The U.S. Air Force will seek congressional approval next year to begin work on a new missile-warning satellite system to replace the Space Based Infrared System (SBIRS), which is being scaled back from at least five to no more than three spacecraft due to technical difficulties that have proven intractable, according to senior Pentagon officials.

The follow-on system, to be competitively selected, will take advantage of more up-to-date sensor and software technology than SBIRS, which is still years away from first launch yet employs programming code that dates back to the 1970s. At the same time, however, the follow-on program will not be as ambitious as SBIRS in terms of performance relative to its predecessor, officials said.

Kenneth Krieg, undersecretary of defense for acquisition, technology and logistics, informed Congress in writing Dec. 12 of the Pentagon's decision to buy no more than three SBIRS satellites, with the third to be contingent on the performance of the first. The action was taken following a review mandated by a U.S. law, known as the Nunn-McCurdy provision, for military programs whose costs grow by 25 percent or more.

This was the latest such infraction for SBIRS, whose projected costs have grown from $2 billion to more than $10 billion since the prime contract was awarded to Lockheed Martin Corp. in 1996. That contract called for the construction of five dedicated satellites in geosynchronous orbit -- one serving as a ground spare -- sensors to be hosted by two classified satellites operating in highly elliptical orbits, and ground equipment. The first of the dedicated satellites, originally scheduled to launch in 2002, is now slated to fly in 2009.

Lockheed Martin has delivered the sensors for the classified satellites, and the payload for the first dedicated satellite is in thermal vacuum testing.

In his letter, a copy of which was obtained by Space News, Krieg said he will not order a third SBIRS satellite until he is satisfied that the first will "perform its mission." That assessment will be made based on performance of the first satellite during ground testing, according to a Defense Department official.

In a briefing with reporters Dec. 13, Air Force Secretary Michael Wynne said the problems with SBIRS are so deep-rooted that staying the course with five satellites would have required extensive redesign work. Defense officials did not have a full appreciation of these problems when they reviewed SBIRS following a Nunn-McCurdy breach back in 2002, he said. Wynne also noted that concerns about a gap in U.S. missile warning capabilities have diminished since then because the existing Defense Support Program satellite system is lasting much longer than expected.

One of the biggest problems with SBIRS lies with its operating software, which is based on a programming language called Ada that was developed in the 1970s, Wynne said.

"Ada is a program that is not popular any longer," Wynne said. "It is a software design that was literally invented around the time that DOS [Microsoft Corp.'s original Disk Operating System] was invented. DOS is no longer even talked about, nor should Ada be, but we still have Ada-based programmers trying to do it."

The Air Force hopes to use a more modern computer language like C+ for the SBIRS follow-on system, Air Force Undersecretary Ronald Sega told reporters during a Dec. 15 briefing at the Pentagon. In addition, the Air Force would like to see upgrades including faster data processors and improved focal plane sensor technology, he said.

"The Department of Defense will work with the Congress to initiate a new program for space-based Overhead Non-Imaging Infrared (ONIR) to generate competition and exploit new technologies," Krieg said in his letter. "Through the restructured SBIRS program and the parallel competitor program, the department expects to gain additional insight that may drive future decisions related to the missile warning and technical intelligence mission areas."

Despite its desire for better capabilities, the Air Force plans to be conservative in its approach to the new satellites, Sega said. The service probably tried to achieve too much too quickly with the SBIRS design, and may pursue the follow-on satellites with an eye toward making improvements over time through a series of block upgrades, he said.

The Air Force plans to seek money to begin designing the new satellites in its 2007 budget request, which is due to the Capitol Hill in February, Sega said. The service likely will ask for significantly more funding in 2008, he said.

Meanwhile, the Air Force has opted against a major SBIRS design compromise that was proposed last year as a way to contain the program's ballooning costs, Sega said. Each dedicated SBIRS satellite is to have two sensors, one for scanning large swaths of territory in a sweeping fashion and one for staring continuously at smaller areas deemed as likely missile-launch sites. There is no staring capability on the Defense Support Program satellites, and the Air Force last year began looking into the possibility of dropping it from the SBIRS design as well. Ultimately, however, officials rejected the idea.

The SBIRS restructuring is a long-term blow to Lockheed Martin. Although the original contract called for five satellites, the expectation at the time was that program would run at least through 2020 and that the company ultimately would build anywhere from 10 to 20 spacecraft.

Steve Tatum, a spokesman for Lockheed Martin Space Systems of Sunnyvale, Calif., said the company is pleased to see the Pentagon going forward with purchase of at least the first two SBIRS satellites because of the program's importance to national security and intelligence.

Lockheed Martin is "confident [that] the groundbreaking work we are performing on the program will be of significant value to future military space initiatives," Tatum said in a written statement.

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