## 2 Review of the PlanetScope (PS) Sensors  
The PS constellation currently includes three generations of satellites (i.e., doves) relevant to remote sensing: Dove Classic, Dove-R, and SuperDoves (Table 1). Each generation comprises multiple \"flocks\", with a flock consisting of a group of satellites launched and placed into similar orbit around the same time. All satellites are in sun-synchronous orbit at 475 km, a 98-degree inclination, and capture data \(\pm\)81.5\({}^{\circ}\), depending on the season. Equator crossing times are between 9:30 and 11:30 am local time. Images are provided in 12-bit (digital number) and 16-bit (radiance) [11]. The first flocks of Dove Classic satellites were launched in 2016, with a payload that included a 4-band (RGB-NIR) 'PS2' sensor with a ground sampling distance (GSD) of about 3.7 m (Table 1). The historical archive of Dove Classic (Dove-C) imagery dates back to 2016; however, during the demonstration phase, PS smallsats were launched primarily from the International Space Station (ISS) into an orbit with a 52-degree inclination at approximately 375 km [12]. The PS data archived from 2016 and early 2017 mainly comprise these ISS platforms before the constellation of near-polar sun-synchronous orbits was built. Therefore, coverage during the demonstration period includes gaps. A shortcoming of the Dove Classic sensors for remote sensing research is the lack of separation in the spectral response for the RGB bands (Figure 2a), making it difficult to use imagery from this oldest generation for precise spectral analyses (e.g., reliable time series of NDVI or Green NDVI). Starting in 2018, a second generation of sensors with improved spectral characteristics, known as Dove-R, were added to the constellation [13]. The spectral resolution of these 'PS2.SD' sensors carried onboard the Dove-R platforms was also 4-band (RGB-NIR), but band separation and definition was improved (Figure 2b). While the spectral clarity enhancements of this second generation are beneficial for the remote sensing community, the shift in band placement between the PS2 (Dove Classic) and PS2.SD (Dove-R) sensors (Figure 2) makes integration of these two products for spectral analyses difficult [10]. In April 2019, the first flock of the third generation PS sensors, SuperDoves, launched. SuperDoves carry a 'PSB.SD' sensor, which can capture eight bands (Figure 2c). Four of these bands (Blue, Green II, Red, and NIR) are generally compatible with the Dove-R \begin{table}
\begin{tabular}{c c c c c c} \hline
**Sensor** & **Instrument** & **No. of Satellites** & **Launch Epoch** & **Swath Size** & **GSD * (m)** & **Spatial Res. **(m)** \\ \hline Dove Classic & PS2 & \(\sim\)36 & 2016–2017 & \(24\times 8\) km & 3.7 & 3.9 \\ Dove-R & PS2.SD & \(\sim\)150 & 2017–2018 & \(24\times 16\) km & 3.7 & 3.7 \\ SuperDove & PSB.SD & \(\sim\)36 & 2018– & \(32.5\times 19.6\) km & 3.7 & 3–12 \\ \hline \end{tabular}
* Ground sampling distance: average at reference altitude of 475 km. ** Orthorectified.  
\end{table} 
Table 1: PlanetScope sensor characteristics. PS2.SD bands, although band widths and placements are not exactly the same (Figure 2b,c). The four additional bands are located in the visible portion of the spectrum (Coastal Blue, Green, Yellow, Red-Edge). At this time, only five PSB.SD bands are available for download: Blue, Green II, Red, Red-Edge, NIR. Importantly, these bands are interoperable with Sentinel-2 bands [14]. The full set of eight bands is anticipated soon. Due to the small PS image swaths (Table 1), mosaicking is often needed and sometimes requires combining multiple sensor generations, especially if a mosaic consisting of images captured on the same day is desired. This task is not straightforward given the differences in band placement and channel width across the generations detailed above (Figure 2). Combining untransformed Dove Classic with Dove-R or SuperDove images for spectral analyses is not advised, which means post-processing of the images is needed.