Using Time-Series Satellite Imagery to Detect Artificial Light At Night: the Case of Luojia-1 and International Space Station

Artificial light at night (ALAN) changes throughout the night and the year within an area. It is crucial to monitor the time-series characteristics of ALAN. In this study, we investigated the changes in ALAN in Hong Kong over time from two medium-resolution remote sensing products: Luojia 1-01 (LJ-1) and the International Space Station (ISS). We validated these images with in-situ night sky brightness measurements. The results show that both the calibrated LJ-1 and the uncalibrated ISS images achieved similar agreements with the in-situ data (R=0.73 and R=0.83, respectively). LJ-1 and ISS images are useful for evaluating ALAN changes over the years and throughout the night, respectively. In early nights (before 23:00), commercial areas were brighter, while in late nights (after 23:00), port facilities and airports were brighter. Finally, we highlight the importance of color composition, time-series observations within one night, and multi-angle observations within minutes for ALAN monitoring.