32 
Further state variables for the environment are given in Table 2 
Designation 
State 
variables 
Reference system 
Explanation 
Unit 
UTC 
turc 
see definition UTC 
Coordinated world time 
s 
Wind relative 
Wxftei 
WyRei 
Ship 
Direction of movement of the sur 
rounding air mass 
m/s 
Depth 
dpthz 
Water 
Distance to ground in z direction 
(below the ship reference point) 
m 
Humidity 
tlAbs 
Absolute air humidity 
g/m a 
Barometric 
pressure 
p 
Air pressure 
hPa 
Air tempera 
ture 
temper 
Air temperature 
deg Cel 
sius 
Water tem 
perature 
tempwater 
Water temperature 
deg Cel 
sius 
Draught 
drghtz 
Water 
Largest distance in z direction to the 
hull 
m 
Air draught 
airdrghtz 
Water 
Largest distance in negative z direc 
tion to the hull (incl. superstructures) 
m 
Table 2: State variables to describe the environment 
4.1.2 Defining the variables used in the INS 
The state variables introduced in the previous section permit an adequate, clear (not over-determined), 
dynamic state description of the physical system ‘ship’. These state variables may not however be 
measured directly in most cases but must be determined indirectly from other variables. For example, 
determining the transverse speed at the reference point from a cross speed which was measured at 
another point on the ship is possible only with the rate of turn (y B ). 
In the following, therefore, is compiled a further catalogue of variables which are generally used in an 
INS. These variables may often be determined directly by measuring devices and used as input infor 
mation by the individual INS functionalities. Further variables are derived from such “primary” variables 
(eg speed over ground). Primary and derived variables do however in their entirety represent an over 
determination of the system ship, which in its application may lead to an inconsistent state description. 
In practice, spatial distances between the measuring points are mostly ignored - as for instance in the 
measurement of position and speed - which, as a result of the increasing accuracy of measurement of 
sensors, may likewise lead to an inconsistent state description in the INS. 
In order to check the consistency of derived variables, all variables not measured directly should there 
fore be reproduced in the state variables established in the previous chapter, for control purposes. In 
this way it is ensured that all variables required for derivation (primary and derived) are determined. In 
Figure 7 an example of such a display in schematic form is represented. The variables used in INS are 
given in Table 3.