In this lecture you will learn about:

• Radar Ranges
• Pulse Length
• Pulse repetition frequency PRF
• Pulse repetition interval PRI

Pulsed radar has to send all the energy for the pulse in one very quick flash, if you could see the pulses you'd see what looked like a strobe light, the typical power in each pulse is around 2000 to 8000 watts. The Pulses travel at the speed of light: 300 metres in one millionth of a second! One of the amazing things about radar is that it can accurately time these pulses and use this information to work out the range.

Pulse Length.

The radar controls precisely how long the pulses are simply by timing precisely how long it transmits for, and this value changes with range. The radar needs much longer pulses for longer ranges, and shorter pulses for shorter ranges - easy to remember. It's important to note at this stage of the course that each time you adjust the range of the radar you are also adjusting the pulse length, this is an important point, as this will effect the way you make use of the radar and there is much more on this later.

Pulse Repetition Interval PRI

Immediately after sending each pulse the radar's receiver must wait sufficient time for it to complete a round trip - to the full extent of the radars range setting and back, this is to ensure the radar does not get any valid return signals confused with false returns from the previous pulse. This 'waiting time' is referred to as the Pulse Repetition Interval or PRI.

The term 'waiting' used above in a radar context is about a thousandth of a second, for a radar that's a long time, enough time for the pulse to travel off the face of the earth.

The Pulse Repetition Interval has to change depending on the range you select, around 750µs for short ranges and up to 1500µs for longer ranges. At the speed of light these intervals allow the pulses to cover around 120 to 240 NM so they are well off the face of the earth before the radar repeats its transmit and receive cycle.

The pulse repetition interval is long on longer ranges and short on shorter ranges.

Pulse Repetition Frequency PRF

This is simply the number of pulses per second that the radar can transmit. On shorter ranges the pulses are shorter, and the PRI is also shorter, so the frequency will increase, so there will be more pulses per second. On longer ranges everything takes much longer, the pulses are longer, the PRI is longer, so the PRF has to come down, so there are fewer pulses per second.