A GENERAL VIEW - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 6

The word real indicates that the reaction of the systems to external events must occur
during their evolution. As a consequence, the system time (internal time) must be
measured using the same time scale used for measuring the time in the controlled
environment (external time).
Although the term real time is frequently used in many application fields, it is subject
to different interpretations, not always correct. Often, people say that a control system
operates in real time if it is able to quickly react to external events. According to this
interpretation, a system is considered to be real-time if it is fast. The term fast ,how-
ever, has a relative meaning and does not capture the main properties that characterize
these types of systems.
In nature, living beings act in real time in their habitat independently of their speed.
For example, the reactions of a turtle to external stimuli coming from its natural habitat
are as effective as those of a cat with respect to its habitat. In fact, although the turtle
is much slower than a cat, in terms of absolute speed, the events that it has to deal with
are proportional to the actions it can coordinate, and this is a necessary condition for
any animal to survive within an environment.
On the contrary, if the environment in which a biological system lives is modified
by introducing events that evolve more rapidly than it can handle, its actions will no
longer be as effective, and the survival of the animal is compromised. Thus, a quick
fly can still be caught by a fly-swatter, a mouse can be captured by a trap, or a cat
can be run down by a speeding car. In these examples, the fly-swatter, the trap, and
the car represent unusual and anomalous events for the animals, out of their range of
capabilities, which can seriously jeopardize their survival. The cartoons in Figure 1.1
schematically illustrate the concept expressed above.
The previous examples show that the concept of time is not an intrinsic property of
a control system, either natural or artificial, but that it is strictly related to the envi-
ronment in which the system operates. It does not make sense to design a real-time
computing system for flight control without considering the timing characteristics of
the aircraft.
As a matter of fact, the Ariane 5 accident occurred because the characteristics of
the launcher were not taken into account in the implementation of the control soft-
ware [Bab97, JM97]. On June 4, 1996, the Ariane 5 launcher ended in a failure 37
seconds after initiation of the flight sequence. At an altitude of about 3,700 meters,
the launcher started deflecting from its correct path, and a few seconds later it was de-
stroyed by its automated self-destruct system. The failure was caused by an operand
error originated in a routine called by the Inertial Reference System for converting
accelerometric data from 64-bit floating point to 16-bit signed integer.