What does coherent integration do in radar reception, and how does it affect SNR?

What does coherent integration do in radar reception, and how does it affect SNR?

• A. Sums magnitudes of samples across pulses, yielding SNR improvement proportional to the square root of the number of pulses.
• B. Delays pulses to align with wind patterns, affecting Doppler.
• C. Sums complex-valued samples with phase alignment across pulses, yielding SNR improvement proportional to the number of integrated pulses.
• D. Only filters noise while ignoring phase, yielding fixed SNR.

Answer: (C)

Preserving and using phase information across multiple radar pulses is what coherent integration does. By converting received echoes into complex (I and Q) samples and aligning their phases so the signal components add constructively, the signal amplitude grows roughly in direct proportion to the number of pulses integrated. Noise, being random in phase, tends to cancel out as you sum, so its growth is only with the square root of the number of pulses. The result is an SNR improvement that scales with the number of integrated pulses, which is much stronger than non-coherent methods that sum magnitudes and only achieve about a sqrt(N) gain. This approach requires stable phase/frequency across pulses and proper compensation for target motion (Doppler); otherwise the phase misalignment reduces the coherent gain.

Preserving and using phase information across multiple radar pulses is what coherent integration does. By converting received echoes into complex (I and Q) samples and aligning their phases so the signal components add constructively, the signal amplitude grows roughly in direct proportion to the number of pulses integrated. Noise, being random in phase, tends to cancel out as you sum, so its growth is only with the square root of the number of pulses. The result is an SNR improvement that scales with the number of integrated pulses, which is much stronger than non-coherent methods that sum magnitudes and only achieve about a sqrt(N) gain. This approach requires stable phase/frequency across pulses and proper compensation for target motion (Doppler); otherwise the phase misalignment reduces the coherent gain.