Atmospheric multiple scattering effects on GLAS altimetry - Part I: Calculations of single pulse bias

Estimates of the effect of pulse stretching on satellite laser altimetry, such as planned for the Geoscience Laser Altimeter System (GLAS), by cloud multiple scattering were made from an analytical method and with Monte Carlo simulations. Altimetry is dependent an the time required for a laser pulse to complete the roundtrip to the surface and return to the transmitter. Since a transmitted Gaussian pulse will be stretched by the effects of multiple scattering, the use of the pulse centroid as the receive time will produce a biased measurement or an apparent delay in the receive time. The magnitude of this delay was found to be dependent an several factors including cloud height, cloud optical depth, cloud particle size, particle shape, and receiver held of view. The delay was found to be largest for low-level clouds with particle radii of 3-20 mum, potentially amounting to altimetry biases of tens of cm. Alternate methods for measuring the receive time, such as a simple Gaussian Bt of the return pulse peak reduce the path delay estimates for all cloud conditions. Since GLAS is a dual mode instrument that includes an atmospheric lidar channel, altimeter measurements that are likely to be significantly contaminated by multiple scattering can be identified.