output voltage is directly proportional to the altitude (Section 201.3 equation 6). Therefore, the compmsation added to the radiation signal will be a logarithmic function of the altitude. The proper compensation voltage for an area contaminated with radioisotopes can be determined by taking two passes over a level section of the area, one at 500 and the second at 1000 feet. The ratio of the readings obtained at the two altitudes can be used to determine the voltage applied to the altitude compensation potentiometers. description of this procedure). (See Section 2.4.4 for a full Once the voltage has been determined and set, the instrument will properly compensate for any altitude over an area with a similar type of contamination. However where these values have previously been determined, the altitude compensation voltage may be set prior to operation. Since the rotation of the indicator shaft of the radar altimeter (APN-22), which has been used with this instrument is not a linear function of altitude, the resistance of the automatice compensation potentiometer (R-Ly) was computed to produce a linear change with altitude. It has been reported that the newer models of the altimeter may have linear output characteristics available thus permitting the use of standard linear potentiometers. 2.1.5 Power Supply All power for the "Top Hat" airborne detector is obtainsd from an internal supply of mercury cell batteries, which have been selected because of their flat voltage discharge characteristic and sharp voltage drop off at the end of useful life. - 17 =