The fourth U.S. Air Force GPS IIF satellite, slated for launch this month, will include a software update designed to address problems found in battery chargers on the Boeing-built spacecraft.

The battery chargers “were experiencing current drop-outs” during production testing, according to Air Force Space Command (AFSPC) officials. Boeing has since developed a software fix that will “govern the operating modes of the battery charger to reduce the likelihood that the charger will experience drop-outs,” the officials say. The software will be uploaded to GPS IIF satellite 4, which will be lofted on an Atlas V May 15, as well as all future IIF vehicles.

“These drop-outs are very short-duration and therefore do not affect the ability to charge the battery,” AFSPC officials say. “Root-cause investigations determined that the current drop-outs were the production of multiple contributing factors present in the power-conditioning unit design.”

A hardware fix, however, also has been readied. It will be implemented on GPS IIF satellite 5, slated for launch in November, and will be added to all subsequent satellites. It will modify the power-conditioning unit to address the contributing factors discovered in the root-cause analysis, the officials say.

AFSPC officials have also identified changes in operating procedures designed to mitigate the likelihood of drop-outs during applicable battery-charging modes.

This problem was found on the ground during testing after spacecraft were already in orbit, and it has not been observed in deployed satellites, the space command officials say. “We are confident that all on-orbit spacecraft are delivering the high-quality mission data expected by our users . . . and the upcoming fixes give us additional confidence that there will be no operational or service-life impacts based on this issue,” they say.

Boeing officials declined to comment on the issue. But this is the latest in a string of problems for the company’s GPS IIF line, which has struggled to be profitable. Last year, the company was working on a fix to the xenon bulb in the cesium clock on GPS IIF satellite 3, which was launched last October.

On the previous satellite, IIF-2, an incident occurred in the cesium clock that “involved trapped air that, when combined with vacuum and high power [in space], caused an event that resulted in a pump failure,” said Paula Shawa, a Boeing representative, at the time. “This failure necessitated higher-than-desired clock maintenance from the ground crew, so it was decided to switch to a rubidium clock” for use in orbit.

The xenon bulb sets the frequency standard for the cesium clock; the IIF design includes a single cesium clock and two rubidium clocks.

Boeing is on contract for 12 GPS IIFs. Pricing for the first nine satellites was fixed about 10 years ago. Air Force officials have said the average contracted price of a GPS IIF is $121 million, though the first satellites cost more than $300 million to build.