Hardware/software design requirements planning: Part 3 - Performance requirements analysis
Environmental Requirements Analysis
One of the most fundamental questions in system development involves the system boundary. We must be able to unequivocally determine whether any particular item is in the system or not in the system.
If it is not in the system, it is in the system environment. If an item is in the system environment, it is either important to the system or not. If it is not, we may disregard it in an effort to simplify the system development. If it is important to the system, we must define the relationship to the system as an environmental influence.
We may categorize all system environmental influences in the five following classes:
1. Natural environment—Space, time, and the natural elements such as atmospheric pressure, temperature, and so forth. This environment is, of course, a function of the locale and can be very different from that with which we are familiar in our immediate surroundings on Earth, as in the case of Mars or the Moon.
2. Hostile systems environment—Systems under the control of others that are operated specifically to counter, degrade, or destroy the system under consideration.
3. Noncooperative environment—Systems that are not operated for the purpose of degrading the system under consideration but have that effect unintentionally.
4. Cooperative systems environment—Systems not part of the system under consideration that interact in some planned way. Generally, these influences are actually addressed as interfaces between the systems rather than environmental conditions because there is a person from the other system with whom we may cooperate to control the influences.
5. Induced environment—Composed of influences that would not exist but for the presence of the system. These influences are commonly initiated by energy sources within the system that interact with the natural environment to produce new environmental effects.
As noted above, cooperative environmental influences can be more successfully treated as system interfaces. Hostile and noncooperative influences can be characterized through the identification of threats to system success and the results joined with the natural environmental effects.
The induced environment is best understood through seeking out system energy sources and determining if those sources will interact with the natural environment in ways that could be detrimental to the system. The natural environment is defined in standards for every conceivable parameter for Earth, space, and some other heavenly bodies.
The challenge to the system engineer is to isolate on those parameters that are important and those that are not, and then to select parameter ranges that are reasonable for those parameters that will have an impact on our system under development. The union of the results of all of these analyses form the system environmental requirements. It is not adequate to stop at this point in the analysis, however.
Systems are composed of many things that we can arrange in a family hierarchy. Items in this hierarchy that are physically integrated in at least some portions of their system operational use, such as an aircraft in an aircraft system or a tank in a ground combat system, can be referred to as end items.
We will find that these end items in operational use will have to be used in one or more environments influenced by the system environment but, in some cases, modified by elements of the system. For example, an aircraft will have to be used on the ground, in the air through a wide range of speed and altitude, and in hangers.
These are different environments that we can characterize as subsets of the system environment definition. The best way to do this is to first define the system process in terms of some finite number of physical analogs of system operation. We may then map the system end items to these process steps at some level of indenture.
Next, we must determine the natural environmental subsets influencing each process step. This forms a set of environmental vectors in three-space. In those cases where a particular end item is used in more than one process, we will have to apply some rule for determination of the aggregate effect of the environments in those different process steps. The rule may be worst case or some other one. This technique is called environmental use profiling.
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