The US Air Force has finalised a deal to purchase three more Northrop Grumman RQ-4 Global Hawk unmanned air vehicles, filling its official requirement for the high-altitude, long-endurance type.

The deal, which brings the air force’s fleet to 37 Global Hawks, is worth $354 million. The last of the three aircraft will be delivered in 2017.

Congress authorised funds to purchase the 11th lot of Global Hawks in fiscal year 2013, but the USAF held off from spending the money while it decided whether to retire the Lockheed Martin U-2 manned reconnaissance aircraft.

The air force considered retiring the Cold War-era U-2 during its 2011 budget negotiations, but ultimately balked because the Global Hawk was and remains unable to carry its optical bar camera: a large wet-film system that is used for treaty verification by the US State Department.

The new contract also includes retrofit kits to add Northrop's airborne signals intelligence payload (ASIP) sensors to two of the USAF's existing RQ-4s, supporting advanced intelligence, surveillance and reconnaissance data collection. There are 18 Block 30 Global Hawks in the fleet, of which only eight lack this multi-intelligence capability, says Mick Jaggers, the company’s Global Hawk programme manager. The ASIP retrofit kits are scheduled for delivery in late 2016 and 2017.

Jaggers prefers to refer to the eventual configuration as Global Hawk B, which is part of the company's one-fleet initiative to create a single model of UAV, capable of carrying a variety of sensor payloads.

A Global Hawk Block 30 carries Raytheon’s enhanced integrated sensor suite that has electro-optical and infrared sensors, as well as a synthetic aperture radar. It still cannot carry the optical bar camera, but Northrop is working on hitching the system to a Global Hawk.

“There are means, and we’re in discussions with the US government right now, to take what we call a universal payload adapter … and make all of the aircraft modular. Whatever sensor you wanted to use, it doesn’t matter,” Jaggers says.

Equipped with a universal payload adapter, the RQ-4 could fly a mission, land and be reconfigured with a different sensor quickly enough not to disrupt its next air tasking order, Jaggers says. All that is required to mount an optical bar camera is routing data and power from the aircraft to the device. Northrop has already tackled attachment and encasement issues, he says.

The Global Hawk, in all its variants, has flown more than 126,000 flight hours supporting diverse global missions. It is capable of capturing detailed surveillance data at altitudes of 60,000ft on sorties that can last up to 32h.