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A1. For arguments and assurance evidence to be available which show that the risks associated with deploying any software used in a safety related ATS system are tolerable.
NOTE: An. denotes a Regulatory (Assurance) Goal and An.n denotes a Regulatory (Assurance) Sub-Goal.
However it is necessary to decompose this Goal into sub-goals that are meaningful regulatory statements that can be comprehended by the Regulatee.
The decomposition can be assisted through an understanding of the goal for software behaviour:
G2. To ensure that any software used in a system only behaves in a manner that is safe and has been predicted.
Another way of saying this is that since the tolerability of the risks associated with deploying systems containing software is established during the system safety process, then the main safety goal for software is to implement those safety requirements allocated to software by the system safety process completely and correctly and to ensure that the implementation of non safety functions does not have an adverse effect on safety.
Therefore the main software safety goal may be divided into three sub-goals; these are:
G2.1 To ensure that the software safety requirements are valid system safety requirements and are necessary and sufficient to achieve the risk tolerability
G2.2 To ensure that safety requirements are implemented completely and correctly
G2.3 To ensure that the implementation contains no functions which have an adverse impact on the safety of the system
In order to be assured that the risk of deploying software is tolerable (goal A1) the arguments and assurance evidence must show that safety goals G2.1, G2.2 and G2.3 have been met. For the assurance evidence to be acceptable it must meet the following criteria:
a) It can be demonstrated that the evidence was collected from the processes and products to which it relates.
b) It has not been altered without the alterations and their justification being made visible.
c) It is available for inspection.
These criteria can be met by the retention and maintenance of all arguments and assurance evidence data and all data used to generate the evidence. They are expressed in the configuration consistency goal for software safety assurance:
A1.1 To ensure that the arguments and evidence, for the safety of the software in the system context, are from: a known executable version of the software, a known range of configuration data and a known set of software products, data and descriptions that have been used in the production of that version (ref 1709)
Assurance that Goal G2.1 has been met is obtained by assuring that software safety requirements are valid and remain valid during software implementation, as expressed in the Validity of Safety Requirements goal for software safety assurance:
A2.1 To ensure that arguments and evidence are available which show that the Software Safety requirements correctly state what is necessary and sufficient to achieve tolerable safety, in the system context (ref 1705)
NOTE: This includes requirements to control hazards identified during implementation.
Assurance that Goal G2.2 has been met is obtained by assuring that the requirements have been correctly and completely implemented as expressed in the Satisfaction of Safety Requirements goal for software safety assurance:
A2.2 To ensure that arguments and evidence are available, which shows that the software satisfies its safety requirements (ref 1706)
However, during the software development process, functions may be introduced which have repercussions on the safety of the ATS system, these will need to be assessed and if necessary, new or changed safety requirements will have to be generated. Otherwise the software safety requirements would no longer be complete and correct.
In order to be assured that the software safety requirements remain complete and correct and, consequently, that objective G2.1 is met for all stages of the development, the software safety requirements must be traceable to the implementation and vice versa. This is expressed in the Traceability of Safety Requirements goal for software safety assurance:
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本文链接地址：CAP 670 Air Traffic Services Safety Requirements 1(89)
