The US Air Force has moved one step closer to acquiring three more RQ-4B Global Hawk Block 30 unmanned aircraft by awarding a $114 million advanced procurement contract to manufacturer Northrop Grumman.

The announcement, made in a Northrop media release, follows efforts by the air force to retire its fleet of Block 30s.

The advanced procurement contract calls for procurement of long-lead items, including three enhanced-integrated sensor suites (EISS) made by Raytheon.

EISS are systems that combine a synthetic aperture radar, ground moving target indication, high-resolution electro-optical digital cameras and infrared sensors, says Raytheon.

In addition, the contract calls for acquisition of three airborne signals intelligence payload (ASIP) systems and three ASIP retrofit kits. Those kits will be installed on aircraft the air force has already purchased.

ASIP systems identify radar and intercept other types of electronic and communications signals.

Northrop says the contract reaffirms the air force's commitment to the aircraft, which has been in service for more than 15 years in the US and abroad.

But air force officials have been eager to retire Block 30 Global Hawks.

In its fiscal year 2013 budget request, the air force said it would extend support of the U-2 programme in order to divest 18 Block 30s. The air force added that developing and sustaining Block 30s costs more money than sustaining U-2 operations.

Block 30s cost $6,710 hourly to operate, compared to the U-2's hourly rate of $2,380, according to the air force's total ownership cost (AFTOC) database, obtained by Flightglobal.

But a senior air force official who requested anonymity told Flightglobal earlier this year that the fiscal year 2013 defence authorisation bill directs the air force to procure three more Block 30s despite assessments that the existing aircraft are sufficient to meet the air force's requirements.

The new aircraft will be "excess to need" and will likely be designated as backup aircraft, said the official.

The defence authorisation bill also directs that Block 30s remain active until the end of FY 2014.