On‐Orbit Radiometric Performance on ICESat‐2

Abstract

The Ice, Cloud, and land Elevation Satellite -2 (ICESat-2) observatory launched on September 15, 2018 with objectives to measure changes in ice sheet elevation, monitor sea ice freeboard, and enable a global assessment of vegetation and canopy height (Markus et al., 2017). ICESat-2 uses green laser light emitted by the Advanced Topographic Laser Altimeter System (ATLAS) as the basis for elevation measurements between 88° north and south latitudes (Martino et al., 2019). ATLAS provides the data necessary for ground processing to calculate the round-trip photon time-of-flight from the observatory to Earth and back again. These components are combined with the laser beam pointing vectors (Bae & Webb, 2017) and the position of the observatory in space (Luthcke et al., 2019) to generate the Global Geolocated Photon data product (ATL03) (Neumann et al., 2020a(Neumann et al., , 2020b) which provides a unique latitude, longitude, and elevation for each photon event telemetered for ground processing and an initial discrimination between a band of signal photons (emitted by ATLAS and reflected by Earth) and background photons (emitted by the sun in the same part of the electromagnetic spectrum). Schematics of the ATLAS transmitter and receiver are found in Neumann et al., 2019, Figures 2 and 3 respectively.The ATLAS telescope collects incoming light and is fiber-coupled to filters which only transmit light of the same wavelength as emitted by ATLAS (532 nm ± 0.15) (Neumann et al., 2019). After passing through these filters, light for a given spot is distributed across the elements of a multi-element photomultiplier tube to convert optical energy to electrical energy to be time tagged (a photon event) for further analysis. The ATL03 product is organized by ground tracks, which refer to the relative location of the 6 beams on the ground. Since the spacecraft orientation changes every ∼9 months in order to optimize the position of the solar array relative to the sun, ATLAS strong and weak spots change their relative orientation (from left to right). We organize our analyses by ATLAS spots which do not change with spacecraft orientation. ATLAS spots are numbered 1-6 indicating laser illumination from the instrument field of view, where 1, 3, and 5 are "strong" spots and have four times the energy of spots 2, 4, and 6, referred to as "weak" spots. Each of the three ATLAS strong spots have 16 detection elements (pixels) which supply 16 individual timing channels. Similarly, each of the three ATLAS weak spots have four independent pixels and timing channels (Martino Abstract NASA's ICESat-2 mission measures Earth's elevation with the Advanced Topographic Laser Altimeter System (ATLAS), a 6-beam photon-counting laser altimeter. The Global Geolocated Photon data product (ATL03) is the primary source of photon information used by surface-type-specific higherlevel products, along with the Atmospheric Layer Characteristics product (ATL09). ATL03 provides time-tagged, geolocated photon heights referenced to the ellipsoid and a paramete...

Radiometric Assessment of ICESat-2 over Vegetated Surfaces

“…Beam 3 had a consistently 10-15% lower radiometric response than beam 1 for all but one of our study areas in no-snow conditions (Congo was the outlier). The underperformance of beam 3 confirms similar observations reported by [10]. Among the weak beams, beam 4 consistently outperformed beams 2 and 6, which was a pattern not reported in [10].…”

“…This phenomenon, however, is also observed over Antarctica ice sheets. In [10], they reported an approximate ~10% (strong beam) and ~40% (weak beam) reduction in signal strength during the Antarctica summer months.…”

“…This photon count criterion was determined to be optimal rate for cryospheric studies during pre-launch analysis. Thus far, the on-orbit radiometry over land ice has maintained the value between 8-9 photons/shot for the strong beams and approximately 2 photons/shot for the weak beams over Antarctica [10].…”

“…Similarly, the outgoing energy of the strong beams are four times the outgoing energy of the weak beams [9]. During the on-orbit commissioning phase of the satellite, the transmit laser energy was adjusted such that the strong beams of ATLAS were detecting on average 9 signal photons per pulse over the highly reflective ice sheets [10]. In [9] they also report strong beam radiometry rates over Antarctica of approximately 9 photons per shot for beams 1 and 5 and 7.5 photons per shot for beam 3.…”

“…This photon count criterion was determined to be optimal rate for cryospheric studies during pre-launch analysis. Thus far, the on-orbit radiometry over land ice has maintained the value between 8-9 photons/shot for the strong beams and approximately 2 photons/shot for the weak beams over Antarctica [10]. Over land, pre-launch studies were estimated, and early on-orbit analysis proved that the laser energy settings optimized for ice sheet reflectance would provide roughly one photon per pulse from both the ground and vegetation strong beam returns due to the lower reflectance.…”