## Appendix A Overview of the SkySat and PlanetScope Push-Frame Imaging System
### PlanetScope  
PlanetScope constellation consists of around 150 individual nano-satellites named \"Doves\", which continuously image the entire Earth surface in two near-polar orbits of \(\sim\)8\({}^{\circ}\) and \(\sim\)98\({}^{\circ}\) inclinations respectively, without requiring tasking or acquisition planning (Figure 3). Each Dove is a CubeSat of a 3U form factor (i.e., 10 cm \(\times\) 10 cm \(\times\) 30 cm) providing an image with a ground sampling distance of \(\sim\)3.7 m at an altitude of \(\sim\)475 km [2; 25].  
PlanetScope has three generations of imaging sensors referred to as Dove-classic (Dove-C), Dove-R and SuperDove (data not available yet). Each has a different sensor plane configuration and optic, but none has a panchromatic band. Basic characteristics of the PlanetScope satellite constellation and sensor specifications are shown in Figure 3.  
The imaging payload of the Dove-C consists of a telescope coupled to a CCD frame camera (\(6600\times 4400\) pixels) equipped with a Bayer-Mask filter. The upper half of the CCD array has an RGB Bayer pattern filter separating the light wavelength into three spectral bands (Bleu, Green and Red), and the lower half allows only the NIR wavelengths. The RGB half of each frame is then combined with the NIR half of the adjacent frame in order to generate the resulting 4-band image.  
The Dove-R imaging sensor consists of the same type of telescope and frame size, but with a different CCD configuration. The sensor focal-plane consists of four distinct bands stripes arranged vertically along-track with each band stripe having a size of \(6600\times 1100\) pixels. The Bayer pattern filter in the Dove-C was replaced with a butcher-block filter providing 4-band images (Blue, Green, Red and NIR). The final composite image is produced by stacking up four frames ahead and four frames behind the anchor frame.  
PlanetScope imagery is captured continuously in a push-frame mode. Each Dove acquires images in a continuous strip of a single frame image known as \"scene\" with approximately 1 sec interval, resulting in \(\sim\)8% along-track overlap between consecutive scenes (Figure 3).  
The Dove constellations are designed to operate in nadir pointing mode. In addition, the distance between consecutive Doves in both orbits is designed in a way so that the longitudinal progression between them over the rotating Earth leads to a void-less scan of the surface and guarantees the daily coverage. To ensure the void-less surface imaging, there is a variation in the view angle from the strict nadir pointing to \(\pm\) 5 degrees off nadir [6; 25]. This variation in viewing angle results in an across-track overlap \(\sim\)40% between subsequent Doves with a time lag of \(\sim\)1.5 min (Figure 3b).  
PlanetScope constellation also involves another overlap depending on the latitude between acquisitions from ascending and descending orbits with a time lag of \(\sim\)1 h (Figure 3c).  
Planet has three PlanetScope imagery products, referred to as an Ortho Tile product, an Ortho Scene product and a Basic Scene product [51]. Ortho Tiles are multiple ortho-rectified scenes in a single strip that have been merged and resampled according to a defined grid size. Ortho Scenes represent georeferenced single-frame images, corrected for systematic distortions due to the sensor and earth rotation and curvature. The Basic Scene product is a Scaled Top of Atmosphere Radiance (at sensor) and a sensor-corrected product, providing imagery as seen from the spacecraft without correction for any geometric distortions. The product is delivered with RPCs derived from external high resolution DEMs and GCPs extracted from a reference Ortho-images (i.e., using the terrain-dependent approach, Figure 1).  
Planet offers free monthly limited PlanetScope images to the scientific research community through the Education and Research Program [2].