12 
The drift angle is found by comparing the Doppler frequency of one 
of the diagonal pairs with the Doppler frequency of the other diagonal pair. 
The ground speed is found by measuring the beat frequency produced when 
reflected ground signals received by the forward-looking beam are mixed with 
those received by the backward-looking beam. 
The system includes a course and distance computer, a Kearfott J-4 
compass, and a specially designed computer to track the aircraft on a set 
course. It provides the aircraft's position continuously and does not require 
ground stations. 
The data from the Doppler navigator is computed and visually 
presented to the pilot instantly. Before the Doppler unit is turned on at the 
beginning of the first flight line of each mission, two parameters, line length 
and magnetic azimuth, are set in the computer. It then begins to indicate 
deviations left or right of track and counts elapsed mileage on a visual 
indicator. During the flight of each line the parameters of the next line are 
introduced into a second circuit of the computer. These are line length, 
magnetic azimuth, and offset (line spacing). 
3.A STATOSCOPE 
The Wild or Zeiss statoscope registers the difference of each 
exposure station relative to an isobaric surface. The statoscope is not an 
absolutely necessary element of the mapping system when the horizon camera is 
used, but it constitutes a valuable addition in certain aerial triangulation 
techniques discussed later in this paper. The slope of the isobaric surface can 
be computed by the following formula: 
■^kfeet = 0.035 v.d. sin«<. sin^P .... 3 
where 
v is the air speed in miles per hour 
d = distance from the starting point 
cL is the drift angle, and 
'P is the approximate latitude.