399.300 
  
  
  
  
Figure 2: Example of an electronic chart 
dian spirals. This type of interpolation is especially 
appropriate for a river and results in a small number 
of interpolation points. 
The appearance of objects represented in the chart 
usually depends on the water level. This dependance 
is linearly interpolated in the chart by assigning a 
slope of the embankment to every spiral or polygon 
element. The value assigned can also be interpreted 
as a measure of accuracy of the element for image 
matching purposes. 
An example of a hardcopy of the electronic chart 
display is shown in figure 2. Two harbors and a bridge 
can be easily distinguished. In the river, the river axis 
and the limits of the navigable water are plotted as 
solid respectively dashed-dotted lines. 
More details about the implementation of the chart 
are given in [4]. 
4 MATCHING IMAGE DATA AND ELECTRONIC 
CHART 
Within this section, a least squares technique for 
matching images and the electronic chart is ex- 
plained. Input to the matching algorithm are predicted 
values of the ship's position and heading. Corrections 
to these predicted values are computed by minimiz- 
ing a weighted sum of distances between image and 
electronic chart. These corrections and their accu- 
racies are used as measurements in a Kalman filter 
described in section 6. 
The matching technique is used for matching radar 
as well as laser scanner images with the electronic 
chart. Furthermore, stationary targets detected by a 
multiple-target tracking algorithm can be matched to 
landmarks in the electronic chart by the same tech- 
nique. 
4.1 Matching radar echo contours and elec- 
tronic chart 
The process of matching the radar contours and the 
electronic chart starts with an estimate of position and 
heading based on previous matching steps. For the 
current radar image, a prediction of the ship's position 
and heading has been computed according to the 
mathematical model of the own ship. Also actual 
measurements from other sensors like a gyroscope 
or GPS may have been processed by a Kalman filter 
algorithm, so this information is already integrated 
in the initial values of the matching process. The 
initial values consist of the global coordinates s,(. 
and s,(_) of the position of the imaging sensor and 
the global heading v... of the ship. The acquisition of 
one radar image lasts some 2.3 seconds, the time for 
one complete turn of the radar antenna. As the ship is 
in general not stationary, the coordinates obtained by 
the matching process are assigned to the time when 
the antenna beam is pointing in the ship's heading 
direction. As mentioned before, the purpose of the 
matching process is the computation of corrections 
As} 
As = As; (1) 
Av* 
to the initial position and heading. 
The standard radars used on commercial vessels 
generate a map-like image of the surroundings. The 
antenna beam has a horizontal beam width of 0.5 to 
1.0 degrees, the vertical beam width is about 15 to 20 
degrees. The antenna performs one complete turn 
within 2.3 seconds. During the turn, radar pulses of 
50 ns length are sent out, the pulses are repeated 
with a maximum frequency of 3 kHz. Thus the raw 
radar image consists of about 7000 radial beams. 
The echo signals received by the antenna are digi- 
tized, filtered and transferred into the main memory 
of the computer. When filtering the raw image, a re- 
duction of the number of rays is also performed. The 
radar data available in the computer are made up as 
a binary image consisting of 720 rays with 256 radial 
pixels. The radial quantization can be chosen as 3 or 
6 m, thus resulting in a maximum sensing distance of 
768 or 1536 m. Currently the interface to the radar 
is redesigned to enable a maximum number of 4096 
radial pixels. 
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