CMRT Signal Processing & Antenna Theory 5 — Questions and Answers
Question 1: In marine radar, rain clutter differs from sea clutter primarily because:
- Rain clutter appears only at short ranges
- Rain clutter is volumetric and can appear at any range within precipitation areas (Correct answer)
- Rain clutter rotates with the antenna
- Rain clutter only affects X-band radars
Correct answer: Rain clutter is volumetric and can appear at any range within precipitation areas
Rain clutter is volumetric, occurring wherever precipitation exists, unlike sea clutter which is surface-based and range-dependent.
Question 2: What is the significance of the radar ambiguity function?
- It predicts antenna mechanical failure modes
- It describes a waveform's joint range and Doppler resolution and ambiguity performance (Correct answer)
- It calculates transmitter efficiency
- It determines the maximum PRF for a given range
Correct answer: It describes a waveform's joint range and Doppler resolution and ambiguity performance
The ambiguity function characterizes how well a radar waveform can simultaneously resolve targets in range and Doppler velocity.
Question 3: Which waveguide mode is the dominant mode used in rectangular waveguides for marine radar feeds?
- TM11
- TE10 (Correct answer)
- TEM
- TE20
Correct answer: TE10
The TE10 mode is the dominant mode in rectangular waveguides because it has the lowest cutoff frequency and propagates with the least loss.
Question 4: A radar system has a false alarm time of 10 minutes. If the detection threshold is lowered, the false alarm time will:
- Increase, producing fewer false alarms per unit time
- Decrease, producing more false alarms per unit time (Correct answer)
- Remain unchanged regardless of threshold
- Increase exponentially with each threshold reduction
Correct answer: Decrease, producing more false alarms per unit time
Lowering the detection threshold allows more noise peaks to exceed it, increasing the false alarm rate and decreasing the mean time between false alarms.
Question 5: What is the function of a circulator in a marine radar transceiver?
- It rotates the antenna at a fixed speed
- It routes the transmit pulse to the antenna and the receive echo to the receiver (Correct answer)
- It circulates coolant through the magnetron
- It provides circular polarization switching
Correct answer: It routes the transmit pulse to the antenna and the receive echo to the receiver
A circulator is a non-reciprocal device that directs transmit energy to the antenna and receive energy from the antenna to the receiver, protecting the receiver from transmit power.
Question 6: Aperture tapering (amplitude weighting) in antenna design is used to:
- Increase antenna gain to its theoretical maximum
- Reduce sidelobe levels at the cost of slightly wider main beam (Correct answer)
- Increase the operating frequency range
- Eliminate the need for a feed network
Correct answer: Reduce sidelobe levels at the cost of slightly wider main beam
Amplitude tapering reduces sidelobes by weighting edge elements less than center elements, but results in a slightly broader main beam and reduced gain.
Question 7: In coherent radar processing, what enables the Doppler frequency of a target to be measured?
- The target's radar cross section variation
- Phase coherence between successive transmitted pulses allows detecting inter-pulse phase changes (Correct answer)
- The antenna rotation rate
- The time-bandwidth product of the waveform
Correct answer: Phase coherence between successive transmitted pulses allows detecting inter-pulse phase changes
Coherent processing maintains phase reference between pulses, allowing the Doppler-induced phase shift between returns to be measured as a frequency.
CMRT Signal Processing & Antenna Theory 5
In marine radar, rain clutter differs from sea clutter primarily because: