Remote sensing refers to the measurement of any variable at distance, without making physical contact.
The term is commonly used to refer to the science of Earth data acquisition through aerial sensor
technologies mounted on aircraft or satellites. The sensors are either active or passive. In the first case
they emit a signal, while in the second they measure reflectance from a signal emitted by a natural source,
such as the sun. In this study, we use data generated from sensors recording electromagnetic radiations
reflected or emitted by the Earth surface. Based on the principle that each natural and human object is
characterized by distinct electromagnetic signature, the spectral intensity for a certain wavelength, or the
comparison between different wavelengths, can be used to infer information such as cloud cover, sea
surface temperature, snow/ice detection, vegetation cover, and land /water boundaries (Kennedy et al.
2009). The sensors are mounted on a satellite and record analogous data over a specific segment of the
electromagnetic spectrum, referred to as the “channel” or “band.” Sensor type and capability depend on
the initial mission of the satellite and the technological state at the time of its conception. The signal
recorded by the satellite is transmitted to ground stations that process and store it. The stations are spread
across the globe and are each responsible for a certain area. Before their use to extract spatial
information, the raw images need a series of prior corrections to take into account geographic distortions,
and contamination due to atmospheric scattering (aerosol, ozone, water vapor, etc.), cloud cover and
cloud shadow, and others.
The four main properties of satellite imagery are the following:
- Temporal resolution / frequency: time interval between the different paths of a satellite over the same
portion of the globe. It ranges from 30 minutes to weeks or months. Images from successive paths
can be aggregated into composite images of lower frequency to reduce cloud and cloud shadow
contamination.
- Spatial resolution: size of the smallest unit that can be detected by the sensor. It is determined by the
Instantaneous Field of View (IFOV) of the sensor – defined as their angular cone of visibility - and
its altitude at a particular place and moment in time. Although the pixel size in an image is related to
the IFOV, they are not strictly identical – the square area represented by a pixel is a useful
approximation of what is typically a Gaussian response function in the IFOV.
- Spectral resolution: smallest wavelength interval that can be distinguished by the sensor. If some
classes like water and vegetation can be differentiated with only broad wavelength ranges, others like
rocks need a much finer resolution. (Canada Center for Remote Sensing 2003)
- Radiometric resolution: corresponds to the number of bits used to code for the magnitude of the
electromagnetic energy in binary format.