GPS Troubles

Since the Global Positioning System [GPS], operated by the US government, first became operational in the early 1990s, its use has grown like topsy.  Originally intended primarily for military use, it has been progressively opened to civilian applications, following executive orders from Presidents Ronald Reagan and Bill Clinton.  Most people today have, or have seen, GPS navigation receivers, either hand-held, or built into automobiles, for example.  Perhaps inevitably, some people become overly dependent on these devices, neglecting more traditional tools like maps, or common sense; as a result, as I’ve noted before, some of them have ended up in situations ranging from the faintly ridiculous to the truly dangerous.

Many people don’t realize, though, the extent to which GPS is used for lots of other things.  In addition to providing the basis for location service, GPS also provides a very accurate time signal.  The applications for which GPS is used include:

  • Cellular Telephony: GPS signals are used to synchronize timekeeping at cell base stations, and also for location of emergency calls (e.g., 911 in the US).
  • Surveying and Map-Making
  • Disaster Relief and Emergency Services
  • Electric Power: GPS time signals are used for phase synchronization.
  • Vehicle and Personal Tracking Systems.  GPS receivers are used to track vehicles and assess tolls in Germany, and are used by many firms to track their delivery vehicles, for example.

Some of these uses create obvious incentives for “fooling” the GPS-based system; toll collection is an obvious example.

According to a “Technology Quarterly” article in the most recent issue of The Economist, this has led to some potentially serious problems with GPS.   The last few years have seen a proliferation of cheap GPS jamming devices, sold via the Internet for as little as $30, which have become popular with toll evaders, and with truck drivers who dislike having their movements tracked by their firms.  Because GPS signals are weak — they are being transmitted from a satellite at a typical altitude of ~20,000 km — they can be jammed easily with a low-power transmitter.  This also makes the jamming transmitters (which are, by the way, illegal to sell or use) difficult to locate.  Accidental interference is also possible, either from malfunctioning transmitting equipment, or as a result of solar activity; but it seems clear that deliberate jamming is becoming more prevalent.

“If you do an internet search on GPS jammers, you get over 300,000 hits, with many of these linking to sites offering them for sale,” says Jim Hammond of the intelligent transport systems working group at Britain’s Association of Chief Police Officers (ACPO). “I’d suggest you don’t get that level of hits for products that nobody buys.”

One approach being taken to address the problem is the development of systems that can identify and locate jamming equipment.  The US Department of Defense has at least one such system in place.

In America there is already a military system to spot GPS interference: the GPS Jammer Detection and Location (JLOC) system run by the National Geospatial Intelligence Agency. According to Navsys, the company that developed JLOC, it involves a network of GPS receivers capable of detecting regions of higher than normal signal levels and low signal-to-noise ratios, either of which can indicate interference.

One difficulty is that, because of the low transmitter power of many jamming devices, a large network of sensors is probably required.  However, another, rather ingenious extension of this idea would take advantage of the GPS receivers built into smart phones, and potentially other mobile devices, to address this issue.

Navsys may have found a way to speed up the process, however. It says it has received encouraging feedback from America’s Defence Advanced Research Projects Agency (DARPA) in response to a recent proposal to develop an app that would turn smartphones running Google’s Android software into JLOC sensors. Members of the emergency services, or even members of the public, would then be asked to download the app and leave it running on their phones.

A complementary, ground-based backup system would also ameliorate the worst effects of accidental or deliberate interference.  There is active interest in developing these alternatives.

… eLoran is another non-satellite-based alternative which has many cheerleaders. It is an enhanced version of Loran-C, which is itself an improved version of the original Loran (“long-range navigation”) system developed in the 1940s.

Some of these alternatives, such as eLoran, can now achieve location accuracies (~10 meter radius) comparable to those achieved by GPS.  They typically use much lower frequencies and much higher transmitter power than GPS, both making them considerably less susceptible to jamming.

GPS is a technology that, like the Internet, we didn’t realize we really needed until we had it; and it is an incredibly valuable tool for many purposes.   But we are once again being reminded that relying too much on GPS, or any other single system, leaves us very vulnerable to certain kinds of disruptions.

 

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