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(225°).
4. Select the distance from the DME to the WP (12
NM).
5. Check and confirm all inputs, and center the CDI needle
with the TO indicator showing.
6. Maneuver the aircraft to fly the indicated heading
plus or minus wind correction to keep the CDI needle
centered.
7. The CDI needle will indicate distance off course of 1
NM per dot; the DME readout will indicate distance in
NM from the WP; the groundspeed will read closing
speed (knots) to the WP; and the time to station (TTS)
will read time to the WP.
VOR/DME RNAV Errors
The limitation of this system is the reception volume.
Published approaches have been tested to ensure this is not
a problem. Descents/approaches to airports distant from the
VOR/DME facility may not be possible because, during
the approach, the aircraft may descend below the reception
altitude of the facility at that distance.
Long Range Navigation (LORAN)
LORAN uses a network of land-based transmitters to provide
an accurate long-range navigation system. The FAA and the
United States Coast Guard (USCG) arranged the stations
into chains. The signal from station is a carefully structured
sequence of brief RF pulses centered at 100 kHz. At that
frequency, signals travel considerable distances as ground
waves, from which accurate navigation information is
available. The airborne receiver monitors all of the stations
within the selected chain, then measures the arrival time
difference (TD) between the signals. All of the points having
the same TD from a station pair create a line of position
7-25
Figure 7-26. A control panel from a military aircraft after LORAN
was first put into use. The receiver is remotely mounted and weighs
over 25 pounds. Its size is about six times that of the LORAN fully
integrated receiver.
(LOP). The aircraft position is determined at the intersection
of two or more LOPs. Then the computer converts the
known location to latitude and longitude coordinates.
[Figure 7-26]
While continually computing latitude/longitude fixes, the
computer is able to determine and display:
1. Track over the ground since last computation;
2. Groundspeed by dividing distance covered since last
computation by the time since last computation (and
averaging several of these);
3. Distance to destination;
4. Destination time of arrival; and
5. Cross-track error.
The Aeronautical Information Manual (AIM) provides a
detailed explanation of how LORAN works. LORAN is
a very accurate navigation system if adequate signals are
received. There are two types of accuracy that must be
addressed in any discussion of LORAN accuracy.
Repeatable accuracy is the accuracy measured when a user
notes the LORAN position, moves away from that location,
then uses the LORAN to return to that initial LORAN
position. Distance from that initial position is the error.
Propagation and terrain errors will be essentially the same as
when the first position was taken, so those errors are factored
out by using the initial position. Typical repeatable accuracy
for LORAN can be as good as 0.01 NM, or 60 feet, if the
second position is determined during the day and within a
short period of time (a few days).
Absolute accuracy refers to the ability to determine present
position in space independently, and is most often used by
pilots. When the LORAN receiver is turned on and position
is determined, absolute accuracy applies. Typical LORAN
absolute accuracy will vary from about 0.1 NM to as much
as 2.5 NM depending on distance from the station, geometry
of the TD LOP crossing angles, terrain and environmental
conditions, signal-to-noise ratio (signal strength), and some
design choices made by the receiver manufacturer.
Although LORAN use diminished with the introduction
of Global Navigation Satellite Systems such as the United
States’ GPS, its use has since increased. Three items aided
in this resurgence:
• In 1996, a commission called the Gore Commission
evaluated GPS’ long-term use as a sole navigation
aid. Although GPS was hailed originally as the
eventual sole NAVAID, which would replace the need
for most currently existing NAVAIDs by the year
2020, the Commission questioned single-link failure
potential and its effect on the NAS. For this reason,
the forecasted decommissioning of the VOR has been
amended and their expectant lifecycle extended into
the future. Additionally, the use of LORAN continues
to be evaluated for facilitating carrying GPS corrective
timing signals.
• The GPS is controlled by the DOD presenting certain
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