Chuck Husick Home

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Chuck Husick: Techno-Talk, July 2005 from BoatUS Magazine (Updated June 2008)

For May 2009 update on Loran and eLoran funding.

Loran C Upgrade On To eLoran

You 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.

sLoran, 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.





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