## 3 Motivating Scenario
### Cluster Construction  
To execute the offloaded task efficiently, the surviving parked vehicles within the monitoring range of a UAV are formed into a cluster as much as possible. The establishment process of the cluster includes the following steps:  
1. Each UAV sends its ID number to the parked vehicles within its coverage range. The surviving parked vehicles then record the received UAV ID information.  
2. A value between 0-1 is randomly chosen by each parked vehicle. For parked vehicle \(n\), if the selected value \(r_{n}\) satisfies the condition that:  
\[r_{n}\leq\frac{p}{1-p(k\bmod\frac{1}{p})} \tag{4}\]  
then parked vehicle \(n\) becomes the cluster head. \(p\) in Equation (4) is the percentage of the number of cluster heads to the total number of parked vehicles, and \(k\) denotes the current number of rounds. Equation (4) aims to enable each node in the cluster to take turn to become the cluster head, with the aim of balancing the load.  
3. After the cluster head is determined, the cluster head transmits a broadcast message including its own ID number and its recorded UAV ID to the remaining non-cluster head nodes. To reduce the delivery overhead of the broadcast message, the dissemination hops of the broadcast message is specified as \(M_{h}\). The remaining non-head vehicles decide whether to join the cluster according to the ID number of the UAV the cluster head belongs. Specifically, for a parked vehicle, if it receives multiple broadcast messages from different cluster heads, and the UAV IDs of these broadcast messages are the same, the vehicle randomly selects a cluster to join in. If the UAV IDs of the received broadcast messages are different, the parked vehicle joins the cluster whose cluster head owns the same UAV ID as this parked vehicle. Our aim is to try to keep parked vehicles within the same UAV coverage range in the same cluster.  
4. Repeat steps 2 and 3 if there are still parked vehicles that have not been added to any cluster. After the above steps, there may still be some isolated vehicles that have not joined any cluster.  
5. When the cluster is too small, the total resources of the cluster may be insufficient for data processing, so that it is necessary to merge several small clusters into a larger cluster. For the cluster whose number of members is no more than the threshold \(C_{th}\), some cluster heads of these clusters are randomly selected as the upper-level cluster heads according to step 2. After the upper-level cluster heads are determined, small clusters are merged according to step 3.  
6. Continue to perform step 5 until the remaining small clusters cannot be merged. Figure 3 shows the cluster establishment process.  
After the cluster is formed, each vehicle member in the cluster periodically sends a message, including its vehicle ID number, its current location coordinates, its remaining energy power, and its current working status, to the cluster head. If no message is received from a member node for some time, the head node considers the node to be corrupted or no long in the cluster.  
The advantages of the cluster are as follows: (1) Message binding: small messages delivered to the same terminal point can be bundled into a larger message by the head node to decrease the data delivery overhead. (2) Vehicle management: parked vehicles can be well managed in the cluster. Moreover, computing resources in the cluster can be allocated uniformly and fairly within the cluster. (3) Results merging: multiple final resultswith small amounts of data can first be merged in the cluster and then returned back to the UAV to reduce the consumption of network resources. In addition, since the probability of a parked vehicle leaving or joining the cluster in a disaster scenario is considerably lower than in a normal urban situation, we believe the structure of the cluster is relatively stable.