Solving the Doppler Dilemma

The trade off between velocity aliasing and range folding which must be made when selecting an operating PRF for a Doppler radar is termed the “Doppler dilemma”. The selection of a 10 cm radar with its inherent additional cost can overcome some of the problem if both long range and high Nyquist velocity is required.

 

Figure 10 illustrates the inter-dependency of the Nyquist velocity and unambiguous range for 10 cm and 5 cm radars.
Figure 10. Relationship between Nyquist velocity and unambiguous range for Doppler radars.
  • The bigger the maximum unambiguous range, the smaller the maximum unambiguous velocity
  • The bigger the maximum unambiguous velocity, the smaller the maximum unambiguous range
The issues caused by the Doppler Dilemma are worse for C-band radars than for S-band radars. It is possible to operate a radar at two PRF’s; a low frequency to collect reflectivity data and a high frequency to collect velocity data. The target returns at the different PRF’s can then be compared to determine true radial velocities and ranges. This is termed “post processing” and can be done in real time with little loss in overall operating performance of a radar. A dual PRF technique is used to increase Nyquist velocity without decreasing maximum unambiguous range and a staggered PRF technique and pulse coding are used to simultaneously increase Nyquist velocity and increase maximum unambiguous range.