Evaluation of strain rates on Ice Stream B, Antarctica, obtained using GPS phase measurements

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Abstract
The “stop-and-go” kinematic Global Positioning System (GPS) technique was used to survey 270 stations twice within a 25 km by 10 km strain grid on the surface of Ice Stream B. One or two geodetic quality receivers operated as reference pivots, while two similar receivers traveled to grid stations. Each station was occupied for 25s. The method is rapid and reliable. Each survey was completed within 2 weeks. Calculated horizontal strain rates are accurate to 1%. Relative vertical velocities are accurate to 20 mm km1 a-1. Maps of the four horizontal velocity gradients, relative vertical velocity and surface elevation are presented. The vertical velocity pattern is used to identify the part of the topography that forms stationary waves and that which is migrating. No strong quantitative link is found between the pattern in horizontal strain rate and surface topography. In particular, there is no evidence that the topography is relaxing toward isostasy. None of the “weir-type” sticky spots, which are commonly observed with other glaciers, is found but there could be two of the “submerged-boulder type”, which cause lateral flow diversion. Evidence for “hot stripes” and zones of preferred crystal orientation is not found.