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Which Aviation GPS Units Are Illegal?
So, which devices are restricted for civilians? Sweet and simple: any GPS receiver that can operate at speeds faster than 500 m/s, or at heights greater than 18 km, are classified as munitions devices, thus making them illegal for civilians to use. Seeing that civilian aircraft don't generally don't go that high or that fast, there are few conflicts with this restriction.
At that, it's not so much that they're illegal as restricted. It's possible to acquire them, but to do so, one must request the appropriate export license from the US State Department. Obviously, the military has full use of such devices, and commercial airplane companies may be licensed for their use as well.
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Why Are Some Aviation GPS Receivers Illegal?
These parameters are chosen specifically to prevent the use of ballistic missiles. Such missiles, if created with GPS recievers, would be incredibly dangerous—and incredibly accurate. The government isn't too keen on this possibility, and so created the parameters that we know of today.
Of course, there are types of missiles that would not be affected by these restrictions—such as cruise missiles, which operate at about the same heights and speeds as many civilian airplanes. However, given the popularity—and importance—of civilian GPS units for safety and navigation concerns, it doesn't look like the government is going to be further tightening restrictions any time soon.
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Has Aviation GPS Been Abused Before?
Nope. The only reported incident of aviation GPS aiding terrorists within the United States were the infamous 9/11 attacks, and that was hardly the fault of the GPS device installed within the planes involved. Other than that, there have been no other incidents, no ballistic missiles with GPS recievers to their target, nothing along those lines—at least within America. Other countries are another issue entirely.
Civilians with perfectly innocent intentions have run into trouble with these restrictions before, however. A common one is with balloon launches, which can reach heights of 100,000 feet—higher than legal GPS recievers can work at. There are many other problems within aviation GPS as well, including a lack of signal reception in many places that are popular with civilian pilots.
For more information about the US government and its policies regarding civilian aviation use of GPS, check out the official GPS website.
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Some countries have more restrictions on the use of GPS in civilian aviation devices than the United States. If you happen to be in another country or plan on flying within one, it's up to you to check out the legalities within the country in question.