After gathering data on Greenland for more than a decade, ESA scientists have reported that the island's ice sheet is actually growing at its interior. Data collection began in 1991 with the radar altimeter instrument on board ESA's ERS-1, followed by ERS-2, and most recently Envisat, which has 10 instruments to measure various properties of the Earth from orbit. Greenland's ice sheet seems to be thickening at a rate of 6.4 cm (2.6 inches) a year above altitudes of 1,500 metres (5000 feet). Below that altitude, the ice sheets are decreasing in thickness.