Space limitations and technical differences in the operation of the ALR/34/35/38 systems preclude a detailed description of the ARDF function in this report. Explicit descriptions of each aspect of the various systems may be found in 469th (Probably should read 460th) TRWM 55-1 and "A Critical Review of the ARDF Operations in SEA" by the 460th Tactical Reconnaissance Wing, both of which are excellent reference sources.

A simplified description of ARDF operations as conducted by the EC-47s in 1970, may, however, be given. Primary considerations in fixing of enemy radio transmissions included knowledge of the aircraft's accurate position and heading, as determined by the Doppler radar system and the C-12 compass, acquisitionof the enemy's signal, and subsequent tactics used to determine his location.

As soon as possible after takeoff, the navigator set the Doppler over a known point, with reference to a Doppler Zero Point. Since the Doppler navigational computer was subject to an accumulation of errors, especially at the low speeds flown by the EC-47, it was necessary to update the system ever 20 to 30 minutes, or immediately following a fix, by flying over a known geographical point and resetting it. This was not always possible, because of night, weather, or terrain which effered no distinctive geographical points, but, in these circumstances, other methods could be used to update the system. For example, Combat Skyspot (MSQ-77 radar), a highly accurate ground-based radar, could "skin paint" the aircraft up to 20 miles, or, if the EC-47 was equipped with an X-band transponder, up to 100 miles, with a high degree of accuracy. Also, TACAN/DME could, within limits, be used to update the Doppler. Unfortunately, neither of these methods was considered as accurate as use of a driftmeter, and CEPs had to be adjusted outward accordingly.

Since the ARDF function was a totally responsive one (transmitters could be fixed only when the radios were emmiting), the first consideration was to position the aircraft so as to pick up signals from the low-powered radios. Most of the fragged areas were approximately 20 nautical miles in radius. Once there, it was up to the navigator to direct the aircraft within the area to provide maximum probablility of picking up enemy radio emmissions.