# Enabling Multi-Mission Interoperable UAS Using Data-Centric Communications
## 5 Functional Validation
The configuration, which is illustrated in Figure 3, guarantees that: (1) every end-user terminal connected through a Wi-Fi access point (provided by a small-sized UAV) gets attached to the same LAN segment, and therefore belongs to the same broadcast domain; (2) the UAV router, the GCS router and the RPIs themselves have a virtual interface to the LAN, hence they can also behave as DDS publishers and subscribers over the broadcast domain (in particular, each aerial vehicle includes a Data Acquisition Unit (DAU) that publishes data using the DDS middleware); and (3) multicast/broadcast data originated by the tactical UAV is sent towards the GCS, where it is replicated by a VXLAN interface towards the small-sized UAVs that provide a Wi-Fi access point (i.e., multiple copies of the same multicast/broadcast data are not transmitted over the UAV-GCS communication channel). With the aforementioned configuration, we used the DAU at the tactical UAV to publish a continuous data stream, emulating the delivery of real-time video from the UAV camera at a constant bit rate of 5 Mb/s (this was supported with the utilization of the RTI Perfest tool, [[https://github.com/rticommunity/rtiperfest](https://github.com/rticommunity/rtiperfest)]([https://github.com/rticommunity/rtiperfest](https://github.com/rticommunity/rtiperfest))). This DAU also offered sensor information from the UAV (e.g., representing temperature, humidity, air pressure, etc.) to interested subscribers, producing telemetry values every 4 s. This feature was provided with the implementation of a specific-purpose DDS application, producing periodic raw sensor values with a configurable time interval. The application was also used to serve telemetry information from the DAUs of small-sized UAVs, emulating the publication of the updated GPS coordinates corresponding to each of these UAVs every 8 s.  
Figure 3: VXLAN and Linux bridge configuration. In our experiments, the small-sized UAVs were positioned on the ground. A mobile ground station (the mission planner in Figure 2) subscribes to the sensor information of the tactical UAV, as well as to the GPS positions of all the small-sized UAVs, specifying QoS requirements that match the publisher offers. The user working at a distant area subscribes to the video stream of the tactical UAV using matching QoS requirements. Figure 4 reflects the telemetry traffic that appears in the network and the DDS signaling traffic coming from the publishers and the subscribers (this traffic has been captured at the equipment of the mission planner). DDS discovery and subscription messages are sent using specific multicast IP addresses. Hence they are distributed over the broadcast domain, and can be observed at the mission planner equipment. Figure 4a shows the different DDS messages sent by the publishers. An endpoint discovery phase takes place between Second 0 and Second 60, with different discovery messages being sent related first to the sensors, then to the GPS data, and then to the video stream. For the sake of clarity, Figure 4a only shows the discovery messages corresponding to the GPS position of one of the RPIs. Figure 4b shows the related messages sent by the subscribers, i.e., the mission planner and the user working at a distant area. In addition, we can see in both figures how the different publishers and subscribers enter a steady state when the subscriptions match the discovery messages. This is observed after approximately Second 20, in the case of the sensor information; after Second 40 for GPS data; and after Second 50 for the video stream (to better observe this effect in our experimental setup, subscriptions have been configured to take place at those instants). Once the corresponding subscriptions have been carried out, the delivery of data starts from publishers to subscribers. This is shown in Figure 4c, which represents the data traffic corresponding to the sensor information and the GPS data received by the mission planner.