BOSTON — The U.S. Air Force is reconsidering its plans to launch two satellites at a time when it begins to deploy a new generation of GPS satellites in 2013, according to the service official overseeing the effort.

Col. Allan Ballenger, commander of the Air Force Space and Missile Systems Center’s Global Positioning Systems Wing, said the service still believes that it can reduce launch costs by launching two GPS 3 satellites at a time on an Evolved Expendable Launch Vehicle (EELV). However, the service is reconsidering the plan due to concern that it might constrain industry from developing the best possible spacecraft, Ballenger said in a May 1 interview.

The Air Force launches its current variant of the GPS satellites, which are built by Lockheed Martin Corp. and are known as GPS 2RM, aboard Delta 2 rockets built by Boeing. Delta 2s are smaller than the medium-lift EELV systems built by United Launch Alliance, a joint venture between Boeing and Lockheed Martin.

The Air Force plans to begin using the EELVs to launch the next variant of GPS satellites, which are known as GPS 2F and are built by Boeing. Those satellites will launch one at a time.

The EELVs might have some additional space that could accommodate a secondary payload adapter ring during GPS 2F launches, an issue that the program office is examining with the Space Development and Test Wing, which finds launches for experimental payloads, Ballenger said.

Ballenger said military GPS users have expressed concern that the dual-manifest strategy could hamper industry’s ability to fit all of the desired capabilities on the GPS 3 satellites, particularly the advanced capabilities that could come from the second and third blocks of spacecraft.

“We’ve heard that loud and clear,” Ballenger said.

Another argument in favor of launching one GPS 3 satellite at a time is that two satellites would not be lost in a launch failure, Ballenger said.

The Air Force expects to award the GPS 3 satellite manufacturing contract at some point around September this year to either Lockheed Martin or Boeing, Ballenger said. The Air Force’s formal request for proposals is expected to be issued May 21, according to a May 8 posting on the Federal Business Opportunities Web site.

The service’s acquisition strategy for the GPS 3 satellites has changed significantly since it began discussing its vision for the constellation in 2000. The GPS 3 prime contract is expected to represent the first prime contract award under the Air Force’s new block approach to satellite acquisition, which was implemented by Air Force Undersecretary Ron Sega shortly after he took office in 2005.

The block acquisition approach is intended to help the U.S. Air Force avoid overreaching for capability with the initial satellites in a constellation, and in doing so dodge the technical problems that have resulted in cost growth and schedule delays across its space portfolio.

This means that the Air Force will phase in the desired capabilities for GPS 3 in three blocks of satellites, rather than include all of its goals on the first satellite that launches in 2013. The same contractor will build each of the three blocks, so the designs for the spacecraft platform that launches in 2013 will have to accommodate the technology that will be a part of the second and thirds blocks of satellites, Ballenger said.

While the first block of satellites, known as GPS 3A, will be somewhat similar to the capability offered by Boeing’s GPS 2F satellites, the GPS 2F platform is not likely to be capable of handling the advanced capabilities envisioned for GPS 3C, Ballenger said.

The GPS 3A satellites will feature upgrades over GPS 2F including a modernized bus, an additional civil signal designed to be compatible with the European Galileo navigation constellation, and additional signal power to overcome enemy attempts to jam the GPS signal, Ballenger said.

The launch timeframe for the two blocks of GPS 3 satellites that are expected to follow GPS 3A is still under discussion, and could be affected depending on whether or not the Air Force sticks to its strategy of launching two GPS 3 satellites at a time. Other factors in the launch schedule will be user needs and constellation health, Ballenger said.

Upgrades that are planned for GPS 3B include satellite-to-satellite communications links, Ballenger said. While previous generations of GPS satellites have included cross-links for a secondary payload that detects nuclear blasts, this would represent the first use of such links for the navigation and timing signals on GPS satellites, according to an Air Force official.

The Air Force hopes to use the cross-links on GPS 3B to add more accurate data to the satellites on a more frequent basis, a capability that will improve the accuracy of information displayed on receivers used by the military as well as civilians, Ballenger said. GPS operators are generally able to update the data provided to each satellite once a day, but the cross links would speed the process so that each satellite could be updated roughly every four hours, he said.

Depending on the speed of progress with the GPS 3 program, the cross-links might be able to be added to the last of the GPS 3A satellites, Ballenger said.

A key upgrade planned for GPS 3C is a doubling of signal power that could help the satellites overcome enemy jamming attempts, as well as a spot beam that would enable the military to boost the power of the satellites’ signals in key areas to further combat disruption, Ballenger said.

Rick Ambrose, Lockheed Martin vice president for surveillance and navigation systems, said in a May 3 interview that the block acquisition approach has given him increased confidence in Lockheed Martin’s ability to meet the goals of the initial GPS 3 satellites, as well as the more advanced satellites that will follow. Ambrose noted that Lockheed Martin has experience with incremental upgrades to GPS satellites, having upgraded its GPS 2R satellites to the 2RM configuration that includes additional jamming resistance and additional military and civil signals.

John Duddy, Boeing’s GPS program director, also agreed that the block approach can help avoid problems with satellite development, and noted that Boeing also has experience with GPS upgrades, having developed the first variants of GPS satellites, and having upgraded its GPS 2F satellites to add more signals and jamming resistance.

The next GPS launch will be a GPS 2RM satellite in late August 2007, followed by two more satellites that are in storage and available to be launched as needed.

The Air Force recently awarded a contract to Lockheed Martin to add an additional civil signal to the last 2RM satellite, to reserve that radio frequency spectrum in case of further delays with Boeing’s GPS 2F satellites.

Air Force Lt. Gen. Mike Hamel, commander of the Space and Missile Systems Center, told reporters last year that the GPS 2F satellites had run into “serious challenges.”

The GPS 2F satellites were scheduled at one time to launch in 2005, but now are not expected to launch before June 2008.

Duddy said Boeing has resolved the issues with GPS 2F, which required redesigning hardware for several signals, replacing a power amplifier, and making the spacecraft wiring more resistant to vibration that could occur during launch.

“We feel the issues are behind us — knock on wood,” Duddy said in a May 7 interview.

The first GPS 2F spacecraft is expected to enter thermal vacuum testing in June, and will be ready to launch next spring, Duddy said. However, the satellite is intended to launch aboard an Atlas 5 rocket, and the Air Force has several other Atlas launches scheduled around the same time, so range availability could cause the launch to slip, he said.

Joe Davidson, a spokesman for the Space and Missile Systems Center, acknowledged the pad availability issue in a May 10 written statement, and described July 2009 as a date in which the Air Force has a “very high” level of confidence for the first GPS 2F launch.