  Most of cosmological observables are light-propagated. I will present
coordinates adapted to the propagation of null-like signals as observed by a
geodesic observer. These "geodesic light-cone (GLC) coordinates" are general,
adapted to calculations in inhomogeneous geometries, and their properties make
them useful for a large spectrum of applications, from the estimation of the
distance-redshift relation, the average on our past light cone, the effect of
the large-scale structure on the Hubble diagram, to weak lensing calculations.
