11.3 Actions performed only once
O
At the start of creating each random feasible solution in the population, the current state of the airport must be used. This state is changed when the state of a gate in the main simulation changes from available to occupied or vice versa.
All the actions described in this section are performed only once in each execution of the optimisation process. These actions are not performed for every population member/solution or every iteration in the optimisation. They are performed in preparation for the process of creating the population of feasible solutions. The current state of the airport is assessed and recorded. The recorded values of the applicable variables are stored in variables that are referred to as “original”.
After each solution in the population has been generated, the variables rep-resenting the current state of the airport in the main simulation must be reset to the original values. The variables that must be reset include the arriving and departing flights with which the flight-to-gate assignment process must start in each solution, the flight with which to end the assignment in each solution and whether each gate at the airport is available or occupied at the start of the op-timisation. For example, the state of a gate (available or occupied) will change over time while creating each solution in the population. However, after that solution is created the state of every gate is again set to the original state, i.e.
11.3 Actions performed only once
how it actually is in the main simulation model that is put on hold while the op-timisation process is performed. This is done in order to create the next solution in the population while considering the correct circumstances at the airport.
11.3.1 Selecting the first arriving flight in the run
O
When the metaheuristic optimisation is started, the flight-to-gate schedule must be calculated for the first arriving flight that have not yet parked at a gate, up to 50 flights after that. The first arriving flight not yet at a gate can be the first flight in the simulation run, which is the case only when the optimisation process is executed for the first time. The first flight in the run can also be the flight arriving 25 flights after the flight for which the optimisation process was executed the last time. Furthermore, this first flight in the run can be a flight that cannot park at the gate to which it was assigned due to the fact that the gate is still occupied by the flight previously assigned to it. Finally, the first flight in the run can be the first flight not yet at a gate, after the gate that was still occupied when it was supposed to be available for use by another aircraft, becomes available.
If the first flight in the optimisation process is the first flight of the simulation run, the variable representing the first flight that must be assigned to a gate is
11.3 Actions performed only once
assigned the value one. If it is not the first flight, it must be determined whether the execution of the metaheuristic optimisation was fired by an arriving flight, or by a departing flight leaving a gate that was supposed to be available and thus caused the execution of the optimisation process. If the optimisation process was activated by an arriving flight, the flight with which to start the assignment schedule calculation is this arriving flight. If the optimisation process was caused by a departing flight, the arriving flight with which to start the calculation of the flight-to-gate assignment schedule is the first flight having a scheduled arrival time larger than or equal to the current simulation time.
Current simulation
time First arriving
flight Arriving
flights
Time
06:00 06:15 06:30 06:45 07:00 07:15 07:30
Figure 11.3: The first arriving flight to consider in the optimisation process
Figure 11.3 illustrates the first arriving flight with which the optimisation process starts. This is the first flight arriving after the simulation time at which the execution of optimisation process is started.
11.3 Actions performed only once
11.3.2 Selecting the first departing flight in the run
O
When starting the execution of the optimisation process, the flight with the earliest departure time, of which the departure time is bigger than or equal to the current simulation time, is the first flight to take into consideration in the optimisation process for departing. If it is the first execution of the optimisation process, this flight is the one with the overall earliest departure time, since no flights have arrived yet. If it is not the first execution, the departing flight with which to start is the first flight having a scheduled departure time plus a buffer time of bigger than or equal to the current simulation time. This buffer time is five minutes and is included to account for the time it takes the aircraft to taxi away from the gate and to allow a little time for delay so that the optimisation process will not need to be executed too often like when an aircraft leaves the gate at which it was parked only a few seconds late.
In Figure11.4, the first departing flight that will be considered in the optimi-sation process is pointed out. This is the first flight departing after the simulation time at which the execution of the optimisation process is started.
11.3 Actions performed only once
06:00 06:15 06:30 06:45 07:00 07:15 07:30
Figure 11.4: The first departing flight to consider in the optimisation process
11.3.3 Assigning the last flight in the run
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As stated before, every time the optimisation process is performed, flights are assigned to gates from the first flight that has not yet parked at a gate to 50 flights after that. The optimisation process thus plans 50 flights ahead. If no delays occur, this is repeated when the 25th flight after the previous first flight of the run arrives. Thus, the process can be described as shown in Figure 11.5.
The process shown in the figure is for an ideal situation, with no delays. If, for example the 32nd flight is delayed due to the fact that the gate to which it was assigned is still occupied by the previous aircraft, the previous calculated
11.3 Actions performed only once
Execution of the metaheuristic
Flight number
0 25 50 75 100 125 150
Figure 11.5: Execution of the optimisation process
flight-to-gate assignment process will not be optimal anymore. The optimisation process must thus be repeated to find the new near-optimal solution. Then the process will be as depicted in Figure 11.6.
Execution of the metaheuristic
Flight number
0 25 32 57 82 107 132 157
Figure 11.6: Execution of the optimisation process with delays
After the first arriving flight in the flight-to-gate schedule has been deter-mined, the flight at which the model should stop assigning gates, i.e. the last flight, can be calculated by adding 50 to the value of the flight number of the first flight in the optimisation process. If the value of the number of this last flight is bigger than the total number of flights in the model, then the last flight in the model becomes the last flight in the calculation of the flight-to-gate assignment schedule. The reason for planning 50 flights ahead is because each flight has an influence on a number of flights arriving after it. It was observed that when this number of flights is set equal to 50, the time it takes to execute the optimisation process is feasible and the results are satisfactory.
11.3 Actions performed only once
11.3.4 Test for available gates
O
Every gate in the model is assessed. If the gate is currently occupied in the simulation model, it is recorded as unavailable in the optimisation process.
However, if the departure time of the aircraft currently at that gate plus the buffer time of five minutes has been reached, i.e. the aircraft is not supposed to be at the gate anymore even though it is, the gate is recorded as available in the optimisation process. This is done since another variable keeps track of those gates that are supposed to be available, but are occupied. Also, it is necessary that these gates appear to be available, since the aircraft by which they are occupied were supposed to have left. In other words, the departure times of these aircraft have already been reached and the departing flight that the optimisation process will start with has a departure time greater than the departure times of those other aircraft. Thus, if the gates that they are occupying are not recorded as available, they will never become available in this execution of the optimisation process since the time in the optimisation process will already have passed the departure times of those flights.