hat
r
in**
Q a o » <
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 &