CoAuthors

Suzanne Dickinson (Polar Sciene Center APL/Univ. of Wash., USA); David Hancock (NASA Goddard Space Flight Center, USA); Leeanne Roberts (NASA Goddard Space Flight Center, USA); John Robbins (john.w.robbins@nasa.gov, USA)

Abstract:
The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) has been developed primarily to measure the height of the Earth’s ice at high spatial resolution but it also provides satellite ocean altimetry unlike any other. To achieve high resolution it uses the Advanced Topographic Laser Altimeter System (ATLAS), a photon-counting, multi-beam lidar pulsing at 10 kHz. At the speed of the spacecraft, each beam of ATLAS illuminates 15 m patch of the surface every 0.7 m of along-track distance. Given the low reflectance of the ocean surface, of all the photons detected by ATLAS, on the order of one photon per pulse returns from the ocean surface. ATLAS determines the apparent height of the reflecting surface for each one of these photons along with the apparent height of a lower density of noise photons. Averaged over along-track distance these heights form a histogram of heights reminiscent of the waveforms of other radar (e.g., CryoSat-2) and analog lidar (e.g., ICESat-1) satellite altimeters. However, we have adopted a different philosophy in processing the ICESat-2 data over the ocean. Rather than retracking a wave form from a footprint, we treat every photon height as an individual point measurement of surface height averaging less than a meter apart, but with a x-y location uncertainty on the order of 10 m. By a histogram trimming method, we determine which of these heights over an adaptively chosen ocean segment length (typically 7-km) are from true surface reflected photons versus noise photons. The resultant received height histogram is deconvolved with an instrument impulse response (IIR) histogram representing the height uncertainty largely associated with the lidar transmit pulse width. This produces an ocean surface height histogram that, with its first four moments, is the primary ICESat-2 ocean product. In addition, the along-track distance heights of the histogram comprising the received histogram are preserved and form a cloud that clearly resolves ocean surface waves. We analyze the spatial series of surface photons heights to characterize surface waves and calculate the correlation of photon return rate and surface height that constitutes the EM sea state bias in the mean sea surface height (SSH). This presentation will demonstrate some of the capabilities of ICESat-2 over the ocean and discuss our initial efforts at calibration and validation.