## 3 Proposed Approach
### Forbidden Sector Detection  
The goal of the forbidden sector detection method is to determine if an adversary aircraft can detect the communication between two friendly nodes. More specifically, it is to determine if the adversary is able to distinguish the main lobe or sidelobes of the incoming signal from electromagnetic noise. To do so, we construct a 3D geometrical model (i.e., a 3D volume) of the radiation pattern, main lobe and sidelobes combined and determine if any of the adversary aircraft are located within the volume. If any such adversaries are found, then the sending node should avoid direct communication and instead try to forward the packet through other friendly nodes unaffected by forbidden sectors. We have used a tetrahedron as a first-order approximation of the radiation pattern, as shown in Figure 4. This provides us with an acceptable tradeoff between accuracy and computation time: the last one being particularly important for reducing simulation time. The tetrahedron is created such that the sender is at the center of the tetrahedron base, and the receiver, Blue 2, is located at the apex of the main lobe. Obviously, this assumes that the receiver is within Tx range from the sender; otherwise, direct communication is not possible. In addition, we assume that when the receiver is located closer than Tx range, the sender can adjust the transmission power so the SNR for received signal is just enough for the sender to decode the data. This reduces the likelihood of leaking data behind the receiver. However, according to information from our aerospace industry partner, the receiver can decode the incoming signal (with an acceptable BER) if its SNR is at least 10 dB. Furthermore, given that a signal can be _detected_ if it is just above the noise floor, when this information is plugged into a simplified path loss model [21], the outcome indicates that a signal can be discerned as far as three times the distance from sender to receiver along antenna boresight. Therefore, we extend the tetrahedron apex at three times the distance from the sender and receiver, which is the _detection range_, the maximum distance at which an adversary can detect the transmission. Similarly, we extend the radius of the tetrahedron base (i.e., the distance from center to each of the three vertices) to 1/30 of the detection range to prevent detection through the sidelobes. Thus, our forbidden sector detector consists of the tetrahedron constructed as described above combined with logic to detect if any adversary is located with the 3D space of the tetrahedron. It is important to point out that when the transmission power is adjusted for the distance to the receiver, the volume of 3D space used by main and side lobes of the signal will change accordingly. Similarly, the tetrahedron is scaled proportionally to the distance to the receiver, but the aspect will remain the same. The left part of Figure 4 illustrates the operation of the detector in the scenario where node Blue 1 wants to transmit to node Blue 2. The red and green nodes are hostile nodes that should not be able to detect the transmission. The right part of Figure 4 represents a cross-section of the figure on the left, as seen from above, and is rotated so that the tetrahedron apex points upwards. The figure also shows the embedded radiation pattern (also a cross-section of the 3D pattern). The Green node is outside the tetrahedron and thus does not pose a detection risk. However, node Red 1 is able to detect the signal from the sidelobes. In addition, although the signal that passes Blue 2 has a very low SNR, it is also detectable by Red 2. Thus, the location of either Red 1 or Red 2 is enough reason to conclude that the transmission from Blue 1 to Blue 2 is located inside a forbidden sector and that an alternative path through an intermediate friendly node would be preferred. The logic used to detect if an adversary is inside the tetrahedron is captured by Algorithms 5-7.