ABSTRACT
The early airborne navigation systems using
ground-based navigation aids consisted of a
fixed-loop antenna in the aircraft tuned to an
amplitude modulated (AM) commercial radio
broadcast station. Pilots would know the location
of the radio station (indeed, it would invariably
have been located close to or even in the town/
city that the crew wanted to fly to). The fixed-
loop antenna was aligned with the longitudinal
axis of the aircraft, with the pilot turning the
aircraft until he received the minimum signal
strength (null reading). By maintaining a null
reading, the pilot could be sure that he was flying
towards the station. This constant turning was
inefficient in terms of fuel consumption and
caused inherent navigation problems in keeping
note of the aircraft’s position during these
manoeuvres! The effects of crosswind
complicated this process since the aircraft’s
heading is not aligned with its track.
