer measurements.
R. W. Lee, "Remote probing using spatially filtered apertures," J. Opt. Soc. Am. 64, 1295-1303 (1974)