, Question 1
An ARPA target is being tracked at a range of 12 NM with a range
discrimination requirement of 30 m. Given a pulse length of 0.05 µs and a
display spot size equivalent to 0.6° of azimuth, which factor fundamentally
limits the achievable range discrimination, and why?
A. Spot size, because it determines the minimum separable azimuth
separation, not range.
B. Pulse length, because the radar cannot resolve targets closer than half
the spatial pulse length (c/2).
C. Beamwidth, because it governs the angular resolution which indirectly
sets range cell size.
D. Receiver bandwidth, because it is inversely proportional to pulse
length and thus directly sets range resolution.
Correct Answer: B - Pulse length, because the radar cannot
resolve targets closer than half the spatial pulse length (c/2).
RATIONALE
Range discrimination is fundamentally limited by pulse length: the
minimum resolvable range separation is c/2, which for =0.05 µs is
about 7.5 m, not 30 m. Spot size and beamwidth affect azimuth
resolution, and bandwidth is linked to pulse length but does not
independently set the physical range resolution limit.
Question 2
During a crossing situation, an ARPA target is acquired at 10 NM and shows a
CPA of 0.3 NM with a TCPA of 6 minutes. If the target's true course is 270°
and speed 18 knots, while own ship steams 000° at 15 knots, what is the most
likely relative motion display interpretation?
A. Target will pass ahead; own ship is the give-way vessel and must alter
course to starboard.
B. Target will pass astern; own ship is stand-on and should maintain
Page 2
, course and speed.
C. Target is on a constant bearing, decreasing range, indicating a risk of
collision requiring immediate action.
D. Target is on a constant bearing, increasing range, indicating no risk of
collision.
Correct Answer: C - Target is on a constant bearing, decreasing
range, indicating a risk of collision requiring immediate action.
RATIONALE
A CPA of 0.3 NM with TCPA of 6 minutes indicates a close-quarters
situation with risk of collision. The relative motion display would
show the target on a steady bearing with decreasing range, which is
the classic collision risk indicator. The other options misstate the
bearing/range trend or the give-way/stand-on determination without
full aspect analysis.
Question 3
An ARPA system operating in sea state 5 with a 4° roll period experiences
frequent track swaps between two close targets. Which processing parameter is
most directly responsible for mitigating this?
A. Decreasing the guard zone size to reduce false associations.
B. Increasing the plot correlation gate size to accommodate maneuvering
targets.
C. Adjusting the tracking gate (association window) to balance between
missed detections and false associations.
D. Reducing the antenna rotation rate to increase dwell time per target.
Correct Answer: C - Adjusting the tracking gate (association
window) to balance between missed detections and false
associations.
Page 3
, RATIONALE
Track swap occurs when the association gate is too large or too small;
optimizing the tracking gate balances missed detections and false
associations. Guard zone size affects acquisition, not tracking
association; correlation gate adjustments and antenna rate changes do
not directly resolve the swap mechanism.
Question 4
A vessel is navigating in a narrow channel with a radar operating at X-band.
The operator observes a target at 0.8 NM with a very large radar cross-section
but no AIS. Which phenomenon best explains the strong return?
A. Multipath interference from the channel walls.
B. Anomalous propagation due to a temperature inversion.
C. Corner reflector effect from the target's structure.
D. Second-trace echo from a distant large vessel.
Correct Answer: C - Corner reflector effect from the target's
structure.
RATIONALE
A large RCS at close range without AIS often indicates a corner
reflector effect, where structural geometry returns a strong signal.
Multipath and anomalous propagation cause false or distorted echoes,
and second-trace echoes appear at incorrect ranges, not as a strong
close target.
Question 5
When using ARPA in a traffic separation scheme, a target is acquired at 8 NM
and displays a predicted CPA of 1.2 NM. After 3 minutes, the CPA decreases to
0.4 NM without any change in own ship's course or speed. What is the most
probable cause?
