Measuring glacier speed. Glaciologist uses a Glo- bal Positioning System (GPS) receiver to measure his position on a glacier in Antarctica. The speed of the ice flow in the glacier is revealed by com- piling such measurements over a period of time. The receiving antenna is at left, whilst the inst- rumentation is in the yellow carrying case. The receiver calculates the user's position by measur- ing the time it takes for signals to arrive from GPS satellites at different points of the sky. By combining many readings the receiver's position may be fixed to within a few centimetres. His cowl protects him from harmful ultraviolet rays in sun- light. Photograph of a Trimble 4000 SSE receiver.
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