Abstract: An all-weather aircraft landing system includes a plurality of ground based passive 90.degree. dihedral reflectors for producing two-bounce reflected signals without ground reflections, and an airborne radar system which may transmit and receive "same sense" circularly polarized radiation, while completely rejecting "opposite sense" polarization returns or else utilizing them to indicate weather conditions. Radar clutter from objects such as rain, buildings and trees which produce opposite sense reflections are rejected by the "same sense" receiver or switched to an "opposite sense" receiver to provide weather/obstacle condition information. By properly orienting a plurality of 90.degree. dihedral angle reflectors of a particular size in a predetermined array pattern and tilt-angle adjacent a runway, the reflections from airborne radar signals are processed and displayed to provide a visual means for determining glide slope deviation and approach vector of the landing aircraft.

Abstract: A pulsed doppler-radar system is utilized to determine if the terrain clearance of an aircraft drops below a predetermined aircraft safety clearance. The system includes a maximum-range doppler-frequency counter which counts the highest doppler frequency received for a maximum range and a range gate which receives and passes doppler frequencies from within a predetermined slant range. A reference-frequency generator generates a reference frequency related to the highest doppler frequency and cooperates with a frequency synthesizer to synthesize a frequency related to the computed doppler frequency for the aircraft safety clearance. The slant-range doppler frequencies are summed with the synthesizer frequency and a critical-frequency detector detects the sum frequency to determine if terrain clearance drops below the predetermined aircraft safety clearance.

Abstract: A doppler-radar system for computing terrain clearance of the projected flight path of an aircraft from a maximum-range doppler frequency and a doppler frequency for a known slant range. The apparatus includes a transmitter/receiver, a maximum-range doppler-frequency counter, a slant-range doppler-frequency counter, and a terrain-clearance computer which receives the outputs of the maximum-range and slant-range doppler-frequency counters to compute a depression angle and terrain clearance and provide an output of the terrain clearance to an indicator.