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unit sends an interrogation signal to the VOR/DME or VORTAC station. The ground station then transmits a
responding signal. The DME unit measures the time elapsed between sending the interrogating signal and receipt
of the response signal. From this information, the DME unit calculates the distance to the station.
Some units also calculate groundspeed and “time to the station”. However, GS and time to the station are only
valid if you are flying directly to or from the station. If you are flying in any other direction, these values will be
incorrect. However, the distance to the station will be correct.
The indicated distance is known asare some distance from the statiodistance will be very close to the gVOR station, the slant range errorNavigation
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For example, if you are flying at 6000 feet, the DME will register 1 NM to the station when you are directly over
the station. See the Aeronautical Information Manual AIM 1-7. DISTANCE MEASURING EQUIPMENT
(DME) for further information.
Automatic Direction Finder (ADF)
Some aircraft are equipped with an ADF receiver. See the Aeronautical Information Manual AIM 1-2.
NONDIRECTIONAL RADIO BEACON (NDB) for more information. They receive radio signals in the medium
frequency band of 190 Khz to 1750 Khz. The ADF receiver can “Home” on both AM radio stations and Non-
Directional Beacons. Commercial AM radio stations broadcast on 540 to 1620 Khz. Non-Directional Beacons
(NDBs) operate in the frequency band of 190 to 535 Khz.
The aircraft equipment consists of two antennas, the ADF Receiver, and the ADF Instrument. The two antennas
are called the (1) LOOP antenna and the (2) SENSE antenna.
The loop antenna can sense the direction of the signal from the station, but cannot discriminate whether the
station is in front or behind the aircraft. The sense antenna can discriminate direction, and solves the ambiguity of
the loop antenna.
The receiver unit has tuning dials to select the station frequency A volume control allows the audible volume to
be controlled for identifying the station. The volume can be reduced to prevent interference with other
communications. You should, however, continuously monitor the identifier while using the NDB for navigation.
The navigational display contains a compass rose dial grend, and a square form on the other end. We will call the aridentification.
There are 2 types of compass rose dials that can exist in the navigational unit. One is a fixed compass rose, called
a “Fixed Card” ADF. Zero degrees is always shown on top of the card. The “Rotateable Card” ADF allows the
compass rose card to be rotated. Interpretation of these displays will be more fully described in later paragraphs.
Non-Directional Beacon (NDB)
Non-Directional Beacons are depicted on aeronautical charts as athe name of the station, the 2 or 3 letter identifier, and the MorsNDBs may be located on the surface of airports, or may be within a few miles from an airport. Sometimes they
are co-located with the Outer Marker in ILS approaches. The NDB provides two principal functions; (1) homing
for VFR operations, and (2) ADF instrument approach capability for IFR operations.
Because the frequency is below and within the commercial AM band, reception is subject to the same
atmospheric disturbances as AM radio, in particular, noise generated by lightening.
Navigation
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ADF Orientation
The pointer end of the ADF navigation unit ALWAYS POINTS TO THE STATION. The degree reading on the
display is dependent on the aircraft heading. In the diagram if the heading of the aircraft changes, the arrow will
always point to the station and the degree reading on the instrument which the pointer indicates also changes..
Fixed Card ADF
The relationship of the aircraft to the stationelements to determining MB. One is the RelativeMagnetic Heading (MH) of the aircraft. This relMB (to the station) = RB + MH
In the example, the MH of the aircraft is 270° as read on the compass. The RB read from the ADF dial is 45° .
Therefore the MB to the station = 270° + 45° = 315°. This equation applies to any problem on the FAA Written
Exam relating to the Fixed Card ADF. If any two values are known, the third can be computed.
Moveable Card ADF
Some aircraft are equipped with an ADF instrument in which the dial face of the instrument can be rotated by a
knob. This is called a Moveable Card ADF. By rotating the card such that the Magnetic Heading (MH) of the
aircraft is adjusted to be under the pointer at the top of the card, the Bearing to the Station (MB) can be read
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