CMRT Radar System Fundamentals 3 — Questions and Answers
Question 1: What is the significance of the 'blind zone' (dead zone) directly around a radar antenna?
- It is the area where the radar beam has maximum power and saturates the receiver
- It is the minimum range within which targets cannot be detected because the duplexer is still recovering (Correct answer)
- It is a regulatory exclusion zone where radar emissions are prohibited
- It is the area affected by multipath reflections from the vessel's own superstructure
Correct answer: It is the minimum range within which targets cannot be detected because the duplexer is still recovering
The minimum range or blind zone exists because the duplexer recovery time after transmission prevents the receiver from processing any echoes returned from very close targets.
Question 2: How does antenna beamwidth affect the azimuthal (bearing) accuracy of a marine radar?
- Narrower beamwidth provides better bearing discrimination and accuracy (Correct answer)
- Wider beamwidth provides better bearing discrimination and accuracy
- Beamwidth does not affect bearing accuracy, only range accuracy
- Beamwidth only affects detection range, not target position
Correct answer: Narrower beamwidth provides better bearing discrimination and accuracy
A narrower horizontal beamwidth means the antenna illuminates a smaller angular slice of the scene, allowing closer targets at different bearings to be resolved and improving bearing accuracy.
Question 3: What does the term 'noise figure' describe in a radar receiver?
- The ratio of output noise power to the noise that would exist if the receiver were perfect (Correct answer)
- The peak transmit power divided by the average noise floor
- The number of false alarms generated per scan
- The gain of the final IF amplifier stage
Correct answer: The ratio of output noise power to the noise that would exist if the receiver were perfect
Noise figure quantifies how much additional noise a receiver introduces compared to an ideal noiseless receiver, directly affecting the minimum detectable signal and thus detection range.
Question 4: In a solid-state radar, what replaces the magnetron as the transmit source?
- A klystron power amplifier
- A solid-state power amplifier (SSPA) using transistors such as GaN devices (Correct answer)
- A traveling-wave tube amplifier (TWTA)
- A Gunn-effect diode oscillator in a waveguide cavity
Correct answer: A solid-state power amplifier (SSPA) using transistors such as GaN devices
Modern solid-state marine radars use gallium nitride (GaN) or similar semiconductor power amplifiers instead of magnetrons, offering longer life, lower peak power with pulse compression, and no high-voltage supplies.
Question 5: What is 'pulse compression' and why is it used in some marine radars?
- Reducing the pulse repetition period to increase data rate
- Transmitting a long coded pulse and processing it to achieve the resolution of a short pulse (Correct answer)
- Compressing the analog video signal before digitization
- Increasing peak power by shortening the pulse duration
Correct answer: Transmitting a long coded pulse and processing it to achieve the resolution of a short pulse
Pulse compression allows a radar to transmit a long, frequency- or phase-modulated pulse to achieve high average power, then correlate the return to produce the equivalent resolution of a very short pulse.
Question 6: What is the purpose of the sensitivity time control (STC) circuit in a marine radar receiver?
- Automatically adjust the PRF based on selected range scale
- Reduce receiver gain at short ranges to suppress sea clutter (Correct answer)
- Increase transmitter power during adverse weather
- Synchronize the sweep timing with the antenna rotation
Correct answer: Reduce receiver gain at short ranges to suppress sea clutter
STC progressively increases receiver gain with time (range) after each pulse, reducing sensitivity at close ranges where sea clutter is strongest and restoring full sensitivity at longer ranges.
Question 7: What does ARPA stand for in the context of marine radar systems?
- Automatic Radar Plotting Aid (Correct answer)
- Advanced Radar Processing Algorithm
- Azimuth Reference and Position Alignment
- Automatic Range and PRF Adjustment
Correct answer: Automatic Radar Plotting Aid
ARPA (Automatic Radar Plotting Aid) is a system that automatically tracks multiple radar targets, computing their course, speed, closest point of approach (CPA), and time to CPA to assist collision avoidance.
CMRT Radar System Fundamentals 3
What is the significance of the 'blind zone' (dead zone) directly around a radar antenna?