Chapter 16 AIRCRAFT INTERCEPTION TACTICS MILITARY AIRBORNE RADAR SYSTEM [MARS] Summary Technical Report of Division 14 NDRC Volume 2

16. 1   EARLY TECHNIQUES

16.1.1   Preradar Techniques

 The earliest techniques involving night fighters in the defense of territory against attacking enemy bombers utilized ground-based early warning systems, searchlights, and day fighters operating at night. The approach of the enemy bomber was detected by the early warning system which determined the course of the bombers. As the bomber approached the target, it was illuminated by the intersection of the beams from three searchlights (manual or radar controlled) which tracked it. When the approaching bomber was detected, fighter aircraft were alerted and directed to the searchlight area, which was then patrolled at an altitude above that of the attacking bomber. The fighter attacked the bomber from above and on the tail as soon as an interception by three of the searchlights had been made. The fighter had the advantage of essentially daylight visibility; the bomber gunner, on the other hand, had impaired visibility as a result of the glare of the searchlight beams. The use of this technique permitted the operation of ordinary day fighters at night.

Such tactics were only adequate for dealing with small raids under conditions of good visibility. Furthermore, concentrations of searchlights were only practicable near important targets, so that this defensive operation could take place only after the bombers had reached their objective. The need for radar controlled and equipped night fighters to intercept the enemy before reaching his target was recognized at an early stage of World War II, and considerable effort was expended on the development of such equipment.

16.1.2   Radar Techniques

The advent and extensive use of ground controlled interception [GCI] and the development of airborne radar systems for the detection of other aircraft resulted in great modification of night fighting tactics. The radar-equipped night fighters were alerted, directed by the GCI to an altitude above that of the raiding bomber, and then vectored (by the GCI) to the bomber. The GCI relinquished direction of the fighter to the radar operator in the interceptor when the target was within the radar range of the fighter. The radar operator then directed the pilot to within a few hundred yards of the bomber, at an overtaking speed of 10 to 30 miles per hour, and to a position above or below (depending upon such factors as light conditions and clouds), and to one side of the bomber. After establishment of visual contact the pilot opened fire at the appropriate range and bearing.

Long wave radar (approximately 200 me) sets were the earliest aircraft interception [AI] equipments used in the manner described above. These sets, developed by the British, gradually evolved into the Mark IV AI system (U. S. version: SCR-540). Later sets used shorter wavelengths (10 cm and 3 cm) and higher power, and some provided for blind firing. The tabulation of AI radars in Chapter 15 summarizes the characteristics of these later sets. Tactics were modified to meet the characteristics of each new type of set.

16.2  GENERAL ANALYSIS OF AI TACTICS

16.2.1  Introduction

An analysis of AI tactics should have two major objects: first, to examine the tactical requirements with the object of making the most effective use of available equipment and second, to arrive at a sufficiently broad understanding of the general problem that more effective equipment can be designed. Although the number of possible tactical situations is very large from the theoretical point of view, experience in World War II showed that the important general situations encountered were GCI controlled missions, free-lance search missions, and free-lance marauder missions against specific targets.

GCI Controlled Missions
With GCI techniques in use at the close of World War II the fighter could be directed from the ground into almost certain radar contact with the enemy. Failures to establish contact could usually be attributed to failures in the radar equipment or in communication. Furthermore, GCI could bring the fighter into contact on a favorable relative course, so that complicated maneuvers were not necessary for

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getting into firing position, except in so far as these were necessitated by evasive action of the enemy.

Free-Lance Search Missions
The independent mission in which the fighter has no prior information concerning the possible location of enemy planes is another important tactical situation. Two distinct stages are apparent — search and interception; each stage requires separate discussion in an analysis of tactics. The interception stage in this situation differs from GCI interception in that a favorable relative course cannot be assumed. The most probable course of an enemy plane when radar contact is established is directly opposite from the fighter's course, the condition least favorable for interception.

Free-Lance Marauding Missions
In a free-lance mission against a known point of enemy air concentration such as an enemy airfield, search is relatively simple. The interception conditions, however, vary considerably with the exact situation, but in general a favorable relative course cannot be counted on. These night missions against enemy airfields were dangerous because of the necessity for maneuvering at low altitude and the accuracy of enemy ground fire. Many such missions were, however, successful in World War II.

These three tactical situations will be discussed in terms of two probability functions: the radar contact probability, which defines the probability of establishing radar contact with an enemy; and the interception probability, which determines the probability that once radar contact is established, the fighter can be brought into firing position. The probability of successful firing will not be discussed.

