Chuck Husick: Techno-Talk, July 2005 from BoatUS Magazine (Updated
For May 2009 update on Loran and eLoran funding, click here.
C Upgrade On To eLoran
may have noticed that your Loran C has been working better than it
ever did in the past. Why? Its performance is being boosted by the
improved quality and strength of the Loran signals from the new transmitters
and timing systems installed as a part of a more than $100 million
improvement program that is now virtually complete. In addition to
preserving and improving the value of your present “legacy” Loran C equipment,
the new transmission system will support new combined Loran C / GPS
receivers that will provide superior navigation information, including
accurate heading data even when your vessel is stationary.
the evolving new standard for the present Loran C system will add
a new feature, the Loran Data Channel. The data channel may be used
to identify the transmitting station, warn of anomalous radio propagation
conditions, provide differential Loran corrections to improve navigation
accuracy and to provide corrections for satellite navigation systems.
In addition, the eLoran data channel will improve the ability of
the system to resist interference and spoofing. eLoran receivers
will operate in an all-in-view mode, automatically acquiring and
tracking the signals of all available Loran stations. The eLoran
system receivers will be able to provide navigators with precise
true north heading information even when the vessel is stationary.
The eLoran system is a vital component of eNavigation.
Not too long ago the mood in Washington was to abandon Loran C
and rely exclusively on the Global Positioning System (GPS). However,
a number of concerns came into focus, including the relative ease
with which the very weak GPS signals might be interfered with.
You may recall the warnings from the U.S. Coast Guard about possible
interference from some types of boat-mounted TV antenna amplifiers.
Accidental transmissions from a Navy vessel in San Diego disrupted
not only the navigation function of GPS in a wide area but also cut
off most of the cellular telephone network in the area since the majority
of cell networks were relying solely on GPS for their timing signals.
Systems that also used Loran timing information remained in service.
Other GPS related concerns include the fact that, unlike Loran C, where
the entire system is located on U.S. soil, the GPS satellites are orbiting
in space, possibly subject to outside interference.
The very large installed base of both marine and aviation Loran C
navigation equipment was also a factor, especially after many aircraft
and boat owners (and especially fishermen who relied upon their many
pages of Loran C TDs) made clear their displeasure with a decision
that would make their perfectly good equipment useless. The result
was a study of the feasibility of upgrading the Loran C system and
the improvements that such a program would make possible.
A decision was made to replace the old Loran C transmitters and timing
systems with new solid state equipment. This upgrade is now
complete in the lower 48 States, all the old transmitters
have been replaced. Alaskan chain transmitters at Kodiak and St.Paul
have been replaced with solid-state equipment. The new transmitters
emit very strong signals, using between 400 and 1600 kW (by comparison
the highest power permitted for an AM broadcast station in the U.S.
is 50 kW).
As with the GPS, the performance of Loran C depends on precise timing
of the transmissions. The new timing system, using atomic clocks
similar to those onboard the GPS satellites ensure that timing accuracy
is within 50 nanoseconds (billionths of a second). These changes
improve the performance of any Loran C navigator and in addition
make a new type of Loran C receiver practical. The new receivers
do not require selection of a specific Loran chain, they work seamlessly
with all available signals. Time and frequency equipment has been
replaced at 20 stations.
The availability of the greatly improved Loran C system makes it
possible to build combined Loran C / GPS receivers that combine the
position fixing information from both sources into a single, highly
precise and robust source of navigation data. One of the most interesting
aspects of the new combined Loran C / GPS receiver system is its
ability to provide precise, true north heading information even when
the vessel is stationary, a capability unavailable with either GPS
or conventional Loran C systems. The availability of speed independent
heading information will be of particular value when it is used to
control a boat’s
autopilot. Combined with known values for local magnetic variation it
will also provide an excellent information source for precisely adjusting
the vessel’s steering compass.
The improved Loran C system brings with it a worthwhile change in
the traditional “E” (electrical) field Loran antenna (typically
a six-foot-long vertical whip) reducing its length to barely two feet.
In addition, new “H” (magnetic) field antennas provide a
number of significant advantages, particularly a high degree of immunity
to interference from static, such as that generated by lightning storms.
The “H” field antenna is flat, typically only an inch or
two high, allowing it to be housed, along with the GPS antenna in a
dome type enclosure only eight inches in diameter and less than six
inches high. As with any new technology, the price of the combined Loran
C / GPS receiver will at first be moderately high, somewhat less than
$1,000 for the first product, the Si-Tex e-LORAN. As we have seen with
all other marine electronic equipment the price will drop as more vendors
introduce products and sales volume increases.