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Subelement A
RADAR Principles
Section 6
Pulse Width - Pulse Repetition Rates
If the PRF is 2500 Hz, what is the PRI?
  • 40 microseconds.
  • Correct Answer
    400 microseconds.
  • 250 microseconds.
  • 800 microseconds.

Hertz is a measure of cycles per second.

PRF = 1 / PRI

Thus, 1/2500 Hz = 0.0004 seconds (or 400 microseconds).

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If the pulse repetition frequency (PRF) is 2000 Hz, what is the pulse repetition interval (PRI)?
  • 0.05 seconds.
  • 0.005 seconds.
  • Correct Answer
    0.0005 seconds.
  • 0.00005 seconds.

To calculate the Pulse Repetition Interval (PRI) when the Pulse Repetition Frequency (PRF) is given, we can use the formula:

PRI = 1 / PRF

Given: PRF = 2000 Hz

Step 1: Calculate the PRI in seconds: PRI = 1 / 2000 Hz PRI = 0.0005 seconds

So, if the PRF is 2000 Hz, the Pulse Repetition Interval (PRI) is 0.0005 seconds.

Mnemonic: "PRF-2000-PRI-0.0005"

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The pulse repetition rate (PRR) refers to:
  • The reciprocal of the duty cycle.
  • The pulse rate of the local oscillator tube.
  • The pulse rate of the klystron.
  • Correct Answer
    The pulse rate of the magnetron.

The pulse repetition rate (PRR) refers to the pulse rate of the magnetron.

In RADAR systems, the magnetron is responsible for generating the microwave pulses that are transmitted through the RADAR antenna. The PRR represents the number of pulses produced by the magnetron in one second, determining the rate at which pulses are transmitted by the RADAR system.

Mnemonic: "PRR - Magnetron Makes Microwaves Move"

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If the RADAR unit has a pulse repetition frequency (PRF) of 2000 Hz and a pulse width of 0.05 microseconds, what is the duty cycle?
  • Correct Answer
    0.0001
  • 0.0005
  • 0.05
  • 0.001

Pulse Width = 0.05 microseconds (0.05 x 10^(-6) seconds) PRF (Pulse Repetition Frequency) = 2000 Hz

Step 1: Calculate the PRI (Pulse Repetition Interval) in seconds: PRI = 1 / PRF PRI = 1 / 2000 Hz PRI = 0.0005 seconds (0.0005 s) or 0.5 microseconds (0.5 ┬Ás)

Step 2: Calculate the duty cycle in decimal form: Duty Cycle = Pulse Width / PRI Duty Cycle = 0.05 x 10^(-6) seconds / 0.0005 seconds Duty Cycle = 0.0001

So, the duty cycle of the RADAR unit is 0.0001 (0.01%).

Mnemonic: "DC 0.0001"

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Small targets are best detected by:
  • Short pulses transmitted at a fast rate.
  • Using J band frequencies.
  • Correct Answer
    Using a long pulse width with high output power.
  • All of these answers are correct.

When dealing with small targets located many miles away from the operating RADAR, it is essential to optimize the RADAR parameters to enhance detection capabilities. Lowering the pulse repetition rate (PRR) allows for a longer time interval between pulses. As a result, the RADAR can listen for longer durations, increasing the chances of receiving weak echoes from distant small targets.

The longer pulse width, achieved by having a lower PRR, helps improve range resolution and enhances the RADAR's ability to detect smaller targets. The longer pulse transmits more energy, increasing the likelihood of receiving reflected signals from distant objects.

Higher power RADARs can boost the strength of the transmitted signal, leading to stronger echo returns from small targets. This increased output power allows the RADAR to detect weaker echoes that might be missed by lower power output RADARs.

Mnemonic: "P.O.W.E.R. - Pulse, Output, Width, Echo, Resolution"

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What is the relationship between pulse repetition rate and pulse width?
  • Higher PRR with wider pulse width.
  • The pulse repetition rate does not change with the pulse width.
  • The pulse width does not change with the pulse repetition rate.
  • Correct Answer
    Lower PRR with wider pulse width.

The relationship between pulse repetition rate (PRR) and pulse width is that a lower PRR is associated with longer pulse width, and a higher PRR is associated with narrower pulse width.

Pulse repetition rate (PRR) refers to the number of pulses transmitted by the RADAR per second. Pulse width, on the other hand, refers to the duration of each individual pulse.

When the PRR is lower, there is more time between consecutive pulses, allowing each pulse to be longer in duration. This results in a wider pulse width. In contrast, with a higher PRR, pulses are transmitted more frequently, and each pulse has a shorter duration, resulting in a narrower pulse width.

Mnemonic: "Lower rate, Longer pulse"

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