What Is Radar?


--How Radar Works--

A Radar Antenna Unit
SOME RADAR SETS have separate antennas for sending and receiving, but the commonest form uses one unit in both functions.



How Radar Pulse is Sent and Received
RADAR WORKS in "complete cycles". Time is allowed for an echo from one pulse to be received before another is sent out.



Three Types of Indication
Three Types of Indication
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 signals. 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 5000 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 focussed 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 signals, 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 signal 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 design 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 pages.


What Radar Can Do

ACTUAL-SIZE PHOTOGRAPH of a radar "scope," taken in a plane flying over Nantucket Island. Circle on scope marks 10-mile radius. Note correspondence with geodetic chart (insert.)

Radar Scope photograph

THIS PHOTOGRAPH ILLUSTRATES the clear definition between land and water provided by radar, as well as the sort of picture presented on one type of radar scope. Such definition, of course, is useful as an aid in navigation 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 signals from reflecting objects will persist for a few moments. Other types of scopes are shown on the following pages, illustrating some of the chief uses of radar. For most of these functions, several radar sets are now available. Details of design, operation and performance of specific sets are listed later in the book.


Radar Aids Ships in Locating Planes & Surface Craft

RADAR ENABLES ships to detect planes out to 100 miles and surface vessels to 28,000 yds, and to distinguish between friendly and enemy craft. It can be used to locate enemy units in night battles, to maintain convoy formation and to navigate.

Radar Controls Ship Gunfire

AFTER LOCATING SHIPS, radar provides range and bearing of targets to the guns, and permit the course of the enemy ship to be calculated more accurately than by any other method. Accurate and continuous determination of range rate allows the gunnery officer to alter fire to take into account the enemy ship's change in course.


Radar Defends Coastlines Against Ships

SHORE-BASED SETS can maintain a round-the-clock seaward search, detecting ships as far out as 45 miles. These sets can direct patrol craft to unidentified ships, can guide friendly ships through mine fields or fog, and can assign any targets to other radars having the specific function of coast artillery gun-laying.

Radar Gives Long-Range Warning of Planes

RADAR CAN DETECT high-flying planes as far away as 135 miles, and permit plotting of their location, approximate course and speed. Enough warning is given to allow fighter planes to take off, thus eliminating the need for a standing air patrol.


Radar Can Direct Searchlights

RADAR GIVES SEARCHLIGHT crews a 40,000-yd. warning and sufficient data so that planes will be struck instantly at least 90% of the time when the lights are turned on -- thus preventing planes from evading the beams.

Radar Directs Anti-Aircraft Fire

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.


Radar On Ground Directs Nightfighters

RADAR ENABLES a control officer at a fighter base to keep track of enemy planes and his own fighters within a range of 45 miles, and to direct fighters by voice to within 2 miles of enemy planes. 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.

Radar In Plane Leads Fighters To Gun Range

A NIGHTFIGHTER can detect an enemy plane by radar when the plane is 7 miles away, and reliability is certain at 4 miles. A CGI should direct a nightfighter to within 20,000 feet of the target, at which point the nightfighter's radar (AI) will permit tracking until the enemy plane is within firing range.


Radar Enables Planes To Detect and Bomb Ships

A RADAR-EQUIPPED PLANE can do blind bombing with the aid of a Norden bombsight attachment that is more accurate at low levels than a Norden bombsight itself in visual day bombing. At 5000 feet a plane with SCR-517 can detect a 10,000-ton ship 45 mi. away, a surfaced sub at 18 miles -- hence, can patrol sea lanes, shepherd convoys, and do reconnaissance in fleet operations regardless of night or bad weather.

Radar Guards Beachheads and Advanced Bases

LIGHT-WEIGHT RADAR SETS may be used in warning a beachhead or advanced base of approaching enemy planes or ships (at night for example), or in directing beachhead operations from the first point of debarkation when landing barges have radar navigation eqauipment. A key for sending messages can be used with portable sets.


Radar Aids Navigation of Planes & Ships

ONE RADAR DEVICE is a beacon (Racon) that emits coded signals in response to radiation from a plane or ship, keeping silent when not actually in use, and able to guide planes to a base or carrier 80 miles distant. Another device (Loran) enables ships to find their position to within 1 mile, as far as 1250 miles from shore.

Beacuse of the clear definition between land and water provided by radar, as is illustrated in the scope photo of an island on page 3 of this book, important navigational aid can be given planes or ships at night, in fog or storm; e.g., in guiding planes to an island or coastal target.

PLANE AT LEFT is at 30 feet. Low-level altimeters measure from 400 to 5 feet. Other radar altimeters can measure absolute altitue as high as 40,000 feet.


Limitations of Radar

As has been seen on the previous pages, 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 signals from clouds, which to an inexperienced operator resemble signals 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 in the intermediate wavelength range are referred to as medium wave. Those employing the longest radar wavelengths (which are still shorter than the waves used by high-frequency radio communications) are designated 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.


Radar in Planning

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 results 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.

Radar in Tactics

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 pages in a military situation never contemplated by its designer.

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 4000 yards. For his first salvo, he used the optical range value. There was no opportunity to fire another.

The Operation of Radar

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 signals 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 signals will even enable fairly accurate estimates to be made of the size and character of enemy forces from the radar signals they present.

Our Radar Equipment

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.S. Radar equipment described on the following pages is about one year in advance of that available to the enemy. The enemy is doing well with what he has got; it is only through conscientious and imaginative use that our better equipment can be made to count.


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