15th Army Air Forces; WWII
15th Army Air Forces; WWII

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Development of Radar Bombing - Appendix VI-XI

APPENDIX VI

ECHO INTERFERENCE SYSTEM

  1. The Echo Interference principle is being incorporated with an H2N type computer and the complete system called Nosmo System by the developing agency in the U.S.  Echo Interference is sometimes called “pulse doppler”.  The material presented here is an extrapolation from verbal and written information received from the U.S.
  2. When the antenna of an operating Mickey set is turned to face directly up the aircraft’s ground track, a change in the interference pattern presented in the A scope may be noted.  The angle between the antenna heading and the aircraft heading is now the drift angle.  This angle is presented in the face of the PPI as the angle between the Lubber Line and the now non-rotating sweep line.  The mechanical cursur may be set approximately over the non-rotating sweep line.  The cursur now represents the ground track of the aircraft and the angle between the cursur and the Lubber Line represents the drift angle.  To be able to control the antenna within the limits needed to detect the change in echo interference, it is necessary to add a micro-control to the antenna control system.  The Servo System designed by Radiation Laboratory to accomplish this gives rapid precise control (to within 1/4o of the drift angle in 15 seconds) of the antenna position.
  3. The procedure for using the Radiation Laboratory Echo Interference drift angle principles for course track might be in this manner (azimuth stabilization off):
    1. After the I.P. turn, the aircraft is turned until the Lubber Line is on the target (aircraft 10 to 50 miles from the target).
    2. With the Lubber Line on the target image, the echo interference control is used to stop the antenna in the position indicating the ground track.
    3. The mechanical cursur is placed upon the stationery sweep which indicates the ground track.  (The aircraft must maintain constant heading).
    4. Automatic rotation of the antenna is resumed.  This replaces the target image back on the PPI.  The lubber line should still be on the target image.
    5. The aircraft heading is now changed until the mechanical cursur is on the target.
    6. The change of heading changed the drift angle.  The drift angle is redetermined, the cursur set, and the target made to track down the cursur by changing the aircraft heading.
  4. The system can be used with azimuth stabilization on, but it is more difficult and less accurate:
    1. Read the drift angle with the lubber line on the target.
    2. Set the cursur on the target.
    1. Turn the aircraft until the angle between the lubber line and the cursur is equal to the angle read in Step a.
    2. Control the aircraft to make the image track down the cursur.
    3. Recomputation of drift may be made by stopping the antenna, reading the drift angle, and setting that angle between the lubber line and cursur.
  2. With azimuth stabilization ON, two conditions must be maintained.  The proper angle between the lubber line and the cursur must be maintained, and the target must be made to track under the cursur.  The system could be improved by adding a clear cover glass with another cursur inscribed in it.  Then, with the lubber line on the target and the perspex cursur on the ground track, the second cursur is placed over the lubber line.  The clear cover is frictionally driven by the perspex so that when the perspex cursur is turned onto the target, the secondary cursur indicates the proper heading of the aircraft and it is only necessary to turn the aircraft until the lubber line lies under the secondary cursur.
  3. Operating with azimuth stabilization ON increases the reliability of target identification but requires the use of a more cumbersome system.

 

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APPENDIX VII

APPENDIX VII

THE BLACK TARGET LINE

  1. In the present VISAR system the target line is an electronic cursur.  The ability to read the scope accurately is decreased somewhat by the characteristics of this line.  The line is created by saturating or nearly saturating one or more sweeps.  The high energy level of the fluorescent particles of the screen causes an effect similar to halation with the result that the line becomes irregular when the radar image is composed of an area of varying light density.  When the target image is small, the target line tends to distort it or even hide it completely.
  2. In order to overcome the difficulty introduced by the use of a white target line, a black target line system has been constructed as illustrated in the accompanying drawing.  The black target line is created in a manner precisely opposite to the method of making a white target line; that is, light is prevented from reaching the screen for one or more sweeps.  The absence of light during these sweeps results in a dark line on the screen.  Such a system is now being tested.  Preliminary trials indicate that it has considerable merit for some targets.  It is obviously of little value when the screen is essentially dark, as when flying over water, but for such cases the white target line is satisfactory.  With the present equipment it is found that one or more revolutions of the sweep are required for the residual light to die out and the black line to become well defined.  The lubber line brilliance control governs the black target line also.  The black line is formed by feeding a negative voltage to the lubber line circuits instead of a positive voltage.

 

APPENDIX VIII

CAMERA CONTROL SYSTEM AND CENTER BRILLIANCE CONTROL

  1. To make a study of radar bombing, such as is being done with VISAR, it is desirable to take simultaneous ground photographs and radar scope photographs.  This permits the scope interpretation to be compared with the run as actually made.  The camera control system that is being used on the VISAR tests is illustrated in the accompanying drawing.  A standard K-22 camera is mounted in the regular B-17 camera well.  The camera is manually leveled by an operator whose sole job is to see that the camera is maintained in a vertical position during the camera operating periods.  The radar recorder camera is a K-24 camera and magazine with a K-22 lens mounted in a special cone.  The cone is attached to the front of the Mickey receiver indicator so that the radar recordings are of the master scope.  The side of the cone is cut away so that the operator may view the front of the scope obliquely.  The relays used were salvaged from wrecked aircraft.
  2. The uneven distribution of light on the face of the PPI makes it difficult to take good navigational radar recordings.  The modification show in Figure VIII-2 has been installed and has undergone partial testing.  Its function is primarily one of the slowing the unblanking during the first part of the sweep and thus reducing the difference in light intensity between the center and the outside.  By reducing the center intensity below the point of screen saturation, it is possible to make visible those signals that were formerly lost.  The testing is not yet completed but it does indicate that the center brilliance control is valuable while navigating and when taking radar scope photographs.  There is some question about its value for bombing, especially as regards the additional complication of using it.

