DAVID VAUGHAN / SCIENCE PHOTO LIBRARY DAVID VAUGHAN / SCIENCE PHOTO LIBRARY
Measuring glacier speed. Glaciologist uses a Global Positioning System (GPS) receiver to obtain a measurement of his position on a glacier in Ant- arctica. The speed of the ice flow in the glacier is revealed by compiling such measurements over a period of time. The receiving antenna is at right, whilst a De Havilland Twin Otter aircraft waits in the background. The receiver calculates the user's position by measuring 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. Photograph of a Trimble 4000 SSE GPS receiver.
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