When transmitter and receiver are moving towards each other...
Refer to figure.
This question refers to the Doppler Effect.
The Doppler effect is where waves have a slightly higher detected frequency when the transmitter and receiver are coming together, but a lower frequency when moving apart (see the above annex, it occurs for EM waves and sound waves).
It is a measurable difference, as we often know the precise frequency that we should be picking up, and so small variations from that can tell us if a transmitter or a reflector is moving towards or away from us, and at what speed.
The Doppler effect occurs everywhere from astronomy (Redshift) to the sound of an ambulance passing us in the street. In aviation, we make use of the effect in modern Doppler VORs, GNSS systems and radars, for MTI (moving target indication) and turbulence detection.
The best way to answer these questions is by thinking of a loud car (an ambulance with sirens on, a car honking its horn, etc.) going past you. The frequency (pitch) of the sound starts high, but decreases as the vehicle passes, and stays low whilst the vehicle is moving away.
Therefore, the frequency is increased when moving together, and reduced when moving apart.
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