Two-frequency transmitting apparatus (S1, S2, LA) for instrument-landing systems is insensitive to so-called difference in depth of modulation (DDM) distortions caused by reflections of a clearance signal from obstacles (H) located near the runway (RW). This insensitivity is achieved by providing different phase shifts to the equal modulation frequencies (90 Hz and 150 Hz) used for the course and clearance signals. The different phase shifts correspond to a phase shift of a common fundamental frequency (30 Hz). The two-frequency transmitting apparatus can be used to specify a localizer course or a glide path.

An ILS (Instrument Landing System) having at least one of the Glide-slope and Localizer functions, each of said functions comprising course and clearance signals. For the course and clearance signals of either function, a single transmitter is supplied, the necessary frequency difference between those signals being obtained by dividing the transmitter output into two signals, one of which is uniquely phase modulated to produce a predetermined frequency offset with respect to the other.

An aircraft landing system employs an array of antennas aligned across the runway, and equipment on board the aircraft employs radio interferometry to measure path length differences between the aircraft and selected pairs of the antennas. The antennas each include circuitry for sideband modulation of a basic FM signal so that each antenna radiates a pair of sidebands displaced above and below the basic signal by a respective number of increments. The FM signal can be carried on an X-band carrier of nominally ten GHz, with a frequency deviation of .+-.250 MHz and a modulating frequency of 20 Hz. Further antennas provide taxiing guidance. Data can be communicated on the system.