Last week in Part 1 we laid out the basics of eLoran and some of its advantages.
By Ev Collier
How we got here
Loran, progenitor of eLoran, was a land-based hyperbolic
radio frequency navigations system developed during World War II that works on
the principle of differential range by pulse timing techniques. Specifically,
what this means is that a pair of transmitting stations, a master and a chain
of slave units, operating as a synchronized pair, simultaneously send out coded
pulses in all directions. The pulse from the closest station arrives at a
shipboard receiver sooner than the pulse from the further transmitter; the
receiver measures this time difference in microseconds. The same time of
arrival of these pulses will also be received and measured by other station
receivers within the chain of transmitting stations whose positions are known.
When connected, these form a hyperbolic shaped line known as
a Loran Line of Position (LOP). To obtain a position fix, navigators used Loran
tables and charts containing accurately plotted lines of position on the
various time differences encountered in a particular area. This same procedure
is followed using two other pairs of transmitting stations to obtain another
LOP. The position of the shipboard receiver is at the intersection of the two
LOPs. Loran-C was capable of absolute accuracies varying from 185 to 463 meters
(0.1 to 0.25 nautical miles) depending on where the observer was within the
coverage area. Repeatable accuracies were sometimes as good as 18 meters (60
feet) but usually better than 100 meters (328 feet).
What really took place that led to the Loran shutdown is not
publically known but the general consensus is that it was a “cost saving”
measure attributed to the Office of Management and Budget. In any case, the
controversy continues to this day. “It made no sense to me,” says Allen
Schneider, Vice President at SI-TEX. “It’s mind boggling that the system hasn’t
moved. Common sense tells you that that’s what we need in addition to GPS!”
Schneider’s comments mirror the feeling of many in the industry.
Position, Navigation and Timing
Loran, although not a satellite-based system, can be
considered to be the first of the PNT (position, navigation, timing) services
provided by the Global Navigation Satellite System, or GNSS. These are systems,
which include GPS, that provide highly accurate and critically needed data to a
wide variety of user communities. Developed initially for use by the military
forces they also provide this critical data to many industries in the private
The data is especially important in the marine industry for
navigating, not only in the open ocean but particularly in congested harbors
and waterways and in search and rescue. In commercial shipping, accurate
position, speed and heading are needed to ensure that the vessel reaches its
destination in the safest, most economical and timely manner that weather and
sea conditions permit. Oceanographers are increasingly using the data for
underwater surveying, buoy placement, hazard location marking and mapping.
Commercial fishing fleets use GPS data to navigate to optimum fish locations,
track fish movements and to ensure that they are in compliance with fishing
GPS data plays a critical role in such systems as the
Electronic Chart Display and Information System (ECDIS) and in the Automatic
Identification System (AIS), and in integrated bridge systems being installed
on commercial vessels of all types, to mention just a few marine applications.
This is in addition to the critical role PNT data plays in agriculture,
construction, transportation, banking and finance, power companies and
utilities and the stock market.
But there are serious downsides as well. GNSS signals–America’s
GPS, Europe’s Galileo, Russia’s GLONASS, etc.–are extremely vulnerable to
interference, intentional and unintentional. It begins with the fact that these
signals, coming from far out in space, must withstand not only great distances
but also such atmospheric disturbances as solar flares and high-energy
electromagnetic fields are extremely weak and ends with the fact that there are
people out there who don’t like us.
GPS signals come from satellites some 13,000 miles away and
have signal strength at the earth’s surface that is much less than that of a
home television signal’s strength. GPS signals at the earth’s surface are
equivalent to viewing a 25 watt light bulb from a distance of 10,000 miles.
Obviously, this weak signal can be easily jammed by a signal of the same or
similar frequency of greater strength.
Jamming and spoofing
Jamming is the intentional electronic interference of GPS,
or other GNSS, signals for the purpose of disrupting the proper operation of
the GNSS device. Reasons for jamming civilian signals vary all over the lot—smuggling,
toll avoidance, automobile and boat theft, to name a few. Jammers like the one below are inexpensive and, although illegal to use in the US, are available on the Internet.
There are many examples of GPS signal jamming, but a more
insidious GPS vulnerability involves intentional spoofing. In a jamming
situation, the system being jammed is aware of the attack. Spoofing is more
subtle—the system is not aware it is being spoofed. In this case, the GNSS/GPS
receiver is fed false position and/or timing information, causing it and its
operators to believe that they are headed toward a desired destination when in
fact they are being misdirected.
In 2013, off the south coast of Italy, a small group took
control of the superyacht White Rose of Drachs (below), an $80 million superyacht’s
navigation system using a homemade spoofing device. Control was successfully
taken from the vessel’s skipper and led off course toward an entirely different
destination. Fortunately, this was accomplished with the skipper’s knowledge
and consent, by a group of researchers from the University of Texas in a test
of a new spoofing device of their design.
Spoofing is more difficult to achieve than jamming and
requires a significantly more complex and sophisticated device than a relatively
simple jammer. Spoofing is most readily accomplished using what is, in effect,
a GNSS/GPS simulator which, uses a signal slightly stronger than the authentic
signal from a satellite. The receiver is made to believe that the fake signals
are authentic and proceeds to calculate erroneous position and time solutions
based on the false signals.
Multi-frequency GNSS receivers will provide jamming
protection through frequency diversity, although intentional jamming often
covers all GNSS frequencies. However, substantial spoofing protection may be
achieved by using a multi-frequency/multi-constellation GNSS receiver since
simultaneous spoofing attacks against GPS, GLONASS, BeiDou and Galileo would be
both difficult and expensive.
Countermeasure techniques for spoofing attacks are complex,
requiring a more intelligent receiver capable of recognizing, remembering and
comparing the characteristics (signal strength, satellite identification codes,
timing intervals, etc.) of the various satellite signals. All of this would
require both hardware and software modifications to the receiver.
Unfortunately, the countermeasures discussed above will not
stop spoofing attacks, but they will alert the user of the attacks and
hopefully force attackers to employ more sophisticated and expensive methods.
All of which demonstrates the desirability—some say the critical necessity—for
a backup system to the GNSS system.
About the author
Ev Collier is an electrical engineer, an avid cruising
sailor and amateur boat builder. He was most recently director of technology
for the Precision Materials Group at GTE. Collier is a member of the Society of
Naval Architects and Marine Engineers, the American Boat & Yacht Council
and National Association of Corrosion Engineers, and the author of The
Boatowner’s Guide to Corrosion.