 Max and Min Ranges of the Primary Radar

CPL Assist
The factors that control the range of primary radar are the pulse recurrence period (PRP) and the pulse width.

You will learn that primary radar works on the basis of multiple pulses of radio signal. A pulse is simply a short burst of signal. Radar uses the fact that a radio signal will reflect from many forms of object (rock, steel, water, etc.), and in doing so, some of the reflection may find its way back to the initiator of the pulse. On a standard primary radar system, the directional (dish) aerial, which transmitted the pulse, temporarily becomes a receiving aerial so as to await any reflected reply. On receiving a reply, the time between the transmission and receipt of the reply is equated to twice the distance to the target, using:

Range = Speed of Propagation x Time / 2

MAX RANGE

It is the period between pulses (while the aerial is in receiving mode), which determines the range. Only those replies that return during that period can be picked up by the aerial. After that period, the aerial will start to transmit the next pulse. The dish aerial, which constantly rotates will also have, by then, moved on from the direction of any late replies.

Because only replies that come back within the PRP can be detected, it can be seen that replies that come at the very end of the period will represent the maximum possible range of the radar. The maximum range can be calculated by putting the PRP into the time value of the Range formula, hence:

MaxR = speed of propagation x PRP / 2

MIN RANGE

Whilst thinking along the same lines, lets look at the signals that come from near objects. If the reflected signal is received whilst the aerial is still transmitting and has not transformed into a receiver, the reply will not be received. This will occur when the object is so close that, the pulse length (which is a time) is greater than twice the time it takes for the signal to return. A typical 3 millionth of a second pulses has a physical length of 900 meters. If it reflects from an object less than 450 meters away, the front end of the pulse will reach the aerial before the back end of the pulse has been transmitted. As the aerial will not yet be in receiving mode, these signals will not be seen. As a result, the Minimum range of the radar can be calculated using

MinR = speed of propagation x Pulse width / 2

Fly-back

Don’t forget to include fly-back in the max range calculation if it is included in the question. Fly-back is the small period of wasted time that it takes, practically to move the CRT’s electron beam back to the beginning of its stroke. If fly-back is given it is often referred to as, e.g., one tenth of the PRP. In this case the one tenth of the PRP should be removed prior to the times inclusion in the Max range formula.

PRF to PRP

Also take note that the PRP is not normally given. The question usually contains the pulse recurrence frequency (PRF). If this is the case, then, divide the PRF into ‘1’:

PRP = 1 / PRF

Then used the PRP in the formula, minus any fly-back.

MaxR = speed of propagation x (PRP - Fly-back) / 2