Development of Radar Bombing - Appendix IV

APPENDIX IV

VISAR CONTINUOUS TRACKING COURSE CONTROL

1. The VISAR continuous tracking course control mechanism built by the Fifteenth Air Force is a control to convert the AN/APS-15 and similar type equipments to continuous tracking control.
2. Presented in the fluorescent face of the PPI is a Lubber Line which is created by the lubber line switch and its associated electrical circuits.  The lubber line switch is actuated by a cam mounted upon the antenna spinner base.  This cam is mounted so that it closes the lubber line switch at the instant that the antenna is radiating directly along the aircraft longitudinal axis.  Thus, the lubber line is created when the antenna has the same heading as does the aircraft.  The lubber line represents in the PPI the heading of the aircraft.
3. The target line switch is electrically connected in parallel with the lubber line switch.  By moving the target line switch in angular position around the spinner in relation to the lubber line switch, a target line is created which is displaced from the lubber line by the angular amount equal to the displacement between the switches.  If the target line is displaced away from the lubber line by the drift angle, the target line will indicate the ground track of the aircraft.
4. When a target is tracking down the course hair of the Norden sight, the sight has been turned from the heading of the aircraft by the drift angle and the course hair indicates the ground track.  The angle between the sight and the aircraft heading is now the drift angle.  This drift angle is indicated on a sector scale mounted on the stabilizer case by a pointer mounted upon the sight head.  By connecting the target line switch to the sight in such a way that as the sight head rotates in relation to the lubber line switch, a target line is produced which will always bear the same relation to the lubber line that the course hair bears to the aircraft heading.  The control of the target line is then exactly the same as the control of the course hair, and the tracking relationships are the same.  The Norden drift knob kills drift by changing the heading of the aircraft until the target tracks at a constant horizontal displacement from the course hair, or until the radar target image tracks at a constant horizontal distance from the target line.  The Norden course knob turns the aircraft until the course hair lies on the target, or until the target line lies on the radar target image.  In this manner, the aircraft course is entirely controlled through either the auto pilot system or through the PDI.  The aircraft control is the same using either the visual or the radar.  The bombing team may use either the visual sight or the radar sight.  The data presentation may be either azimuth stabilized or not as the operator need consider only the relation of the target line and the radar image.  The system is fully coordinated; i.e. any information inserted by either the radar or the visual is immediately usable by the other.
5. Of the several possible methods of producing the target line and its control, the following three were considered.  A step-by-step repeater system which would actually transmit angular displacement of the sight head to a target line switch mounted on the antenna mount.  This was discarded because of the complexity of the repeater system and the difficulty of mounting the switch on the proper arc.  A selsyn system of transmitting the sight rotation back to the antenna position was not built because small error selsyns were not available.  A system of continuously rotating autosyns for the transmission of the antenna position to the target line switch mounted upon the bombsight was adopted because the error introduced by the rotational drag could be calibrated out.  This meant that the system could only be used for fast or slow scan, not sector scan, and the scan speed for which the mechanism was calibrated would have to be used.  These disadvantages were not considered important in the model to be built for testing the target line principles.
6. The target line switch system now employed uses an autosyn transmitter driven by the antenna.  A ratio of five to one is used to reduce the angular errors.  See Figures IV-1, V-12 and V-4 for mounting details and schematics.  This autosyn transmits the antenna position information to the unit mounted upon the sight (Figures V-5, V-6).  The target line switch unit contains an autosyn indicator which is driven by the transmitter (Figure V-9).  The indicator drives a rotor (Figure V-10).
7. The switch contacts mounted upon the contactor disc ride against the face of the rotor.  The position of the contacts is changed as the sight is turned by the gear which meshes the sector mounted on the stabilizer case (Figure V-10).  Thus, as the sight is rotated, the contactor is turned through an angle five times as great.  This changes the relative position of contact of the rotor conductor and the contactor contact.  The rotor conductor represents the antenna position in relation to the contactor.  If the contactor is adjusted with the sight set on zero drift so that the target line switch is closed at the same time as the lubber line switch is closed, the target line and the lubber line will occur at the same PPI position.  As the sight is turned in relation to the stabilizer, position of closing the switch is changed, and the target line is generated at a position removed from the lubber line.  Manual switches are provided so that the target line and/or the lubber line may be turned off as desired (Figures V-1 and 3).
8. Because the rotor of the target line switch unit turns five revolutions for every revolution of the spinner, five target lines are potentially generated.  Four of these lines are prevented by mounting a cam operated switch upon the antenna mount so that the target line contacts are energized only during sixty degrees of the antenna rotation.  The target line is calibrated by lifting the contactor drive gear against the retaining spring and turning the gear until the target line is within one tooth of being correct.  The teeth are engaged and the knurled ring on the autosyn indicator mounting cap is turned until no error exists.  Calibration is best made with the sight turned for twenty degrees of drift.  The target line and the lubber line are both turned on

and the target line adjusted until the angle between the two is twenty degrees as indicated upon the face of the PPI.  Zero position is checked by alternately turning the target and the lubber lines on and observing their positions.  If the positions coincide, the calibration is right.  The circuit which generates the target line requires a few degrees of rotation of the antenna to recover.  Therefore, if both the target line and the lubber line are turned on, and one of them is generated a few degrees before the other, the second one will not show.  An autosyn power switch has been added to remove the power from the system when it is not being used.

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