The acronym RADAR comes from Radio Detection and Ranging. We have already seen that a radio waves can be reflected by objects, just as visible light waves can be reflected. Some of the waves will be reflected directly back towards the transmitter. A sensitive receiver tuned to the transmitted frequency can detect the reflected waves, and therefore the object reflecting them. If the waves are transmitted in short bursts or pulses then it is possible to measure the time between the transmission of tho original signal and the reception of the reflection. Because electromagnetic waves travel at a virtually constant speed through a particular medium, such as air, the time from transmission to reception is proportional to the distance the signal has travelled and therefore the range of the object from the transmitter. This was the earliest form of radar use.
The reflectivity of an object depends on the shape it presents to the signal beam, like a mirror reflecting light. However, it also depends to a considerable extent on its size relative to the wavelength of the radio signal. Ships, for example, are large and can reflect quite long wavelength (hence low frequency) signals. Raindrops, however, only reflect very short wave length (hence very high frequency) signals.
By sending a narrow, directional beam of waves it became easy to determine whether the reflecting object (target) was contained in the beam. To determine the actual direction of the target, the beam had to be moved around, so the antenna was turned mechanically.
The Time Base
A basic reflected radar signal can be displayed on a screen.The width of the screen represents the distance out to which the operator wishes to detect a target. A stream of electrons pointing at the screen is made to move in a straight line along it at a speed proportional to the speed of radio waves across that distance. It therefore simulates the pulse of radar energy transmitted outwards. After reaching the end of that distance (the maximum range), the electron stream is brought back to the beginning again for the next pulse (fly-back). A reflected signal from a target is engineered to make a mark on this timebase by deflecting the electron beam at 90° to it, and an operator can see the target’s range by comparing it with fixed marks representing ranges from the transmitter.
A basic radar only shows ranges, but once a target is detected it is possible to move the antenna until the signal is no longer received, which will give an indication of the target’s bearing from the receiver. Moving the timebase with the antenna will show that direction, as the target will ‘paint when it is within the polar diagram of the antenna, and will slowly disappear (fade) when no reflected signal is received.
A moving target will change its position between sweeps. If it is desired to provide further information on the movement of a particular target, a separate antenna can be moved back and forth in a sector scan (reciprocat ing) so that the target is illuminated for a much longer period. If the scan vertical, the display can measure height.
Precision approach radars use the sector scanning technique with two antennas to monitor the target in both the horizontal and the vertical.
Use of Radar
The use to which a radar is to be put determines the design parameters of the transmitter and antenna. As already stated, the size of the target often determines the signal frequency. For example, a weather radar requires wavelength of about 3 cm to reflect large raindrops while ignoring small ones, hence a frequency of about 10 GHz. Similar but usually slightly longer wavelengths (to avoid saturation by rain returns) at corresponding frequencies can resolve detail from larger objects, such as shapes of aircraft on an airfield ground movement radar.
Modern Radars do not always require the antennas to rotate physically in order to sweep the beam. A number of transmitting or receiving antennas within a large array can be electronically switched by a computer in such a way that the resultant beam direction changes as if the aerial was actually moving. Nor do modern radars require to display the actual received signals. It is usual for a computer to convert the received signals to digital inputs which can be processed before display on a computer monitor.