Transmitter apparatus for glide path guidance includes two aerials spaced apart (preferably so that the perpendicular bisector of a line joining the aerials is parallel to the desired glide path) and means for feeling them with distinct but related frequency-modulated waveforms so as to yield at least two distinguishable cross-correlation products whose relative amplitudes will vary differentially with displacements from the desired glide path. One aerial may be fed with a first carrier frequency modulated with a first modulation signal and the other aerial with a second carrier frequency modulated with second and third modulation signals, the second modulation signal being a frequency modulated waveform amplitude-modulated at 90 Hz., the first modulation signal being the said frequency-modulated waveform delayed by a time delay to , and the third modulation signal being the said frequency-modulated waveform delayed by a time delay 2to and amplitude-modulated at 150 Hz. A monitor receiver has inputs connected to inputs mounted on reflectors of the two aerials.
