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Use of “all−in−view” stations by a receiver is made
possible due to the synchronization of LORAN
stations signals to UTC. This new generation of
receivers, along with improvements at the transmitting
stations and changes in system policy and
operations doctrine may allow for LORAN’s use in
nonprecision approaches. At this time these receivers
are available for purchase, but none have been
certified for aviation use.
2. The basic measurements made by certified
LORAN receivers are the differences in time−of−arrival
between the Master signal and the signals from
each of the secondary stations of a chain. Each “time
difference” (TD) value is measured to a precision of
about 0.1 microseconds. As a rule of thumb,
0.1 microsecond is equal to about 100 feet.
3. An aircraft’s LORAN receiver must recognize
three signal conditions:
(a) Usable signals;
(b) Absence of signals, and
(c) Signal blink.
4. The most critical phase of flight is during the
approach to landing at an airport. During the
approach phase the receiver must detect a lost signal,
or a signal Blink, within 10 seconds of the occurrence
and warn the pilot of the event. At this time there are
no receivers that are certified for nonprecision
approaches.
5. Most certified receivers have various internal
tests for estimating the probable accuracy of the
current TD values and consequent navigation
solutions. Tests may include verification of the timing
alignment of the receiver clock with the LORAN
pulse, or a continuous measurement of the signal−
to−noise ratio (SNR). SNR is the relative strength of
the LORAN signals compared to the local ambient
noise level. If any of the tests fail, or if the quantities
measured are out of the limits set for reliable
navigation, then an alarm will be activated to alert the
pilot.
6. LORAN signals operate in the low frequency
band (90−110 kHz) that has been reserved for marine
navigation signals. Adjacent to the band, however,
are numerous low frequency communications
transmitters. Nearby signals can distort the LORAN
signals and must be eliminated by the receiver to
assure proper operation. To eliminate interfering
signals, LORAN receivers have selective internal
filters. These filters, commonly known as “notch
filters,” reduce the effect of interfering signals.
7. Careful installation of antennas, good metal−
to−metal electrical bonding, and provisions for
precipitation noise discharge on the aircraft are
essential for the successful operation of LORAN
receivers. A LORAN antenna should be installed on
an aircraft in accordance with the manufacturer’s
instructions. Corroded bonding straps should be
replaced, and static discharge devices installed at
points indicated by the aircraft manufacturer.
d. LORAN Navigation
1. An airborne LORAN receiver has four major
parts:
(a) Signal processor;
(b) Navigation computer;
(c) Control/display, and
(d) Antenna.
2. The signal processor acquires LORAN
signals and measures the difference between the
time−of−arrival of each secondary station pulse
group and the Master station pulse group. The
measured TDs depend on the location of the receiver
in relation to the three or more transmitters.
2/19/04 AIM
Air Navigation Aids 1−1−23
FIG 1−1−16
First Line−of−Position
(a) The first TD will locate an aircraft
somewhere on a line−of−position (LOP) on which the
receiver will measure the same TD value.
(b) A second LOP is defined by a TD
measurement between the Master station signal and
the signal from another secondary station.
FIG 1−1−17
Second Line−of−Position
(c) The intersection of the measured LOPs is
the position of the aircraft.
FIG 1−1−18
Intersection of Lines−of−Position
3. The navigation computer converts TD values
to corresponding latitude and longitude. Once the
time and position of the aircraft are established at
two points, distance to destination, cross track error,
ground speed, estimated time of arrival, etc., can be
determined. Cross track error can be displayed as the
vertical needle of a course deviation indicator, or
digitally, as decimal parts of a mile left or right of
course.
e. Notices to Airmen (NOTAMs) are issued for
LORAN chain or station outages. Domestic
NOTAM (D)s are issued under the identifier “LRN.”
　
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