Since in GCI controlled missions the fighter can be directed into almost certain radar contact with an enemy, the radar contact probability is essentially unity; consequently, it is not discussed further.

16.2.2 Radar Contact Probability

In studying radar contact probability on a freelance search mission against an unknown distribution of enemy targets, certain simplifying assumptions can be made without seriously limiting the applicability of the results. It will be assumed, first of all, that the maximum range of the radar is large compared to the altitude range in which targets are expected. This means that the coverage can be considered to be cylindrical, and the problem is essentially reduced to two dimensions. Secondly, it will be assumed that the targets are moving at random, and that the average density of targets is uniform in the region considered. The fighter will be assumed to be moving with constant speed in a fixed direction.

The analysis in the present section is largely based on that given by Dr. H. M. James in a report discussing the contact probability for a bomber flying through a uniformly patrolled region. The results derived in this report apply equally to the discussion of a fighter flying through a random distribution of bombers.

A number of important results established in this report are summarized below with all derivations omitted.

1. Most contacts will be made in the forward hemisphere. If u is the speed of the fighter and v the speed of the enemy aircraft, the per cent of contacts made in the forward hemisphere is given as a function of v/u in Table 1. It is assumed that the fighter's radar has a 360-degree coverage.

Table 1. Per cent of forward contacts as a function of speed ratio (Assuming 360-degree coverage).
v u	Per cent forward contacts
0.8	92.8
0.9	91.0
1.0	89.2
1.1	87.2
1.2	84.8

 

This means that little is gained for AI work by scanning the rear hemisphere, except to provide some tail-warning against enemy fighters.

2. Aircraft coming into radar contact are most likely to have a relative course angle of 180 degrees with respect to the fighter. The distribution of these encounters for 30-degree intervals in the case of an AI set viewing only the forward hemisphere is given in Table 2.

Table 2. Distrubution of encounters according to relative course.
Relative course angle (degrees)	Fraction of encounters haveing angle in this range (per cent)
0-30	2.2
30-60	7.8
60-90	14.4
90-120	20.8
120-150	26.0
150-180	28.8

 

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Thus 24.4 per cent will be heading away from the fighter and 75.6 per cent toward the fighter. These results are important in determining subsequent tactics. The independent interceptor must expect that most contacts will be made at large course angles, which presents a difficult problem because of the difficulty of turning into a homing course without losing the enemy. This problem will be further discussed in Section 16.2.3.

3. When an enemy plane is detected at extreme range, its most probable course is directly toward the observer. The nearer the bearing is to straight ahead, the greater is this probability. Thus, if any action is to be taken while an enemy is still at extreme range, before his course is determined, the choice of tactics should be made on the assumption that the enemy is proceeding directly toward the observer.

Radar contact probability for a free-lance marauding mission against a known point of enemy air concentration is in general higher than for free-lance search missions. For, in marauder missions, the radar contact probability is dependent largely upon the ability to navigate to a fixed target where the density of enemy aircraft is expected to be higher.

 

16.2.3 Probability of Interception

Interception probability after radar contact is established is discussed here for a fixed-gun fighter only, since practically all interceptor aircraft used in World War II were of this type. The standard tactical procedure for this type of fighter was to approach the enemy from behind on an approximately parallel course. Interceptions are more difficult if the enemy is initially on an opposite course from the fighter when radar contact is established (as is most probable in the random search case, Section 16.2.2) than if accurate GCI has brought the fighter into a parallel course behind the enemy.

James 2 has given a detailed theoretical treatment of the dependence of interception probabilities on the relevant factors involved. He makes the following assumptions: (1) the two aircraft are at the same altitude (thus reducing the problem to two dimensions) ; (2) the fighter pilot keeps his plane directed at the target whenever possible (homing tactics); (3) maximum rate of turn is used to bring the target dead ahead; (4) the approach is made at constant airspeed; (5) loss is considered to occur if the target gets out of the range of vision of the fighter. Some of the more important results of this treatment are included in the following discussion. Derivation of the results is not within the scope of this book.

Effect of Range
For accurate GCI work the radar range need only be sufficiently large to ensure contact while under ground control. This means that the radar range must be greater than the GCI error radius. Greater range of the fighter radar will, of course, decrease the time which GCI must spend on a given fighter, and hence increase the traffic-handling ability of a given ground control station. This may be extremely important in the event of large raids.

