Development of Radar Bombing - Fifteenth Air Force

Historical Documents

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 Development of Radar Bombing - Appendix I
 Development of Radar Bombing - Appendix II
 Development of Radar Bombing - Appendix III
 Development of Radar Bombing - Appendix IV
 Development of Radar Bombing - Appendix VI - XI

SECTION II – BOMBING METHODS AND TECHNIQUES

SECTION II – BOMBING METHODS AND TECHNIQUES

1. Introduction.

Numerous techniques are available for utilizing the bombing capabilities of the Mickey equipment.  The bombing technique of this Air Force has undergone several changes in the past and will be changing again in the future.  The trend is toward closer coordination and cooperation between the radar operator and the bombardier; that is to say, a more closely coordinated operation of the Mickey bomb sighting and the visual bomb-sighting equipment.  The system of Multiple Angle Prediction (sometimes termed PFF-Synchronous) that is at present used by this Air Force was developed as an initial step in the direction of a completely coordinated and synchronous system of bombing.  Present development is aimed at improving the bombing accuracy of the radar equipment and improving the coordination between visual and radar bombing techniques.  The importance of such development is illustrated by the following diagram which shows the advantages to be gained when bombing under different conditions of visibility.  The diagram is not a precise representation of bombing accuracy but is as fair an assessment as is possible on the basis of data at present available.

2. Definitions of Bombing Techniques.

The changes that have been and still are being made in the equipment and techniques of radar bombing make it desirable to present certain definitions of bombing techniques in order to clarify the terminology in this report.  Four techniques of bombing are defined as follows:

 

1. Visual Bombing.  Performed with conventional visual methods employing either the Norden M-Series or the Sperry S-Series bombsights with their connected auto pilot control of the aircraft.
2. Blind Bombing.  Performed without any visual sighting either on the target or on reference points.
3. Coordinated Bombing.  Performed by coordinating the blind bombing system and the visual bombing system.  There are several degrees of coordination possible.  Fully coordinated bombing would utilize all of the capabilities of both the visual system and the radar system.  A more limited degree of coordination is obtained when information is furnished the visual system by the radar (our present method) or when information is furnished to the blind bombing system by the visual system.  In each of these two methods the flow of information is one way only.
4. Offset Bombing.  Performed by determining course (or bomb release point) by a method other than sighting on the target.  The usual form of offset bombing is to sight on a point whose location with respect to the target is known and by means of suitable corrections thereto make the allowance necessary to place the bombs on the target.  Bombing of this type can be done with radar as well as with visual sighting methods.

 

3. The Bombing Problem.

The interworking of the visual bombing system with the technique of radar bombing makes it desirable for this discussion to introduce certain well known principles of visual bombing.  All bombing equipments and bombing techniques are directed toward solving the bombing problem.  The bombing problem itself is usually resolved into three components, range determination, course or track determination and the vertical relations between the aircraft and the target.

1. The effect of the velocity of approach of the aircraft toward the target is taken into account by computations of range and range rate, that is, the range of the aircraft from the target and the rate at which the range is changing.  Range and range rate computations are made with computer systems (bombsights, etc.) that are carried within the aircraft and the results are combined with the proper factors to determine the point of bomb release.
2. When bombing in a moving air mass, the effect of the component of the air velocity perpendicular to the ground track of the aircraft must be determined.  Track determination is done either with or without a mechanical computing system and in effect is accomplished by successive corrections to the flight path of the aircraft such that it travels along a path that includes a proper bomb release point.
3. The vertical relations between the aircraft and the target are usually expressed in terms of the time of fall of the bomb when bomb release is accomplished from an aircraft flying at constant altitude.

4. Methods of Range Computation.

The following methods of computing the proper bomb release point along the track of an aircraft are applicable to either visual or blind bombing, or to a combination of these two, that is, coordinated bombing.

