8-5A: Range, Pulse Width, PRF

Question 1

8-5A3

The minimum range of a RADAR is determined by:

A. The frequency of the RADAR transmitter.
B. The pulse repetition rate.
C. The transmitted pulse width.
D. The pulse repetition frequency.

Answer 1

ANSWER C
The minimum range at which a RADAR can detect a target is determined by the pulse duration, sometimes called pulse width or pulse length. As the RADAR operator switches down to a close-in scale, the RADAR automaticallyshortens its pulse width to as little as 0.1 microsecond. A 0.1 microsecond pulse overs approximately 150 yards of range on the display. Small boat marine REDARS use even shorter pulses for target detection as close as 75 feet.

Question 2

8-5A5

For a range of 30 nautical miles, the RADAR pulse repetition frequency should be:

A. 0.27 kHz or less.
B. 2.7 kHz or less.
C. 27 kHz or more.
D. 2.7 Hz or more.

Answer 2

ANSWER B
First calculate the amout of time for a RADAR wave to travel 30 nautical miles out and back. 30 x 2 x 6.17 = 370.2 microseconds. The maximum pulse repetition frequency required to allow that signal to return is 1 divided by the pulse time in seconds or 1/0.0003702 = 2,701 Hz = 2.701 kHz or less. Watch out for answers that say “or more.” They would not work because the pulse repetition rate would not have enough time for the pulse to return the the receiver before another pulse was sent. As a target gets closer, you want to be able to detect progressively small amounts of movement, which span a greater subtended angle for a given speed. High PRFs are also unusable where ambiguity might be a problem if you use a RANGE GATE to eliminate “impssible solutions.” Why would you want to do this? High PRFs give you more effective power. If you use averaging techniques; sometimes useful for very weak targets. Range GATES have to be used with caution however, so as not to inadvertantly blank the real thing.