continuously and automatically correct the aircraft reading to ground
level.
The altitude compensation consists of a radio altimeter
arranged to provide a voltage inversely proportional to the height
above the ground.
A potentiometer is coupled to the indicator shaft
of a radar altimeter, Figure 8, to develop the required voltage for
altitude correction.
Barometric altimeters are unsuitable, since
they indicate altitude from an arbitrary references, usually sea
level, and do not account for variation of the earth's surface.
A
rager gitimeter gives the true altitude from the surface.
Manual altitude compensation is possible should the radar
altimeter fail.
Manual operation is useful only when the terrain
is fairly level and when it ia possible to closely estimate the
distance from aircraft to ground.
Then. by flying at a constant
barometric altitude. ground level data can be obtained.
2olo4
Altitude Compensation Circuit Description
The detectcr signal 1s coupled into the compensation circuit
through switch, S-1H (Fig. 6).
The altitude compensation voltage
is developed by potentiometer R-l,, in automatic operation, and by
potentiometer R=37 in manual operation.
I+ is added to th detector
voltage to produce a signal proportional to the radiation intensity
at ground level.
This level is measured by a vacuum tube voltmeter
circuit having a one volt full scale sensitivity.
(Section 3.2.1).
A voltage corresponding to the maximum (1500 ft) altitude is
developed across the altitude compensation potentiometer by
battery Be7 through calibration resistor R-38.
2 15.
The potentiometer