Doppler Frequency Comparison
Since the Doppler frequency (few Hertz) is small relatively to the transmitted frequency (much MegaHertz), therefore a phase comparison is more easily to carry out than a direct frequency comparison.

The Doppler  frequency shall be calculated at a transmitted frequency of 2975 MHz
and at a
radial speed
of the airplane of 650 kilometers per hour.  

By the relation λ=^{c0}/_{f} we calculate the wavelength or puts this relation into the formula at once.  

The light speed is known, but take the measurement unit of meters per seconds into account.  

We put the frequency of the radar sets transmitter into the formula and shorten with the decades of the denominator at once.  

Notice that we need the radial speed in meters per second (m/s). Therefore we multiply by 1000 (meters) and divide by 3600 (seconds)  after shortening the value 3.6 is left in the denominator. 
The rest of work doing at the pocket calculator: the Dopplerfrequency is 3581 Hz. Although the radial speed was very high anyway, this value is very small compared one with the transmitted frequency. E.g. if the radial speed drops less than 10 km/h, then the Dopplerfrequency becomes smaller than 110 Hz! At sufficiently good edge steepness favourable filters for such frequencies are digitally structured only.