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down to the fix position (Δ) to determine how far you are from the fix (48 NM in this example; Figure
5.15.)
5.16.4. Repeat the procedure as necessary to keep your progress updated.
164 AFPAM11-216 1 MARCH 2001
Figure 5.13. Setting Present Position on MB-4 Computer.
Figure 5.14. Setting Fix on MB-4 Computer.
AFPAM11-216 1 MARCH 2001 165
Figure 5.15. Solution to Fix-to-Fix and Distance to Fix.
Section 5D— Communication
5.17. Long and Short Range Communication. Air-to-ground communications can be achieved
through the use of many types of radio equipment. High frequency bands (HF, VHF, UHF) are relatively
static-free and are less susceptible to outside interference than lower frequencies. It must be
remembered, however, that the higher the frequency, the more nearly the transmission will follow an
LOS path. As frequency increases, therefore, communication range decreases.
5.17.1. Long Range:
5.17.1.1. Systems used for long-range radio communications between aircraft and ground stations may
be either amplitude modulation (AM) or single sideband (SSB) transmissions. Single sideband
transmitters concentrate all available power into one sideband; therefore, SSB is much more efficient
and has greater range than an AM transmitter of the same power.
5.17.1.2. Although HF ground waves attenuate rapidly, sky waves at these frequencies are capable of
transmitting at distances up to 12,000 miles or more, depending on ionospheric conditions. HF
equipment is used mostly in remote areas where VHF or UHF communication is not possible because of
the great distance which must be spanned.
5.17.2. Short Range Air-to-Air and Air-to-Ground. Short range air-to-air and air-to-ground
communications are confined to the VHF and UHF bands. VHF channels are spaced at 25 kHz intervals
from 116 to 151.975 mHz, and UHF channels are spaced at 50 kHz intervals from 225.0 to 399.9 mHz.
Most UHF and HF transceivers have a manual frequency selection capability in addition to a number of
preset channels. Transmission and reception are accomplished with a single antenna.
166 AFPAM11-216 1 MARCH 2001
5.17.3. AN/APX-64 Transponder. The AN/APX-64 (Figure 5.16) provides for Mark X Identification
Friend or Foe (IFF) with selective identification feature (SIF) in normal operating modes. Two
additional modes allow more specific identification by controlling agencies. Mode C works in
conjunction with the aircraft altitude computer and gives automatic altitude reporting to ground radar.
Mode 4 provides a secure IFF capability for military aircraft. A typical IFF set has the following
features:
Figure 5.16. IFF/SIF Transponder.
5.17.3.1. IFF Master Control Knob. This is a five-position detented rotary switch: (1) OFF, (2)
STBY—No transmission capability. Warmup period, (3) LOW—Receiver sensitivity is reduced and
only local interrogations are answered, (4) NORM—Full range operation, and (5) EMER—Causes
automatic transmission of emergency reply signals when interrogated by Mode l, Mode 2, or Mode 3/A.
5.17.3.2. IFF Mode Switches. Four three-position toggle switches are used to select the desired
operating modes. Mode 1 is the aircraft security identity mode. Mode 2 is for unit identity. Mode 3/A is
for normal air traffic control identity. Mode C is the altitude reporting mode. The three positions are
identical for all four toggle switches: (1) OUT—Mode disabled, (2) ON—Allows normal response to
mode interrogations, and (3) TEST—Spring-loaded for in-flight test of selected mode. Test
interrogations are generated, the transponder response is analyzed, and a positive indication illuminates
the test light.
5.17.3.3. Code Selector Wheels. (1) MODE 1—Sets any one of 32 possible codes, (2) MODE 2—Sets
any one of 4,096 possible codes. Not located on this control unit, and (3) MODE 3/A—Sets any one of
4,096 possible codes.
AFPAM11-216 1 MARCH 2001 167
5.17.3.4. Identification of Position Switch. A three-position toggle switch that enables IDENT replies
(This action illuminates the return on the controllers scope more brightly): (1) IDENT—Spring-loaded
switch which initiates a 30-second reply, (2) OUT—OFF, and (3) MIC—Initates a reply for 30 seconds
when the MIC button is depressed and the interphone transmitter selector knob is set to UHF.
5.17.3.5. Radar Test Monitor Switch. Used by ground maintenance and in conjunction with the test
lamp to monitor all responses in any SIF mode.
5.17.3.6. Test Lamp. Lights in conjunction with proper mode and/or test conditions and RAD
TEST/MON SW interrogations.
5.17.3.7. Mode 4. Mode 4 is a classified transponder system which provides secure IFF capability to
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