Passive coherent radar systems rely on which source of RF illumination?

Passive coherent radar systems rely on which source of RF illumination?

• A. Military satellites
• B. Civilian transmitters of opportunity
• C. Dedicated air defense radars
• D. Random noise sources

Answer: (B)

Passive coherent radar works by using external RF signals as the illumination source rather than emitting its own. The system captures a copy of the transmitted waveform (the reference) from a separate receiver, while another antenna picks up reflections of that waveform from targets. By coherently comparing the received reflections with the reference signal, the radar can detect targets and measure their Doppler shifts even though there’s no dedicated transmitter on the radar platform itself. Civilian transmitters of opportunity, like broadcast towers (FM/TV, etc.) or other common RF sources, are ideal in this setup because they provide strong, stable signals that are widely available and legally accessible for illumination. This makes passive coherent radar economical and covert, since the radar doesn’t own or operate the transmitter. Military satellites could serve as illumination, but they’re not the typical or most practical choice for everyday passive radar use due to access, tracking, and regulatory complexities. Dedicated air defense radars emit their own signals, which would make the system active rather than passive. Random noise sources wouldn’t provide a stable, known waveform needed for coherent processing, so they can’t serve as reliable illumination.

Passive coherent radar works by using external RF signals as the illumination source rather than emitting its own. The system captures a copy of the transmitted waveform (the reference) from a separate receiver, while another antenna picks up reflections of that waveform from targets. By coherently comparing the received reflections with the reference signal, the radar can detect targets and measure their Doppler shifts even though there’s no dedicated transmitter on the radar platform itself.

Civilian transmitters of opportunity, like broadcast towers (FM/TV, etc.) or other common RF sources, are ideal in this setup because they provide strong, stable signals that are widely available and legally accessible for illumination. This makes passive coherent radar economical and covert, since the radar doesn’t own or operate the transmitter.

Military satellites could serve as illumination, but they’re not the typical or most practical choice for everyday passive radar use due to access, tracking, and regulatory complexities. Dedicated air defense radars emit their own signals, which would make the system active rather than passive. Random noise sources wouldn’t provide a stable, known waveform needed for coherent processing, so they can’t serve as reliable illumination.