## 3 Materials and Methods
### Satellite Data and Processing  
Remote sensing imagery from the PlanetScope constellation, Sentinel-2A/B, and Landsat 8 satellites were employed in our ML modeling approach to infer a power plant's operational status through the identification of emitted visible water vapor plumes. A sample image of a power plant from each satellite is shown in Figure 3. A description of each satellite and imagery processing step is provided below. PlanetScope. Planet Labs' PlanetScope satellite constellation consists of approximately 130 individual satellites, called \"Doves\", with the first launch of this constellation in 2014 [35]. The PlanetScope constellation provides daily revisits with an equatorial crossing time between 7:30 and 11:30 a.m. (Planet, 2022). Each PlanetScope satellite images the Earth's surface in the blue, green, red, and near-infrared (NIR) wavelengths (\(\sim\)450-880 nm), with the exception of the \"SuperDove\" instrument which includes additional wavelengths [36; 37]. The swath width is 25 km. PlanetScope PSScenes were downloaded via the Planet Labs API, providing a spatial resolution of \(\sim\)3 m, and including eight additional Usable Data Mask (UDM2) image quality bands. Sentinel-2. The European Space Agency's Sentinel-2 mission comprises two satellites: Sentinel-2A launched in 2015 and Sentinel-2B launched in 2017 [38; 39]. Each Sentinel-2 satellite has a 10-day revisit time with a 5-day combined revisit and an equatorial crossing time of \(\sim\)10:30 a.m. [40]. Both satellites are equipped with a multispectral (MSI) instrument that provides 13 spectral band measurements, blue to shortwave infrared (SWIR) wavelengths and (\(\sim\)442 nm to \(\sim\)2202 nm) reflected radiance and, depending on the band, provides measurements at 10 m-60 m spatial resolution [39; 40]. The swath width is 290 km. We downloaded Harmonized Sentinel-2A/B Level-1C Top of Atmosphere (TOA) products from Google Earth Engine (GEE). Landsat 8. The Landsat 8 mission was built and launched by NASA and is operated by US Geological Survey (USGS) [41; 42]. Landsat 8 was launched in 2013 and has a 16-day revisit with an equatorial crossing time of 10 a.m. (\(+/-\) 15 min). Landsat 8 is equipped with two instruments: Operational Land Imager (OLI) and the Thermal Infrared Sensor (TIRS). OLI has nine spectral bands ranging from \(\sim\)430 nm to \(\sim\)1380 nm with a spatial resolution of 30 m. TIRS has two bands spanning 10,600 to 12,500 nm with a spatial resolution of 100 m that is upscaled to 30 m. [43]. The swath width is 185 km. We downloaded the Landsat 8 Collection 2, Tier 1 TOA from GEE. Each satellite has multiple bands with different spatial resolutions. Lower-resolution bands were upsampled to match the highest-resolution band for that satellite. Each band also has a different distribution of pixel values, which can cause instability during model  
Figure 3: The Ninghai power station in China as seen from Landsat 8 at 30 m spatial resolution **(left)**, Sentinel-2 at 10 m spatial resolution **(center)**, and PlanetScope at 3 m spatial resolution **(right)**. These images only represent the visible bands (red, green, and blue). training. For this reason, we standardized the radiance values in each band to a mean of 0.5 and a standard deviation of 0.25, placing most pixel values between 0 and 1.