A. ARPA tracking error due to sea clutter.
B. The target has altered course or speed.
Page 4
An ARPA target is being tracked at a range of 12 NM with a range
discrimination requirement of 30 m. Given a pulse length of 0.05 µs and a
display spot size equivalent to 0.6° of azimuth, which factor fundamentally
limits the achievable range discrimination, and why?
A. Spot size, because it determines the minimum separable azimuth
separation, not range.
B. Pulse length, because the radar cannot resolve targets closer than half
the spatial pulse length (c/2).
C. Beamwidth, because it governs the angular resolution which indirectly
sets range cell size.
D. Receiver bandwidth, because it is inversely proportional to pulse
length and thus directly sets range resolution.
Correct Answer: B - Pulse length, because the radar cannot
resolve targets closer than half the spatial pulse length (c/2).
RATIONALE
Range discrimination is fundamentally limited by pulse length: the
minimum resolvable range separation is c/2, which for =0.05 µs is
about 7.5 m, not 30 m. Spot size and beamwidth affect azimuth
resolution, and bandwidth is linked to pulse length but does not
independently set the physical range resolution limit.
Question 2
During a crossing situation, an ARPA target is acquired at 10 NM and shows a
CPA of 0.3 NM with a TCPA of 6 minutes. If the target's true course is 270°
and speed 18 knots, while own ship steams 000° at 15 knots, what is the most
likely relative motion display interpretation?
A. Target will pass ahead; own ship is the give-way vessel and must alter
course to starboard.
B. Target will pass astern; own ship is stand-on and should maintain
Page 2
, course and speed.
C. Target is on a constant bearing, decreasing range, indicating a risk of
collision requiring immediate action.
D. Target is on a constant bearing, increasing range, indicating no risk of
collision.
Correct Answer: C - Target is on a constant bearing, decreasing
range, indicating a risk of collision requiring immediate action.
RATIONALE
A CPA of 0.3 NM with TCPA of 6 minutes indicates a close-quarters
situation with risk of collision. The relative motion display would
show the target on a steady bearing with decreasing range, which is
the classic collision risk indicator. The other options misstate the
bearing/range trend or the give-way/stand-on determination without
full aspect analysis.
Question 3
An ARPA system operating in sea state 5 with a 4° roll period experiences
frequent track swaps between two close targets. Which processing parameter is
most directly responsible for mitigating this?
A. Decreasing the guard zone size to reduce false associations.
B. Increasing the plot correlation gate size to accommodate maneuvering
targets.
C. Adjusting the tracking gate (association window) to balance between
missed detections and false associations.
D. Reducing the antenna rotation rate to increase dwell time per target.
Correct Answer: C - Adjusting the tracking gate (association
window) to balance between missed detections and false
associations.
Page 3
, RATIONALE
Track swap occurs when the association gate is too large or too small;
optimizing the tracking gate balances missed detections and false
associations. Guard zone size affects acquisition, not tracking
association; correlation gate adjustments and antenna rate changes do
not directly resolve the swap mechanism.
Question 4
A vessel is navigating in a narrow channel with a radar operating at X-band.
The operator observes a target at 0.8 NM with a very large radar cross-section
but no AIS. Which phenomenon best explains the strong return?
A. Multipath interference from the channel walls.
B. Anomalous propagation due to a temperature inversion.
C. Corner reflector effect from the target's structure.
D. Second-trace echo from a distant large vessel.
Correct Answer: C - Corner reflector effect from the target's
structure.
RATIONALE
A large RCS at close range without AIS often indicates a corner
reflector effect, where structural geometry returns a strong signal.
Multipath and anomalous propagation cause false or distorted echoes,
and second-trace echoes appear at incorrect ranges, not as a strong
close target.
Question 5
When using ARPA in a traffic separation scheme, a target is acquired at 8 NM
and displays a predicted CPA of 1.2 NM. After 3 minutes, the CPA decreases to
0.4 NM without any change in own ship's course or speed. What is the most
probable cause?
A. ARPA tracking error due to sea clutter.
B. The target has altered course or speed.
Page 4
