RTCM Report
eLoran: Its time has come (and gone?)
By Bob Markle, President
There were many dismayed people around the RTCM water cooler when the Coast Guard announced it was shutting
down the Loran-C system. Jules Rutstein and David Hayden raised their objections in the last edition of MEJ. Why
should we care about an “obsolete” navigation system when we have GPS? (Better make that GNSS—Global Navi-
gation Satellite System to include the Russian GLONASS system and future European Galileo and Chinese Compass
systems.) Well, we’re not interested in Loran as a respected historical artifact. We’re not afraid of newfangled GNSS
systems. We’re not even opposed to bird sanctuaries (the presumed fate of the Coast Guard’s Loran Support Unit in
Wildwood, NJ).
Operating an upgraded automated
Loran-C system would cost about
$12 million annually compared to
$135 million to shut it down.
Ironically, Congress has already
provided $160 million for upgrades
and development of eLoran.
Let’s start with President Bill Clinton’s Presidential Decision Directive/NSC-63 (PDD- 3) on Critical Infra- structure Protection,
signed on May 22, 1998 (that’s right,
before 9/11). Among other things, it
directed the Department of Transportation
(DOT), in consultation with the Department
of Defense, to undertake a thorough evaluation of the vulnerability of the national
transportation infrastructure that relies on
GPS. DOT’s Volpe National Transportation
Systems Center published the vulnerability
assessment in August 2001. Its overall
finding was that GPS is vulnerable to interference and disruption, and that independent backup systems and procedures were
needed in critical applications. Several of
the recommendations pointed to a modernized Loran-C system, which the Coast Guard
had already started to develop.
So why have the civil uses of GNSS
become so important? Navigation is the
most obvious use for us. But many businesses now rely on GNSS-based “
telematics” systems to monitor and route their
fleets of vehicles. GNSS navigation systems
depend on precise timing, so applications that also require precise timing, such as cellular
telephone communications, have come to depend on the GNSS timing signal. Emergency
services use GNSS locations to respond to 911 calls. Motorists with GPS units no longer
need to carry maps. A GNSS outage or significant degradation could have a severe safety
and economic impact beyond maritime and air navigation.
Enhanced Loran (eLoran) promises to be the ideal companion Position, Navigation,
and Timing (PNT) system for GNSS. eLoran uses precise cesium clocks to provide timing
comparable to GNSS satellites. That allows receivers to use all available Loran signals as a
single network, rather than having to use signals only from a particular chain of stations
consisting of a master and several secondaries. eLoran adds a data channel to the pulsed
Loran signal. This data channel includes precise timing information equal to that of GNSS,
as well as navigational correction data. It’s differential Loran that along with providing
corrections to improve Loran’s position accuracy also includes information such as the
delay that the Loran signal is experiencing as it travels over different kinds of terrain. An
eLoran receiver that makes use of this information can achieve accuracies of 10 meters or
so, depending upon the user’s location in the network. The data channel is invisible to
Loran-C receivers, so those legacy receivers continue to operate as they always have.
