A photocopy of this document is in the Navy Department Library.
The principle upon which radar operates is simple. Briefly, it consists of sending out a short pulse of radio energy and receiving a portion of the same energy reflected back by objects in its path.
The energy sent out by a radar set is the same as that transmitted by ordinary radio. But unlike an ordinary radio set a radar set literally picks up its own als.
Thus the radar set transmits a short pulse of energy, receives its echoes, then transmits another pulse and receives its echoes. This out-and-back cycle is repeated from 60 to times a second. The extremely short time interval between the sending of the pulse and the reception of its echoes can be measured very accurately--even to one ten-millionth of a second. Since the speed at which the radio energy travels is known it is the same as that of light the range of any reflecting object can be obtained.
Furthermore, the transmitted energy is focused in a beam which can be pointed in any chosen direction.
Hence the bearing of a reflecting object is also obtained. The echoes received by the radar appear not as sound als, as is the case with ordinary radio, but as marks of light on a flourescent tube.
The face of the tube--or scope, as it is usually called-may be marked with a scale of miles, or degrees, or both. Hence from the position of any al on the scope an observer can tell at a glance the range and direction of the corresponding target. The three commonest types of scopes are shown in the sketch at the left.
Just as there are different types of scopes, there are many kinds of radar sets. They vary in size from those that can be carried by hand to those that weigh several thousand pounds.
The de of a radar set depends upon the job it is intended to do, and radar serves many functions. The chief of these are described in the following s. Circle on scope marks mile radius.
Note correspondence with geodetic chart insert. Such definition, of course, is useful as an aid in of planes and ships, in observing the movements of ships and convoys, in detecting targets at night, in shelling or bombing enemy vessels or coastal targets, or in mapping work. This PPI scope has a flourescent screen so made that the als from reflecting objects will persist for a few moments.
Other types of scopes are shown on the following s, illustrating some of the chief uses of radar. For most of these functions, several radar sets are now available.
Details of de, operation and performance of specific sets are listed later in the book. It can be used to locate enemy units in night battles, to maintain convoy formation and to navigate.
Accurate and continuous determination of range rate allows the gunnery officer to alter fire to take into the enemy ship's change in course. These sets can direct patrol craft to unidentified ships, can guide friendly ships through mine fields or fog, and can as any targets to other radars having the specific function of coast artillery gun-laying. Enough warning is given to allow fighter planes to take off, thus eliminating the need for a standing air patrol.
MOBILE RADAR provides range for anti-aircraft guns, day or night, more accurately than optical height finders, and can give continuously and automatically the elevation and azimuth of target regardless of its visibility. From this range the fighter can, in daytime, complete the interception visually, or at night, the fighter can track the target to gun range by means of another radar set AI carried in his own plane.
A CGI should direct a nightfighter to within 20, feet of the target, at which point the nightfighter's radar AI will permit tracking until the enemy plane is within firing range. At feet a plane with SCR can detect a 10,ton ship 45 mi.
A key for sending messages can be used with portable sets. Another device Loran enables ships to find their position to within 1 mile, as far as miles from shore. Because of the clear definition between land and water provided by radar, as is illustrated in the scope photo of an island on 3 of this book, important al aid can be given planes or ships at night, in fog or storm; e.
Low-level altimeters measure from to 5 feet. Other radar altimeters can measure absolute altitude as high as 40, feet.
As has been seen on the s, radar can be useful in almost any military situation. But the successful use of radar demands an understanding of its properties.
Radar has several inherent limitations: it cannot "see" under water, for example, nor very far beyond the line of sight. Its range is affected to some extent by atmospheric conditions.
Radar sometimes records als from clouds, which to an inexperienced operator resemble als from aircraft or ships. It is still a new weapon, rushed through production and into field use in so short a time that frequent instances of faulty operation and breakdown are encountered. Further, the performance and operation of individual radar sets vary greatly.
