When a phenomenon is observed this is always done with some kind of sensor. Even if the observation is made by a person without using any tool we can still say that this person was the sensor who observed the phenomenon. The result of an observation is nearly always depending on what sensor used and on how that sensor is used, therefore knowing about the used sensor for all your observations is important for interpretation of the observation results.

Each feature type can be observed by different types of sensors delivering observation results of the same structure but having different sensor properties. As features are instances of feature types Sensors are instances of sensor types. Each sensor is assigned to a specific domain. Each observation stored for a feature can be associated with a specific sensor that is in the same or a superior domain as the feature. The Application Server allows to store data for sensor itself, for specific time spans and fore each observation performed with that sensor.