Transponder based landing system

A landing approach system for aircraft having a transponder which includes means for automatically transmitting encoded altimeter information when said aircraft transponder is interrogated, said system comprising:

• a ground based interrogator/receiver unit for interrogating said aircraft transponder and adapted to receive response signals therefrom including said encoded altimeter information, the electronic center of said land based transponder antenna being aligned with the center line of a prescribed landing runway;
• means in circuit with said ground based unit for processing said response signal for respectively determining the azimuth of said aircraft relative to said runway, and the range of said aircraft to the point of touchdown on said runway;
• means in circuit with said ground based unit for decoding said encoded altimeter information to produce signals corresponding to the elevation reported from said aircraft;
• means responsive to the output of said range determining means for generating signals corresponding to a ground distance and a vertical approach path for said aircraft whereby a glide path for said aircraft is prescribed, said vertical approach path signals being a fixed proportion of said ground distance signals;
• and
• means for comparing said vertical approach path signals with said aircraft elevation reported signals to produce error signals corresponding to the difference therebetween;
• said azimuth, range and error signals comprising the information for guiding said aircraft along said glide path.