You've seen the news reports: Russian-made jammers may be used against U.S. GPS-based weapon systems in Iraq, and there have been warnings about this technology in the hands of terrorists. U.S. Representative Joseph R. Pitts (R-Penn.), a former Air Force officer, has a page on the topic on his Web site. How serious is the threat of GPS jamming, and what can we in the GA community do about it?
After a week researching this topic, I have to tell you that it's pretty serious -- regardless of what the Russians may or may not have sold to terrorists. An anonymous author has provided a detailed technical description of how to build a GPS jammer at a hacker's Web site. The U.S. Air Force itself has been conducting GPS jamming exercises -- one such exercise is scheduled for March 10-14, 2003, in the Yukon MOA near Fairbanks, Alaska. There has been at least one well-documented case of unintentional jamming, and the U.S. Federal GPS Policy makes clear that GPS, while now supervised by a joint inter-agency governing board, is subject to control by the National Command Authority. That means the President can decide to jam the entire system if he decides it is necessary.
Sensitive = Vulnerable
GPS is particularly vulnerable to jamming, because the receivers are exquisitely sensitive -- they have to be, to receive the extremely weak signal from orbiting satellites. A relatively low-powered jammer, transmitting static on the GPS frequency band, can overpower legitimate GPS signals over a wide area -- as much as a 100 kilometer circle at just 1 watt radiated power. (By comparison, typical general aviation Mode-C transponders radiate hundreds of watts.)
GPS receivers are so sensitive that there have even been cases of unintentional jamming. In one case no less than three separate jamming signals were being generated by VHF/UHF television antenna preamplifiers on boats in California's Monterey Bay. The signals from the preamplifiers were strong enough to completely jam GPS within a one-kilometer radius at sea level. The authors of the report speculate that a similar source, if transmitted using an omni-directional antenna, could interfere with aviation GPS receivers at a range of 50 kilometers.
Despite speculation in the press, military systems such as JDAM are designed to resist jamming. Among other things, they use directional antennas designed to see only the sky, and they have backup navigation capabilities so that they can continue to function even if they are jammed. Unfortunately, GA GPS systems are not jam-resistant, though IFR certified receivers do have one feature that can help: Receiver Autonomous Integrity Monitoring (RAIM), which is designed to raise a flag under conditions where you cannot trust the navigation data coming from the receiver. Jamming is one of several conditions that can cause a RAIM flag to be raised. VFR GPS receivers do not have RAIM -- but then, you're never supposed to use a VFR GPS as your only method of navigation.
How Much Risk?
How serious a risk does GPS jamming pose to GA pilots? Michael Braasch, an Associate Professor of Electrical Engineering at Ohio University, and an avionics expert, says, "The fears about jamming are probably overblown -- the world may seem a little more unsafe than it was a few years ago, but inadvertent jamming is a problem, and GPS systems can malfunction. To be a prudent navigator you should be using all available means of navigation to cross-check your position. LORAN, RNAV, VOR/DME and looking out at the terrain periodically are all good options."
According to Braasch, the RAIM feature of IFR approach-approved GPS units should provide a warning if jamming or other problems cause the GPS position to become inaccurate: "RAIM is basically a form of fault detection. It looks at the ranges from all the satellites in view, and checks to see if they're consistent with each other. If one is inconsistent, it's ignored. You have to have at least four satellites to get a GPS location. To do fault detection, you need five, which will give you redundancy -- but won't give you fault isolation; for that you need at least six satellites." Braasch notes that jamming isn't the only situation that can cause RAIM to issue a warning: "The distribution of satellites in the sky determines whether you have enough redundancy to make RAIM work -- if you have two satellites in the same part of the sky, that's not as good as if they're well separated in both azimuth and elevation. That's why many of the more advanced IFR units, like Garmin's GNS430 & GNS530, have a RAIM prediction feature to tell you if an adequate number of satellites will be in view on a given flight plan."
The situation for VFR GPS receivers is more problematic, according to Braasch: "Sometimes a VFR GPS will give you a message about low signal strength, poor GPS coverage, or low accuracy, or too few satellites in view. If you drop below four satellites, a lot of VFR GPSs will drop into a 2-D position, where it assumes you're at constant altitude -- you're at reduced accuracy at that point, and as soon as you start descending or climbing, the position will be false. The catch is that a VFR GPS isn't required to issue any warning at all -- it may just continue to report an inaccurate position, so it's vital for the pilot to cross-check what the GPS says with other forms of navigation."
What can you do to protect yourself? IFR pilots should have a fallback plan in case of a GPS malfunction: Have an alternate airport with something other than a GPS approach, and an alternate flight plan using other navigation means (such as VOR-DME navigation along published airways) in case GPS becomes unavailable for direct-to navigation. VFR pilots should always cross-check their position using visual references on a sectional chart or ground-based navaids.
Braasch -- an instrument-rated pilot himself -- is appalled at the way some pilots have become GPS-dependant. "Just the whole business of situational awareness -- you get in a little trouble, and ATC asks where you are, and you have no idea -- you're 200 miles from some fix. With people filing direct-to, situational awareness is actually decreased. It's easy to get lulled into a sense of invulnerability. What if you have an emergency and the GPS 'Nearest Airport' function is not available?"
Incidentally, jamming is by no means the only event that can make a GPS receiver deliver incorrect data -- signals from the GPS satellites can be corrupted, too few satellites can be visible from certain positions to deliver an accurate position, and receivers can malfunction (which has happened to me, personally). Cross-checking what your GPS thinks your position is against a visual or electronic reference is always a good idea.
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