CMRT Radar System Fundamentals 5 — Questions and Answers
Question 1: What does the term 'antenna gain' mean in the context of a marine radar antenna?
- The physical size of the antenna aperture measured in square meters
- The ratio of the antenna's radiated intensity in its principal direction to that of an isotropic radiator (Correct answer)
- The total transmit power delivered to the antenna feedpoint
- The voltage amplification provided by the antenna preamplifier
Correct answer: The ratio of the antenna's radiated intensity in its principal direction to that of an isotropic radiator
Antenna gain (in dBi) expresses how much more power the antenna concentrates in its main beam direction compared to a hypothetical isotropic radiator; higher gain means a narrower, more focused beam.
Question 2: In a marine radar system, what is the purpose of the automatic frequency control (AFC) circuit?
- Lock the local oscillator frequency to maintain a constant IF despite magnetron frequency drift (Correct answer)
- Automatically adjust the PRF to avoid range ambiguity
- Control the antenna rotation speed relative to vessel speed
- Stabilize the display sweep frequency against power supply variations
Correct answer: Lock the local oscillator frequency to maintain a constant IF despite magnetron frequency drift
The magnetron's frequency can drift with temperature and aging; AFC continuously compares the IF to the desired value and adjusts the local oscillator to keep the difference (IF) constant.
Question 3: What is the 'radar equation' fundamental relationship between detection range and transmitter peak power?
- Range is proportional to peak power (R ∝ P)
- Range is proportional to the fourth root of peak power (R ∝ P^0.25) (Correct answer)
- Range is proportional to the square of peak power (R ∝ P²)
- Range is inversely proportional to peak power (R ∝ 1/P)
Correct answer: Range is proportional to the fourth root of peak power (R ∝ P^0.25)
The radar range equation shows R_max ∝ (P_t)^(1/4); to double detection range, peak power must increase by a factor of 16, making power a relatively inefficient way to extend range.
Question 4: Which IMO regulation requires most SOLAS vessels to carry a marine radar capable of operating on the 9 GHz band?
- SOLAS Chapter IV (Radio Communications)
- SOLAS Chapter V (Safety of Navigation), Regulation 19 (Correct answer)
- COLREGS Rule 7 (Risk of Collision)
- STCW Chapter VIII (Watch-keeping)
Correct answer: SOLAS Chapter V (Safety of Navigation), Regulation 19
SOLAS Chapter V, Regulation 19 specifies navigational equipment requirements by vessel size and trade area, mandating radar (9 GHz X-band) for vessels above specified tonnage thresholds.
Question 5: What is 'target glint' and how does it affect radar measurements?
- Steady, high-amplitude return from a corner reflector; improves range accuracy
- Rapid fluctuation of a target's echo amplitude and apparent position due to interference from multiple reflection points on the target (Correct answer)
- Bright return from the target's radar-absorbing paint that disappears at short range
- A fixed false echo caused by antenna sidelobe illumination of a large target
Correct answer: Rapid fluctuation of a target's echo amplitude and apparent position due to interference from multiple reflection points on the target
Glint occurs when a target with multiple scattering centers produces a composite echo whose amplitude and apparent angle of arrival fluctuate rapidly, introducing bearing and range measurement errors.
Question 6: What is the typical function of a radar transponder (RACON) when interrogated by a vessel's radar?
- Re-transmit the vessel's own signal back to confirm two-way communication
- Respond with a coded signal that appears as a distinctive Morse-code dash extending from the transponder's position on the radar display (Correct answer)
- Provide AIS position data superimposed on the radar picture
- Suppress nearby clutter to highlight the vessel's position
Correct answer: Respond with a coded signal that appears as a distinctive Morse-code dash extending from the transponder's position on the radar display
A RACON (radar beacon) receives the interrogating radar pulse and transmits a reply swept across the radar frequency, producing a distinctive coded symbol (Morse character dash) that helps mariners identify specific marks or hazards.
Question 7: What is the significance of the 'noise floor' in a radar receiver for determining minimum detectable signal (MDS)?
- The noise floor sets the upper limit of the receiver's dynamic range
- A target's echo must exceed the noise floor by a sufficient signal-to-noise ratio for reliable detection (Correct answer)
- The noise floor determines the maximum transmit power allowed by regulations
- The noise floor is only relevant during calibration and does not affect operational detection
Correct answer: A target's echo must exceed the noise floor by a sufficient signal-to-noise ratio for reliable detection
The receiver's thermal noise floor establishes a baseline below which signals cannot be distinguished from noise; for reliable detection a target return must exceed the noise floor by several dB (typically 10–15 dB SNR).
CMRT Radar System Fundamentals 5
What does the term 'antenna gain' mean in the context of a marine radar antenna?