GROL Marine Electronics and Radar Systems 1 — Questions and Answers
Question 1: What is the primary frequency band used by marine radar systems?
- HF (3–30 MHz)
- VHF (30–300 MHz)
- X-band (9–10 GHz) (Correct answer)
- AM broadcast band (535–1705 kHz)
Correct answer: X-band (9–10 GHz)
Marine radar primarily operates in the X-band (around 9–10 GHz) because this frequency provides good target resolution and is reflected well by water surfaces and vessels.
Question 2: What does ARPA stand for in marine radar terminology?
- Automatic Radar Plotting Aid (Correct answer)
- Advanced Radio Position Antenna
- Auxiliary Radar Processing Amplifier
- Automated Range and Position Analyzer
Correct answer: Automatic Radar Plotting Aid
ARPA stands for Automatic Radar Plotting Aid, a system that tracks targets and calculates their course, speed, and closest point of approach (CPA).
Question 3: Which type of radar display shows the ship's own position at the center with north always pointing up?
- Relative motion display
- True motion display (Correct answer)
- Head-up display
- Course-up display
Correct answer: True motion display
A true motion display moves the own-ship symbol across the screen to reflect actual movement, with north fixed at the top and sea clutter stationary.
Question 4: What is the purpose of the sea clutter (STC) control on a marine radar?
- To increase transmitter power near land
- To reduce echoes caused by wave returns close to the ship (Correct answer)
- To eliminate interference from other radar stations
- To adjust the antenna rotation speed
Correct answer: To reduce echoes caused by wave returns close to the ship
The Sensitivity Time Control (STC) reduces the gain of the receiver for close-range returns, suppressing strong echoes from nearby sea waves that would otherwise obscure targets.
Question 5: What is the typical pulse repetition frequency (PRF) for a short-range marine radar setting?
- 100–200 Hz
- 500–800 Hz
- 1,500–4,000 Hz (Correct answer)
- 10,000–20,000 Hz
Correct answer: 1,500–4,000 Hz
Short-range marine radar settings use higher PRFs, typically 1,500–4,000 pulses per second, allowing more frequent updates at short ranges while keeping pulse width narrow.
Question 6: What is the function of the magnetron in a conventional marine radar transmitter?
- To amplify received radar echoes
- To rotate the antenna
- To generate the high-power microwave pulses (Correct answer)
- To convert AC power to DC for the display unit
Correct answer: To generate the high-power microwave pulses
The magnetron is a high-power vacuum tube oscillator that generates the microwave frequency pulses transmitted by the radar antenna.
Question 7: In marine radar, what does 'minimum range' refer to?
- The farthest distance the radar can reliably detect a target
- The closest distance at which the radar can detect a target (Correct answer)
- The range at which beam power is reduced by half
- The distance at which sea clutter is automatically suppressed
Correct answer: The closest distance at which the radar can detect a target
Minimum range is the closest distance a radar can detect a target, determined mainly by the pulse width—longer pulses have greater minimum ranges because the transmitter must stop transmitting before echoes can be received.
