2-cm GPS altimetry over Crater Lake

Differences in electromagnetic path delay, between direct Global Positionin g System (CPS) signals and those reflected from the surface of Crater Lake, have led to lake surface height estimates with 2-cm precision in 1 second. This is the first high-precision altimetric demonstration with CPS from su fficient altitude (approximate to 480 m) to probe fundamental experimental errors, which bear on future air- and spaceborne passive CPS altimetry. It also serves as the first demonstration of a new approach to altimetric remo te sensing in the coastal region, an area that is poorly measured by conven tional radar altimetry. Time-series analyses suggest that tropospheric and thermal noise fluctuations dominate the altimetric error in this experiment . Estimating the differential delay from several simultaneously visible sat ellites may enable tropospheric error estimation and correction. Thermal no ise on the reflected signal will be reduced with fully polarimetric observa tions arid larger antenna apertures.