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2.3 Target Selection Procedure
It is of prime importance to establish a procedure for target selection in order to avoid an overloaded collision avoidance display in congested waterways as shown in Fig. 5. Relevant selection criteria to display collision danger sectors are; DCPAi(L) < Nx DCPAlim(N is an integer multiply) and 0 < TCPAi(T) < TCPAlim. An additional selection parameter can be derived from Eq. (1) when Vo > VTi
which states that no PPC exits if (2) is satisfied for λ= I. λ > 1 reflects a safety margin. The HSC navigator is therefore in control of the collision risk when surrounding vessels are of displacement hull type with limited speed capability.
Fig.5 |
VCASS display in congested waterways if no target prioritizing (except TCPAi > 0) is applied. |
Fig.6 |
VCASS display in congested waterways if target prioritizing is applied. Here, DCPAi > 3 x 0,3 n. miles, 0 < TCPAi < 3 min., V0 = 38 kts., VTi = l2kts.,and λ= 1.5. |
2.4 Algorithm and Program Structure
The algorithm and actual display of the VCASS are programmed with C++ by using the Borland C++ Builder development environment. It provides a suite of rapid application development design tools, including programming wizards and application and form templates.
Vessel position and movement data are broadcasted from SEATRAS [5] through Ethernet and forms the key inputs to VCASS. The anti-collision parameters and co-ordinates of the collision danger region to all targets are calculated at every time step according to the following scheme [3] that represents the core algorithm of VCASS:
・read UDP data string containing traffic data
・get total number of vessels (ShipNum) and length of string (StringLength) and set user-defined target selection parameters (DCPAlim, TCPAlim and λ)
・for(int i = 0; i < StringLength; i++){
○get vessel data:
identification number, Xo, XTi, V0, VTi
}
●for(int j 0;j < ShipNum;j++){
○calculate relative velocity vector VRi
○calculate TCPAi, and DCPAi,
○calculate Xki; k = {A,B,C,D}
○if target i satisfy the selection criteria:
DCPAi < Nx DCPAlim and 0 < TCPAi < TCPAlim
and α0 < λsin-1(VTi/V0); V0 > VTi
・display collision danger region
}
Fig.7 |
Simulation architecture of VCASS. The simulator adopts User Datagram Protocol(UDP) to ensure a robust simulation system. |
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