Image Processing Reference
- the moment of acquisition is the measurement that corresponds to a date set by
the clock. Usually, the error made for this data is ignored;
- the moment of transmission when the data is available to be fused, for example;
- the processing time that corresponds to the time interval between the acquisi-
tion moment and the transmission moment. This time may or may not be constant
depending on which processes are applied;
- the sampling period, which corresponds to the time interval between two acqui-
sitions. Usually, this period is constant.
Figure 11.1. Time related quantities associated with a sensor
The essential time constraints are the acquisition frequency of measurements and
the delay caused by the processing time. If the frequency is high enough and if the
delay can be neglected, we can consider that all of the information that can be used
in the fusion system is constantly available. It is worth noting that in most fusion
applications, temporal constraints are neglected. They are dealt with mostly in military
applications for detecting and tracking targets [ANK 01, APP 98, PAO 94], in mobile
robotics [KOS 93, PRU 99] and in the context of intelligent vehicles [ETE 94, JOU 99,
TRA 93, TRA 94].
Fusion uses data provided by different sources with temporal characteristics that
may be different. In the context of temporal fusion, information provided by the same
source may also be fused, but at different times.
11.3.1. Fusion of distinct sources
In the ideal case, the sources of information are synchronous (same period and
same acquisition time) and provide their information with the same processing time.
The data available can then be immediately fused.
With real applications, the data is rarely synchronous and cannot be fused directly.
Here are the major causes: