Tutorial
3. Radio Navigation
Radio navigation provides the pilot with position information from ground stations located worldwide. There are
several systems offering various levels of capability with features such as course correction information, automatic
direction finder and distance measuring.
Most aircraft now are equipped with some type of radio navigation equipment. Almost all flights whether cross-country
or "around the patch" use radio navigation equipment in some way as a primary or secondary navigation
aid.
Automatic Direction Finder (ADF)
ADF is the oldest
radio navigation system still in use. ADF uses Non-Directional Beacons (NDBs) that are simply AM-radio transmitters
operating in the Low and Middle Frequency (L/MF) Band from 190 to 535 kHz. These frequencies are below the standard
broadcast band. All ADFs can also home in on AM broadcast stations. Pilots can listen to the radio and navigate
also. The ADF indicator has a compass rose and an indicating needle. The needle automatically points to the station.
"Homing" means following the needle. "Crabbing" to track to the station is more efficient.
Crabbing is a method of flying in which the horizontal axis of the airplane is not parallel to the flight path.
ADFs have a "HDG" knob where the pilot can dial in the aircraft heading.
Very High Frequency Omni-directional Range (VOR)
The VOR station
transmits two signals, one is constant in all directions, and the other varies the phase relative to the first
signal. The VOR receiver senses the phase difference between the two frequencies and the difference identifies
360 different directions or "radials" from the VOR. The aircraft is on one, and only one, radial from
the station. The system does not provide distance information.
When the appropriate VOR frequency is entered into a navigation radio, the VOR indicator connected to that radio
is used to find where the aircraft is relative to the VOR station. The vertical needle called a Course Deviation
Indicator (CDI) on the VOR indicator shows whether the aircraft is right or left of the chosen course. A "To/From/Off"
indicator indicates whether the aircraft is on the "to" or "from" side. If the aircraft is
"abeam the station", an "off" indication is given. To fly toward the station, the Omni Bearing
Selector (OBS) is turned until the CDI is centered with a "to" indication. The pilot then flies that
heading. To find out where the aircraft is located from that station, center the needle with a "from"
indication. If a radial is dialed into the VOR indicator, the CDI will be right or left of the center and either
a "to" or a "from" indication will be seen. The heading of the aircraft does not matter.
Distance Measuring Equipment (DME)
DME as its name states is an electronic device that measures "slant range" from the DME station. Slant
range is a measure of an aircraft's position relative to the DME station that incorporates the height of the aircraft,
its angle from the ground station and its unknown ground range based upon a 90° angle. The farther the aircraft
is from the station and the lower the aircraft's altitude, the more accurate the distance reading. An aircraft
could be directly over the DME station at an altitude of 10,500 feet above ground level (AGL) and the DME would
correctly indicate the aircraft is two miles from the station.
Instrument Landing System (ILS)
An aircraft on an
instrument landing approach has a cockpit with computerized instrument landing equipment that receives and interprets
signals being from strategically placed stations on the ground near the runway. This system includes a "Localizer"
beam that uses the VOR indicator with only one radial aligned with the runway. The Localizer beam's width is from
3° to 6°. It also uses a second beam called a "glide slope" beam that gives vertical information
to the pilot. The glide slope is usually 3° wide with a height of 1.4°. A horizontal needle on the VOR/ILS
head indicates the aircraft's vertical position. Three marker beacons (outer, middle and inner) are located in
front of the landing runway and indicate their distances from the runway threshold. The Outer Marker (OM) is 4
to 7 miles from the runway. The Middle Marker (MM) is located about 3,000 feet from the landing threshold, and
the Inner Marker (IM) is located between the middle marker and the runway threshold where the landing aircraft
would be 100 feet above the runway.
The VOR indicator for an ILS system uses a horizontal needle in addition to the vertical needle. When the appropriate
ILS frequency is entered into the navigation radio, the horizontal needle indicates where the aircraft is in relation
to the glide slope. If the needle is above the center mark on the dial, the aircraft is below the glide slope.
If the needle is below the center mark on the dial, the aircraft is above the glide slope.
LORAN-C
 |
| LORAN-C stations across the contiguous United States |
 |
|
