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.