Abstract
The ice, cloud, and land elevation satellite-2 (ICESat-2) is providing global elevation measurements to the science community. ICESat-2 measures the height of the Earth’s surface using a photon counting laser altimeter, ATLAS (advanced topographic laser altimetry system). As a photon counting system, the number of reflected photons per shot, or radiometry, is a function primarily of the transmitted laser energy, solar elevation, surface reflectance, and atmospheric scattering and attenuation. In this paper, we explore the relationship between detected scattering and attenuation in the atmosphere against the observed radiometry for three general forest types, as well as the radiometry as a function of day versus night. Through this analysis, we found that ATLAS strong beam radiometry exceeds the pre-launch design cases for boreal and tropical forests but underestimates the predicted radiometry over temperate forests by approximately half a photon. The weak beams, in contrast, exceed all pre-launch conditions by a factor of two to six over all forest types. We also observe that the signal radiometry from day acquisitions is lower than night acquisitions by 10% and 40% for the strong and weak beams, respectively. This research also found that the detection ratio between each beam-pair was lower than the predicted 4:1 values. This research also presents the concept of ICESat-2 radiometric profiles; these profiles provide a path for calculating vegetation structure. The results from this study are intended to be informative and perhaps serve as a benchmark for filtering or analysis of the ATL08 data products over vegetated surfaces.
Highlights
In 2018, NASA launched the space-based laser altimetry mission ice, cloud, and land elevation satellite-2 (ICESat-2)
The ground radiometry was the number of labeled generated based on the ATL08 photon classifications to allow a comparison between the ground and canopy radiometries
The ground radiometry was the number of labeled ground photons per outgoing shot averaged across each 100 m ATL08 segment
Summary
In 2018, NASA launched the space-based laser altimetry mission ice, cloud, and land elevation satellite-2 (ICESat-2) This mission was the result of the priority recommendation of the National Research Council Decadal Survey in 2007 to dedicate a mission to continue elevation time series in the cryosphere. Depending upon the surface reflectance and atmospheric conditions, up to 16 photons per outgoing shot could be detected on the 4 × 4 detector array for the strong beam and 4 photons on the 2 × 2 detector array for the weak beam. These maximum photon detections are indicative of saturation conditions until the detector satisfies the dead-time recovery period. The size of each ICESat-2 footprint on the ground has been determined to range between 10–12 m in diameter [3,4]
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