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4.2.6.1.2 Determination derived state variables 
As mentioned above, variables which are calculated from other state variables are assembled here. On 
the level represented in DFD 5.1.2 can be found the course and speed over ground or through water, in 
any one particular process. It should be made clear that these variables (i.e. amount and direction of 
movement in the particular reference system) are closely linked with one another in functional terms. 
This is also recognisable from the fact that these “value pairs” have to be based on the same state vari 
ables (see DFD 5.1.2.1/5.1.2.2). 
Data flow diagram 5.1.2.1 shows two fundamental methods of determining course and speed over 
ground: 
• Determining of amount and direction of the current movement by means of time-related deriva 
tion of the position information 
• Calculation of CMGx and SOGx from the components of speed In the longitudinal and trans 
verse reaction of the ship. 
DFD 5.1.2.4: The diagram shows, as already mentioned above, the process of dead reckoning for de 
termining the current ship position from a starting position and the course and speed. It should be borne 
In mind that course and speed over ground - although necessary here in principle - are not always avail 
able (especially since these values have to be determined by a method which is independent of the 
controlling navigation method). If this is the case, the course and speed through water are used to de 
termine the position. This naturally leads to a greater level of uncertainty with regard to the position 
determined by the process. In the event of a short-term failure of the controlling navigation method, it is 
nevertheless more stable to change over to dead reckoning since in this way - in contrast to the selec 
tion of an alternative position sensor - the continuity of the track control process is ensured (there is no 
“jump” in position). In practice, there exist three alternatives for implementation of the process of dead 
reckoning: 
1. Starting dead reckoning by the user. The starting position for the process may in this case ei 
ther be manually input (alphanumeric/graphic) or the current position of a position sensor is 
transferred. 
2. Automatic start of dead reckoning after sensor failure. In this case the “last valid position” of the 
failed sensor is adopted as a starting value. The question remains as to what the last valid 
sensor position is. Normally only the “valid” flag of the sensor is evaluated for this purpose. 
Thus uncertainty remains whether this datum is not the “first invalid position” of the sensor. 
3. Start of dead reckoning at the beginning of automatic track control with the sensor position valid 
at the time. This method fulfils the minimum requirements for automatic track control, i.e. 
monitoring of the controlling navigation method by a second independent method. Here the 
difficulty arises that the position determined In this way becomes less accurate with increasing 
duration of automatic track control. Thus a constant plausibility test for the process of dead 
reckoning and in certain cases a restart of the process are necessary.No full text available for this image