1. Fixed Angle Release.  In visual bombing methods this technique utilizes a bombsight which is set at a predetermined angle so that the intersection of the target and the cross-hair indicates the proper bomb release point.  In radar bombing, this technique uses a pre-set electronic range circle on the scope so that the intersection of the target image by the range circle indicates the bomb release point.  It is obvious that for high altitude bombing the fixed angle technique is rather crude and it is not used by this Air Force either in visual or radar bombing.
2. Multiple Angle Prediction.  This method of predicting the bomb release point essentially utilizes a series of predetermined fixed angles.  There are certain advantages in using this technique rather than a single fixed angle.  This is particularly true when doing coordinated bombing.  A method of multiple angle prediction is at present being used by this Air Force in coordinated bombing^*.  Multiple angle readings are made by radar and furnish data to the visual bombing computer which controls the release of the bombs.
3. Continuous Tracking Prediction.  This method of predicting the bomb release point makes use of synchronous range (or synchronous “rate”) computations.  In visual bombing, the continuous tracking of the target with the bombsight cross-hair computes the proper release point when the ballistic data is pre-set into the bombsight.  For proper tracking, the relation of the cross-hair to the aiming point must be examined for two properties.  First, the relative motion between the cross-hair and the target must be zero.  This indicates that the correct rate of closure of the aircraft with the target has been established and from that the proper dropping angle is determined.  Secondly, the displacement of the cross-hair with respect to the target must be zero.  This indicates that the position of the aircraft along the path of approach has been correctly set into the computer.  In Mickey techniques, range and range rate comparisons are made between the target image on the scope and the bombing circle.  Coordination permits a flow of information from the radar to the visual computer.  It is preferable to have one computer into which both visual sighting and radar sighting feed data and from which both can receive the tracking formation needed.  In this case, radar could supply all the information in blind bombing but would receive assistance when partial cloud cover permitted glimpses of the target, or a bomb run could be started by Mickey and completed by visual sighting.

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^* “PFF-Synchronous Bombing – Fifteenth Air Force”, report by 1st Opr Analysis Sec, 20 Sep 44.

 

4. Range-Offset.  Offset bombing is applicable to any of the foregoing methods of determining the point of bomb release.

1. In applying range-offset to fixed angle bombing, an aiming point other than the target may be selected and either a new angle computed on the basis of the position of the aiming point with respect to the target or a timing system could be used to make the proper allowance for the correct time of bomb release.
2. Several methods are possible for applying offset bombing to the method of multiple angle prediction.

1. By sighting with the standard method on an aiming point other than the target and applying a correction to the dropping angle.
2. By sighting on an aiming point other than the target but making no correction to the dropping angle.  In this case predetermined sighting angles must be adjusted so that the dropping angle that is computed is correct for placing the bombs on the target.

3. A method of synchronous range-offset would require that the computer determine the proper bomb release when the range component of the distance between the aiming point and the target is set into it.  With the present bombing computers, offset bombing is accomplished by synchronizing on an aiming point and then displacing the sighting angle (or radar slant range) by an amount necessary to place the bombs on the target.

 

5. Methods of Track Determination.

Methods of determining the correct ground track of the aircraft are described in this section.  In order to establish the proper flight path it is necessary to make allowance for the true airspeed of the aircraft and the velocity of the wind.  In most cases the determination of the correct path of flight is made by a sequence of corrections so that the path does not traverse a smooth curve but rather a series of straight line segments.

1. 1. Continuous Tracking.  In this method, continuous tracking of the target feeds the proper information into a computer which establishes the course.  In radar bombing the target image is continuously compared to a “target line”^* which is a controlled electronic cursur which is the analogue of the bombsight course hair.  The computing mechanism of the bombsight determines the proper course^**.
   2. Pre-Set Drift, Semi-Tracking.  A number of types of this method are usuable.  The method most frequently used is that for Mickey blind or coordinated bombing.  From navigational information, the drift angle for the ground track is determined.  This angle is then set between the lubber line and the azimuth index marker and the aircraft is turned in heading until the target tracks down the index marker.

