The PRETTY, Passive REflecTometry and dosimeTrY, mission will peer ahead to the horizon to receive signals from Global Navigation Satellite System (GNSS) satellites visible just above it. Two patch antennas on its forward face will pick up the same signal from the same satellite – or rather one signal that has reached it through space and the equivalent signal that has reflected off Earth’s cryosphere or oceans.
Using a highly advanced version of ‘spot the difference’ PRETTY will compare the twin signals onboard to derive ice and sea height to an accuracy of at least 50 cm from 550 km orbital altitude.