CMRT Signal Processing & Antenna Theory 3 — Questions and Answers
Question 1: What is the primary advantage of pulse compression in marine radar?
- Increases antenna gain
- Achieves fine range resolution without requiring a very short transmitted pulse (Correct answer)
- Eliminates sea clutter completely
- Doubles the maximum unambiguous range
Correct answer: Achieves fine range resolution without requiring a very short transmitted pulse
Pulse compression transmits a long modulated pulse and compresses it on receive, achieving the range resolution of a short pulse with the energy of a long pulse.
Question 2: Which of the following best describes the polarization of a typical marine radar antenna?
- Circular polarization
- Horizontal polarization (Correct answer)
- Vertical polarization
- Elliptical polarization
Correct answer: Horizontal polarization
Marine radar antennas use horizontal polarization because it provides better sea clutter rejection compared to vertical polarization.
Question 3: When a radar receiver is described as having a noise figure of 6 dB, this means the receiver adds noise equivalent to:
- 6 times the thermal noise at room temperature
- Quadrupling the input thermal noise power (Correct answer)
- A 6 dB signal loss before amplification
- Reducing sensitivity by a factor of 6
Correct answer: Quadrupling the input thermal noise power
A 6 dB noise figure means the SNR degrades by a factor of 4 (quadrupling noise power) through the receiver.
Question 4: In FMCW radar used in some marine systems, the target range is determined by:
- Time delay between transmitted and received pulses
- Measuring the frequency difference between transmitted and received signals (Correct answer)
- Counting pulse repetition intervals
- Analyzing the Doppler shift only
Correct answer: Measuring the frequency difference between transmitted and received signals
In FMCW radar, the frequency difference (beat frequency) between the linearly swept transmit and receive signals is proportional to target range.
Question 5: What effect does increasing radar antenna height have on maximum detection range?
- No effect since range depends only on transmitter power
- Extends the radar horizon and increases maximum range (Correct answer)
- Reduces clutter by narrowing the beam in elevation
- Decreases range due to increased ground reflection
Correct answer: Extends the radar horizon and increases maximum range
Greater antenna height extends the radar horizon because range to the horizon is proportional to the square root of antenna height.
Question 6: A CFAR (Constant False Alarm Rate) processor in marine radar is designed to:
- Maintain constant transmitter output power
- Adaptively set the detection threshold based on local noise and clutter levels
- Keep the false alarm rate constant regardless of sea state (Correct answer)
- Fix the PRF regardless of range scale
Correct answer: Keep the false alarm rate constant regardless of sea state
CFAR processors adaptively adjust the detection threshold so that the probability of false alarms remains constant across varying clutter conditions.
Question 7: The effective aperture of an antenna is related to its gain by which relationship?
- Ae = G × λ
- Ae = G × λ² / (4π) (Correct answer)
- Ae = G / λ²
- Ae = 4π × G × λ²
Correct answer: Ae = G × λ² / (4π)
Effective aperture Ae = Gλ²/(4π), linking an antenna's ability to capture power to its gain and operating wavelength.
CMRT Signal Processing & Antenna Theory 3
What is the primary advantage of pulse compression in marine radar?