# Integrating Inland and Coastal Water Quality Data for Actionable Knowledge
## 3 Overview of Observational Methods and Platforms
### Remote Sensing  
#### 3.3.1 Passive Remote Sensing  
Space-based, optical observations provide a unique capability to obtain spatially resolved water quality information over large geographical scales and at relatively high temporal frequencies [37; 38; 39; 40]. The use of satellite data for mapping water quality parameters in inland and coastal systems can be traced back to the early 1970s, when the Landsat satellite program captured the first observation of ocean color from space. Over recent years, the exploitation of satellite data has increased significantly, as the spatial, spectral, and radiometric characteristics of satellites with ocean-color capability have improved considerably. Space agencies and, increasingly, private companies operate several satellite missions for aquatic and terrestrial environmental monitoring. Remote sensing systems provide calibrated top-of-atmosphere radiance or reflectance with varying spectral, temporal, and spatial sampling resolutions. This radiometric signal enables a direct retrieval of optically active water quality properties, such as phytoplankton, nonalgal particles (or turbidity), and colored dissolved organic matter, as well as physical properties, such as surface temperature [18]. Polar-orbiting, multispectral ocean color satellite missions provide high temporal revisit data (typically global coverage at 1 to 3 days) at moderate spatial resolutions (0.3 to 1 km), and, as such, these sensors are well suited to the operational monitoring of large lakes and coastal waters. Ocean color satellite band settings are optimized specifically for aquatic applications, and, although challenges remain, significant research efforts have been invested in the development of improved algorithms for the atmospheric correction and retrieval of water quality information from such data [41; 42; 43; 44; 45; 46; 47; 48; 49]. The design of new ocean color sensors with extended spectral range (UV to SWIR) and hyper-spectral capabilities, such as PACE (Plankton, Aerosol, Cloud, ocean Ecosystem), and improved temporal resolution (multiple observations per day), such as the geosynchronous GLIMR (Geosynchronous Littoral Imaging and Monitoring Radiometer), will enable the identification and tracking of advanced aquatic biological and ecological indicators to guide sustainable management in inland and coastal waters [50; 51]. For application to inland and coastal waters, measurements from ocean color sensors are often complemented by satellite optical imagery from high spatial resolution sensors primarily designed for terrestrial observations (such as the Landsat-8/OLI--Operational Land Imager (OLI) and the Sentinel-2 MSI--MultiSpectral Instrument). Although the design of these sensors is not optimal for the observation of inland and coastal waters, with coarser spectral, radiometric, and temporal resolutions compared with heritage and existing ocean color missions (e.g., VIIRS--Visible Infrared Imaging Radiometer Suite), their higher spatial resolution (10 to 30 m), enables to observe small-scale variability in optically active water quality parameters, such as chlorophyll-\(a\) and total suspended matter in waters. The recent emergence of micro- and nano-satellite constellations (e.g., PlanetLabs Doves; [52]) that provide data at even higher spatial (\(<\)5 m) and temporal resolutions (daily) hold considerable promise for water quality monitoring, particularly for smaller waterbodies, although considerable research is required to evaluate their radiometric stability [53] and develop robust approaches to atmospheric correction and retrieval of water quality parameters.