## 4 Pheromone-Based Alarm Delivery Concept
### Study about the Influence of the Movement Pattern  
From the analysis of the previous simulation set, a conclusion can be drawn in relation to the observed results: the movement pattern of the mobile nodes has a direct influence on the cost of the proposed pheromone-based alarm delivery. This statement is based on the observed differences in the results both of the trail-search and trail-follow mechanisms, when we consider the increase in the number of threats. This fact changes the movement pattern from the random-walk with a number of changes in the movement direction to a random-walk with longer straight line segments. This finding motivated the execution of a third simulation set. In this new set, the previous results using a random-walk for the UAVs' movement pattern with the above described parameters are compared to results achieved by a similar random-walk pattern, but with longer movement steps. The movement steps of this new set are of the size of the maximum length of the pheromone trails, _i.e._, 5 km, instead of the movement step size originally used, _i.e._, 500 m. This setup will prevent the UAVs from changing their direction while flying during the lifetime of a trail, unless they reach the edges of the surveillance area or in case they have to change their direction and move towards a threat just informed by a new alarm. For this simulation set an area of 10 km \(\times\) 10 km is used as scenario, in which two UAVs patrol the area and variations with the same number of threats were tested, _i.e._, 1, 3, 5, and 7 threats. The assessed metric is the same as in the previous simulation set, namely the cost per threat for both parts of the pheromone-based approach, _i.e._, trail-search and trail-follow mechanisms. We check whether the number of threats does not influence the cost results, thus confirming the conclusion about the influence due to the movement pattern. Figure 15 presents the results achieved for the extended movement step compared to the original one. As can be observed in the results achieved with the extended movement step, the number of messages required by both parts of the coordination strategy is almost the same for the different number of threats, within a statistical variation. The point that presented the difference from the simulation variations from three to five threats observed with the original movement step is not  
Figure 15: Results of the cost metric due to the trail-search and trail-follow mechanisms and to the total (trail-search plus trail-follow) for the simulations with 2 Unmanned Aerial Vehicles for both original and extended movement step: (**a**) one threat; (**b**) three threats; (**c**) five threats; (**d**) seven threats. observed in the results with the extended step (from Figure 15b to Figure 15c). This confirms the following: (a) the number of threats does not influence the results; and (b) in fact the observed differences in the previous simulation set are due to the change in the movement pattern that is created by the threat handling in the simulations with the original movement step. Moreover, a decrease in the values of the coefficient of variance can be noticed for both the trail-search and trail-follow mechanisms, which confirms that they became more deterministic in the results for the extended movement step than for the original step. For the total number of messages sent, there is no big difference between the results originally achieved when compared to the ones with the extended movement step. In relation to the variability of the results, Figure 15 shows that it is smaller with the extended step, as values of the coefficient of variance that comes together with the values of the averages are smaller for the extended step, which is consistent with the results for the trail-search and trail-follow mechanisms.