97 
2 Modelling approach 
The modelling method used here must fulfil the following requirements: 
Capacity to support a hierarchical view on systems (in this case considering tasks, functionalities and 
functions) 
High flexibility, e.g. no restrictions with regard to the field of application considered 5 . 
Simple notation, i.e. simple for developers and users to learn and use 
Support for a “functional system view”. 
It was found that the method of structured system analysis ensures excellent fulfilment of these re 
quirements. In particular, this method offers the possibility of representing large and complex systems 
in a readily comprehensible manner. This is achieved by having various aspects of the system repre 
sented by different sub-models. For the purpose of modelling the integrated navigation process the 
following sub-models are used: 
The context diagram represents the system’s reference to its surrounding environment. In this dia 
gram, the interfaces of the system to the “outside world” are specified, i.e. incidents to which the system 
must react, and the corresponding system responses. 
Data flow diagrams represent the input and output information and the functional dependences for 
each functional unit considered. 
The data dictionary specifies in detail the data flows represented in the data flow diagrams. 
The structured system analysis is based on the concept of recursive breakdown of a complex system 
into comprehensible sub-systems. For the purpose of modelling the functionality of the system consid 
ered, the function-orientated breakdown already carried out in part A is used in this case. In the course 
of this, the starting point used is the system as a whole, in a top-down approach. Taking the system 
tasks as a basis, the assigned functionalities are refined for such time until the level of the elementary 
individual functions is reached. The result of this progressive refinement becomes clear in the hierar 
chical structure of the data flow diagrams (see Section 3). This approach is supplemented by an object- 
orientated breakdown of the information flows present in the system. 
21 Context diagram (CD) 
The context diagram forms the starting point of a structured system analysis. The main purpose of this 
diagram is to clearly define the “object” of the analysis and identify the boundary of the system (Figure 
8). In the context diagram, furthermore, are represented the external units with which the system com 
municates. The comparison of the context diagram with the “top level” data flow diagram (Figure 10) 
shows that the “system” delimited in the data flow diagram is represented in the context diagram by one 
process. The number and type of data flows beyond the system boundary must be identical in the two 
diagrams. 
5 A large number of extended analytical methods have originated from the areas of data bank development or 
development of business applications, and are only conditionally applicable to the modelling of other processes.