For independent interceptions, the probability of successful interception increases with the range of the radar. Furthermore, the rate of increase of interception probability with range is also an increasing function of range. The increase with range is most rapid when the two planes are initially approaching each other.

The greater the angle of radar vision, the more valuable will be an increase in range. On the other hand, if the available rate of turn is increased, less stress need be laid on range.

Speed Advantage
Too great a speed advantage, especially in the late stages of pursuit, leads to a decrease in interception probability. As the target is approached, speed advantage should be reduced to at most 20 per cent or preferably 10 per cent.

As the radar range is decreased, there is increased danger of losing the target if the speed advantage is too great.

Available Rate of Turn
For GCI interception, the rate of turn is principally important in those phases of the pursuit when the target is taking evasive action.

For independent interceptions, where the initial relative course angle can be expected to be large, the probability of a successful interception increases rapidly with the maximum rate of turn of the fighter. In night fighting there is a tendency for pilots to restrict themselves to relatively low turning rates; this should be avoided so far as possible. However, the advantage of increased rate of turn falls off rapidly as radar range is increased.

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Angle of Radar Vision
For GCI interceptions a large angle of vision is valuable both in ensuring radar contact, and in following evasive action.

Wide-angle vision is even more important in the case of independent interceptions where unfavorable initial courses can be expected. Increase in angle beyond 180 degrees is probably not worth the added weight required, but increase from 120 to 180 degrees greatly improves the chance of interception.

Persistence of Pursuit After Target Disappearance
The target sometimes will go out of the range of vision (in angle) of the pursuer at some stage of the pursuit. This situation is most likely when a small rate of turn is available. In this case it is very important that the pursuer not give up, but continue in his circular course (at maximum rate of turn). Such persistence may add as much as 30 per cent to the probability of interception.

The greater the available radar range, the more important is this persistence. The greater the available rate of turn, the smaller will be the increase in target range during the time that the target is out of sight.

16.3 SPECIAL PROBLEMS IN AI TACTICS

In Section 16.2 AI tactics were discussed from a general point of view. In this section we will discuss the effect of certain special factors on tactics.

16.3.1 Firing Methods

Once a fighter is on the tail of an enemy aircraft either visual firing or blind firing (if the radar is equipped with this facility) may be used. Blind firing has several advantages over visual firing: (1) the range information is much better than any visual estimate, especially when made at night; (2) radar firing can begin at greater ranges, thus reducing the danger of detection and defensive fire by the bomber; (3) the total time of approach is decreased; (4) the angle about the tail of the enemy aircraft, in which night fighters are expected at any stage during their approach, is increased; (5) the turning rates required to keep the night fighter's sights on the bomber are decreased. Visual firing, on the other hand, has the big advantage of better target identification, but this factor is much less vital if adequate radar identification equipment is available.

AI radars, such as the AN/APS-6, which have blind firing directed by a conical scan, restrict the final closing operation to a direct tail chase. The condition that the target be centered on the G scope, and remain centered, is that the line of the fixed guns of the fighter (and thus of the conical scan axis) remains pointed at the target. Enemy evasive action is very likely to be successful against this type of installation.

Night fighters equipped with turrets such as originally planned for the P-61 would, of course, allow much more flexible tactics in the closing stages of pursuit than were possible with fixed gun fighters. Automatic following, such as provided by the AN/APG-1 system, allows easier following of evasive action than is the case with nontracking radar sets.

16.3.2 Nature of Target

Tactics should be planned with all possible knowledge of the enemy target in mind. The most important factors to be considered are: the coverage and range of defensive radars (such as tail-warning); the nature and extent of enemy defensive fire power; the speed of the enemy planes; and the probable tactics which the enemy will use in avoiding pursuit. Prior knowledge of any of these factors will affect both GCI and AI techniques.

The tactics employed in combating missiles, of the guided or unguided type, depends upon the type of missile. Missiles such as the German V-l buzz-bomb are easily visible at night, so that radar-equipped night fighters are not required. The tactic adopted in this case consisted of cruising at an altitude several thousand feet higher than that at which the V-l's were flying; when a buzz-bomb was detected, the fighter dived toward the missile, thereby obtaining a speed advantage.

