Geoscience Reference
In-Depth Information
1969. Camille was one of the most powerful cyclones ever to hit the US. The alert,
which was raised thanks to the information provided by satellite observations,
undoubtedly saved the lives of hundreds of people. The advantage of satellites is
linked to the high frequency of their observations and also to the fact that they
monitor the spatial dynamics of meteorological phenomena, in other words they
highlight any changes that occur in the weather, good or bad [BAD 95]. Since the
1970s it has become possible to produce a spatialization of thermal data and cloud
cover so that an accurate weather forecast can be made. It is also believed that these
satellites have increased the length of a weak forecast period (of 5 days) by up to 2
days. Satellites used in climatology are most commonly used for the following: for
monitoring cloud cover (by using the ISCCP program), for estimating precipitation
levels from METEOSAT data, for monitoring drought, and for monitoring of the
phenomenon known as El-Niño.
3.2.2.3.
From the satellite to the image: methods and importance
Waves are ranked in relation to their wavelength in a vacuum. In the
electromagnetic spectrum, the human eye is only able to see a limited range of
wavelengths, visible waves range from 0.4-0.7 μm. Within the electromagnetic
spectrum the shortest wavelengths are the ultraviolet waves and the longest
wavelengths include infrared waves, radio waves, etc. All of these different
wavelengths occur whenever there is a specific energy level in place.
Meteorological satellites measure a fraction of the waves, which have a short
wavelength, in order to find out the albedo of the surfaces. In addition, these
satellites measure the geographical information necessary for analyzing the radiation
that exists on a given part of the Earth's surface. The second wave type measured is
the infrared waves because the frequency at which these waves are emitted depends
on the temperature of a particular surface (band width 10-12μm). Other
wavelengths are important for obtaining information about the Earth's surface or
atmosphere: near-infrared waves are used to monitor vegetation, medium infrared
waves are used to measure absorption by atmospheric water, and microwaves are
used to measure the water content of water droplets that are found in clouds, etc.
The satellite detectors (radiometers) are sensitive to the energy of the wave that
they are measuring: calibrating the detectors means that it is possible to record
values of certain wavelengths or a particular region of the electromagnetic spectrum.
This idea of calibration defines the purpose of a channel, or it can also be used to
define the spectral resolution of a sensor. Depending on the distance from the
satellite to the surface of the Earth and how often the data is collected by the
satellite's radiometer, the distance for any two successive measurements will be
different: this is known as the sensor's primary spatial resolution, and it is here that
the pixel (picture element) is defined. The pixel is the smallest element of a satellite
image and corresponds to a radiometric measurement. For example, the satellite
SPOT-4 has a spatial resolution of 10 m in panchromatic mode, and 20 m in
multiband mode. The LANDSAT-TM satellite has a resolution of 30 m, whereas the
METEOSAT-3 satellite has a resolution that ranges from 2.5-10 km.
