Russia suspected of jamming GPS signal in Finland
Finnish Prime Minister Juha Sipila has said the GPS signal in his country's northern airspace was disrupted during recent Nato war games in Scandinavia.
He said he believed the signal had been jammed deliberately and that it was possible Russia was to blame because it had the means to do so.
Finland is not a Nato member but joined the war games which began last month.
Norway also reported GPS problems during the exercises near Russia's north-western borders.
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"It is difficult to say what the reasons could be but there are reasons to believe it could be related to military exercise activities outside Norway's borders," Wenche Olsen, director of the Civil Aviation Authority of Norway, told the Barents Observer earlier this month.
Russia is also suspected of jamming the GPS signal in Norway's border area last year when it held its own war games.
Relations between Nato and Russia have been strained since Russia annexed Crimea from Ukraine in 2014.
How serious was the disruption?
The Finnish region of Lapland and northern parts of Norway close to the Russian border were affected, with the Norwegian regional airline Widerøe confirming its pilots had experienced GPS disruption, Germany's DW news site reports.
However, the airline pointed out that pilots aboard civilian aircraft had other options when a GPS signal failed.
"This is not a joke, it threatened the air security of ordinary people," said Mr Sipila, who is himself an experienced pilot.
"It is possible that Russia has been the disrupting party in this. Russia is known to possess such capabilities."
How could Russia block the signal?
GPS is a global navigation system originally devised by the US military which works by sending signals from satellites above the Earth back down to receivers.
"Technology-wise, it's relatively easy to disturb a radio signal, and it's possible that Russia was behind it," Mr Sipila was quoted as saying.
Russia's electronic warfare capability has impressed many Nato commanders, the BBC's Jonathan Marcus wrote last year.
The country has its own, lesser-known global navigation system, called Glonass.
Why were the war games held?
Nato's biggest military exercise since the Cold War, codenamed Trident Juncture, rehearsed how the US-led alliance would respond to the invasion of an ally.
All 29 Nato members, as well as Finland and Sweden, were involved and it took place a few hundred miles from Norway's border with Russia.
At one point in the exercises, a Russian maritime reconnaissance plane flew past a US warship, the USS Mount Whitney.
The exercises began on 25 October and ended last Wednesday.
Just after they ended, an oil tanker collided with one of the Norwegian warships involved, in a fjord in southern Norway. The warship had been repeatedly warned of its collision course with the tanker, the BBC was told.