WASHINGTON — More than 8,000 deployed military GPS receivers experienced compatibility problems with the most recent upgrade to the timing and navigation constellation’s ground control segment, but the U.S. Air Force has implemented an interim fix while it validates a permanent solution.

When the Air Force upgraded the GPS Operational Control Segment Jan. 11, some military users began reporting their systems were losing GPS signals. The problem was isolated to a specific type of GPS receiver known as the Selective Availability Anti-Spoofing Module (SAASM) deployed in at least 86 U.S. weapons systems that were having trouble authenticating a new messaging format implemented as part of an upgrade, the Air Force disclosed in an April 30 posting on the Federal Business Opportunities website. The posting has since been removed.

The service on Feb. 12 contracted on a sole-source basis with the receivers’ manufacturer, Trimble Military and Advanced Systems of Sunnyvale, Calif., to help Air Force Space Command’s GPS Wing track down the affected hardware and implement a fix.

Trimble identified two models of SAASM receivers that were affected and developed a temporary software solution that was implemented in critical U.S. platforms, Air Force spokesman Joe Davidson said May 4 in an e-mailed response to questions. Trimble has now completed development of a permanent solution for one type of receiver and is close to a permanent solution for the other, Davidson said. Both solutions must pass through the GPS Wing’s security certification process.

When the problem surfaced, the GPS Wing assembled a User Equipment Crisis Action Team to contact military users around the world, including in Iraq and Afghanistan, to identify whether they had any of the affected Trimble receivers in their equipment.

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”In some cases, users do not know whether they have one of the affected Trimble receivers because the card/form factor has been integrated into another box or higher level assembly, or the GPS receiver is in an inaccessible location,” the Air Force said in the April 30 contract notice.

The Air Force said the urgent nature of the problem justified the issuance of a $900,000 sole-source, cost-only contract to obtain Trimble’s help tracking down the affected receivers. The company’s work on this contract is nearly complete, Davidson said.

The problem forced the services to temporarily suspend the use of certain weapons platforms, the April 30 posting said. The U.S. Army, for example, was prompted to suspend the use of Excalibur 155-millimeter guided artillery shells that rely on GPS to enhance their accuracy. “If the SAASM receiver within the unit doesn’t pass the correct information to the precision guided munitions, the round could fall on friendly troops or civilians,” the Air Force posting said.

The only other affected weapon system the Air Force identified by name was the U.S. Navy’s X-47B, an unmanned combat aerial vehicle demonstrator built by Northrop Grumman. According to the Air Force, the Navy’s X-47B program office is losing almost $1 million per day while the vehicle is grounded.

Navy Capt. Jeffrey Penfield, the service’s Unmanned Combat Air System program manager, said the aircraft was affected for a short time and no issues remain.

“A GPS anomaly that resulted in navigation problems for the X-47B system was identified during testing in January 2010,” Penfield said in a May 5 e-mailed response to questions. “Within one month, the GPS Joint Program Office distributed a solution that was successfully tested on the X-47B and surrogate aircraft. The GPS issues have been resolved, with no further problems noted.”