Instrument Radiometric Correction of Laser Signals and Background Noise for ICESat-2 Photon-Counting Lidar

Recently, an increasing number of studies have progressively explored the radiometric properties of ICE, Cloud, and land Elevation Satellite-2 (ICESat-2), enabling this satellite and its payload to provide support for not only geometric but also radiometric applications. However, due to potential changes in the optical throughput and electronics of the instrument during flight, the essential radiometric corrections are needed for quantitative radiometric applications. In this study, using ICESat-2 signal and noise data on icesheet and desert surfaces, which have a relatively stable reflectance and clear sky, a radiometric correction method is proposed to describe the changes in instrument parameters, i.e., to estimate how much the signal and noise should be exactly scaled. With instrument and environmental parameters provided by the ICESat-2 ATL04/ATL09 product, signal and noise models are used to calculate the theoretical signals and noise levels. The theoretical predictions are then compared with the actual measured signals and noise results by ICESat-2 to obtain the scale factors (or radiometric correction factors) for signal and noise, respectively. The results indicate that all three photon counting electronics (PCEs) (corresponding to three laser pairs) exhibit very close scale factor values (i.e., ~1.9), i.e., ICESat-2 receives both signal and noise nearly double the expected values. The noise scale factors
$F_{\text {noise}}$
over icesheet are expected to yield the most accurate scale factors because
$F_{\text {noise}}$
will change very little with and without layers. We also analyze the annual average radiometric drift of ICESat-2, which indicates the decreases of ~2.5% (
$F_{\text {signal}}$
) and ~2.2% (
$F_{\text {noise}}$
) from 2019 to 2022. This study demonstrates the consistency of signal and noise in radiation and also helps to understand the radiation closure and data fusion between the passive background noise and active laser signal for a spaceborne lidar.