APPENDIX IX

APPENDIX IX

SYSTEMS OF SCOPE PRESENTATION EXPANSION

 

  1. This appendix is intended to point out the need for a study that was initiated here but was not carried to completion because of limitations of time and material.  Expansion of the scope presentation gives greater relative motions of the radar images which for bombing means the ability to set in better corrections.  It may also give better definition but not necessarily so since some targets tend to “break up” when expanded.  This may be due to a variety of causes but all are related to the difficulty encountered in differentiating between nearly equal energy returns from area targets.  Scope expansion is also an aid in identifying certain types of targets.  The need for expansion is demonstrated by the acceptance of continuous expansion on the 20 mile range on the present Mickey equipment.
  2. The studies of the VISAR system brought out four kinds of expansion control that might be desirable.  The first is that of placing the 20 mile expansion control in the bombing operators position.  The second is that of automatic expansion with the Norden computer.  This system has not been tried but it was thought that the expansion potentiometer could be connected with the proper switching so as to be controlled by the Norden computer in such a way that the scope is automatically expanded as the target is tracked.  A system such as this would have advantages in that the expansion is smooth and continuous and there is no tendency for the operator to get lost.
  3. The third method considered is that of using a fixed mechanically inscribed circle on the face of the scope and expanding the presentation behind the circle.  There are a number of disadvantages to this system that make it seem rather undesirable.  The fourth method, which is thought to be the most desirable, is a system of using two scopes.  In addition to the regular navigational scope, an expanded scope could be employed, probably a B scope.  The intersection of the target line and range circle on the navigational scope could be made to indicate the center of the expanded section of the B scope.  Such a system can be constructed so that tracking could begin at least 40 miles from the target and be continuous from there to bomb release point.  The navigational scope would always be available for target identification while the B scope would be available for tracking.  A workable system of this type could certainly improve the accuracy of radar bombing.

APPENDIX X

APPENDIX X

Proportional Angle Determination of Uncorrected Drift and Heading Components

      1. The proportional angle method for determining the uncorrected drift and heading components determines the direction and approximate magnitude of the angle through which the aircraft must be turned in order to cause the aircraft to pass over the aiming point.  This method is applicable to Mickey Radar bombing.  Figure X-1 gives an example of proportional angle correction when the correction is four to one, i.e., quadruple angle correction.
      2. The mechanical cursur is placed on a target as it crosses a given range reading mark.  The target is observed as it moves toward the center.  When it crosses the three quarters mark (that is, when it has traveled one fourth of the way to the center), the cursur is again placed on the target and the angular displacement 0 noted.  Four times 0 is the angle Ø through which the aircraft heading should be changed in order to cause the aircraft track to pass approximately over the target.  Good accuracy requires using the range unit computer and ground ranges.
      3. The derivation of the equations for the proportional angle system assumes that the length r of triangle ABC is equal to the radial difference of the two circles, and that the value of the tangent of an angle is equal to the numerical value of the angle expressed in radians.  Both assumptions are legitimate since the errors introduced by them are small in comparison to the accuracy obtainable with the equipment.
      4. The triangle of Figure X-1 then becomes the triangle of Figure X-2.  And,

a. Tangent 0 = d/s

b.  Tangent Ø = d/r

c.  d = s tan 0

d.  d = r tan Ø

e.  r tan Ø = s tan 0

f.  tan Ø = (s/r) tan 0

g.  Ø = (s/r)0

The angle Ø1 (equal to Ø) is the angle through which the aircraft heading must be changed to correct for the drift.

The angle 01(equals to 0) is the angle through which the aircraft heading must be changed to correct for the amount drifted off during the drift determining period. Then,

h.  A = Ø1+ 0 = s/r0 + 0 = (s/r + 1) 0

These equations apply to double, triple, quadruple, or any other proportional angle change.  Applying them to the quadruple angle correction case,

  1. s = 3r
  1. A = (3r/r + 1) 0 = 40.  The change in aircraft heading must be equal to four times the central angle swept by the cursur.

 

APPENDIX XI

THE WILLOWDALE TRACKING RANGE

 

  1. The Fifteenth Air Force has recently completed setting up and putting into operation an SCR 584 gun-laying equipment for the tracking and plotting of practice bombing aircraft.  Studies are now beginning to determine the limitations and accuracy of this tracking range.  Test procedure and the analysis of the results have not yet been standardized.
  2. The VISAR equipped B-17 was flown on this range several times and the results have been plotted and presented in Appendix II of this report.  The runs were made to indicate the approximate ground track and the order of magnitude of the corrections made during the run and not trying to determine the accuracy inherent in the VISAR system.  The attached maps show the targets and their grid, and the I.P.’s now being used.