## 4 Extended Simulation Analysis
In first case, the UAV is configured to only collect data from ground sensors and does not communicate with the Sink at all. In later cases, the UAV is configured to send one message of 70 bytes periodically after 200 ms, 500 ms, 1 s, 2 s, 3 s, 4 s, and 5 s to the Sink using its extended transmission power. The message sent by the UAV could be a telemetry packet or data collected from ground sensors. All along the path of one complete lap of 1650 m at speeds of 5, 10, and 15 m/s, different numbers of messages were transmitted by UAV to the Sink, depending on the size of period the UAV used (e.g., 200 ms, 500 ms, 1 s, 2 s, 3 s, 4 s and 5 s) which we call the **Flush Back Period**. In this case, we present in Figures 13-15, the averages of the _transmitted_, _delivered_, and _re-transmitted_ variables. The _transmitted_ denotes the total number of messages that were transmitted, including delivered and re-transmissions attempts. The _delivered_ variable is the number of messages successfully delivered and _re-transmitted_ is the number of failed re-transmissions that did not receive the ACK. Similarly, the re-transmission rate of telemetry data (sent by UAV to BS), with respect to the speed of UAV and Flush Back Period, is shown in Figure 16.  
Figure 14: UAV to BS Communication at 10 m/s with respect to different Flush Back Periods. Figure 13: UAV to BS Communication at 5 m/s with respect to different Flush Back Periods. It can be noted here that the successful delivery rate (from UAV to BS) increases with decreasing the UAV Speed and increasing Flush Back Period. The majority of the re-transmission attempts are in areas where the UAV is furthest from the BS. This behavior is also confirmed by our practical experiment  
Figure 16: Re-transmission Rate of UAV to BS Communication for different UAV speed and Flush Back Period. Figure 15: UAV to BS Communication at 15 m/s with respect to different Flush Back Periods. shown in Figure 6. It must be noted that we do not consider how cluster members communicate with CH, although the communication between cluster members and CH is very important and it can affect the communication between UAV and CH. As described before, we use a simple and intelligent way for the CH, knowing the UAV speed and hence its approximated contact period, to instruct all non CH nodes to remain in sleep mode until the UAV is gone. To measure the goodput, we repeated the simulation experiment several times with different UAV speeds and with a different Flush Back Period for one complete lap of 1650 m. It is assumed that each of the cluster Heads has infinite data to transmit to the UAV. In the first case, the UAV is configured to only collect data from the underground sensors and not to communicate with the Sink at all. In later cases, the UAV is configured to send one message of 70 bytes periodically after 200 ms, 500 ms, 1 s, 2 s, 3 s, 4 s, and 5 s to the Sink. A very slight increase in the network goodput can be seen in Figure 17, when the UAV also sends data back to the Sink. This is due to the additional messages the UAV sends to Sink. In the case of short Flush Back Periods, the data rate is slightly decreased because the UAV spends more time sending messages to the Sink. It can be seen here that the data rate with and without sending telemetry data is roughly same. This shows the viability of using a single transmitter for a mobile data collector in a WSN. Furthermore it can also be noticed that the larger the distance or gap between clusters of sensors, the higher the goodput. The gap between sensor clusters, in which the UAV is not busy communicating with the underlying CH, can be used to flush collected data back to the Sink, and hence increase the goodput. In this way, an intelligent mechanism can also be implemented to vary the Flush Back Period so as to use the available bandwidth wisely. It should be noted that the UAV and Sink talk on a communication channel other than the one used for communication between UAV and sensor network.