Algorithm 2 Prioritized-based contact-duration frame selection mechanism (PCdFS) Algorithm.

Input: Initial deployed information of nodes and UAVs, ${\mathbb{S}}_{kB}^1$, ${\mathbb{S}}_{kB}^2$, $N_p$, $N_{node}$.

Output:$N_p$, $N_{node}$

1: for $\forall S_i\in {\mathbb{S}}_{k_B}^1$, $\forall S_j\in {\mathbb{S}}_{k_B}^2$ do

2:        Make a judgement for sensor $S_i$ and $S_j$: which priority area does them in;

3:        Calculate the contact duration between $S_i$ and the $UA{V}_1$, $S_j$ and the $UA{V}_2$, respectively;

4: end for

5: For $UA{V}_1$ (and $UA{V}_2$), $t_k$ allocated to the one (e.g., $S_{i_k}$, and $S_{i_k}\in {\mathbb{S}}_{k_B}^1$ for $UA{V}_1$, and $S_{j_l}$, $S_{j_l}\in {\mathbb{S}}_{k_B}^2$ for $UA{V}_2$) which is within the higher priority area; When more than one node within the same high priority area, $t_k$ allocated to the one (e.g., $S_{i_k}$ for $UA{V}_1$, and $S_{j_l}$ for $UA{V}_2$) which has the shorter contact duration with the UAV.

6: In $t_k$, $S_{i_k}$ and $S_{j_l}$ send packets to $UA{V}_1$ and $UA{V}_2$ respectively;

7: Update $N_p$, $N_{node}$;

8: return $N_p$ and $N_{node}$;