The objective of sensor fusion is to determine that the data from two or more sensors correspond to the same phenomenon. A sensor is an electromechanical device that translates the energy of a physical phenomenon into a signal. When discussing sensor fusion, it is convenient to use the terms physical realm and digital realm.

The phenomena occur in the physical realm. The sensor reacts to the phenomena to produce an electronic signal, either analog or digital. The signal is interpreted into data and it exists in the digital realm. Thus, we would say that the sensor translates a phenomenon in the physical realm into a corresponding data in the digital realm.

There would be no need to think in terms of the digital realm without the Internet. Before the Internet, a signal would exist inside a single isolated machine or a well-defined isolated system. With the advent of the Internet, a single processor can access the signals from any number of sensors. Thus, the challenge of sensor fusion is predicated upon the existence of an Internet, a universal network of networks.

Before the Internet, the association between two or more sensors would only occur within a single well-defined system. The engineers would be able to determine and define the interdependencies of the sensor signals in the design of the system. With the Internet, we can now access the signals of several sensors on an ad hoc basis. This capability is useless, however, without some means to determine the interdependencies of the sensors.