# Integrating Inland and Coastal Water Quality Data for Actionable Knowledge
## 3 Overview of Observational Methods and Platforms
### Remote Sensing
#### 3.3.2 Active Remote Sensing  
Some of the weaknesses of optical images can be compensated by active remote sensing observation. Synthetic Aperture Radar (SAR) has been used successfully to quantify physical properties of seawater such as biogenic films, phytoplankton scums, and to detect oil spills and bathymetric features in shallow waters [68; 69]. SAR provides high spatial resolution (10 to 20 m) measurements of the backscatter signal of the ocean surface with a low frequency (typically 12 to 35 days). The use of SAR can be traced back to 1992, with the launch of ERS-1. Since then, several technological advances, both in spatial and temporal resolution, have been achieved making the use of radar remote sensing an additional tool to complement optical remote sensing for the monitoring of the health of a water body. Active sensors measuring through the microwave portion of the electromagnetic spectrum enable the acquisition of images day and night regardless of weather conditions. Lately, LiDAR (Light Detection and Ranging) method has attracted a lot of attention [70; 71; 72; 73], as it can overcome some limitations of optical imagers only day-time observations of near-surface features under clear sky conditions. Airborne LiDAR has been used extensively in coastal waters for studying fish schools [74], and phytoplankton layers [75], and geometric changes in shorelines and sedimentary environments. Studies using spaceborne LiDAR (CALIOP) have shown that it is possible to estimate the backscattering coefficient at 532 nm over the global ocean [76; 77; 78]. Using the ICESat-2 spaceborne LiDAR, [PERSON] et al. [79] retrieved two-dimensional (depth-resolved) distributions of particulate backscattering (at 532 nm), linking to particulate organic carbon (POC) and phytoplankton carbon biomass distributions in the coastal Antarctic ocean. Originally designed for monitoring the ocean and successively for mapping ice sheets and sea ice, the recent advances in radar altimetry technology have enhanced the resolutions and accuracy of altimetric data making them an interesting alternative for inland water measurements [80]. Radar satellite altimetry provides information on the Earth's surface dynamical features of heigh sea and its changes over time. Crucial for the regional and global monitoring of sea level rise impact, radar altimeter measurements provides useful information for mapping a wide range of parameters including sea surface height, sea surface wind speed and significant wave height, but also vertical land motion estimating, bathymetric computations, and marine geoid modeling [81; 82]. Altimetry data have also demonstrate to improve the estimation of lake ecological status and river discharge when merged with optical satellite imagery [83; 84] although measurements for small-medium rivers remains challenging due to the poor spatial sampling and temporal resolution of satellite altimetry [85].  
Such information from new active sensors, including airborne and space-based LiDAR, would help improve understanding of water quality and ecological changes in inland and coastal waters and would complement measurements from high spectral, spatial, and temporal imaging technologies.