Abstract: An airplane or other vehicle is to be used for repeated traverses over an assigned area successively in respective different ones of a plurality of parallel paths displaced laterally one from another. Path alignment apparatus carried by the vehicle cooperates with delivery means that dispenses a material along each of the paths in a swath of predetermined width. The material includes an agent that presents characteristic radiation at a predetermined frequency. A sensing means is oriented to view the path of approach of the vehicle for detecting the radiation and developing a discriminated signal which distinguishes between presence and absence of the radiation. Developed by range-finding means is a distance signal that represents location of the vehicle relative to the location of the path being approached.