WASHINGTON — The goals of the U.S. Defense Department’s $22 billion GPS modernization effort could be accomplished more quickly and at less cost than currently expected by allocating more resources to receiver upgrades and leveraging a commercial satellite program, a new study concludes.

According to the Congressional Budget Office (CBO) study, the Pentagon could reduce the GPS system’s vulnerability to signal jamming by equipping planned military receivers with better antennas and inertial navigation systems, the ability to receive data from the Iridium satellite communications system, or both. Any of the three alternative approaches would yield a significantly more robust system eight years sooner than the 2026 date now anticipated, the study said. Cost savings would range from $1.3 billion to $3.3 billion, depending on the alternative selected, the study said.

The report, dubbed “The Global Positioning System for Military Users: Current Modernization Plans and Alternatives” and released Oct. 28, assumes the U.S. Air Force presses ahead with its next-generation GPS 3 satellites, now in development. But whereas the current GPS 3 plan would introduce upgrades on successive batches of satellites, the CBO scenario has the Air Force purchasing 40 spacecraft with the same |capability.

The U.S. Air Force GPS modernization effort, which dates back to 2000, was driven by the vulnerability of relatively weak signals from the basic constellation of 24 satellites to jamming. The approach has been to equip the satellites with more signals and transmit those signals at higher power levels.

These enhancements, including the addition of a new M-code signal available only to military users, have been introduced in incremental fashion on the GPS satellites currently being launched. The Air Force’s long-term modernization strategy is centered on GPS 3, a multibillion-dollar next-generation constellation under development by Lockheed Martin Space Systems of Denver, and an overhaul of the GPS ground control system led by Raytheon Intelligence and Information Systems of Garland, Texas.

According to the CBO study, the Pentagon’s modernization plan, which also includes the purchase of hundreds of thousands of receivers capable of processing M-code signals, will cost about $22 billion from 2012 to 2025. That estimated price tag assumes the Air Force will have to buy, at a cost of $3.6 billion, eight more GPS 3 satellites than currently planned to reap the full benefit of the system’s most advanced capabilities.

The Air Force intends to buy 32 GPS 3 satellites in three successive blocks, each with improved capability: eight of the initial GPS 3A versions; 16 of the more powerful GPS 3B variants; and eight GPS 3C spacecraft featuring spot beams to concentrate coverage of selected areas. According to the CBO study, however, 16 of the GPS 3C variants will be needed to fully realize the enhancement the spot beams are intended to provide.

In the CBO’s alternative scenarios, the Air Force would purchase 40 GPS 3A satellites. The alternatives also each assume that modernization of the ground control segment continues and that the Air Force purchases the same number of M-code receivers. The cost estimate for the features common to all three alternative scenarios is about $18 billion.

In the first alternative scenario, the Air Force would equip the receivers with new antennas capable of filtering out jamming signals, the CBO study said. The receivers also would be equipped with small inertial navigation systems to reduce the position-location errors caused by interference and give users a basic navigation capability even if GPS signals were unavailable, the CBO says.

In the second and least-expensive scenario, GPS satellite signals would be retransmitted to modified receivers via the 66-satellite communications constellation owned by Iridium Communications of McLean, Va., a scheme based upon an existing demonstration program dubbed High Integrity GPS, or iGPS. Signals from the low-orbiting Iridium satellites would be less susceptible to jamming than the transmissions from the GPS satellites, which orbit at a much higher altitude.

The third and most expensive alternative is a combination of the two: GPS receivers equipped with both the improved antennas and the ability to receive Iridium signals.

The inertial measurement units would be included in all three |scenarios.

The CBO conceded some weakness to its alternatives. Outfitting GPS receivers with new antennas and inertial navigation systems would add to their weight and power consumption requirements, making them less useful to military forces operating on foot, at least until the technology can be miniaturized. In addition, the High Integrity GPS option would put the Air Force in a position of dependence on a commercial operation, the CBO said.

Warren Ferster is the Editor-in-Chief of SpaceNews and is responsible for all the news and editorial coverage in the weekly newspaper, the spacenews.com Web site and variety of specialty publications such as show dailies. He manages a staff of seven reporters...