Figure 1: Frequency counter up to 1.3 GHz
It is quite simple: The oscillations per second are counted and the result is the frequency...
... too bad that the radar does not transmit a whole second in a row!
But most frequency counters can already start the counting process at the beginning of the transmission and then only need the information on how long it has to be counted (duration of the transmission pulse!). The result is extrapolated by the device and the oscillations per second (i.e. in Hertz) are displayed.
But if the device cannot do this, then exactly this calculation must be made on paper with the known duty cycle according to the rule of cross-multiplication.