Bi-static Microwave Presence Sensor
This is a very common form of radar proximity detector.
This device is a presence sensor in that it can detect both moving and nonmoving targets.
In this type of system, there are separate antennas for the receiver and transmitter. They may or may not be collocated, but there must be a high degree of isolation between them to prevent the receiver from picking up any energy at all unless an object is present in the field of regard.
When an object is present in the field of regard, it reflects signal back into the receiver antenna. This signal, is amplified and then filtered.
If the signal passes through the fitter, then it can be used to phase lock the PLL.