Since 2001, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven(AWI) has operated a purpose built, unique HEM system to map sea ice thickness in theArctic and Antarctic. To determine the sea ice thickness from the measured EM field we usean empirical curve fitting approach. The master curve is tuned for every flight leg usingmeasurements over open water between the ice floes in leads or polynyas. The level icethickness, which is a key value for sea ice science and climate research can be derived with anaccuracy of a few centimeters.