16.3.3 Evasive Action

The evasive action frequently employed by bombers is to change altitude and course (by 5 or 10 degrees) at intervals of 1 to 2 minutes. Thus, the weaving character of the target course may cause difficulty for a night fighter in the final stages of approach. An evasive maneuver very difficult for the night fighter to follow is a sudden sharp turn through 360 degrees by the bomber to throw the night fighter off the bomber's tail. This maneuver is especially

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Figure 1. Simple form of range clock	Figure 2. Standard range clock

difficult to follow if the fighter is close to the bomber at the time the bomber initiates this evasive action. Another maneuver which is very effective is to peel off in a steep dive to left or right. In general, however, large angle turns will not be encountered by the defending fighter, because a bomber employing such tactics would require extra fuel at the expense of bomb load.

16.3.4 Effect of Ground or Sea Clutter and Countermeasures

The quarry can be lost during the approach because of disappearance in the altitude signal, disappearance in the ground or sea clutter, or the effects of window or chaff. A discussion of the altitude signal and sea and ground clutter is given in Section 15.1.6. References 9 and 10 contain a detailed discussion of sea return and altitude effects in the AN/APS-6 system, with particular reference as to how the tactics might be affected for this type of system.

Window or chaff is sometimes used to protect attacking bombers against defending interceptors. The object of using window is to present the equivalent of a large number of reflecting dipoles so as to effectively jam the GCI and AI radars protecting the territory under attack. Window is dropped from several of the attacking aircraft in an attempt to confuse the GCI and AI radars. Narrow beam and short pulse durations in the radar systems are the most effective means of penetrating this interference, although it is possible, in principle at least, to drop sufficient window to cause loss of the target altogether.

16.4  THE RANGE CLOCK

In any interception the night fighter must get onto the course of the bomber and convert the approach to a tail chase hi the most efficient manner possible. James 6 has devised a range clock which enables a night fighter to get on the course of the target with a minimum number of radar observations. The use of the range clock reduces the approach to a target to the following steps: (1) turning until the target is dead ahead, (2) flying a straight course at a standard airspeed, and (3) executing a single turn at a standard rate through an angle determined in advance.

A range clock, in its simplest form (Figure 1), consists of a moving hand, a marker having a fixed position during each run, and two scales on the dial face. The outer scale is a range scale calibrated in miles; the inner scale is calibrated in degrees and indicates the point at which the turn is to start. For application to large angle interceptions two additional markers are added to the range clock, producing the so-called standard range clock (Figure 2). These two additional markers allow the radar operator to determine whether the interception is nearly a head-on interception, and provide a method of hand lingsuch interceptions.

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The operation of the simple range clock is begun with both hand and marker in the zero position. When the starting knob (upper left corner) is pulled out the hand can be set to any range indicated on the outer scale. The marker is automatically set at one-half of this range.

Referring to Figure 3 we will assume that the fighter is at P after having turned toward the target which is now dead ahead at range PT. The clock is set at range PT and started; the pilot flies a straight course (P to T). As described in

 

Figure 3. Approach of fighter to target using range clock procedure

 

James' report, "the hand of the range clock will move toward smaller ranges at a rate equal to the standard airspeed agreed on in advance for use in these approaches; thus it will indicate at any instant the distance still to be traversed before the point T is reached.

"When the clock is started the marker remains fixed. When the hand has reached the marker the pursuing plane will be at point O (Figure 3), halfway to point T, while the target will be at 0', nearly an equal distance from T. At this moment the radar operator will observe the bearing angle /3 to the target.
"By doubling the angle /3 the radar operator can determine the angle between the courses of the two planes, which is the angle through which the pursuing ship must turn. He should then at once pass this information to the pilot."

Doubling the bearing angle to obtain the angle between the courses is exact if the speeds of the target and pursuer are equal; if not, correction may have to be made for greater precision.

To determine the time at which this turn should be begun, the radar operator will now refer to the inner scale. This is calibrated in terms of the angle between the courses of the planes; the calibration depends on the rate of turn to be used. When the moving hand reaches the calibration corresponding to the angle 2/3 already determined, the turn should be started as soon as possible. In practice there will be a lag in the actual starting of the turn; for best results the radar operator should call for the turn somewhat before the hand actually reaches the calibration. The correct amount can be learned by experience.

The simple form of range clock procedure described above is effective, except when there is a very large angle (greater than 130 degrees) between the courses of the planes, and an appreciable speed difference. In order to deal adequately with all interceptions at very large angles, two more markers must be added to the range clock, thus producing the standard range clock (Figure 2). Use of this standard clock is fully described in James' report.

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U.S. Night Fighter Radars of WWII