The US Air Force has completed the first phase of a two-stage expansion of the GPS navigation-satellite constellation that will increase coverage and improve signal availability and accuracy for military and civil users alike.
The “Expandable 24” initiative involves repositioning of satellites in three of the six orbital planes used by GPS and is possible because there are 35 satellites in space of which 31 are set to healthy, more than required for the basic 24-spacecraft operational constellation.
Several of the extra satellites have been flying in tandem with older spacecraft, and the repositioning and reconfiguration of the constellation will increase the number of satellites in view from any point on the Earth. For the military, this will improve the probability of signal acquisition in mountainous terrain – ie Afghanistan – and provide more robust availability and better accuracy for all users.
The first phase, begun in January 2010 and completed on Jan 18, involved the repositioning of four satellites: SVN24, 26 and 30, all GPS IIAs, and SVN49, a GPS IIR-M spacecraft. SVN49 took 351 days to complete its repositioning.
The second phase begin in August 2010 is to be completed in June. It involves repositioning another three satellites (SVN46 and 56, which are IIRs, and SVN55, a IIR-M) and will result in “the most optimal geometry” in the 42-year history of GPS, the Air Force says.
(NOTE: updated with correct information on the satellites repositioned.)