If you're the type that thinks the rules governing personal electronic devices on commercial flights are hogwash, here is an interesting read.
And here is information regarding VOR and ILS, the two systems cited as most vulnerable to interference, as well as GPWS.
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There are numerous reports in the Aviation Safety Reporting System (ASRS) database of navigation errors which ceased after the crew asked for PEDs to be turned off. Although the VOR system was most often cited, the instrument landing system (ILS) was reported to have been affected 17 times, radar altimeters 10 times, autopilot systems eight times, ground-proximity warning systems seven times and an engine fuel controller once, in data through March 2001. Significantly, in a number of cases, the cockpit crew verified that the PED was the source of the error by having it turned back on and seeing the navigation error repeat, before requiring that the device be secured.
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With support from the FAA, assistance from three major airlines, and approval from the FAA and Transportation Security Administration, we flew a spectrum analyzer on board 37 revenue flights along the Eastern Seaboard. Key findings are that onboard cellular activity is appreciable, signal activity was observed in the GPS band at levels that could result in interference, elevated broadband noise was observed on many occasions in the VOR/ILS band, and passenger use of electronics including wireless devices is occurring at prohibited times including during approach.
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And here is information regarding VOR and ILS, the two systems cited as most vulnerable to interference, as well as GPWS.