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shown here are the course indicator (Figure 5.8), the radio magnetic indicator (RMI) in Figure 5.9, and
the bearing direction heading indicator (BDHI) in Figure 5.10. The course indicator has eight significant
features: (1) TO-FROM indicator, (2) glide slope and course warning flags, (3) course selector window,
AFPAM11-216 1 MARCH 2001 157
(4) marker beacon light, (5) glide slope indicator, (6) heading pointer, (7) course deviation indicator
(CDI), and (8)course set knob.
Figure 5.7. VOR Nav Control Panel.
Figure 5.8. Course Indicator.
158 AFPAM11-216 1 MARCH 2001
Figure 5.9. Radio Magnetic Indicator.
Figure 5.10. Bearing Distance Heading Indicator.
5.12.3. Indicators. The TO-FROM indicator shows whether the radial set in the course selector window
is to or from the station, and the CDI represents this radial. If the aircraft is to the right of the radial, the
CDI is displaced to the left of center on the course indicator. The glide slope indicator is similar to the
CDI but represents the glide slope transmitted by an instrument landing system (ILS). If the glide slope
indicator is below the center of the course indicator, the aircraft is above the glide slope. The glide slope
and course warning flags inform the user that either the glide slope indicator or CDI is inoperative, or
that signals received are too weak to be used. The heading pointer indicates the difference, left or right,
between the aircraft MH and the radial set in the course selection window. The marker beacon light
AFPAM11-216 1 MARCH 2001 159
flashes when passing over a marker beacon (such as, outer marker of the ILS). The RMI is a bearing
indicator, usually with two pointers and a movable compass rose. The compass rose rotates as the
aircraft turns, indicating the compass heading of the aircraft under the top of the index at all times.
Therefore, all bearings taken from an RMI are magnetic. Consult the specific technical order as to which
pointer is the VOR.
5.12.4. BDHI. The BDHI is similar to the RMI because the needles provides MB information.
Additional information concerning the BDHI is in the TACAN section.
5.13. Tactical Air Navigation (TACAN). The TACAN system was developed to provide the
crewmember with information needed for precise positioning within 200 NM. As with VOR, TACAN
provides an infinite number of radials radiating outwardly from the station. In addition, distance
measuring equipment (DME), an integral part of TACAN, provides continuous slant-range distance
information. TACAN operates in the UHF band and has 126 channels available in the X-band pulse
code. Development of pulse coding has given ground equipment the capability of an additional 126
channels in the Y-band. The station identifier is transmitted at 35-second intervals in international Morse
code. Airborne DME transmits on 1025-1150 mHz; associated ground-to-air frequencies are in the 962-
1024 mHz and 1151-1213 mHz ranges. Channels are separated at 1 mHz intervals in these bands.
5.13.1. Ground Equipment. The ground equipment consists of a rotating-type antenna for transmitting
bearing information and a receiver-transmitter (transponder) for transmitting distance information.
Permanent ground stations are dual transmitter-equipped (one operating and one in standby) installations
which automatically switch to the standby transmitter when a malfunction occurs. Each station has a
ground monitor which is set to alarm at a radial shift of 1o from the alignment to magnetic north (MN).
This alarm is usually located in the base control tower or approach control, and sets off a light and
buzzer to warn when an out-of-tolerance condition exists. It is possible to select a TACAN station and
get erroneous DME and azimuth lock-on when the station is undergoing maintenance. This can be
detected by an absence of signal identifier. Checks of en route or radio NAVAIDs may be made by
consulting NOTAMs prior to flight or by contacting air traffic control for advisories when airborne.
5.13.2. Airborne Equipment. The airborne equipment also contains a multichannel transmitter-receiver
(transceiver). Bearing information is automatically obtained with the correct channel selected. Distance
is determined by measuring the elapsed time between transmission of interrogating pulses of the
airborne set and reception of corresponding reply pulses from the ground stations. This sequence is
initiated by the aircraft transmitter and requires about 12 microseconds per NM round trip. Since the
DME gives a readout of slant range rather than ground range, a correction has to be applied to the
reading. (See slant range table for correction.)
5.13.2.1. DME is designed to provide reliable information to a maximum distance of 199 NM,
dependent on aircraft equipment and LOS. Accuracy is plus or minus one half NM or 3 percent of the
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