In unusually good radar propagation conditions (such as strong ducting), you notice an echo displayed at a short range that does not correspond to any visible or plausible target that close, while a genuine large vessel is known (e.g. from AIS or previous observation) to be well beyond your radar's selected maximum range. Your examiner asks you to explain this.

A. A second-trace (or "multiple-trip") echo occurs when a target lies BEYOND the theoretical maximum unambiguous range for the current PRF setting, but under abnormal propagation conditions (such as super-refraction or ducting) its echo still returns to the scanner — just not before the NEXT pulse has already been transmitted. Because the radar times every echo assuming it belongs to the most recently transmitted pulse, this late-arriving echo from a genuinely FAR target is displayed at a falsely SHORT range, as though it were a close target. This is the direct consequence of the PRF/maximum-unambiguous-range relationship: the timing electronics have no way of knowing the echo actually corresponds to an earlier pulse.

B. A second-trace echo is a genuine close-range target that has simply been duplicated by a software display glitch and has no relationship to PRF, propagation conditions, or timing.

C. A second-trace echo occurs only from reflections off the ship's own structure and is displayed at the correct range but an incorrect bearing — this describes an indirect echo, not a second-trace echo.

D. A second-trace echo always displays a distant target at a falsely LONGER range than its true position, rather than a falsely shorter one.

A. Nothing — a test lift can proceed immediately after repairs

B. Testing should wait until cargo operations begin

C. The area within the grab's swing radius should be cleared of personnel before the lift

D. Only the crane operator needs to be aware the lift is happening

A. Who must remain clear of the exclusion zone while the lift takes place

B. The lunch schedule for the deck team

C. Only the weather forecast for the next watch

D. The cargo manifest for the next port