## 2 Background and Related Work
### Related Work  
[PERSON] and Kumar [PERSON] and [PERSON] (2018) have written a survey of flying ad hoc networks (FANETs) for UAVs. The authors conclude that such networks can be categorized into different types. One important type of network is self-organizing networks, which is what we consider in this paper. One important research area is medium access control (MAC) protocols for FANETs ([PERSON] et al., 2018). The MAC protocols for FANETs are challenging for many reasons, e.g., the high speeds of the aircraft create a highly dynamic topology. Some FANETs use directional antennas. Networks with directional antennas offer many advantages, including higher communication capacity through non-interfering simultaneous communication in different directions ([PERSON] et al., 2018). Another advantage with using directional antennas is that it becomes easier to avoid detection by hostile nodes; HDARP benefits from this advantage. The challenges on the MAC level in networks using directional antennas have been studied by several researchers ([PERSON] et al., 2018; [PERSON] et al., 2018).  
In this paper we focus instead on the network layer where the MANET routing protocols are located. [PERSON] et al. (2018) have conducted a survey of routing protocolsfor FANETs. The authors note that at least 60 routing protocols for FANETs have been suggested. However, most of these 60 protocols are designed for UAVs in non-military contexts, and none of the 60 protocols consider leakage of packets to adversary aircraft, which is the main focus in our case.  
We consider communication in an airborne tactical network (ATN) [9] where there are hostile aircraft. Previous studies on ATNs have, however, not considered the presence of hostile aircraft and the implications and restrictions that the presence of hostile aircraft have on routing [10; 11; 12].  
[PERSON] et al. [13] consider ad hoc networks with hostile nodes, which they call detection systems. The authors define a routing algorithm called MinDP (Minimizing Detection Probability). Compared to routing protocols that do not consider hostile nodes, MinDP reduces the detection probability by 74%. This study differs from our study in several aspects: the nodes in the ad hoc network are not mobile, only two-dimensional space is considered (we consider routing in three-dimensional space), only the main lobe of the directional antenna is considered (we also consider sidelobes) and the evaluation is based on simplistic assumptions about a quadratic two-dimensional space with random placements of nodes (we use mobility data from realistic flight scenarios used by the defense industry).  
In mobile ad hoc where the nodes can have a high relative speed, geographical routing is sometimes used [14; 15]. Geographical routing is based on the idea that the source sends a message to the geographic location of the destination. Geographical routing uses the position, speed and (sometimes) acceleration of the nodes in the ad hoc network. Three common approaches when the destination node cannot be reached in a single hop are to select the next node as the node that is closest to the destination node, as the node that the nearest node that makes forward progress, or as the node that is closest to a straight line from the sender to the destination [16; 17]. HDARP is a geographical routing protocol in the sense that the routing decisions are based on the location, speed and acceleration of friendly and adversary nodes. However, the routing decisions are based on an adaptation of [PERSON]'s shortest-path algorithm (see Section 3.3).