The U.S. Air Force has begun work on some of the technologies that would not be included aboard its initial Space Radar surveillance and imaging satellites under the revised program plan unveiled in early February.

The efforts are focused on advanced solar-cell and battery technologies that are critical to a key aspect of the Space Radar’s mission — detecting moving ground targets in areas obscured by darkness, the Air Force said in a written response to questions. The initial satellites will be equipped with proven solar-cell and battery technology, a compromise that will reduce program risk but limit the system’s surveillance capabilities.

The Space Radar’s missions include mapping, taking high-resolution images and detecting moving targets on the ground, day or night and in all kinds of weather. Congress has been reluctant to fund the program due to concerns that it is risky and stands a good chance of exceeding its advertised cost, which is in the billions of dollars.

To allay those concerns, the Air Force has adopted an incremental approach that would deploy key system capabilities incrementally, rather than all at once. Gary Payton, deputy undersecretary of the Air Force for space programs, said in a Feb. 7 interview that among the technologies that are no longer targeted for deployment on the first round of Space Radar satellites are lithium-ion batteries and advanced solar cells.

These technologies would better enable the satellites to conduct nighttime operations, when their solar arrays are not directly receiving sunlight, Payton explained. While some of this technology exists today it is not qualified for spaceflight, he said.

According to a written response to follow-up questions provided by Air Force Maj. Regina Winchester, a spokeswoman for the service, the Air Force Research Laboratory expects to award contracts for design work on advanced solar-cell technologies in April. Potential study contractors include Emcore Corp. of Albuquerque, N.M., and Boeing Co.’s Spectrolab division of Sylmar, Calif., according to the Air Force statement.

Meanwhile, the Aerospace Corp., a federally funded research and development center in El Segundo, Calif., that assists the Air Force on space programs, is leading testing activities on lithium-ion batteries to evaluate performance and endurance, according to the statement. Payton said lithium-ion batteries, commonly used in cellular phones today, may be the best candidate for powering the advanced Space Radar satellites.

The National Reconnaissance Office has operated radar spy satellites for decades, but these spacecraft are used primarily for taking imagery, experts said. The Space Radar satellites will require more power because they also will conduct near-continuous surveillance for moving ground targets, they said.

“It’s a matter of how much you want to see, and how long you want to see it,” said James Lewis, director of technology and public policy at the Center for Strategic and International Studies, a think tank here. Lewis likened the difference in power requirements between today’s radar imaging satellites and the Space Radar satellites to that between a still-photo camera and one used to film movies.

The initial Space Radar satellites could prove highly useful to the military even with a relatively rudimentary moving-target surveillance capability , Lewis said. Satellites, which are not constrained by overflight restrictions or vulnerable to ground fire, will complement increasingly capable manned and unmanned aircraft as part of an integrated surveillance architecture, he said.

Lewis said the incremental technology strategy has been employed successfully in the past, notably on the U.S. Corona optical spy satellite program. Attempts to leapfrog technological steps are part of the reason for skyrocketing costs and delays on programs like the Future Imagery Architecture spy satellite system, he said.

Jeff Grant, vice president for business development at Northrop Grumman Space Technology , said the Space Radar ultimately could prove as useful to the military — and possibly to civilian users — as the GPS navigation satellites, which had a beginning similar to that of Space Radar. Northrop Grumman Space Technology of Redondo Beach, Calif., is leading a team competing against Lockheed Martin Space Systems of Denver to build the Space Radar satellites.

The GPS program initially had lukewarm support from both the Pentagon and Congress, and began with a small constellation that has since grown in size and capability, Grant said. It was difficult at the time to envision how indispensable GPS would become to both military and civilian users , Grant said.

Studying ground movements, even in a limited fashion , can vastly increase the Pentagon’s knowledge of a region , Grant said. That was the case, Grant said, with the Joint Surveillance and Target Attack Radar System, the airborne radar that was rushed into the field as a prototype in the 1990s.