A campaign GPS network of 14 benchmarks was established at Redoubt in 1991 and reoccupied in 2001, 2008, 2009, and 2010. The first CGPS station was installed by the PBO 28 km NE of Redoubt in 2006. This station recorded subtle motion radially outward from the volcano from early May 2008, which then reversed with the onset of explosive activity in 2009. In response to unrest at Redoubt, the U.S. Geological Survey Alaska Volcano Observatory converted 4 of the campaign GPS sites to temporary CGPS stations during early 2009 prior to the first magmatic eruption. GPS observations are split into time intervals reflecting changes in eruptive behaviour:
(1) inter-eruptive period (June 2001–August 2008) – no clear deformation signal until May
(2) precursory phase (August 2008–March 2009)  – subtle inflation from May onwards
(3) explosive phase (March 2009–April 2009)  – inflation signal is reversed
(4) effusive phase (April 2009–June 2009) – continued deflation
Deformation during the precursory phase is consistent with inflation of a deep magmatic source modelled as a Mogi source with volume increase of ~0.1 km and depth of ~22 km. During the explosive and effusive phases, deformation is attributed to a shallower, mid-crustal source. In the explosive phase, the best-fit deflation source is a prolate spheroid centered at ~9 km depth with a volume change of ~- 0.05 km³. During the effusive phase, the best-fit deflation source is a Mogi source at a depth of ~9 km with a volume change of ~-0.003 km³ (Cervelli et al., 2009; Grapenthin et al., 2012).