Global navigations satellite systems (GNSS) like GPS, Galileo, GLONASS and Beidou are used for monitoring of civil structures and natural phenomena since the 1980s. However, most of the implementations were either short term or realized in a very controlled environment. In this article we report about the experiences of more than 20 years of GNSS based monitoring of a large scale object. In 1999 the GNSS monitoring system was developed and installed at a deep-seated gravitational slope movement in Austria and it is still operating. The system gradually evolved during the years of operation to maintain a reliable operation and to take new developments of information and communication technology (ICT) into account. This included the change from radio communication to UTMS, the maintenance of the hardware (revision of GNSS stations, revision of solar collectors, exchange of batteries) and maintenance of software (Firmware updates of GNSS receiver, software updates, change-over to new operation systems). Especially the constant changes in the used software system results in maintenance time, which is often underestimated.

From these experiences we derive lessons learned which can be a guideline for other long term GNSS based monitoring systems.