Multiple Reflections Inside the GLAS Footprint: Estimation of the Altimetry Bias and a Roadmap towards a Correction Scheme

ICESat's primary mission statement is to use laser ranging to estimate the altitude (hence thickness) of the Earth's ice sheets at better than 10 cm per shot, leading to a mapping of the trend at about 1 cm per year accuracy upon temporal averaging. This ambitious goal is dictated by global warming science considerations. Laser altimetry and/or ranging can be done either by detection of the earliest photon returned from a short but intense pulse or by tracking the shift of the maximum of a longer pulse. In altimetry mode, GLAS uses the latter algorithm which makes it vulnerable to spurious shifts caused by multiple reflections in the unresolved terrain. We show that, for GLAS's instrumental characteristics and typical reflection and roughness properties of ice, the resulting negative altimetry bias can be commensurate with the mission's precision goal. The bias will furthermore have a seasonal cycle which may become aliased into the annual mean. Finally, we will argue that, based on our modeling results, GLAS has within its product portfolio much of what is needed to correct the bias.