Transcript LORAN C - Seamanship Tutor

LORAN C
By Farhan Saeed
Loran C
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Loran-C is a hyperbolic radio navigation system.
The systems operate on the principle that the difference
in the time of arrival of signals from two or more stations,
observed at a point in the coverage area, is a measure
of the difference in distance from the point of observation
to each of the stations.
Loran-C employs time difference measurements of
signals received from at least three fixed transmitting
stations. The stations are grouped to form a 'chain' of
which one stations is labelled the master (designated M)
and the others are called secondary stations (designated
W, X, Y, or Z).
Loran C
For a given master-secondary pair of
stations, a constant difference in the time
of arrival of signals defines a hyperbolic
Line Of Position (LOP).
 Second master-secondary pair results in a
second LOP.
 The position fix is achieved by observing
the intersections of the two LOPs on
specially latticed Loran-C charts.
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Master / Slave
W
X
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M
Z
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Y
M is the master
station.
W, X, Y and Z are
known as secondary
stations (or slaves).
Master / Slave
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The master transmits a set of 8 plus 1 pulses.
The pulses are received at the ship and at W, X, Y and
Z.
When the ship receives the first master pulse, it starts a
timing clock.
When the secondary stations receive the first master
pulse, they wait for a short time known as a coding
delay and then each transmits a similar set of 8 pulses.
The ship receives the pulses from W, X, Y and Z and
times the interval between receiving the master pulse
and receiving each of the four secondary pulses.
Coding Delay
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The coding delay is such that the ship will
always receive the master station pulse first,
then W pulse, then X pulse then Y pulse and
finally Z pulse.
The coding delay also is such that the pulses do
not overlap as they are received.
After a short interval of between one twentieth to
one tenth of a second, the master station
transmits another set of pulses and the cycle
repeats.
Time Difference
The position of the ship determines the
time differences
 If we know the time differences, we know
the ship’s position.
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Ninth Pulse
It enables the Loran receiver to identify the
master station.
 It is used to transmit warnings if any
station is not transmitting correctly. The
warnings trigger alarms in the Loran
receiver.
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Time Difference Measurement
Pulse matching
 Cycle matching
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Group Repetition Interval
Each chain sends its pulses at a specified
Group Repetition Interval (GRI).
 There are several different intervals. Each
is a few hundreds of microseconds less
than 50,000, 60,000, 80,000, 90,000 or
100,000  seconds.
 Examples;
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49900  sec
 59300  sec
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known as Station 4990
known as Station 5930
Time Difference Measurement
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Uses the third cycle of the received pulse
because;
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The start of the received pulse may be too weak to be
heard
The master and secondary signals may not be
received at the same strength.
It is possible to accurately identify the time when the
third cycle ends and time this point.
This part of the pulse arrives at the ship before there
can be any sky wave interference.
Accuracy
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The accuracy of the Loran system
depends upon:
The accuracy of measuring the timing delays
(0.1  sec).
 The angle between the Loran lines of position
(LOP).
 The position of the ship in the Loran coverage
area, that is whether the position is near the
base line or the base line extension.
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Additional Secondary Factor (ASF)
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The Latitude/Longitude computation in many
receivers is based upon a pure seawater
propagation path.
Over land distances signals travels at a slower
speed.
For those receivers that accommodate the
correction it is called an Additional Secondary
Factor (ASF) correction, and this is applied
automatically when the receiver computes the
latitude and longitude.
eLoran
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Enhanced Loran, or eLoran, is independent of
GPS but fully compatible in its positioning and
timing information, and its failure modes are very
different.
eLoran is based on the existing low frequency
Loran-C infrastructures that exist today in the
United States, Europe, and Far East, and in fact
throughout much of the northern hemisphere.
It is an internationally recognized positioning and
timing service, the latest evolution of the low
frequency long-range navigation (Loran-C)
radionavigation system.
eLoran
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Why eLoran?
GPS is vulnerable to disruption, and it doesn't
work everywhere - entering a tunnel or parking
garage or even traveling down a narrow city
street: the navigation system generally alerts to
"loss of satellite reception."
Some interruptions of cell-phone operations or
losses of other services for no apparent reason
have been the result of GNSS interference.
eLoran
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Perhaps the most exciting changes from Loran-C to
eLoran are the new operating concepts.
All transmitters are timed directly to UTC, so that a user
may use all eLoran signals in view and may combine
them with GNSS signals for robust position and time
solutions.
Each transmitter includes a messaging channel; this is
an in-band signaling channel that allows the eLoran
signal to also carry information to improve the user's
solution.
Very much like GPS this messaging channel provides
transmitter identification, time of transmission, differential
corrections, and authentication and integrity signals.
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Any Questions ?