# Detection of Surface Water and Floods with Multispectral Satellites
## 2 Multispectral Satellite Remote Sensing for Flooded Area and Wetland  
### 2.0.1 Inundation Mapping  
Since the mid-1970s, several optical satellites have been monitoring the Earth's surface. The Landsat program represents the oldest and longest-running satellite mission, operating since July 1972, when Landsat 1 was first launched. Over the years, new satellites equipped with different instruments were placed in orbit, and the most recent mission, represented by the Landsat 9 satellite, was recently launched in September 2021. Satellites still in operation are Landsat 7, Landsat 8, and the aforementioned Landsat 9. Optical sensors onboard these satellites capture images at different spectral (i.e., wavelengths and bandwidths) and spatial resolutions, with a common revisit time of 16 days.  
The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument operates on the Terra and Aqua satellites, respectively launched in December 1999 and May 2002, and captures images in 36 spectral bands. Despite its coarse spatial resolution (see Table 1), MODIS has a daily revisit time, which is one of the most important factors when data obtained during the flood event are needed for flood extent delineation [35].  
Sentinel-2 is a constellation formed by two twin satellites, S-2A and S-2B, respectively launched in June 2015 and March 2017, that together ensure a revisit time of 5 days. A multispectral instrument (MSI) mounted on Sentinel-2 provides images in 12 spectral bands (see Table 1). Together with Landsat, Sentinel-2 constitutes the family of medium-high spatial resolution multispectral satellites.  
The National Oceanic and Atmospheric Administration's (NOAA) Advanced Very High-Resolution Radiometers (AVHRR) sensor has been equipped on polar orbiting weather satellites since 1978. Since then, different instruments onboard several satellites have been imaging the Earth. The last version, AVHRR/3, currently operating on NOAA-15, -18, and -19 satellites, acquires daily images in six spectral bands (from the visible to the infrared portions of the electromagnetic spectrum) with a spatial resolution of 1.1 km, making it possible to perform flood mapping at very large scales.  
These are examples of passive remote sensing programs offering free-of-charge data. In this work, we reviewed the use of medium-high-spatial resolution imagery for flooded area and wetland inundation mapping. In addition, as reviewed by [PERSON] et al. [40], Sentinel, Landsat, and MODIS are the most significant Earth observation satellite missions, based on the remote sensing impact index (RSIF) built by the authors, that will likely still be on the front line for environmental monitoring. For these reasons, we focused on the Landsat, MODIS, and Sentinel-2 satellites in our review.  
Table 1 illustrates the main spectral and spatial resolution characteristics of Landsat 4- and 5-Thematic Mapper (TM), Landsat 7-Enhanced Thematic Mapper Plus (ETM+), Landsat 8-Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS), Sentinel-2 MSI, and Terra-Aqua MODIS instruments, with a particular focus on spectral bands commonly used in surface water monitoring studies. In addition, references and links to the data portals from which it is possible to visualize and freely download multispectral images from these satellites are provided in the same table.