Coverage of (e)Loran in the Latest NIST Technical Notes #2187 and 2189

November 2021 – There is a lot of coverage of (e)Loran in the latest NIST Technical Notes #2187 and 2189, available at these links: https://doi.org/10.6028/NIST.TN.2187 and https://doi.org/10.6028/NIST.TN.2189. Full disclosure, although NIST did not reach out to UrsaNav to provide direct input/comment to their publications (not unusual), they used a lot of UrsaNav text, graphics, and pix in their discussions of Loran and eLoran. These documents are an incredibly rich source of timing information for the public and private sectors. NIST (and USNO) has always been a fan of eLoran, and Loran-C before it, so we were glad to see they still consider it a valuable alternative to GNSS. As you may recall, Loran-C was a DOD system so its timing was “sourced” from the USNO (via “hot clocks”) and the USNO monitored its relationship to UTC and provided notices to users of any offset(s) and drift rates. This is similar to the function that the USNO provides for GPS.

Note that NIST did not mention that (e)Loran can operate in a Two-Way Low-Frequency Time Transfer (TWLFTT) mode. TWLFTT can provide a terrestrial, layered time-transfer network wherein time is transferred between any two (e)Loran transmission sites that can receive each other’s signals. UrsaNav tested this theory as part of our first CRADA with the USCG Research and Development Center, which started in 2012. Note that TWLFTT is completely “sky-free”.

Note also that there has been a lot of “hay” made from (e)Loran competitors that there are no miniature LF receivers. This is simply not true. Even at the 60kHz signal that WWV uses (40 kHz lower than Loran), timing receivers are easily chip-scale. See page 148 of NIST 2187. Further miniaturization is not an issue of science or technology; it’s simply a matter or market demand.

There were a couple of statements in NIST 2187 that we found extremely troubling, and which we are bringing to the attention of the authors:

“Beginning in the early 1990s, the Loran-C stations were synchronized with GPSDCs, with the Hewlett-Packard GPS Smart Clock used as the primary reference at many station sites after about 1997. This direct synchronization to GPS did, of course, invalidate the premise that Loran-C, and the proposed eLoran system that followed, were GNSS independent, which made them unsuitable as a GPS backup system.”

GPS was not fully operational until 1995. GPSDCs were NOT used operationally at any North American Loran-C stations, and at no other international Loran-C stations of which we are aware before 1999. The USCG did test various methods of distributing UTC to the Loran stations, including TWSTT and GPS Common Mode, starting around 1993, but these tests were never moved into full operation. The USCG started installing Time-of-Transmission Monitors (TTM) at the transmission sites in the late 1990’s and early 2000’s. The initial installations were at only at few test sites, and then only at the Master transmission stations. It was not until later that the TTMs were installed at all Loran-C stations in North America as part of the redesign of the Frequency Standard Rack. The TTM included a GPS disciplined quartz oscillator, but the system was not directly connected to the station timing generation equipment, and was not used to control the transmissions. The TTM only reported the difference between the transmission times and UTC, as referenced against a local, survey-grade GPS Timing Receiver. It is not correct to suggest that these GPS Smart Clocks were used as “the primary reference”; they were an external reference against which the three Primary Reference Standards at the transmission sites were compared, originally manually and then via software. The primary reference for generating the timing of Loran transmissions has always been the three rubidium or cesium standards.

Starting in 2003, the first version of the Timing and Frequency Equipment (TFE) was installed at North American Loran-C stations as part of the Loran Modernization and Upgrade Program. Although not intended, this first version did not provide sufficient separation between GPS and the transmitted Loran signal. The software allowed the transmission sites to be steered without bound using GPS as an input. This software error was subsequently corrected in later versions of the TFE.

A well-designed eLoran system would have no direct connection to any external timing source, whether it be GNSS, TWTT, microwave, or fiber. All of these sources might be monitored, but there would be an air-gap between them and the triple ensemble of Primary Reference Standards. The system would be purpose-built to not allow an external timing reference to be able to “walk” the station’s timing away from UTC, and the system could operate for a defined period without an external reference of any sort. The fact that a modern eLoran transmission site might use GPS or another GNSS as external references is not the same as saying they are directly synchronized to them.