The circuit shown is not radar. I've seen similar circuits work, but the descriptions in the article, and by karyoker, are wrong. I did find two schematic errors: TR3 emitter is missing (connected to R4 & C5) and R6 not labeled (it's across C8).
IC3 generates an ultrasonic signal, and receives what is hopefully a steady return signal amplitude. (The frequency is of no concern). It is amplified by TR3 and IC1, and then rectified by voltage doubler D1 and D2, charging C8. R6 (not labeled) discharges C8 when the signal changes. C10 and R7 form a high pass filter, to detect when the voltage on C8 (representing the return signal amplitude) changes. So a moving object in the field causes a changing voltage on IC2. P1 sets the AC gain of IC2. As the voltage on IC2 changes, these changes are rectified by voltage doubler D3 and D4. When the doubler output reaches the threshold of IC3, it latches and triggers the alarm connected to TR1.