Aircraft Direction Finding

Ground Direction Finding

 The earlier system was called the Radio compass. This navigation method is still with us today although called the Automatic Direction Finder [ADF].

The earlier fuselage fixture was a roof mounted Direction Finding [D/F] loop aerial. This could be manually rotated until the correct signal [null] is heard in the earphones, which indicates the loop is pointing at the transmitting ground station. There was a 180' ambiguity in this. The US Navy's Air Navigation manual [P.282] uses the following example;

"if the signal bearing points over the right wing, the ground station could also be over the port wing. If a constant heading is flown for a few more minutes, the bearing changes thus indicating the real direction of the transmitting station."

The ground stations that could be tuned into by the D/F include public radio Broadcast Stations [BS] as well as aviation Homing Beacons once called "Homers" and now called NDB stations. The 1940 Adelaide / Sydney Aviation Strip Map listed some of the Broadcast Stations pilots could have used.

The range of these Homing Beacons depends on the need. For locations in mid ocean, these can be several hundred miles by day or can be as little as 15nm as at Calga near Sydney NSW. [ERSA 25 March1999]

Some long-range locations include;

  1. Norfolk Island between Sydney and Fiji. This NDB 268 has a range of 300nm by day and 120nm by night.

  2. Cocos (Keeling) Island in the northern Indian Ocean. The NDB range at Cocos is 400nm by day and 150 night.

  3. Forrest in mid desert on the Adelaide / Perth route has an NDB range of 90nm by day and 75 by night. [ERSA November 2002]

  4. The 500 ksc Easter Island's homing beacon, near Chile in the Pacific Ocean, had a range of 300 nm in 1951. [Taylor P.251]

On May 31 1928 Charles Kingsford-Smith’s "Southern Cross" departed Oakland airport on its way to Australia. The range of the US Army's radio beam that Warner, the radio operator, used to maintain course, was 700 miles.

The arrival of "Southern Cross" at Hawaii was also assisted by a radio beacon, as was the departure for Fiji. The Hawaiian departure nav aid was possible due to the US Army Signal corps rotating the Mauri beam and aligning it with the planned course. [Gwynn-Jones P.102]

The type of gauge fitted in Sir Gordon Taylor's Consolidated Model 28-1 [commercial version of the Navy PBY-1] NC777 when he flew it across the Indian Ocean in 1939 was different to the present day ADF arrangement. There were two needles on the dial. They were supposed to rise up and cross on a bearing which indicated the direction of the aircraft's head in relation to the ground station. [Taylor P.128]

Pictures of the radio compass in Australian DC3 aircraft in the 1950's indicate a single needle gauge similar to the ADF of today.

Today this onboard D/F method is automatic and thus is called the Automatic Direction Finder [ADF].


Page last updated on 03/01/04

Dead Reckoning
Night Beacons
Celestial Navigation