CMRT Radar System Fundamentals 4 — Questions and Answers
Question 1: What frequency band do most commercial marine navigation radars operate in for short-range coastal navigation?
- S-band (2–4 GHz)
- X-band (8–12 GHz, typically 9.2–9.5 GHz) (Correct answer)
- C-band (4–8 GHz)
- Ku-band (12–18 GHz)
Correct answer: X-band (8–12 GHz, typically 9.2–9.5 GHz)
X-band radars operating at approximately 9.3–9.5 GHz are the most common marine navigation radars because their shorter wavelength provides finer resolution and suits smaller antennas.
Question 2: What is the radar horizon, and what factor primarily determines it for a given installation?
- The maximum range set by the operator; determined by the selected range scale
- The geometric line-of-sight distance to the sea surface; primarily determined by antenna height (Correct answer)
- The range beyond which clutter exceeds target returns; determined by sea state
- The range at which the radar beam intersects the ionosphere; determined by frequency
Correct answer: The geometric line-of-sight distance to the sea surface; primarily determined by antenna height
The radar horizon is approximately 1.22 × √(antenna height in feet) nautical miles; since radar waves travel slightly beyond the geometric horizon due to refraction, antenna height is the dominant factor.
Question 3: Which parameter directly controls the range resolution of a pulsed radar?
- Antenna rotation rate
- Pulse width (pulse duration) (Correct answer)
- Transmitter peak power
- Antenna gain
Correct answer: Pulse width (pulse duration)
Range resolution equals c × τ / 2, where τ is the pulse width; shorter pulses resolve closely spaced targets at the same bearing more precisely.
Question 4: What is 'rain clutter' and which radar control primarily suppresses it?
- Returns from precipitation; suppressed by the FTC (fast time constant) or differentiation circuit (Correct answer)
- Interference from another ship's radar; suppressed by the interference rejection filter
- Multipath returns from rain-moistened superstructures; suppressed by STC
- Sidelobe returns caused by rain; suppressed by reducing transmit power
Correct answer: Returns from precipitation; suppressed by the FTC (fast time constant) or differentiation circuit
Rain clutter consists of returns from precipitation filling large areas of the display; the FTC (rain clutter) control differentiates the video signal to enhance target edges and reduce the smeared clutter background.
Question 5: In a waveguide transmission line used in marine radar, what is the primary advantage over coaxial cable at microwave frequencies?
- Waveguide is less expensive and easier to install than coaxial cable
- Waveguide has lower attenuation and higher power-handling capability at microwave frequencies (Correct answer)
- Waveguide supports both TEM and TE modes, allowing more signal channels
- Waveguide is flexible and can be routed around tight bends more easily
Correct answer: Waveguide has lower attenuation and higher power-handling capability at microwave frequencies
At X-band frequencies, coaxial cable has unacceptably high resistive losses; rectangular waveguide carries the signal as a guided wave in air with much lower attenuation and can handle the high peak powers of radar transmitters.
Question 6: What is the function of the local oscillator (LO) in a superheterodyne radar receiver?
- Generate the transmit pulse at the radar operating frequency
- Produce a frequency slightly offset from the received signal to create an intermediate frequency through mixing (Correct answer)
- Provide the timing reference for the pulse repetition interval
- Amplify the received echo before it enters the mixer
Correct answer: Produce a frequency slightly offset from the received signal to create an intermediate frequency through mixing
The local oscillator generates a CW signal that the mixer combines with the incoming RF echo to produce the fixed intermediate frequency (typically 60 MHz) at which the bulk of receiver amplification occurs.
Question 7: What causes 'indirect (false) echoes' on a marine radar display?
- Noise bursts in the receiver that exceed the detection threshold
- Radar energy reflected from the ship's own structure before reaching a real target, creating a ghost return (Correct answer)
- Doppler frequency shifts from fast-moving targets misinterpreted as range errors
- PRF aliasing that places a distant target's echo on a closer range ring
Correct answer: Radar energy reflected from the ship's own structure before reaching a real target, creating a ghost return
Indirect echoes occur when the radar beam strikes part of the vessel's superstructure (masts, funnels) and is deflected to a target, returning along the same indirect path and appearing on the display at an incorrect bearing or range.
CMRT Radar System Fundamentals 4
What frequency band do most commercial marine navigation radars operate in for short-range coastal navigation?