The most important factors in the performance of a set are the wavelength used and the proficiency of the operating and maintenance crews; in the set descriptions which follow the range of wavelength is indicated. Sets in the shortest wavelength range are called microwave. Those employing the longest radar wavelengths which are still shorter than the waves used by high-frequency radio communications are deated long wave.
Most radar equipment now issued uses a long or medium wavelength. Sets employing these frequency ranges usually have a long range, which increases with the size of the antenna and the height at which it is mounted.
The definition and angular accuracy, however, are poor. Choice of a location for these sets is most important, since the presence of hills, buildings, or other reflecting objects within view of the radar site will cause permanent echoes on the scope.
Long wave ground or ship sets, too, give poor protection against distant surface vessels of low-flying planes. Usually, there are also gaps in the coverage against aircraft within the extreme detection range.
Microwave equipment has the advantage of sharper definition and greater angular accuracy. It also offers better coverage of objects at low elevation angles, and is less dependent upon careful choice of site. Most microwave sets now in production do not have as great a range against high-flying aircraft as that of long wave equipment.
Gaps in coverage are less important in microwave sets, but there is likely to be poorer coverage against high-altitude planes. The ability of radar to "see" under any conditions of visibility and to give continuous accurate values of target range and direction has considerably altered the military picture; but the full capabilities of this new facility can be realized only when all concerned appreciate its possibilities and requirements.
First, the commanding officer and his staff must have such an appreciation.
They must understand that radar permits undertakings of a wholly new type to be carried out. For example, ships can be navigated at night at high speed in narrow waters with precision and safety, and can engage an unseen enemy with guns or torpedoes. By the use of radar beacons, aircraft can home with an accuracy measured in yards on an objective distant 80 miles or more.
The commanding officer and his staff must appreciate that the employment of radar by the enemy must always be assumed, and they must understand what this means in terms of their plans. Darkness, smoke, or fog must no longer be relied on for certain cover.
Aircraft approaching an objective must no longer fly as high as possible to escape detection; better will attend a very low approach or one which makes use of the radar "shadows" cast by mountains obstructing radar beams. The use of countermeasures to confuse or defeat the enemy's radar must be considered in planning crucial operations. It must be understood that radar cover against air attack must be planned for the very earliest stages of a new occupation.
Only by the timely alerting and the defensive facilities provided by radar will it be feasible to deal with the sustained air attack which can be mounted by a determined enemy. The tactical officer in the field must understand the power that radar gives him.
Its use is so new that tactical doctrine is still being worked out, and an ingenious officer will sometimes be able to employ one of the sets described in the following s in a military situation never contemplated by its deer. For example, there is the case of a patrol bomber equipped with ASE, which located enemy ships and attacked them unsuccessfully.
A squadron of motor torpedo boats not equipped with radar was available, and the aircraft was directed to cruise in the area and direct the PT boats to the target, using the general situation plot provided by the radar.
The PT boats were thus enabled to make a successful attack. Full reliance must be placed on radar in order that it may be wholly effective. In the Hood-Bismarck engagement, for example, the gunnery officer of the Hood had his choice between using optical and radar range information, which differed by yards. For his first salvo, he used the optical range value. There was no opportunity to fire another.
Finally, the officer in charge of the actual operation of radar equipment must be resourceful, skillful, and well-informed. As has been remarked, technical difficulties with the equipment are all too frequent, and must often be dealt with without the help of adequate spares or test equipment. The radar officer must realize that the reliance which will be placed on the information provided by his equipment will largely depend on the reliability with which it has been made available.
He must be prepared at all times for the employment of countermeasures by the enemy, in an attempt to render our radar partly or wholly useless. In general, such measures can be dealt with by an experienced and resourceful radar officer. He must understand that the proper interpretation of the als presented by the radar equipment is most important.
Spurious echoes from clouds must be recognized as such, the reliability of IFF indications must be maintained to avoid false alarms, and highly skilled interpretation of als will even enable fairly accurate estimates to be made of the size and character of enemy forces from the radar als they present.
Given this understanding and skillful use of radar by all concerned, we shall have the advantage of the enemy in this field. Experts assert that the state of technical development of U. ZIP: 50022