WASHINGTON — Significant technology upgrades proposed by the U.S. Senate for the fifth and sixth satellites in the Pentagon’s missile warning program could cost more than $800 million and cause delays leading to gaps in coverage, according to an Air Force report.

In the report accompanying its version of the Defense Authorization Act for 2014, the Senate expressed concern that Space Based Infrared System satellites five and six would launch with 30-year-old technology and inquired about the feasibility of replacing the image-recording focal planes on the satellites’ main sensors with modern digital focal planes. Although that language did not find its way into the conference report that accompanied the final version of the bill, which was signed into law in December 2013, it still required a reply from the Air Force.

That response, in the form of a report dubbed “SBIRS 5/6 Focal Plane Technology Insertion” and given to congressional defense committees in May, said the technology insertion program is feasible. “However, the costs are higher and the schedule will stretch out beyond the need dates adding risk to the operational capability of the constellation,” the Air Force said.

The Air Force report said the technology upgrade would add at least $424 million to the cost of the satellites and delay their delivery by nearly two years. The report noted that the change would negate much of the savings the Air Force expects to realize by buying the satellites together rather than serially.

Officials with the Air Force and SBIRS prime contractor Lockheed Martin Space Systems of Sunnyvale, California, have said the block buy of SBIRS GEO 5 and 6 saved about $1 billion. The recently ordered satellites are expected to cost $1.9 billion combined and be available for launch in the 2020s.

Congress had previously authorized the Air Force to spend $3.9 billion to purchase those satellites, called GEO-5 and GEO-6.

The Air Force also considered a deluxe upgrade option that in addition to modernizing the focal planes would improve the resolution of the sensors by 20 percent. That scenario, which would entail design changes, would add $859 million to the satellites’ price tag and delay their delivery by more than three years, the Air Force said.

“The Air Force does not recommend incorporating these changes due to large cost impacts and resultant schedule slips beyond the operational need dates,” the report said.

The report does mention 11 smaller technical updates that are being made to the satellites, several of which focus on the payload sensor

When fully deployed, the SBIRS system will include four dedicated satellites in geosynchronous orbit, infrared sensors hosted on classified satellites in highly elliptical orbits, and a network of ground stations to receive, process and distribute the data. SBIRS is the replacement for the Defense Support Program constellation of satellites that have provided missile warning since the early 1970s. The SBIRS GEO 5 and 6 satellites are expected to help replenish the constellation.

The Senate’s concerns stem from the fact that the design of the SBIRS satellites has remained essentially static since they were developed starting in the late 1990s.

“The committee understands that SBIRS GEO 5 and 6 [are] to use much of the same technology as in earlier versions found in GEO 1 and 2,” the Senate wrote in its report. “If such an approach is used, that means that GEO 5 and GEO 6 when launched in the 2020 timeframe will be using, for example, focal plane technology developed in the 1990s, which is 30 years old.”

SBIRS GEO-1 was launched in May 2011 and was followed in March 2013 by SBIRS GEO-2. The third geosynchronous SBIRS satellite, GEO-3, is notionally slated to launch in 2015, with the fourth to follow about a year later.

The Air Force said some of the new technology being developed under the SBIRS modernization account would best be incorporated into a follow-on missile warning system. That system, the service said, could put tactical and strategic missile warning capabilities — each SBIRS satellite has both — on separate satellites.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.