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inaccurate. Under changing atmospheric conditions, corrections applied to PA readings to obtain TAs
are only approximate. In addition, any error made in determining the terrain elevations results in a
corresponding error in the absolute altitude.
3.16. Radar Altimeter (High-Level). A typical high-level radar altimeter is designed to indicate
absolute altitude of the aircraft up to 50,000 feet above the terrain, land, or water. This altimeter does not
warn of approaching obstructions, such as mountains, because it measures altitude only to a point
directly below the aircraft (Figure 3.19).
Figure 3.19. Typical High-Level Radar Altimeter.
3.16.1. A typical set consists of the radar receiver-transmitter, height indicator, and antenna. The
transmitter section of the receiver-transmitter unit develops recurring pulses of radio frequency (RF)
energy which are delivered to the transmitter antenna located either flush-mounted or on the underside of
the aircraft. The transmitter antenna radiates the pulsed energy downward to reflect off the earth and
return to the receiver antenna on the aircraft. The time consumed between transmission and reception of
the RF pulse is determined only by the absolute altitude of the aircraft above the terrain since the radio
wave velocity is constant.
3.16.2. The receiver antenna delivers the returned pulse to the receiver section of the receiver-transmitter
unit where it is amplified and detected for presentation on the indicator unit. The radar altimeter
indicator displays absolute altitude which is used in pressure pattern navigation, terrain clearance, or as a
backup for the PA.
104 AFPAM11-216 1 MARCH 2001
3.17. Radar Altimeter (Low-Level). This type altimeter provides a dial or digital indication of the
altitude of the aircraft above the terrain. It is designed to eliminate the necessity of adding antennas or
any other equipment external to the surface of the aircraft. This equipment may also be used in
conjunction with automatic pilot or other devices requiring altitude limit data.
3.17.1. Systems vary widely, but typically include a receiver-transmitter, height indicator, and electronic
control amplifier. The height indicator contains the only operating control on the equipment. This
instrument normally gives altitude readings up to 35,000 feet. If the instrument has an analog scale, the
markings are usually logarithmic, graduated for the low altitude portion of its range. A variable altitude
limit indicator system is included to provide an indication of flight below a preset altitude.
3.17.2. To operate the equipment, turn the ON-LIMIT control to ON. After warmup, the terrain
clearance of the aircraft within the range of 0–20,000 is read directly from the single pointer on the
indicator as shown in Figure 3.20. This pointer can be preset to any desired altitude by the ON-LIMIT
control and is used as a reference for flying at fixed altitudes. The altitude can be maintained by
observing the position of the pointer with respect to the small triangular marker instead of the actual
altitude scale. In addition, a red light on the front of the indicator lights up when the aircraft is at or
below the preset altitude. To turn off the equipment, it is only necessary to turn off the ON-LIMIT
control on the indicator.
Figure 3.20. Low-Level Radar Altimeter.
Section 3F— Temperature
3.18. Basics. Determination of correct temperature is necessary for accurate computation of airspeed and
altitude. Temperature, airspeed, and altitude are all closely interrelated, and the navigator must be
familiar with each in order to work effectively and accurately.
AFPAM11-216 1 MARCH 2001 105
3.19. Temperature Gauges. The temperature gauge most commonly used in the Air Force employs a
bimetallic element. The instrument is a single unit consisting of a stainless steel stem which projects into
the airstream and a head which contains the pointer and scale. The sensitive element in the outside end
of the stem is covered by a radiation shield of brightly polished metal to cut down the amount of heat
that the element might absorb by direct radiation from the sun. The bimetallic element (called the
sensitive element) is so named because it consists of two strips of different metal alloys welded together.
When the element is heated, one alloy expands more rapidly than the other causing this element, which
is shaped like a coil spring, to turn. This, in turn, causes the indicator needle to move on the pointer dial.
Temperatures between –60 oC and 50 oC can be measured on this type of gauge.
3.20. Temperature Scales. In the United States, temperature is usually expressed in terms of the
Fahrenheit scale (oF). In aviation, temperature is customarily measured on the centigrade, or Celsius
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