The Pentagon’s chief arms buyer last week approved the next step in a multibillion-dollar competition to replace the 16 existing Joint Surveillance Target Attack Radar System aircraft, or JSTARS, which have flown over 100,000 combat hours and are approaching the end of their service life.

James said in a statement that the Air Force’s plans would allow replacement of one of the highest priority weapons programs for U.S. military commanders at an affordable cost.

Three teams are competing for the work: Northrop Grumman (NOC), which built the existing planes and is teaming with General Dynamics (GD); Lockheed Martin (LMT), which is working with Raytheon (RTN) and Canada’s Bombardier; and Boeing (BA).

Air Force Major Robert Leese said Kendall’s approval of the so-called Milestone A phase of the program would allow the Air Force to exercise options on contracts previously awarded to the three teams to conduct system functional reviews, preliminary design reviews, and subsystem prototype demonstrations.

He said the Air Force would also begin efforts to award up to two contracts for work on new radar systems, and was on track to release a draft request for proposals for the engineering and design phase of the program in early 2016.

“JSTARS recapitalization is absolutely necessary for the Air Force to continue providing a combat-proven capability to the warfighter,” Leese said, noting that the cost of maintaining the planes was growing as the fleet aged.

Senator Johnny Isakson and 12 other senators urged Defense Secretary Ashton Carter in a letter dated Friday to protect funding for the JSTARS program in the fiscal 2017 budget request that is being finalized this week.

“Due to high demand over the years and increasing maintenance requirements, it is becoming increasingly difficult and costly to keep aircraft available to meet operational and training requirements. This leaves a growing capabilities gap that is unacceptable,” the senators said.