 

5. 3. Proportional Angle.  This method is used as a refinement to a pre-set drift run.  The following quadruple angle method is one example of this type.  The target image is observed in relation to the azimuth index marker while the aircraft travels one-quarter of the distance from the starting point to the target.  The angular amount that the azimuth index marker must be displaced to remain upon the target is approximately one-third of the uncorrected drift angle (see Appendix II).  To correct the aircraft heading, prevent further drift and to correct for the amount already drifted, the heading must be changed by an amount equal to four times the indicated azimuth index angle.
   4. Echo Interference.  Certain pulse echo interference phenomena are usable with proper antenna control to give a Mickey indication when the antenna is facing directly up the ground track of the aircraft (see Appendix VI).  The angle between this antenna position and the heading of the aircraft is the drift angle.  The drift angle may be used for presetting the drift or may also be used to give successive corrections to the flight path.
   5. Track-Offset.  This method is accomplished by establishing a ground track whose position with respect to the proper ground track is known.  A predetermined correction is then made to put the aircraft on the correct course.  The difficulty of executing good track-offset bombing with present bombing equipment makes it an unsatisfactory method.
6. Comparative Considerations in Methods of Range Determination.

The different methods of determining the proper bomb release point have certain advantages and certain disadvantages.  The methods are discussed in the following paragraphs with particular reference to their shortcomings.

1. Fixed Angle Bombing.  The shortcomings of this method of visual bombing are well known and it is not used at medium or high altitudes.  The disadvantages of this method of radar bombing are not so well known.  The inability to determine the ground speed of the aircraft is one drawback.  The difficulty of judging when the target image is properly intersected with the range circle is also a possible source of bombing errors.  The center brilliance of the scope, the central distortion and the area compression make it very difficult to work with the target image in the region of bomb release.  The use of an expanded bomb release circle on the scope with the introduction of a suitable time delay for releasing bombs will avoid some of these difficulties.  However, as the bomb circle is expanded farther from the center of the scope it becomes more difficult to make an accurate reading because of the increased distance from the target at the time of reading.  The angular rotation of the sweep of the scope moves the target image in a series of jumps.  These jumps together with a changing aspect of the target are a considerable disadvantage, particularly when the sweeps are not rapid.  The simplicity of this method of radar bombing is, of course, the biggest point in its favor.

 ^* “A New System for PFF Course Synchronization”, report by 1st OAS, 14 Oct 44.

^** See Appendix IV.

 

2. Multiple Angle Prediction.  The following discussion is of the method of bombing used by this Air Force (see Appendix IX).  The same difficulties of reading that are mentioned in paragraph a. are also encountered in this method.  It does have the advantage, however, of making several angle readings.  The adverse effect of the jumping of the target image and the change in target aspect is to a certain extent averaged out by making a number of readings.  Unsatisfactory antenna pattern and the need for scope expansion and receiver gain changes, however, detract from the accuracy of the readings.  The coordination is only one way as information is relayed only from the radar to the bombardier and there is no way for the bombardier to distinguish rate corrections from range corrections except by noting the results of several corrections.  This method, therefore, requires a considerable amount of judgment and intuition on the part of both radar operator and bombardier.
3. Continuous Tracking.  With this method of bombing, the accuracy of range determination would be as good as in visual bombing if the scope could be read as accurately as the bombardier can scan the terrain with the bombsight.  It is obvious that the greatest factor limiting the accuracy of this type of radar bombing is scope interpretation.  Continuous tracking will, however, average out to a considerable extent the effects of the changing aspects of the target and the jumping of the target image.  The fact that both the scope range circle and the bombsight cross-hair, are controlled through the bombsight computing mechanism permits a high degree of coordination between visual and radar sighting and permits each of these to take advantage of the sighting and corrections made by the other.
4. Offset Bombing.  There is no inherent advantage in substituting another aiming point for the target itself.  It is an advantage only when the target cannot be used directly.  Some radar targets cannot be easily detected and there is also a possibility that radar camouflage may be encountered.  In such cases it would be necessary to resort to offset bombing in order to attack the targets.  A satisfactory radar offset bombing method should be synchronous and have a computing system so that it is not necessary to make arbitrary corrections during the bomb run.

