Fusion of Non-Simultaneous Elements of Information: Temporal Fusion - Information Fusion in Signal and Image Processing - page 237

by the fusion process based on criteria that have to be optimized. It is also possible
to define a forgetting coefficient which makes it possible to minimize or eliminate the
effect of older measurements.
11.3.3. Temporal registration
Sensors observe quantities that vary with time. In order to be able to fuse the data,
we have to ensure that it is obtained from measurements that correspond to the same
acquisition time. If this is not the case, a temporal registration mechanism has to be
implemented in order for the measurements which appear to be acquired at the same
time. In multi-sensor fusion, this mechanism will only be implemented if the tem-
poral characteristics of the sensors are too different. In single sensor fusion, the data
always has to be registered. This registration mechanism is the major characteristic of
temporal data fusion.
11.4. Dating measurements
Whether in single source or multi-source fusion, evaluating variable data requires
taking into account the time and particularly the moment when the measurement was
taken. Evaluating the acquisition times of measurements is one of the major problems
of temporal fusion [ALL 01]. In the most favorable case, the sensors are physically
close to one another and the acquisition of measurements is supervised by the same
electronic device. The measurements and their associated data are then dated by a
single clock common to all of the sensors. On the other hand, in the most common
case, the sensors operate independently and all have their own clocks. In this case,
it is necessary to account for this multiplicity of clocks. Here are the most common
solutions.
Registering the clocks. At certain times, a master clock sends a registration signal to
all of the slave clocks so that they may be synchronized. Between two synchronization
times, the delays between the clocks are considered to be small and to cause only few
mistakes in the measurements.
Common clock. The sensors have the possibility of operating with the help of an exter-
nal clock which will be the common clock. This requires for the clock signal to travel
through the entire device.
It is sometimes very difficult to determine the acquisition date, particularly when
the measurement provided by the sensor is evaluated from a time measurement. This
is the case, for example, when using an ultrasound sensor for which the distance mea-
surement is determined from the time of flight of the wave traveling from the sensor's
transmitter, to the object being studied and finally to the sensor's receiver. The acqui-
sition date is considered equal to either the wave's emission date or its reception date.