Define unambiguous range and its relationship to PRF.

• A. The maximum range that can be measured without range aliasing; it is inversely related to PRF
• B. The minimum range that can be measured; independent of PRF
• C. The range where Doppler shift is zero
• D. The range resolution times PRF

Answer: (A)

Unambiguous range is the farthest distance at which a target’s return can be assigned to a specific transmitted pulse without range aliasing. In pulsed radar, echoes from targets beyond this limit can be misinterpreted as coming from closer ranges because the next pulse may arrive before the previous echo is properly resolved. The relationship to PRF is inverse: increasing the pulse repetition frequency reduces the unambiguous range, while decreasing PRF increases it. The practical formula is R_unamb = c / (2 × PRF), where c is the speed of light. For example, a PRF of 1,000 pulses per second gives about 150 kilometers of unambiguous range.

This matches the idea that the unambiguous range is the maximum range free from aliasing and that it shrinks as PRF grows. The other options mix in concepts that don’t define unambiguous range: minimum range is determined by hardware and other factors, Doppler-related zero-shift concerns velocity, and range resolution times PRF isn’t the standard description of unambiguous range.