# Open Collaborative Platform for Multi-Drones to Support Search and Rescue Operations
## 3 Open Collaborative Platform
### Collaborative Searching Path Planning
#### 3.3.3 Return-to-Home  
As in most SAR operations, searching drones need to return to (or re-collect by) SAR teams. Thus, we assume a drone needs to return to the starting point (e.g., initial takeoff point) or a specific location (e.g., a rendezvous point) for changing its battery, uploading data, or simply retrieving the device. Since the Krypto module records the trajectory information and other status information in real-time, a _Flight Distance_ is calculated as how long and how far the drone has been flown. Next, the _Average Power Consumption_ (\(APC\)) in _meters_ per _Watts_ (\(m/W\)) is calculated based on the recorded information as follows:  
\[APC\ =\ \textit{Flight Distance}\ /\ \textit{Power Consumed}, \tag{2}\]  
Since a drone needs to return to a specific location (e.g., home location), a _Return Distance_ is calculated based on the (Euclidean) distance between the specific location and the drone's current location, see Figure 10. Thus, the _Remaining Distance_ (in _meters_) is calculated as follows:  
\[\textit{Remaining Dist.}\ =\ \textit{Remaining Power}\ \times\ \textit{APC} \tag{3}\]  
Based on our previous experiments in [4], the _maximum flight distance_ consumed only 80% of power and is applied to ensure the drone is able to fly back in the case of downwind or crosswind. The maximum distance is calculated as follows:  
\[\textit{Max. Distance}\ =\ \textit{Max. Speed}\ \times\ 80\%\ \textit{Battery Operating Time} \tag{4}\]  
Figure 9: Distributed searching path with multiple drones.  
According to (4), the _maximum flight distance_ (_Maximum Distance_) for DJI Phantom 2 is as follows:  
\[Max.\ Distance\ =\ 36\ (km/hr)\ \times\ 18\ (min)\ =\ 10.8\ (km) \tag{5}\]  
Therefore, if the total distance between the _Flight Distance_ and the _Return Home Distance_ is greater than the _Maximum Distance_ (4) and (5), Krypto will return to the specified location. Notice that the decision of returning home can be decided on Krypto or the Open Platform. In the case the mission is completed while additional power remains in the drone, a new mission can be requested by [PERSON] to the open platform, see Section 3.2.2.  
For the _Centralized_ approach, [PERSON] needs to determine if it is able to complete sensing the next row on its searching path. We defined the _Required Distance_ for this approach is as follows:  
\[\begin{array}{ll}\text{{Required Dist.}}&=\ Distance\ to\ next\ row\ +\ Length\ of\ the\ row\\ &+\ Return\ Home\ Distance\ from\ the\ end\ of\ the\ row\end{array} \tag{6}\]  
For example, in the case of three drones joining SAR operation (see Figure 8c), the _Required Distance_ is shown in Figure 11. Therefore, if the _Required Distance_ is greater than the _Remaining Distance_, Krypto will decide to return home (or to the specified location).  
\[\begin{array}{ll}\text{{Required Dist.}}&=\ Distance\ to\ next\ sub\_region\\ &+\ Distance\ to\ sensing\ the\ sub\_region\\ &+\ Return\ Home\ Distance\ from\ the\ sub\_region\\ \end{array} \tag{7}\]  
Figure 11: Required distance for centralized approach.  
Figure 10: Return home distance.  
For example, if we have two drones in the SAR operation, the _Required Distance_ is shown in Figure 12. Similarly, if the _Required Distance_ is greater than the _Remaining Distance_, Krypto will decide to return home.