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...small jamming devices are increasingly available on the internet.
Low-power, hand-held versions that cost less than £100 can run for hours on a battery and confuse sat-nav receivers tens of kilometres away. Higher-power versions can do far worse, and at both GPS and mobile phone frequencies.
What is more, receivers can be "spoofed" - not simply blinded by a strong, noisy signal, but fooled into thinking their location or the time is different because of fraudulent broadcast GPS signals.