Up to this point, it is important to note that as a variety of different sensors available for capturing of RS data, this creates a number of possibilities for data fusion to provide better capabilities for scene interpretation. This is known as "Multi-concept" in Remote Sensing, which basically includes multitemporal, multispectral, multiscale etc.
Retrievability as a concept is worth looking at in this path for the fact that as observations are made with multiple sensors, then they provide more information, with different characteristics being measured.
Besides that, we will consider Multi-concept as an item in this learning path worth of discussion -- if data fusion is to provide better capabilities for scene interpretations.
Finally this learning path will also introduce us to the concept of GEOSS -- which stands for Global Earth Observation System of Systems