-3- 
A considerable advantage lies in the fact that useful statoscope data can 
he obtained under nearly all topographical and meteorological conditions. 
APR 
The airborne profile recorder (APR) is a combination of a statoscope 
(hypsometer) and a radar altimeter. 
In its present form it supplies a continuous graphical registration of two 
parameters - the radar clearance and the difference between radar clearances 
and the height deviation of the aircraft from an isobaric reference surface, 
the latter representing the absolute ground height with respect to the 
isobaric reference surface. 
The characteristic of the hypsometer component is identical to that 
described in the foregoing paragraph. 
Radar altimeter 
The radar altimeter registers the approximate vertical distance between the 
aircraft and the ground. 
Due to the relatively wide beam of the conventional equipment (1®), the 
reflections of an area with a diameter of about 1*5of the flying height are 
recorded by the equipment. The registration corresponds to a weighted mean 
of all reflected energy, which can lead to unreliable data in the case of 
mountainous terrain (steep slopes) and over forested areas. 
In addition to the graphical registration, a special camera takes at short 
intervals (usually 6 per airbase and at each exposure of the survey photo 
graphy) registering photos parallel to the radar beam, allowing the 
identification of these selected profile points on the survey photography. 
The corresponding APR-Data are identified by an index on the registration 
chart. 
The above mentioned influences of the terrain makes it necessary to evaluate 
the individual APR data (by stereoscopic inspection of the nature of the 
APR-points on the survey photography) before they are used. If necessary, 
they have to be classified in different categories of reliability and 
consequently used with different weights in the aerial triangulation. 
The accuracy of the radar altimeter data will also be influenced by a tilt 
of the aircraft. (A tilt of 3° causes an error of about 1$ of the flying 
height).