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Fig. 10 Installation of Doppler Navigator 
in a DC-3 Aircraft 
DOPPLER NAVIGATIONAL SYSTEM 
DOPPLER — 
4 RADAR BEAMS 
Fig. 11 
The Doppler navigator, or navigational system, measures the ground 
speed and drift angle of an aircraft in flight. The system (Fig. 10) controls 
each flight in a predetermined direction, thus maintaining specified overlap 
between flight lines. It also triggers the aerial camera at predetermined 
distances and hence it establishes the forward overlap. 
The Doppler navigational system is self-contained and does not 
require any ground stations. It demands only one assumption of basic informa 
tion, that we know the starting point. Electronic waves transmitted from the 
aircraft to the ground are reflected and received again in the aircraft. 
Because of the relative motion of the aircraft to the ground, the frequency of 
the reflected signal differs slightly from the transmitted one. This difference 
in frequency is due to the Doppler effect. 
The Radan-Doppler navigational system employed by us is manufactured 
by General Precision Laboratories of New York. A compact antenna-receiver- 
transmitter, mounted in the aircraft (Fig. 11), transmits four beams of pulsed 
microwave energy toward the earth: two at a time in diagonal pairs, with left- 
front, right-rear; then right-front, left-rear, by means of a specially designed 
slotted-planar-array antenna. Echoes of these signals, shifted in frequency by 
an amount proportional to the aircraft's ground speed, are received back in the 
plane. 
11 
mm &