7. Comparative Considerations in Methods of Track Determination.

The different methods of determining the proper flight path of the aircraft have certain advantages and disadvantages.  The methods are discussed in the following paragraphs with particular reference to their shortcomings.

1. Pre-Set, Semi-Drift.  The mechanical cursur used with this method does not permit many refinements in killing drift.  By inspection the operator must determine if drift is present and must estimate the amount of drift and the magnitude of the correction necessary.  This method of killing drift and establishing course by judgment is too crude to give a very high degree of accuracy.  The use of the mechanical cursur may also introduce errors due to parallax, off-centering of the sweep and distortion in the scope presentation.  Aircraft turn-controls have been added so that the radar operator can correct the aircraft heading directly; this is an advantage but it does not eliminate the various shortcomings of this

system.

2. Proportional Angle Method.  This method is subject to the same criticisms mentioned in paragraph a. except that it is not necessary for the operator to estimate the magnitude of the course correction.  He estimates the amount of uncorrected drift angle and makes a correction that is a predetermined multiple of it.  With this system, a deviation in aircraft heading would be interpreted as drift--which is undesirable.  The use of an auto pilot turn-control that permits the radar operator to govern the heading of the aircraft would eliminate this undesirable feature.
3. Echo Interference.  In the form in which originally proposed, this system would have the shortcomings of the mechanical cursur as regards parallax, distortion and centering.  It does, however, climinate the necessity for estimating magnitude of drift.  This system does not require the operator to determine how much or how fast but only requires that he match.  This has the advantages of continuous matching but it applies only with azimuth stabilization OFF and not with azimuth stabilization ON.  The early stages of the correcting may cause formation wandering and fairly sharp turns if a large amount of drift is present.  It may be necessary to use special techniques for these large drift angles.  Approximately 15 seconds of target identification time are lost for each drift angle determination.  In common with the two previously discussed systems, this method of drift determination is difficult to coordinate with the visual sighting system.
4. Continuous Tracking.  The accuracy of this method of killing drift and establishing course is limited only by the accuracy with which the scope can be interpreted.  The controls and procedures are the same as with visual bombing since the electronic target line is the analogue of the bombsight course hair.  Because the target line is a gyro stabilized electronic cursur, parallax, off-centering or smear distortion do not introduce errors.  A very considerable advantage is the high degree of coordination that is possible between the bombardier and the Mickey operator.  Although in principle the capabilities of this system are equal to those of the visual sighting system, the difficulty encountered because of the relatively poor definition of the present scope will seriously limit its accuracy.  The degree of definition presently available with radar does not limit the accuracy of the continuous tracking method any more than it does the accuracy of the other methods.  However, since in this method poor definition is the only significant limitation, it assumes relatively greater importance.
5. Offset Bombing.  Good track offset bombing is difficult to achieve unless the mechanism to accomplish this is built into the system.  A limited amount of track offset might be accomplished with the appropriate technique when using the echo interference system.  This can also be accomplished with the other methods of bombing but, in general, the arbitrary corrections that must be made during the bomb run are too difficult to execute satisfactorily in combat.

SECTION III – VISAR SYNCHRONOUS BOMBING SYSTEM

SECTION III – VISAR SYNCHRONOUS BOMBING SYSTEM

1. 1. 1. VISAR Bombing System.

A completely synchronous and continuous tracking radar bombing system (VISAR), with properties as described in the previous sections of the report, has been designed.  Personnel of this Air Force have constructed a test model which has been installed in a B-17 aircraft.  A test program (described in Appendix I) is under way to evaluate this bombing system.  Some results of the tests are presented in Appendix II.  Although the testing program is only in its first phase, the few runs that have been completed and analyzed indicate that this synchronous radar bombing system is both usable and desirable.  The test operators are all combat personnel and their reaction to using this system is very favorable, particularly as regards the continuous tracking system for killing drift and establishing course.

