Hi Josep.
Except GRASS you may also try GDAL, OSSIM, ORFEO Toolbox (OTB), wxGIS to orthorectify the imagery.
A few links (translated, originals in Russian):
http://translate.google.ru/translate?hl=ru&sl=ru&tl=en&u=http%3A%2F%2Fgis-lab.info%2Fqa%2Fortho-wxgis.html
http://translate.google.ru/translate?hl=ru&sl=ru&tl=en&u=http%3A%2F%2Fgis-lab.info%2Fqa%2Forbview3-ortho-gdal.html