FCC Commercial Element 8 pool, section 38

Subelement D

Display & Control Systems

Section 38

Miscellaneous

838D1

The heading flash is a momentary intensification of the sweep line on the PPI presentation. Its function is to:

A.
Alert the operator when a target is within range.
B.
Alert the operator when shallow water is near.
Correct Answer
C.
Inform the operator of the dead-ahead position on the PPI scope.
D.
Inform the operator when the antenna is pointed to the rear of the ship.

838D1

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838D2

The major advantage of digitally processing a RADAR signal is:

A.
Digital readouts appear on the RADAR display.
Correct Answer
B.
Enhancement of weak target returns.
C.
An improved operator interface.
D.
Rectangular display geometry is far easier to read on the CRT.

838D2

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838D3

In order to ensure that a practical filter is able to remove undesired components from the output of an analog-to-digital converter, the sampling frequency should be:

A.
The same as the lowest component of the analog frequency.
B.
Two times the highest component of the analog frequency.
Correct Answer
C.
Greater than two times the highest component of the sampled frequency.
D.
The same as the highest component of the sampled frequency.

838D3

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838D4

Bearing resolution is:

Correct Answer
A.
The ability to distinguish two adjacent targets of equal distance.
B.
The ability to distinguish two targets of different distances.
C.
The ability to distinguish two targets of different elevations.
D.
The ability to distinguish two targets of different size.

838D4

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838D5

The output of an RC integrator, when driven by a square wave with a period of much less than one time constant is a:

A.
Sawtooth wave.
B.
Sine wave.
C.
Series of narrow spikes.
Correct Answer
D.
Triangle wave.

838D5

When an RC integrator is driven by a square wave with a period much shorter than the time constant (RC), the output waveform takes the shape of a triangle wave.

To understand why this happens, let's consider the behavior of an RC integrator. An RC integrator consists of a resistor (R) and a capacitor (C) connected in series. When a voltage input signal is applied, the capacitor charges and discharges according to the time constant (τ = RC), which is the product of the resistance and capacitance.

In the case of a square wave with a period much shorter than the time constant, the input voltage changes rapidly from one state to another before the capacitor has enough time to fully charge or discharge. As a result, the capacitor cannot fully respond to the input changes, and its voltage output increases or decreases gradually, creating a linear ramp-like waveform. This ramp waveform, when plotted, resembles a triangle.

Mnemonic: "Swift Rise, Gradual Fall"

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838D6

How do you eliminate stationary objects such as trees, buildings, bridges, etc., from the PPI presentation?

A.
Remove the discriminator from the unit.
Correct Answer
B.
Use a discriminator as a second detector.
C.
Calibrate the IF circuit.
D.
Calibrate the local oscillator.

838D6

How do you eliminate stationary objects such as trees, buildings, bridges, etc., from the PPI presentation?

Use a discriminator as a second detector.

To eliminate stationary objects such as trees, buildings, bridges, etc., from the PPI (Plan Position Indicator) presentation in a radar system, you can use a technique called "MTI" (Moving Target Indication). The MTI technique uses a discriminator as a second detector to filter out signals from stationary objects, leaving only the signals from moving targets.

Here's how it works: The radar receiver uses two detectors, one after the other. The first detector captures the radar returns as usual. The second detector, known as the discriminator, compares the current radar returns with the previous ones. It identifies and removes signals that remain stationary or have very slow changes over time (indicative of stationary objects like buildings and trees).

The remaining signals with significant changes are likely coming from moving targets, such as vehicles or aircraft. By eliminating stationary returns, the PPI presentation becomes clearer and allows radar operators to focus on detecting and tracking moving targets more effectively.

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