The objective of STARS task 5.1 is to review the existing navigation augmentation systems and services in terms of performances achieved and application environments, and to analyse the experimentations and projects already performed in railway applications related to the use of these systems and services. This report aims to draw the state of the art of existing monitoring system (as used in aeronautics) and past experiences of railways when using such concepts. The focus only concerns GNSS-based positioning systems and do not pretend to present the global systems developed to insure the complete safety level of railway signalling.