## 6 Analysis and Discussion  
Our experiments show that the HDARP protocol avoids adversary detection by minimizing the amount of leaked packets. The HDARP protocol uses situation awareness (SA) data when making routing decisions. The pilots of fighter jets need SA data for completing their missions. This means that SA data are used for two purposes: providing the pilots with mission critical information and for routing communication packets so that they cannot be detected by an adversary. The HDARP protocol prevents leaking packets to an adversary by proactively trying to avoid transmitting in forbidden sectors. A consequence of this strategy is that a packet sometimes cannot be sent along a direct line between the sender and the destination. In such cases, the packet needs to be sent via an intermediate node (see Figure 3). As a result of this, the number of hops will increase in some cases, and as a consequence, the average packet life time will also increase when we avoid leaking packets. Our experiments quantify this effect for the two scenarios considered. It turns out that compared to most protocols that do not avoid leaked packets, the average packet life time increased significantly for the more complex of the two scenarios (the BVR scenario). However, experts in the area of fighter jet design consider packet leakage to be a major tactical disadvantage from an operational perspective, and the cost in terms of longer average packet life time is therefore regarded as acceptable, depending on scenario at hand. Our experiments show that using directional instead of omnidirectional antennas will reduce the number of leaked packets by several orders of magnitude. Similar reductions of the detection probability when using directional instead of omnidirectional antennas have been observed by other researchers [45]. The HDARP protocol only works for directional antennas. However, when using directional antennas, simultaneous senders using the same frequency will not detect that the receiver suffers from destructive interference. As a consequence of this, some packets will be lost, resulting in a somewhat lower packet delivery rate for HDARP compared to the best of the omnidirectional protocols. Directional antennas can receive two packets on the same frequency simultaneously if the difference in angle of arrival (AOA) between the two senders is big enough. Our simulator does not consider the angle of arrival, which means that there will be destructive interference regardless of the AOA difference between the senders. This means that the packet delivery rates for HDARP shown in Figure 8 are pessimistic and that the cost of using directional antennas and avoiding adversary detection in terms of a somewhat decreased packet delivery rate is acceptable. The total number of sent packets is relatively small for the HDARP protocol. In fact, it is almost at the same level as the protocol with the smallest number of sent packets (DSR-UU). This means that SA data are a relatively efficient way of representing topology information about the mobile ad hoc network. As discussed previously, the pilots need SA data for completing their mission in any case. If we compensate for this, the difference between the number of packets sent by the HDARP protocol and the number of packets sent by DSR-UU becomes even more marginal. This means that the cost of avoiding adversary detection in terms of an increase in the number of sent packets is minimal and clearly acceptable. In this paper, we base our evaluation on data from realistic flight scenarios used by the aerospace industry. Unfortunately, many studies of mobile ad hoc networks use very simplistic and unrealistic workload models, e.g., nodes with random movement patterns in a rectangular area [46]. This project was conducted in close cooperation between researchers from academia and aerospace industry experts, which facilitates a high level of realism in the evaluation.