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Phase modulation...
  • A
    results in a change of polarisation of the modulated signal.
  • B
    is a modulation form used in GPS where the phase of the carrier wave is reversed.
  • C
    results in a change of amplitude of the carrier wave.
  • D
    is a modulation form used in ILS where the phase of the navigation signals is changed with 90° or 150°.

As the name implies, Phase Modulation, PM uses variations in phase for carrying the modulation.Phase modulation works by modulating the phase of the signal, i.e. changing the rate at which the point moves around the circle.This changes the phase of the signal from what it would have been if no modulation was applied.

In other words the speed of rotation around the circle is modulated about the mean value.To achieve this it is necessary to change the frequency of the signal for a short time.In other words when phase modulation is applied to a signal there are frequency changes and vice versa.Phase and frequency are inseparably linked as phase is the integral of frequency.

Note: For the ranging codes and navigation messages to travel from the satellite to the receiver, they must be modulated onto a carrier frequency, by varying the phase of the signal ( phase modulation).As soon as a data signal will be modulated onto the carrier wave, the phase of the carrier wave is reversed by 180°.
The receiver will detect this reversal and is able to reconstruct the data.

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