LORAN-B, D and F

LORAN-B was basically a phase comparison variation of Loran-A and an attempt to re-design the system properly plus add some new features. Wartime development of the Loran-A system had been rushed. Even in ground wave range, it was never a very accurate system.

One shortcoming was it's pulse length which was approximately 40 microseconds. It was noted however, that Loran was providing usable, steady ground wave signals to at least 400 miles out to sea. In the original design, no attempt had been made to achieve phase coherence in the transmitted pulses, which made phase comparison within the pulse (as in Loran-C) impossible. With Loran-B the transmitters were redesigned to achieve this function and a new receiver was developed as well. Phase comparison increased accuracy to about half a cycle, (about 0.25 microsecond) a considerable improvement. Another new feature of the system had the transmitters operating in chains instead of pairs, thus allowing simultaneous measurement of two time differences.

Later on, phase comparison systems at 2 MHz became enormously successful for offshore survey work and in the 1980's one company, apparently never having heard of Loran-B, proposed it as a new idea to pulse its 2 MHz CW phase comparison system to eliminate sky wave and ambiguity problems!

William Roland, a retired Coast Guard Officer living in Panama City Beach, Florida recalls some of the history of this system. "Loran-B, designed only for ships, was operated experimentally at the USCG Electronics Engineering Center (EECen), and its predecessor, the USCG Test Station in Chesapeake Bay, Maryland from approximately 1948 to 1955.  The unique feature of Loran-B was that, although it operated on the same frequencies as Loran-A, the timing control and the timing measurements were made on the pulse carrier, after cycle identification from the pulse envelope.  The process made for very precise timing measurements.  However, at the land-sea interface, there was considerable phase distortion.  This resulted in major differences in the phase to envelope timing depending on the azimuth from the transmitter.  There were probably instrumentation problems resulting from the temperature and vibration sensitivity of components which were available at the time.  In any case, the work was abandoned in favor of the Loran-C system which was first taken up by the EECen in the late 1950's and which continues to this day".

LORAN-D was a short range, high accuracy, low power, tactical system designed for use as a bombing aid by the United States Air Force. It used "portable" transmitters and much shorter baselines than Loran- C, plus a sixteen pulse transmission instead of eight. Loran-D modified the Loran-C pulse pattern by interleaving an additional 8 pulses after each pulse of the current Loran-C pulse pattern. To interleave the additional pulses within the standard group interval, Loran-D used a modulation scheme known as Supernumary Interpulse Modulation (SIM). The additional interleaved additional pulses were coded in a pattern to which the normal Loran-C receiver was non-responsive. Thus conventional Loran-C receivers designed to receive only the 8-pulse Loran-C pattern operated on the imbedded Loran-C portion of the Loran-D signal.

In standard Loran-C, the pulses of a Loran group are spaced 1000 uS apart for all Secondaries. The Master's pulse spacing is the same for the first 8 pulses. Having an additional 9th pulse located 2000 uS after the 8th pulse identifies the Master station. The most obvious spacing to interleave additional pulses between the standard Loran-C pulses is to use the previous Loran-D format. In Loran-D the spacing was 500 uS apart. The original Loran-D pulses peaked at 80 uS, with the standard sampling point located at 60 uS. Later, Loran-D pulses were transmitted identically to Loran-C.

The system was used for several war exercises in Europe in the 1960's and later. One station was temporarily erected in the UK.

LORAN-F  - At the same time that the U.S. Air Force was developing Loran-D, Motorola experimented with the Multi-User Tactical Navigation Systems (MUTNS), a continuously pulsed pseudorandom coded low frequency navigation system used for drone control. It was sometimes referred to as Loran-F in proposals but this was not an official designator. Loran-D came out ahead during system evaluations, therefore no further work was done on MUTNS.

REFERENCES:

1) The Journal Of Navigation - Chapter 4 ; W.F. Blanchard, Royal Institute of Navigation; Vol. 44, No. 3; Sept. 1991. Used with permission.

2) William Roland, UCSG (Ret'd) Panama City Beach, Florida

3) Enhanced Loran-C Data Channel Project (white paper)
    www.uscg.mil/hq/lsu/webpage/papers/Bonn_LSU_final.htm

4) Loran Support Unit - RD2 Don Befort

5) The Aviator's Guide to Loran-C", John F. Good, Aksunai Press, Wakefield,  MA;  1984, pg 89.

6) Charles A. "Chuck" Schue, III; Vice President of Operations; W R Systems, Ltd. Fairfax, VA
 

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Feb 01/01