1.1.1 The need for satellite observations
Earth remote sensing, by definition, is the science of acquiring information about the Earth without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing, and applying that information [CCMEO, 2013]. Satellite remote sensing started in 1960s after Sputnik-1, the world first man-made satellite, that was launched into space by the Soviet Union on 4 October 1957. Since then, satellite technology has devel-oped quickly, especially over the last few decades, providing valuable knowl-edge for numerous fields of Earth sciences (for example, geography, oceanogra-phy, glaciology, forestry, agriculture, and hydrology).
Applications of satellite remote sensing focusing on land surface hydrology began with the successful launch of the first Earth Resources Technology Satellite (ERTS-1, later renamed as Landsat-1) on 23 July 1972. It opened a new era for hydrological studies since scientists had a new source of valuable satellite obser-vations useful for researches of hydrological dynamics and processes [Pietroniro and Prowse, 2002]. Compared to hydrological data provided by in situ measure-ments, satellite remote sensing observations have obvious advantages. First, in situ measurements can provide longer data records, however, they contain lim-ited information about the spatial dynamics of hydrological parameters (surface water, for instance). In contrast, satellite observations can provide not only re-gional, but also global observations at different wavelengths and spatial resolu-tions, with uniform quality and rapid data acquisition. Second, the distribution of the stream gauge network is not uniform, very dense in developed countries (in the US, for example), but very sparse in developing countries (especially in African countries), or even not available in remote areas [Alsdorf et al., 2007].
Satellite observations can access to remote or inaccessible areas, and provide reg-ular observations all over the globe. Third, in situ measurements are often un-available for scientific purposes due to geopolitical reasons, but satellites can ac-quire information over countries that are not willing to share data (that is very important for military applications). For these reasons, nowadays, in situ data
are normally used for calibration and validation of methodologies using land surface satellite observations. Despite all these advantages, satellite remote sens-ing has limitations. A satellite system is very expensive, and it takes a long and complicated process to develop, build, test, launch and operate the system. As a consequence, many countries have their own satellites with the ability to provide observations, but only a few countries can master satellite technology (the US, Russia, Japan, France, German, Italy, India, etc). Different satellites provide dif-ferent spatial resolutions, and the applications can be affected due to large uncer-tainties and errors in the measurements. It is also difficult for data interpretation, especially with radar images.
In land surface satellite remote sensing, there are two types of sensors: passive and active. Depending on the observed surfaces, the interactions between the sur-faces and passive/active sensors will be different. Passive sensors measure sun-light radiation reflected from the Earth surfaces, or thermal radiation naturally emitted from objects. Active sensors create their own electromagnetic energy that is transmitted from the sensor toward the target to be investigated. The incoming energy interacts with the target producing a backscattered energy that is reflected back to the sensor for measurements. Observations acquired by measuring sur-face reflectance from the Sun is known as optical observations. Therefore, optical imagery relies on the Sun, and can be subjected to the presence of clouds. Optical observations are often used for applications related to surface water monitoring, disaster monitoring, inundation mapping, or land surface change detection [Owe et al.,2001]. Active sensors emit radar beams that are not blocked by clouds, and radar imagery can be acquired at any time, under all weather conditions and are independent from the Sun [Owe et al.,2001].
1.1.2 The atmospheric transmission windows
The presence of different types of molecules in the Earth’s atmosphere puts lim-itations on the spectral regions that can be used in land surface remote sensing.
Each molecule has its absorption bands in various parts of the electromagnetic spectrum. As a consequence, only wavelengths outside the main atmospheric ab-sorption bands can be used in remote sensing of the Earth surface, and these
wavelength regions are known as the atmospheric transmission windows. Some wavelengths easily pass through the atmosphere, other wavelengths are blocked or absorbed partly to totally by the atmosphere. Figure1.1shows details of the at-mospheric transmission windows from radio to X-ray wavelengths. High energy wavelengths (Ultraviolet, X-rays, and Gamma-rays) are absorbed by the ozone in the Earth’s upper atmosphere. The visible wavelengths are not blocked by the Earth’s atmosphere, but they can be scattered by dust and clouds. In the in-frared ranges, some wavelengths are blocked by the atmosphere, but others can pass. Similarly in the microwave ranges, the Earth’s atmosphere is transparent at some wavelengths, but not at others. Finally, in the radio ranges, the Earth’s at-mosphere is totally transparent to most of its wavelengths. Based on these atmo-spheric transmission windows, satellite remote sensing instruments are designed to operate in one or more windows where wavelengths can pass through the Earth’s atmosphere to observe the Earth surface. More details of the atmospheric transmission windows can be found in remote sensing books, for exampleElachi and van Zyl[2006].
FIGURE 1.1: The atmospheric transmission windows from radio to X-ray wavelengths. Figure created by NASA (https://earthobservatory.nasa.
gov/).