If you increase pulse repetition frequency to a level too great for the depth of field, the result will be:

• A. Increased side lobes
• B. Increased grating lobes
• C. Range ambiguity
• D. Increased electronic noise

Answer: (C)

The main idea is how pulse repetition frequency (PRF) limits the unambiguous imaging depth in a pulsed system. After each pulse, you wait for echoes to return. The time between pulses is 1/PRF. To correctly assign each echo to the right pulse, the round-trip travel time from the deepest reflectors must be less than this interval. If you raise PRF so high that echoes from the farthest targets take longer than the time between pulses to return, echoes from different pulses can mix together. You end up unable to tell which pulse a given echo belongs to, so the depth measurement becomes ambiguous—this is range ambiguity.

Other phenomena like side lobes, grating lobes, or electronic noise are not the direct consequence of increasing PRF beyond what the depth allows; they stem from beam pattern geometry or receiver electronics, not from the timing issue that causes range ambiguity.