Abstract: 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.

Abstract: A ground station for VOR navigation systems with at least one transmitter and several radiators, which ground station radiates azimuth-dependent and azimuth-independent information. The azimuth-dependent information consists of the wave amplitude-modulated with 30 Hz in the field and generated by the superposition of the nondirectionally radiated VHF carrier on a VHF figure-8 directional pattern rotating at 30 Hz. The azimuth-independent reference wave consists of a 30-Hz wave which is contained as frequency modulated in a 9,960-Hz subcarrier wave amplitude-modulated on the VHF carrier.

Abstract: 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.