# Advances in Remote Sensing and Propulsion Systems for Earth Observation Nanosatellites
## 3 Earth Observation
### Inland Water Monitoring
Among the four variants of satellite constellation configurations explored, the authors' proposal of four satellites on four orbits, including two reverse orbits, emerged as the most efficient, achieving full coverage of the area of interest within a concise timeframe. This constellation promises to enhance water quality monitoring by increasing measurement frequency and providing rapid access to comprehensive river section data. Future research will extend to optimizing orbit models for flood risk monitoring, showcasing the potential of satellite constellations in safeguarding inland waters and beyond. [PERSON] et al. [29] assess the radiometric quality of Remote Sensing Reflectance (Rrs) from the commercial PS CubeSat constellation for inland water studies. The experimental evaluation results across various lakes in the Lower Amazon Floodplain show that while PS bands exhibited satisfactory accuracy in turbid lakes, results were less favorable in clear water environments. However, spectral dependence was observed in Rrs retrievals for turbid lakes, with more robust metrics in visible bands, notably the red band (R\({}^{2}\) = 0.83). Moreover, significant correlations were established between water quality parameters and PS Rrs, particularly in the red band (R\({}^{2}\) > 0.74). The authors suggest CubeSats's potential applicability for fine-scale monitoring of inland waters. [PERSON] et al. [30] propose an approach to monitoring fine-scale, subseasonal fluctuations in Arctic-Boreal surface water using high-resolution CubeSat sensors, overcoming the limitations of traditional satellite remote sensing. The authors analyze individual observations of changing surface areas for thousands of lakes in Northern Canada and Alaska, revealing broad-scale declines in lake areas across diverse terrains. This study highlights the significance of CubeSat imagery in capturing short-lived phenomena such as floodplain inundation and aquatic vegetation phenology at a local scale. [PERSON] et al. [31] propose an approach using the PS constellation to track river ice floes, serving as indicators of water surface velocities in northern rivers. Despite the challenges posed by cloud cover and low sun angles in high latitudes, the authors succeed in a significant advancement in remote sensing of river dynamics, providing frequent and expansive coverage compared to current methods. The proposed method is promising for enhancing the understanding of river systems, especially when integrated with satellite measurements of river area, surface height, and slope. [PERSON] et al. [32] propose that small satellites, such as the PS constellation and other satellite constellations like RapidEye (RE), possess immense potential to enhance the mapping of water turbidity in both inland and coastal waters. While the PS constellation consists of CubeSats, the RE constellation comprises larger satellites weighing over 150 kg. These systems boast a high spatial resolution of 3 m and moderate temporal resolution of daily observations, thereby significantly improving the current data sources regarding spatial and temporal coverage. The authors employed the Dark Spectrum Fitting (DSF) atmospheric correction method, ensuring accurate surface reflectance retrieval even at small sub-scene scales. The reflectance data from visible bands exhibited excellent agreement with in situ measurements, particularly in the green and red bands. Furthermore, the remote estimation of water turbidity, primarily derived from the red band, showed promising results comparable to autonomous measurements in various water bodies. The findings suggest that CubeSats from the PS and RE constellations could be a valuable tool for mapping water turbidity in both inland and coastal waters. Table 5 summarizes the methods used and evaluation metrics for each study regarding inland water monitoring.