For the better part of the past decade, U.S. Air Force officials have gone back and forth on how to modernize a fleet of surveillance planes so old that the original airframes — Boeing 707 airliners — have been out of production since 1979.

The 17 planes now in service, known as the E-8C joint surveillance target attack radar systems (JSTARS), have been workhorses around the world and average 48 years in age. It is not uncommon for a portion of the fleet to get grounded for maintenance issues, increasing the pressure on the Air Force to replace them with more reliable airplanes.

“These are critical aircraft,” Air Force Chief of Staff Gen. David Goldfein said in congressional testimony earlier this year.

Goldfein and other senior officials repeatedly have been pressed by lawmakers to explain the way forward for this small but mighty portion of the military’s arsenal.

The Air Force is embarking on a $7 billion program to replace the JSTARS fleet with modern planes by 2024. To meet that target, it has to select a team of contractors in the coming months to start building prototypes. Officials have said a decision will be made sometime between October and March. Congress already appropriated $128 million in fiscal year 2017 for research and development of a JSTARS replacement and the Air Force requested $417 million for 2018.

JSTARS operates as an airborne sensor and command center. The aircraft are packed with surveillance cameras and communications systems. The fleet has flown more than 130,000 hours in combat zones around the globe since 2001. They have been in great demand, and due to their age, require costly depot overhauls.

NATO Commander Army Gen. Curtis Scaparrotti recently told Congress that the planes are “particularly important against a large force like Russia so that we do have good information, so we understand movement and change.”

The Pentagon arguably created its own JSTARS shortage problem when, in a 1997 strategic review, it recommended the requirement be cut from 33 to 13 aircraft. The thinking at the time was that the world was entering an era of peacetime. Several years later, the Air Force moved to buy a replacement, the E-10 MC2A, but the program ran into technical and budgetary trouble and was terminated in 2007.

With the Air Force finally poised to make a move on the JSTARS recapitalization, the defense industry’s largest firms are pulling out all the stops to ensure a win. Teams led by Boeing, Lockheed Martin and Northrop Grumman are presenting the Air Force with three airframe choices: A Boeing 737 airliner, a Gulfstream G550 and a Bombardier Global 6000 business jet. Because of the Air Force’s insistence that the new system must be relatively low maintenance, all three bidders are advertising their systems as long-term money savers.

A "source selection evaluation board" of senior military and civilian leaders will make the final decision on which company gets the contract. It’s never disclosed who is on these boards and membership varies from program to program. But, for a program this significant, it will likely include high-ranking officials.

Boeing’s offer, a militarized 737 airliner, is larger and more expensive than a business jet, but the company has argued that the military in the long run would save on maintenance costs by tapping into Boeing’s global logistics and supply chain. Officials said a larger aircraft also would give the military spare room to add new features if it so chooses in the future, without having to worry about the space constraints of a smaller jet.

“That’s our argument,” Stan Deal, president and CEO of Boeing Global Services, told RealClearDefense. The company officially opened the global services business July 1 to focus entirely on aircraft support and maintenance, both in the commercial and government markets.

“We want to be better positioned to help our customers when it comes to sustainment,” Deal said. Boeing is confident about its chances in the JSTARS program, he said. Another selling point for the 737 platform, noted Deal, is that it would give the Air Force commonality with the Navy’s P-8, a militarized 737 used for maritime surveillance. “We have made a compelling case,” Deal said. “We’re not going to lay down easy on this one.”

Industry consultant Tom Captain said the JSTARS recapitalization should be a test case for how the Pentagon weighs the benefits of long-term savings versus upfront costs. It boils down to “pay me now or pay me later,” Captain said in an interview. If an airplane is more expensive to buy but saves money over its service life, it is worth considering, he said. The Defense Department has a terrible track record keeping its aging aircraft fleets in working order, which explains the runaway operations costs. The so-called “mission capable rates” of some U.S. military fleets are under 50 percent, meaning that more than half the fleet is unavailable. This is a serious problem, said Captain. “It will be a complete embarrassment when we have to fight.”

The next step for the Air Force will be to narrow the field of bidders from three to two for the $2.7 billion development phase of the program.

Northrop Grumman, the incumbent JSTARS prime contractor, designed the system for the purpose of keeping tabs on the movements of enemy tanks during the Cold War, “and we’ve continued to add many new capabilities to track and target a much more elusive enemy,” the company noted on its website. For the replacement program, it is offering a militarized Gulfstream business jet, and joined forces with Gulfstream’s parent company, General Dynamics, and L3 Technologies.

Retired Air Force Lt. Gen. Thomas McInerney, an industry consultant, has promoted the Gulfstream as the option that would “lower costs while improving performance and effectiveness.”

“Will the U.S. Air Force decide to replace the large 707 aircraft with another large aircraft or select a business jet for this mission?” McInerney asked in a recent RealClearDefense op-ed. He touted the Gulfstream as more desirable for wide-area surveillance missions as it flies at higher altitudes than the 737. “You get a larger coverage area, you literally see over and beyond difficult terrain like mountain ranges, and you have much more effective radar performance.” He said an increase in altitude from 41,000 to 47,000 feet allows the radar to see almost 20 percent more of the battlefield.

Northrop Grumman has projected that the G550 business jet will cost 60 percent less to own and operate than the existing E-8C fleet. Over a 20-year period, that would amount to $10 billion in operations and support costs. McInerney also noted that large airliners require longer runways whereas the G550 weighs 80,000 pounds less than the 737 and can operate from more airports around the world.

Lockheed Martin also is proposing a business jet, albeit one larger than the Gulfstream. Its team includes Bombardier, Raytheon and Sierra Nevada Corporation. The Bombardier Global 6000 is “less expensive to operate than modern airliners” in part because of its fuel efficiency, Lockheed Martin’s program manager, Dave Rapp, said in a statement. The company has submitted two JSTARS recapitalization proposals to the Air Force. One includes Raytheon’s newest electronic ground surveillance radar, and the other offers a different radar made by Northrop Grumman Mission Systems.

Rapp said the Global 6000 has been previously modified for military use into a configuration similar to what the Air Force wants for the JSTARS recap. He said the aircraft is the “right size” — larger than the G550 but smaller than the 737. “Analysis based on both civil and military operations indicates the Air Force could save over $1.3 billion by choosing the Global 6000,” Rapp said.

Everyone associated with this program agrees the Air Force faces a tough decision. When contractors were asked to submit proposals, the Air Force purposely chose to not be too prescriptive and gave companies some leeway to design a system according to their best judgment. The clock is ticking and the generals now need to make up their minds.

Sandra Erwin is a national security and defense reporter for RealClearDefense. She can be reached at serwin@realcleardefense.com. Follow Sandra on Twitter @Sandra_I_Erwin.