Rudder 
Engine 
ectories of other 
of radar images. 
ır image located 
oreign vessel. A 
sults in estimates 
of these vessels. 
f the position and 
third task of the 
the vessel. The 
put to the control 
1d engine throttle 
is supervised by 
The results of the 
olor display. The 
m by means of a 
enting or overrid- 
of the integrated 
Nledge deposited 
ymputer. These 
dashed boxes in 
for the electronic 
dels for the own 
(nowledge bases 
navigator himself 
oses. For exam- 
route knowledge 
to the navigation system. 
The navigation system has been tested success- 
fully in numerous trials. The test have been carried 
out at the institute's test vessel "Falke" (16 m, 15 t), 
the measuring ship "Neckar" (30 m , 200 t) and the 
commercial vessel "Neuenstein" (105 m, 1900 t). Re- 
cently a first test on the push tow "L16" (total of 185 
m, 10000 t) has been undertaken. 
3 THE ELECTRONIC CHART 
3.1 Requirements of the electronic chart 
The structure of the electronic chart conforms to the 
distinct requirements within the integrated navigation 
system. The format of the database for the storage 
of the electronic chart is determined mainly by the 
demands of real-time processing. Furthermore, the 
comparison of the chart with the radar image requires 
a specific type of spatial access to the data of the 
electronic chart. This access sets out from a point 
given in absolute coordinates. The database returns 
the nearest objects in the chart and their Euclidian 
distance to these input coordinates. The distance 
is calculated as the minimum of the distances to all 
points on the contour of the spatially distributed chart 
object. Several hundreds of these complex accesses 
are usually necessary for the comparison of one radar 
image with the chart. Therefore, one of the accesses 
has to be accomplished within a few milliseconds. 
The data structure of the database for the chart has 
to be object-oriented and flexible with respect to mod- 
ifications. Object-orientation is necessary for the op- 
timal interpretation and classification of the structures 
in a radar image. Flexibility is an obvious necessity 
for an experimental system, where changes due to 
new findings have to be feasible in a compatible and 
easy manner. 
3.2 Objects in the electronic chart 
Two different coordinate systems are used for the 
chart. One of them is an absolute coordinate system. 
For practical reasons, this system is the German offi- 
cial Gauss-Krüger system, although any similar sys- 
tem can be employed as well. The second coordinate 
system is used for the purpose of fast access to the 
data of the electronic chart. These so-called river 
coordinates employ the river axis as a reference line. 
The river axis is a virtual line along the river defined by 
the river authorities. It ideally consists of straight lines 
and curves of constant radius fitted together without 
bends. The position on the river is given by the kilo- 
meter along the axis and an offset perpendicular to 
the axis. This coordinate system is optimal for the 
purpose of access to objects in the neighborhood of a 
given point. The objects in the database for the elec- 
tronic chart are sorted according to their longitudinal 
65 
  
| | Real Objects | Virtual Objects | 
  
  
  
  
Spirals River Bank Ideal Guiding Line 
Limits of 
Navigable Water 
River Axis 
Polygons | Bight 
Bridge 
Embankment 
Ferry 
Groyne 
Harbour 
Island 
Lock 
Mooring 
Overhead Line 
Point of 
Embarkation 
River Mouth 
Points Buoy Altitude 
Landmark Branch 
Level Station 
Mean Current 
Message 
River Identification 
  
  
  
  
  
Table 1: Objects in the electronic chart 
position in the river. Selecting an object with a known 
position only requires searching the database linearly, 
which can be implemented as a very fast operation. 
A two-dimensional search, on the other hand, would 
be time-consuming and hard to implement under the 
given real-time constraints. 
The classes of objects held in the database are 
listed in table 1. Real and virtual objects are dis- 
tinguished. Real objects are objects visible in the 
waterway such as the river banks, bridges, locks etc., 
whereas virtual objects express information relevant 
primarily for ship guidance, e.g. limits of the navigable 
water, ideal guiding lines, altitude. 
The object classes in table 1 are also sorted ac- 
cording to their structure into objects for points, poly- 
gons and spirals. Point-like objects do not have an 
extent in the chart. Polygon objects represent struc- 
tures in the chart with a limited extent, which can be 
depicted by one or more polygons with a bounded 
number of points. There is a one-to-one correspon- 
dence between point-like as well as polygon objects 
and a single entity in the chart. 
Spiral objects, on the other hand, do not correspond 
to a single entity. An arbitrary number of these ob- 
jects together describe a structure in the chart. This 
structure is oriented along the river, with every object 
equivalent to one interpolation point for the structure. 
Between two interpolation points the distance to the 
river axis is interpolated linearly, resulting in Archime- 
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