When trying to work out what might be causing such a high level of interference, the project engineers noticed that most of it occurred during the week, dropping off at weekends, which ruled out solar weather events that occur more randomly.

Jamming was also most marked during rush hour which pointed to commercial vehicles being the culprits rather than, as previously suspected, vehicle thieves trying to foil security tracking systems.

"The pattern of behavior suggests it is likely to be civilian-sourced jamming and most likely the evasion of tracking within commercial vehicles for moonlighting activities or for other non-work purposes," said Charles Curry, project head and Chronos Technology founder.

The best guess is that van drivers want to hide unauthorized use of delivery vans, using jammers to confuse the central tracking software now used by all major delivery networks to optimize the supply chain.

A year ago, the government-funded Sentinel Project first reported on the issue of GPS interference, using its network of sensors placed on bust roads to log what had previously been a largely anecdotal problem.

And problem it is; GNSS is critical for aviation and air traffic control, shipping, and specific applications such car tracking to deter theft; consumers also use it for SatNav. Jamming can cause significant problems for these applications.

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