Precision guidance of aircraft throughout a great volume of positions, and with spectrum economy, is achieved with coordinated scanning of coarse and fine coverage localizer antennas and coarse and fine glide-path antennas. The scan coordination is predetermined for sequential traversals of the useful scan sectors, microwave transmission through the several antennas being effected in the sequence of the scans. Each such transmission includes modulation not only in accordance with the antenna scan angle but also with the direction of movement in the scan sector. The aircraft receiver means detects the aircraft position by the detected modulation and/or reception timing of each directionally transmitted beam. General position is determinable from reception of the wide-angle (coarse) scanning beams. Precise guidance in the vicinity of a predetermined low-angle glide path is derivable from the beams transmitted through the narrow scan antennas.
Where a signal reflective fixed obstruction is present in a subsector of a navigation angle sector in which guidance is required, scanning beams are employed including a main scan which is scanned over most of the navigation angle sector and an auxiliary scan which is scanned only over the obstruction subsector. The scan signals are received and correlated by the aircraft to distinguish obstruction subsector signals to thereby avoid reflection interference.
High accuracy guidance signals are produced over a wide angle by sequencing in a predetermined sequence through a series of narrow angular sectors while electrically scanning the transmitted signals within each sector.
