ope is limited 
there is con- 
then possible. 
tion does not 
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at statoscope 
———— 
negative film. 
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ord, produced 
ing film. The 
1 is in actual 
its limit; this 
'arried out by 
|gress. 
SURVEY NAVIGATION, CORTEN 77 
3.8. Radar altimeter. Airborne profile recorder. 
Principle. 
a. A self-contained radar, its narrow beam vertically stabilized by gyro, measures the 
airplane's ground clearance. 
b. A microbarometer measures the airplane’s flying height variations with respect to 
isobaric surface. 
If these two measurements are combined, the reflecting surface’s profile with respect 
to a datum parallel with the isobaric surface is found. In order to convert this into useful 
information, corrections must be applied such as: 
the isobaric surface’s slope with respect to survey datum plane, 
terrain reflectance characteristics, 
instrumental errors, ete. 
As a result, a profile of the terrain is obtained. 
Instruments available. 
RVTD Radiovysotomer, constructed by the Central’nyj Nauéno-Issledovatel’skij In- 
stitut Geodezii Aeros-emki i Kartografii (CNNGAiK). It is used in combination with the 
statoscope and with photographic recording. 
Airborne Profile Recorder Mk 5, Canadian Applied Research Ltd. 
Narrow Beam Altimeter EA-NBA-2, Electronic Associates Ltd. 
The U.S.S.R. radar altimeter, consisting of an oscillator unit, transmitter-receiver 
unit and indicator unit, displays its indication on a circular scope screen which is synchro- 
photographically recorded. 
The APR, in its improved version Mk 5 contains a hypsometer as a sensitive pressure 
altimeter. This contains toluene, the boiling temperature of which is measured by means 
of a thermister. Its sensitivity, particularly at the greater flight altitudes, is equivalent 
to approx. 30 em flying height difference. 
The indicator supplies a continuous record of the ground clearance, the airplane’s 
height variations and the terrain profile. 
Remarks. 
In Canada, operational research was carried out on the APR’s proper ground loca- 
tion, on the proper choice of elevation points, and on the determination of the isobaric 
surface. 
The inner accuracy of the instrument was found to be better than 1 m, but external 
influences such as the precise location error, the elevation point choice within the illu- 
minated area, etc., augment this error to m, — = 1,5 m at shorter distances and from 
lower altitudes, and to m, — * 3 m in aerial triangulation strips with flying heights of 
9.000 m. 
In U.S.S.R., the radar altimeter is used: 
— for small scale topographic contour mapping with the topographic stereometer; 
— as auxiliary in radial triangulation establishing planimetric control (increase of 
ground control point efficiency by a factor 1,5...2 through reduction of scale error 
accumulation) ; 
— for elevation control extension as auxiliary data in aerial triangulation; 
— for compilation of semi-controlled mosaics; 
— for evaluation of position determination in flight by means of radar ranging at each 
camera station as is applied for 1:100.000 mapping 
and is proposed for use in topographic mapping 1 : 25.000 and 1 : 10.000.