## 4 Pheromone-Based Alarm Delivery Concept
### Scalability Analysis
In Figure 12, a similar behavior is noticed, but with a remarkable decrease from setups with three threats to those with five threats, which is maintained in setups with seven threats, and, finally, in Figure 13, a remarkable decrease from setups with five threats to those with seven threats can be observed. In the case of Figure 11 this denotes the situation when the setups with one UAV pass from the variation with just one threat to the others with more threats. In the case of Figure 12, the transition for the setups with two UAVs is from the variation with three to more threats, while in Figure 13 the transition for the setup with four UAVs is from the variation with five to the one with seven threats. This observed effect can be explained by the increased mobility that the increasing number of threats imposes in the different setups. Considering the adopted movement model, the UAVs fly according to a random walk pattern, changing their heading angle after each movement step, as explained above. With the parameters used for the performed simulations, during a trail life time the UAVs may change their movement direction up to 10 times. This fact may create a number of pheromone overlapping areas in a trail, which as a result reduces the percentage of the total area that is effectively covered by a trail. Figure 14 depicts an example of trail with overlapping regions (in gray color), while Figure 14b shows another trail without overlap. Notice that for routes of the same size, overlapping regions are counted twice. This fact reduces the amount of sensor nodes covered by trails with overlapping regions compared to those without overlap. When the UAVs receive an alarm and are requested to handle a threat, they perform a straight line movement from their current positions to the position in which the threat was detected. This movement  
Figure 14: Area covered by a trail: (**a**) with overlap (parts colored in gray); (**b**) without overlap. is like the one presented in Figure 14b, thus creating trails without overlap and increasing the percentage of the area covered by a pheromone trail. In the setups with 1 UAV, for the variations with one threat the UAV will be performing its random movement, following a movement such as the one exemplified in Figure 14a. However, for the variations with more threats, after having received the first alarm the UAV will fly straight to the threat, as the example in Figure 14b, thus increasing the pheromone trail size, which increases the probability of an alarm finding a trail while performing a trail-search, thus reducing in average the number of messages used by this mechanism. In many cases new alarms informing about other new threats will arrive within or close to the time window in which the UAV is already moving towards the most current threat to handle it. In this case, the trail will not present overlaps, or less overlaps compared to the trail left by a UAV performing the ordinary random-walk. For the setups with two and four UAVs a similar reasoning can be performed, but for a number of threats that exceeds the number of UAVs, which means that the UAVs will be receiving new alarms while going to handle or just after handling a previous one. In these cases, as they have moved in straight line, their trails have no or less overlaps and are then easier to find by the trail-search mechanism. Notice that for these two setups, with two and four UAVs, the number of threats immediately higher than the number of UAVs, three and five, respectively, do not stress much this observed behavior, but this becomes clearer for five and seven threats. This is explained by the fact that, with just one threat more than the number of UAVs, the average results do not change much, but when the difference is greater, the observed difference appears clearly, as it becomes more probable that the UAVs will have pending alarms in queue to be handled. Analyzing the cost due to the trail-follow mechanism, it is possible to observe that the number of UAVs do not influence this mechanism.