The functions of the VLTI are activated under the 4 following specific operational modes:

• Observation Support Mode: provides all the functions to support the observer in defining and preparing an observing program. It is an off-line mode, i.e. it is performed in advance and does not involve hardware functions.
• Observation Mode: provides all the functions to prepare and carry out the observation run, including the reconfiguration of the Interferometer.
• Stand-By Mode: provides all the functions necessary to control and to monitor the status of the Interferometer when no observation is being executed.
• Maintenance Mode: provides all the functions to perform maintenance actions at the Interferometer level, such as internal alignment and calibrations. It includes also testing facilities to assess the full functionality of the system.

 The concept of operational mode is more related to the way the user is dealing with the VLTI, than to the internal state of the VLTI. This becomes evident when the observer is preparing an observation: this task is usually done off-line, at his/her Institute or in Garching, and has no relation at all with the current situation of the VLTI, which might be operating and observing or completely off.