Page 329 - 360.revista de Alta Velocidad - Nº 5

Control and Maintenance of Railways through Satellites.
Is it possible?

• Optical satellites are characterized by their high spatial resolution and the range of the
electromagnetic spectrum they capture, being influenced by the presence of clouds in the
imaging.
• Satellites with synthetic aperture radar technology are not conditioned by the presence of
clouds, since their operation is based on the emission of electromagnetic pulses and the
capture of the rebound.

Both images of optical satellites and those obtained by radar are subjected to rectification
processes such as orthorectification or interferometry.

In addition to classification based on the function they perform, another differentiation in
the typology of satellites can be made depending on their orbit. The orbit of a satellite is
characterized, firstly, by the centre of rotation (geocentric or heliocentric). In the geocentric
satellites, the orbit is classified according to the height, where HEO (High Earth Orbit) exist,
above the 35,786 km; MEO, between 20,000 and 35,786 km; and LEO, below 20,000 km (Gaetano
Hadad & Blanco Arias, 2009). The latter usually get better resolutions, so they will be those
studied in the present work. LEO satellites are divided into orbits synchronized with the Sun,
or polar.
The type of orbit and its altitude will characterize the period of passage through the same
point, and therefore the frequency of capture of images at the same point that they will have.
There is a large amount of satellite data applicable to different fields, given the large amount of
information they generate, although there is still some difficulty in interpreting and processing
such information. Some of the possible applications are described below.
One of the areas where significant progress has been made is the control of pollutants produced
in industrial sectors, allowing the monitoring of columns of contaminants produced in farms
and fires. In addition, by observing the concentration of different gases, it is possible to
support air quality prediction models, including the generation of new models, and to estimate
anthropogenic emissions, thanks to the ability of these satellites to measure the number of
molecules of certain gases, like NOx, SO2 in scarce emissions and NOx, CO and CH4 in larger
emissions, existing between the Earth's surface and the satellite. These measurements allow
the creation of maps of temporal evolutions of emissions, from daily to annual scales, and the
monitoring of concentrations (Duncan et al., 2014).

To develop these tasks, agencies like NASA have developed satellites such as Terra, Aura or
GOES. However, this use has certain limitations, such as the lack of information regarding
the vertical distribution and composition of these columns. However, they do not seem to be
applicable in our field, since there are no large concentrations of pollutants in the railway
system.
The use of satellites to control soil moisture is mainly focused on improving weather forecasts,
drought control and the possibility of associated fires, predicting possible floods and improving
water use on farms (NASA, 2004b). This satellite typology uses synthetic aperture radar (SAR)
systems for the detection of moisture from the surface layers of the earth. SARs are so-called
active sensors, which record high spatial resolution images (Sillerico, Marchamalo, Rejas, &
Martínez, 2010). These radars measure the electromagnetic radiation of any body that is at a
given temperature (Universidad Politécnica de Cataluña, 2009). This radiation will depend on
the composition of the soil, so applied techniques of interferometry can obtain the soil moisture
and salinity of the oceans. SAR interferometry is an applicable technique in different areas. It
involves the use of a satellite to record two or more images of the same area at different time
points. When comparing the different images, it is possible to detect any changes that may
have occurred in that period of time (AIRBUS, 2012).The most important satellites in this field
are the SMAP of NASA and the SMOS of the ESA. They differ from satellites with radar technology
because of their ability to take pictures in different spectra.

International Congress on High-speed Rail: Technologies and Long Term Impacts - Ciudad Real (Spain) - 25th anniversary Madrid-Sevilla corridor 327