Boeing has fixed an issue with cesium clocks aboard GPS IIF satellites, and is prepared to launch the third example on 4 October.
An issue identified earlier, which may have shortened the usable lifespan of the GPS satellites, was traced to a xenon bulb with higher than expected pressure. The bulb is standard equipment aboard GPS' highly accurate cesium clocks, says Boeing, but inflating it to standard pressures could potentially shorten the satellite's lifespan.
The issue is solved, Boeing says, by drilling additional holes in the satellite and allowing more time in the vacuum to gas to escape. While the permanent fix will be launched with the fourth satellite, a temporary solution was put in place for space vehicle three (SV-3).
Boeing is contracted to build twelve GPS-IIF satellites, of which six have been built. Two have been launched, one sits on the launch pad atop a ULA Delta IV launch vehicle, and three are in storage awaiting a launch date from the US Air Force (USAF), which purchases and operates the GPS constellation.
GPS IIF is a bridge capability between existing satellites and Lockheed Martin's GPS III, which remains under development.
The GPS IIFs are the first satellites with a fifth signal frequency, called L-5, which will be used for aerial navigation and search and rescue. At least four satellites are required for basic coverage.