The navigator's viewfinder operates directly from the airborne 
computer. The viewfinder is primarily an instrument for updating 
the navigation system, or checking the automatic performance of the 
navigation system. It has several operating modes. For example, 
if the geographic coordinates of a landmark are known, these coord 
inates may be manually inserted into the navigation station controls. 
The computer system compares these coordinates with the calcu 
lated position of the aircraft. When the landmark is in range of the 
viewfinder, it is automatically aimed at the landmark. This mode 
of operation drastically reduces the effort of the navigator in 
searching for poorly defined features of the terrain. Once the land 
mark has been identified, the navigator visually tracks it as the air 
craft passes by. If the aircraft is under SHIRAN control, the 
calculated aircraft position will generally exceed the accuracy that 
can be obtained from a sighting by the viewfinder. If the aircraft is 
relying strictly on the inertial navigation system, any drift of the 
inertial system may be removed while the navigator tracks the known 
landmark. 
Another mode of operation of the viewfinder is tracking an un 
known terrain feature. If such a landmark is tracked, the computer 
calculates the latitude and longitude. After this is accomplished, the 
latitude and longitude can be inserted into the computer so that the 
same landmark will be picked up during the adjacent flight strip, and 
used to eliminate any drift of the inertial system that occurred be 
tween the two sightings. Such unknown landmarks can be photo 
graphed by the use of a polaroid camera which is part of the 
navigator's viewfinder system. Terrain features surveyed in by this 
technique are of course not as accurate as photogrammetric deter 
mination. However, they are useful for future reference as a check 
on gross position and as an accurate check on the relative positions 
on two adjacent flight paths. In addition to the strictly navigational 
functions of this viewfinder, it can be used as a back-up for the photog 
raphers viewfinder for measuring drift angle, and for direct control 
of the camera mounts.