eLoran: More accurate & less vulnerable but not a done deal yet, Part 2
1/9/2017


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 sector.

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 regulations.

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.

 

GNSS/GPS vulnerabilities

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.

 

Countermeasures

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.

 

 

 

 

 

 


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