Three years ago, LFV, which provides air navigation services for civil and military aviation in Sweden, received only a few reports of planes flying without their transponders switched on. Last year, there were 29 instances reported, and this year, there have been eight to nine cases, according to LFV.

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"Rather often, these have been Russian planes," says Olle Sundin, the director general of LFV. "There are also NATO planes, but the ones without transponders have primarily been Russian planes that have carried out signals intelligence."

He noted that since Russia began increasing armaments, they have noticed a lot of transponder-less air traffic flying from the direction of Estonia and Finland, and then continuing down alongside Gotland and over the southern Baltic.

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Transponders send out precise information about a plane's location. In international air space, it is permitted to fly without sending out this information, but ethical rules demand that military planes make way for civil flight traffic. Sundin says most agree that good ethics were not exercised in the air space over international waters in the majority of instances last year.

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