1. The design of a radar begins with consideration of its intended use, that is, the function to be performed by the radar as a whole. The uses generally divide into three categories:
1) Warning and surveillance of activity, including identification.
2) Aids to the direction of weapons, that is, gunfire control and searchlight control.
3) Observation of terrain echoes or beacons for navigation and control of bombing.
2. There is nothing mysterious or complex about radiolocation. It rests on the foundations of ordinary radio theory, and is a technique based on the transmission, reception, and interpretation of radiofrequency pulses. Considered as a whole, it must be admitted that even the most elementary of radar equipment is difficult to visualize, but thus us simply due to the fact that so many (normally) curious circuits and pieces of apparatus are gathered together under one roof. No particular circuit or detail of the equipment is in itself especially difficult to understand, and once the elements are known the complete assembly is no longer mentally unmanageable.
3. The word "radar" is derived from the phrase "radio direction-finding and range", and it may be more expressive than the older "radiolocation", or it may not. Finding the position of an aircraft or a ship by means of radio covers a very wide field of electronic application; covers, in fact, the whole area of radio direction-finding (R.D.F.) from the elementary bearing-loop to the principle of the reflected pulse which represents the latest principle of the technique. In this article the term will be used to cover only those methods of detection, which depend upon the reflected pulse, the characteristic (by popular opinion) which distinguishes radar from all other methods of position finding in that no co-operation is required on the part of the target. We shall not dwell, therefore, upon the older and more familiar methods, which depend upon the reception at two or more points of a signal, transmitted by the body under location itself.
4. The actual equipments in use which employ the reflected pulse principle are greatly varied from the point of view of physical appearance, but their basic principles are the same.
5. First, let us tabulate and briefly analyze the problem to be met. The aim of radar is to find the position of a target with respect to a fixed point on the ground - say the position of an aeroplane or a barrage balloon with respect to the radar equipment situated in a field a mile or so away. Three quantities must be measured in order to define the position of the aeroplane or the barrage balloon: first, the slant range, the length of the most direct line drawn from the radar site to the target; second, the angle of bearing, i.e. which point of the compass the target occupies; third, the angle of elevation. When the target is an aeroplane, these three quantities are continuously varying so that the problem of position finding is somewhat complicated by the fact that the radar equipment has to "follow" as well as find. In the case of barrage balloon, things are not quite so difficult, and the three important factors may be found at leisure.
6. Radar works in this way. Powerful transmitters send out a stream of radio-waves, called "pulses", at the rate of perhaps - one thousand every second. These pulses shoot through the air at a speed of one hundred and eighty-six-thousand miles a second till they hit an object, perhaps a ship, an aeroplane, an iceberg or a hill. These pulses cannot go through the object; and so they immediately shoot back at the same speed, in the opposite direction, and hit the aerial of the radar. The moment this happens, a spot or line of light appears on the radar screen, showing the position and distance of the object.
7. Today radar is used for many peaceful purposes all over the world. Modern ships of all nations carry radar to help them sail in bad weather, when it is possible to see for a distance of perhaps only a few hundreds yards. In the past, the captain usually had to anchor his ship at such times and wait for conditions to improve. But today a ship with radar can sail in almost any weather conditions, with no danger of an accident.
8. Radar also helps to guide aeroplanes across the world. Radio signals called beams are transmitted into the sky from land radio stations at a number of points along their course. The pilots can hear these signals on their radio and see them on their radar screen. The beams are like sign-posts in the sky. A pilot guides his aeroplane towards one beam, and then flies on to the next beam. At the end of his journey, he will land his aeroplane with the help of radar. Trained men at the airport watch the aeroplane on their radar screens and tell the pilot by radio the course to follow, they tell him when and by how much to reduce height, and give any other orders that may be necessary.
9. In many countries radar controls the speed of cars on roads where there is a speed limit. They hope that radar will help them reduce the number of accidents on the roads. Thousands of people all over the world lose their lives in road accidents every year. The police can now watch the speed of the cars on their radar screens and stop motorists from driving too fast.
10. Radar is used to forecast the weather, and to study the moon and stars. Powerful radar telescopes are used to study the stars. They are also used to study scientific experiments in space.
III. After-text Exercise
1. Agree or disagree with the next statements:
1) Radiolocation rests on the foundations of ordinary radio theory, and is a technique based on the transmission, reception and interpretation of radiofrequency pulses.
2) The aim of radar is to find a position of a target without respect to a fixed point on the ground.
3) Powerful transmitters send out a stream of radio-waves, called "pulses". These pulses can go through the object.
4) Radio signals called beams are transmitted into the sky from land radio stations at a number of points along aeroplanes course.
5) Powerful radar telescopes are used to study the stars.
2. Find the Information about:
1) finding the position of an aircraft or a ship by means of radio;
2) three quantities which must be measured in order to define the position of an aeroplane or the barrage balloon; 3)using radar by modem ships;
4) guiding aeroplanes by means of radar;
5) other peaceful uses of radar.