by Clliiffford Appell
Last month, I explained how LORAN (LOng RAnge
Navigation) worked as a navigational aid system and
how it was originally implemented using vacuum tube
transmitters. This month, we will continue with the story
of the retirement of the world's most powerful vacuum
tube LORAN transmitter of 1.6 megawatts at the US
Coast Guard station at George, WA. The vacuum tube
transmitter went on air in September of 1976 and was
officially taken off air on December 8, 2003. After
switching the antenna transmission line from the vacuum
tube transmitter, a new solid-state transmitter was
operating within about two hours. Read on to learn about
the new solid-state concept and LORAN's future.
The Solid-State Transmitter
The differences between the vacuum tube
transmitter(s) and the new solid-state transmitter are like
Figure 1. The frequency/control equipment for the solid-state transmitter.
The two left racks are the TCS, then there is the TFE, the AUX rack with
three cesium beam frequency standards, and two RAIL racks on the right.
Love that alphabet soup! Photo courtesy ETC K. Anderson.
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night and day. This should be the case because of a 25
year period to improve transmitting efficiency and
frequency/control techniques from computer technology.
The transmitter known as the Accufix 7500 is
manufactured by Megapulse of North Billerica, MA.
Megapulse has supplied earlier versions of the Accufix,
known as the AN/FPN- 64, which were of lesser output
power capability for USCG LORAN stations, primarily in
the inland areas of the US. Megapulse has recently
finished supplying units to the nation of Japan for their
LORAN chains.
The new frequency/control equipment is shown in
Figure 1. At the left are two Transmitter Control
Subsystems (TCS) — one active and one standby. The
TCS controls precise timing of the transmitted LORAN
pulses and also monitors the “quality” of those pulses.
Third from the left is the Transmit Frequency
Equipment (TFE), which generates George, WA’s
LORAN rates of 9,940 and 5,990 as the old
equipment in Figure 2 of last month’s article did for
the vacuum tube transmitter. To the right of the
TFE is the AUX rack containing three Hewlett
Packard HP-5071A Cesium Beam Frequency
Standards. Each Cesium costs over $28,000.00
and they are the reason the LORAN signal is so
repeatable with time to its users. In a one day
period, HP rates the frequency stability as better
than 3 x 10-14. You don’t get that kind of stability
from a TCXO (Temperature Compensated Crystal
Oscillator), which is a common option in our HF
amateur radio equipment.
Two racks on the far right (only a portion of
one is shown) are the RAIL (Remote Automated
Integrated LORAN) equipment, similar to what
was used for the vacuum tube transmitter. The
RAIL — consisting of commercial grade Dell
computers — provides timing corrections, system
alarms, a means to input “commands” to the TFE
and TCS, and also acts as a communications tool
to the control monitors. For the Canadian West
Coast Chain ( 5,990 rate) the control monitor is
SEPTEMBER 2004
