2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202189A reference frame consists of: a reference space, a time scale and a spatial metric. The geometric structure induced by these objects in spacetime is developed. The existence of a class of spatial metrics that are rigid, have free mobility and can be derived as a slight deformation of the radar metric, is shown.30 pagesGeneral Relativity and Quantum Cosmologytext