NATO will replace its ageing fleet of AWACS surveillance planes, in service since the Cold War in the 1980s, with a militarized version of the Boeing 737 commercial jet, the alliance said on Wednesday, in a deal likely worth billions of euros.

Acting like a flying radar tower, the AWACS (Airborne Warning and Control System) jets with the distinctive, nine-meter wide radomes on their backs have been NATO’s eyes in the sky since 1982.

With their rotating radar, the current modified Boeing 707 jets can detect aircraft at a distance of more than 400 kilometers (250 miles).

They are capable of monitoring an area of some 300,000 square kilometers (115,000 sq miles), a territory the size of Poland, according to NATO, and can also detect ground targets such as ships.

To replace the old AWACS jets, NATO aims to purchase six Boeing E-7 A Wedgetail planes, with the contract to be signed in 2024 and the first jet ready for operational duty by 2031.

“Surveillance and control aircraft are crucial for NATO’s collective defense and I welcome allies’ commitment to investing in high-end capabilities,” NATO Secretary-General Jens Stoltenberg said.

“This investment in state-of-the-art technology shows the strength of transatlantic defense cooperation as we continue to adapt to a more unstable world.”

The alliance did not specify why it chose to replace its fleet of 14 AWACS planes with only six Wedgetail jets, but it said the new aircraft would have better capabilities and be more expensive than their predecessors.