A low RPM provides a longer dwell time onto the target which allows to integrate more number of echo pulses resulting into a higher SNR (the noise is random and therefore it decreases after summation while the signal does not). A higher SNR means a longer range.

Thanks for the response sir. Can you please elaborate as how high SNR means a longer range (in KMs). High SNR can lead to greater dynamic range which again relates the greater range of signal levels.

If you have a high SNR at one range, the SNR will be smaller but useable at a longer range, because the signal is smaller but the noise remains the same.

As said, In radar, high antenna rotation rate can be good in some respects but this then decreases the number of return pulses on target during a beam pass for integration. This then decreases the SNR. To reverse this, the PRF (pulse rep frequency) is increased restoring the integration gain but then reducing the maximum 'unambiguous' range. Longer range detections are there but ambiguous unless other techniques are introduced. The SNR at any one range for any one target is to first order, unchanged.