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As of 08 January 2010, it appears that LORAN-C will be
decommissioned in the United States, possibly beginning as early as February,
with all stations of the air by the end of the Federal Government's Fiscal Year,
01 October 2010.
I won't go into the details of how LORAN-C works, as it is
well covered elsewhere, other than to note that it's a pulse-based system,
operating at 100 KHz with positioning based upon time of arrival calculations
from signals received from several stations. The stations, arranged in groups or
"chains" all transmit on the same frequency, with the individual
transmitters being distinguished by slightly different pulse rates.
This page provides a few spectrum analyzer images of
LORAN-C signals, as received in Clifton, VA, near Washington DC. The signals are
captured with an HP 8568B spectrum analyzer and a Clifton Laboratories Z1501C
active antenna.
First, the classic spectrum view. The nominal bandwidth of
the LORAN-C signal is 20 KHz. Bandwidth is normally defined as the "20 dB"
bandwidth, i.e., 99% of the transmitted energy is found within the specified
bandwidth. Still, when operating with hundreds of kilowatts of power, as the
LORAN-C stations do, there is appreciable energy outside the 99% bandwidth, as
is evident in the image below.
The 8568B's plot image is captured with a Prologix
USB-GPIB adapter and KE5FX's excellent plotter emulation software, 7470.EXE. |
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Switching to zero span, which sets the spectrum analyzer to non-sweeping
"receiver" mode, we can view the data pulses. Looking at the image below, at
least three different pulse sequences are visible, distinguished by their
amplitude. |
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Increasing the sweep speed to 2 ms/division, the individual
pulses can be seen. Each pulse is 200 us duration, and 1000 us between pulses,
with each station transmitting 8 pulses. In most cases the "master" station in
each chain transmits a 9th pulse, delayed by one pulse interval, i.e.,
there's a "missing" pulse between the 8th and 9th pulses. |
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I've numbered the pulses to make the master stand out. Note
that pulses 1...7 have a second signal almost overlapping, but not quite. The
numbered pulses are slightly weaker, perhaps 2 or 3 dB, below the nearly
overlapping pulses. |
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The image above shows at least five different stations,
distinguished by their signal strength. |
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I've labeled the stations A...E in the image above.
Of course, when it comes to distinguishing stations to
determine position fixes, stations are identified not by differences in
amplitude, but rather differences in the pulse group repetition period. |
The plot below measured the interval between the 1st and 8th pulse of a master
transmitter. There are 7 1000 us time intervals between the 8 pulses, and
the measured time is 6660 us, or 951 us between each pulse about 5% short of the 1,000 us
actual interval. Most of this error comes from the difficulty in positioning the
markers at the same point within the start and end pulse. The spectrum analyzer
isn't really intended for this purpose and it's quite decent performance to
measure within 5% with this simple approach. |
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