Page 330 - 360.revista de Alta Velocidad - Nº 5

Mayoral, Juan. Jiménez, Pablo Lorenzo. Fernández-Sánchez, Gonzalo. Jardí, Ignacio.

Flexibilization of regulations that prevented high resolution images from space has allowed
resolutions of up to 0.30 m to be reached in panchromatic mode (Satellite Imaging Corporation,
2015), characterized by an observation made in a single spectral band located in the visible
part of the spectrum electromagnetic (Spot Image, 2005).

The applications of the high resolution images in the railway maintenance can vary, from the
control of the existing vegetation near the track, until the observation of changes of colour in
the infrastructure that can indicate different problems.
Within this field there is a very wide offer, so that only cite some with different resolutions and
operated by different companies:
• WorldView-3: resolution of 0.30 m.

• WorldView-2: resolution 0.46 m.
• Pleiades: 0.50 m resolution.
• Spot 6/7: resolution of 1,50 m.

• KazEOSat-1: resolution of 1.00 m.
• Kompsat-3: resolution of 0.70 m.
• RapidEye: resolution of 5m.

Satellites capable of detecting surface movements on slopes are based on the same technologies
as soil moisture detection, i.e. SAR satellites, differing from these at the wavelength at which
they emit. While satellites whose objective is to measure soil moisture emit in wavelengths
around 21 cm and a frequency of 1.4 GHz, band L of the electromagnetic spectrum, the
satellites used in the detection of surface movements work in variable wavelengths depending
on the type of terrain and in the environment of 3.1 cm and 9.6 GHz (c and x bands of the
electromagnetic spectrum).
From the techniques of interferometry, distance data are obtained between the satellite and
the terrain surface, calculated by measuring time and time lags. This technique, also known
as InSAR, of combination of 2 interferometric passages allows to measure phase differences in
two satellite steps in the same zone. These differences can be due to the different position
between the two satellite trajectories or because of a displacement of the observed area, so
this technique allows the detection of movements and deformations of the Earth's surface.
The signal received by the sensor is influenced by parameters such as atmosphere and surface
movement (Sillerico, Marchamalo, Rejas, & Martínez, 2010).
Radar images are black and white, although colour images can be produced by combining 3
independent images of different dates, forming a composite image (European Space Agency,
2010).

From the InSAR technology, the differential SAR or D-InSAR interferometry is generated, which
allows the generation of maps of the terrain displacement, from “n” steps of the satellite
through the same zone. For the processing D-InSAR different techniques already exist. The most
used is to obtain displacements from the areas away from the study area, which are assumed
to be free of movement and with constant geometry. The DINSAR technique allows to eliminate
some of the parameters that influenced INSAR interfeormetry (Sillerico, Marchamalo, Rejas, &
Martínez, 2010).

The resolution of this type of images depends on the size of the images taken, varying this in
each one of the satellites. Some of the satellites available today are:

1. Terrasar-X: its resolution in the X and Y planes vary between 0.25 m and 3 m, while their
accuracy in the measurement of subsidence can reach 3.1 mm.

328 360.revista de alta velocidad