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