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\begin{document}

\title{ICESat-2 Photon Classification: Finding Signal Photons in the ATL03
Geolocated Photon Data Product}



\author[1]{Anita Brenner}%
\author[2]{Tom Neumann}%
\author[3]{Jeffrey Lee}%
\affil[1]{Sigma Space Corporation}%
\affil[2]{NASA Goddard Space Flight Ctr.}%
\affil[3]{Stinger Ghaffarian Technologies, Inc., NASA Goddard Space Flight Center}%


\vspace{-1em}



  
  \date{May 22, 2024}


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\begin{abstract}
ICESat-2 carries NASA's next-generation laser altimeter, ATLAS,
(Advanced Topographic Laser Altimeter System), designed to measure
changes in ice sheet height, sea ice freeboard, and vegetation canopy
height. ATLAS contains a photon-counting lidar which transmits green
(532-nm) pulses at 10kHz. Each pulse is split into 3 pairs of beams (one
strong and one weak). Approximately 1014photons per pulse travel from
ATLAS through the atmosphere to reflect off the Earth's surface. Some
return back into the ATLAS telescope where they are recorded. Photons
from sunlight and instrument noise at the same wavelength are also
recorded. The flight software time tags all photons within a 500m to 6
km range window and generates histograms. Using the histograms, it
selects a telemetry window which varies from 20m over flat surfaces to
hundreds of meters over rougher terrain. ATL03 contains the time, height
(relative to the WGS-84 ellipsoid), latitude and longitude of every
photon within the telemetry window. The basic challenge is to determine
which of these photons were reflected off the surface. We have developed
an algorithm that identifies these signal photons and assigns a
confidence level (low, medium, or high) to each signal photon based on
the signal to noise ratio. We present an overview of the signal
identification algorithm and show the results on actual ICESat-2 data
over ice sheet, sea ice, vegetated, and water surfaces. Higher level
ATLAS products work with aggregations of the photons in order to
determine the ellipsoidal height of the Earth, canopy height and
structure, and other quantities of geophysical interest.%
\end{abstract}%



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