EQUIPMENT UPGRADING

THE DOPPLER SYSTEM

For any aircraft moving through space to "fix" accurately the position of a captive radio target on the ground, it was as important for the people in the aircraft to know their exact position as it was to have a readable (LOPs). These would be meaningless if the position and heading of the aircraft were not known with extreme accuracy. To achiev this objective, EC-47 aircraft employed two primary navigation systems - the C-12 fluxgate compass system to provide accurate heading indication and the Bendix APN-179 Doppler radar naviational system to provide position over the ground.

The Doppler system consisted of three seperate but interrelated units. The Doppler radar was composed of an antenna, fluch mounted on the underside of the fuselage, a frequency tracker and receiver-transmitter located in the radio rack, and a ground speed and drift indicator at the navigators station.

With the system operating, four separate beams of energy were radiated toward the earth's surface at 8,800 mhz. The signal was reflected back and picked up by the receiver-transmitter (RT) unit, having been shifted in frequency by an amount proportional to the aircraft's displacement from its originally set latitudinal and longitudinal position. The frequency tracker, using the RT information, provided inputs to the ground speed and drift indicator and to the Doppler computer, which integrated the information with that provided by the C-12 compass and relayed it to the computer controller. This procedure provided the operator with the aircraft's position to within +or- 0.1 nautical miles under "normal" conditions, i.e. equipment accurate, Doppler set-point reasonably updated, weather conditions reasonable, ground speed over 100 knots, and drift not exceeding 12 to 15 degrees. (Figure 5 depicts the operation of the APN-179 Doppler system.)

THE C-12 COMPASS SYSTEM

The C-12 compass system provided an accurate heading reference to the Doppler radar system, the ALR-34/35/38, and remote indicator on the aircraft. Useful at any latitude, the system used the gyro mode at higher latitudes and the magnetic mode at lower latitudes; in SEA, therefore, it was used only in the magnetic mode. The induction compass transmitter (flux valve, or flux gate) located in the right wing of the aircraft, electrically detected the horizontal component of the earth's magnetic field to provide the basic magnetic signal to the system. The directional gyroscope provided the basic heading stabilization for the C-12 system. With the system in magnetic mode, if the system heading did not agree with that of the induction compass, a heading error signal was developed. Applied to a slaving power amplifier, it produced an ooutput to drive the gimbal of the directional gyro until the error signal went to zero. With ofhte possible errors compensated internally, the C-12 was accurate at mid-latitudes to within 0.25 degrees at speeds up to 500 knots.