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.

Capt  Harry Hodges (Ret'd) provides this additional info about the Loran 'D' system. "  I installed and operated a Loran-D system for the Utah Test and Training Range (UTTR) based out of Hill AFB.  I was part of the 6514th Test Squadron and the system was installed to support Unmanned Aircraft testing on the UTTR.  I recall we installed the system around 1975 and it was operated until late 1978 (approximately).  The system, built by Megapulse (Mass), consisted of a transmitter using an Hewlett Packard  beam clock and a 150 foot  antenna along with a ground plane located at Little Mt, UT, Nephi, UT and Montello, NV.  (I can't recall the power level used) . A control station located at Hill AFB,  managed the signals.  We were monitored by some signals folk out of Colorado and our stability was considered to be pretty high. particularly after the Cs timing drift across the clocks was adjusted.

Loran-info has some odd data about the 'D' chain in the last document of this page.  They list it as a Loran-C mini-chain, but it was really Loran-D.

The stations were unmanned and controlled over telephone lines and modems.  Equipment was installed in mini-buildings (10'x20') running from commercial power with a supplementary window AC from summer.  Winter required no heating unless the equipment stopped from power failure, a cooling fan was used during the winter.  If power did fail in the winter, we had to travel to the site to restart because the equipment had a "too-cold" shutdown.

We did a lot of mapping over the range to get estimates of navigational accuracy; we were attempting to measure UAV navigational capabilities using Loran to a CEP < 50ft.

The system was to allow us to test UAV for deployment to SEA where the USAF had a Loran-D system.  By the time we completed our testing on Loran equipped UAV, SEA was winding down.  I heard that our mini-chain was dismantled sometime after I retired in '79 and the system was shipped to the Phillipines, but I have no direct knowledge of that.  I do recall a call from folk paying for the telephone lines (~1980) asking if those lines were still needed - a ghost circuit I believe such services are called".

He goes on to say "Tactical Loran-D was useful for more precise positioning.  Utah had some odd propagation features mostly related to the conductivity of the Great Salt Lake.  We never quite got to using TOA direct use, but I understand that Loran-D allowed that capability.

My fondest memories were when I was able to get TD to Lat/Long calculations done via the HP-65 calculator.  I worried that problem for quite awhile because of the limited number of registers and steps, but eventually got it done.  I was quite proud.  We used correction data by 'calibrating' the UTTR area.  We first tried to use an RF-4C aircraft with it's ARN-92 Loran equipment and direct photography of known geodesic points.  Sadly the RF-4C that happened to be at Hill AFB at the time was the original integration prototype aircraft which wasn't fully functional.  We actually did the calibration by taking a receiver into the field, setting up on a lot of USGS survey markers and recording data.  That way we reduced conductivity position uncertainty to approximately  5ft.  The calibration varied over time, however, because rain over the desert changed conductivity allowing accuracy to  approximately 15ft.  Most of the area had excellent crossing angles so geometric dilution of precision was low ".

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.


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)

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

7) Harry Hodges <hhodges(at)compuserve.com> for Loran D input.

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Aug 28/15