1. 1. 2. VISAR Equipment.

Details of the equipment are given in Appendices III and IV.  The synchronous rate is achieved by controlling the scope range circle through the Norden bombsight computer and through a potentiometer.  The model constructed here is the forerunner of H2N.  Its mechanism is somewhat different than H2N since only linear potentiometers were available.  The synchronous drift and course system introduces an electronic target line on the face of the scope.  The target line is controlled through the drift computer of the Norden bombsight and through a transmitting system.  The range circle and the target line intersect on the face of the scope to form electronic cross-hairs which are the analogues of the bombsight cross-hairs.  The control and behavior of both of these sets of cross-hairs is the same since they both are controlled through the same computing system.  Any corrections that are made by either radar or bombsight are simultaneously incorporated in the readings of both.

1. 1. 3. Operation of VISAR.

The arrangement of the bombing equipment in the test aircraft is such that all sighting operations are performed by the bombardier.  A remote scope is installed next to the bombsight so that the bombardier can sight and synchronize through the bombsight telescope or by means of the radar scope.  Scope expansion, receiver gain and relative brilliance of target image, bombing circle and target line are controlled by the radar operator.  There are certain disadvantages to this system.  The fact that the bombardier must divide his attention between the scope and the bombsight telescope may make it difficult to utilize all of the radar and visual sighting capabilities.  It may, however, be satisfactory for the bombardier to concentrate on the scope and refer to the bombsight telescope only when informed by the navigator that visual sighting is possible.  There is, of course, a question of eye adaptibility involved when alternately looking at the scope and through the bombsight.  Another possible method of operation is for the equipment to be arranged so that the radar sighting and synchronizing is performed by the

radar operator and the visual sighting is performed by the bombardier.  The chief disadvantage of this system is that with the present equipment the visual synchronizing would be more or less independent of the radar synchronizing and special signalling systems would be required to achieve any degree of coordination.  Still another possibility would be to use the equipment essentially as at present installed and have the navigator perform the radar sighting and the bombardier perform the visual sighting.  This method would achieve a high degree of coordination since both the radar sighting and the visual sighting would work through the same computer mechanism of the bombsight.  The best method of using VISAR bombing is still to be worked out.  The answer will, of course, depend on such factors as, possible locations for the pieces of equipment, training of personnel, coordination between visual and radar sightings, etc.  The system to be adopted should not add any complexities that are not more than made-up by other simplifications.  Any calibration that is required in flight should be simple and easy to perform.  Dependability and serviceability of the equipment should be high.  With the present equipment, 20 mile continuous expansion has been of aid and off-center PRI and continuous expansion coupled to the bombsight would be additions that would be helpful.  An expanded B scope would also be of aid since some targets show up better with expansion while others are better without expansion.  Echo interference drift angle indication could be added to the equipment if it is found desirable to do so.  Echo interference is reported to be included in the Nosmo equipment.  Appendix VI on echo interference is based on somewhat scanty information that has been received from Radiation Laboratory at Massachussets Institute of Technology.

1. 1. 4. Conclusions.

There are still a number of problems to be worked out before it will be known just what is the best method of coordinated bombing.  It is believed that a system incorporating the advantages of VISAR completely synchronous bombing is desirable.  The criterion of a bombing system, of course, is whether it will produce better combat bombing results than the system which it is to replace.  It is hoped that the tests now underway will give at least a partial answer to this for VISAR bombing equipment.

ROBERT N. DAVIS,

Operations Analyst.

 

Approved by:

GEORGE W. HOUSNER,

Chief Analyst.

 

VANE T. WILSON,

Major, Air Corps,

Commanding.

 

 

 

 15th AFHQ