## 2 Background
### Crewed vs. Uncrewed Probe-and-Drogue Refueling  
Aerial refueling has been an integral capability supporting both strategic and tactical aircraft within the DoD for decades [1]. Historically, it has relied heavily on the skills of highly trained human pilots for successful execution [43]. Improvements in both UA and modern sensor, robotic, and control technologies could now enable A3R in UA with an acceptable level of assurance and risk. To complete A3R, the UA must be able to perform all the functions of the pilot beginning at the initial rendezvous between the UA and the tanker aircraft and ending with the UA safely separating from the tanker to continue onto the next portion of the programmed mission. Using the current NATO guidance for A3R, the phases and tasks have been organized into five phases and seven tasks [44]. To enable a UA to complete A3R, at a high level, the UA and tanker must complete the following seven tasks (many of these tasks are supported by pilot skills in conventional aerial refueling):  
Phase 1. Transitioning from Execution Portion to A3R Portion of Mission. 1. Establish communications. Prior to initiation of refueling, communications and datalinks must be established between the UA and the tanker, and any supporting entities that are involved, such as the Ground Control Station (GCS) and Aerial Vehicle Operator (AVO). The GCS receives all pertinent data related to the A3R and the AVO monitors the performance of the UA (the AVO is analogous to a remote safety pilot). As technology advances, the role of the AVO will likely diminish as additional assurance in the system becomes available. 2. Determine relative position. Determine the initial position of the UA relative to the tanker. Phase 2. UA Receiver Joins on Tanker. 3. Decrease separation to astern proximity. Once cleared by the tanker, decrease the separation of the two aircraft in a safe and predictable manner. Phase 3. UA Transitions from Astern to Engagement. 4. Transition to computer vision (CV). Once cleared by the tanker, arrive at the astern position to transition from position keeping provided by a data link or navigation aid to position keeping provided by a CV system. Object identification is required to provide location information to the guidance, navigation, and control (GNC)system; this information will then be translated by the UA to decrease the distance between the probe tip and the coupler, once cleared to contact by the tanker. 5. Position keeping during refueling. Transition back to relative position keeping upon successful engagement to allow fuel to transfer within the FTZ. Phase 4. UA Receiver Separates from Tanker. 6. Initiate separation after refueling. Once refueling is complete and cleared by the tanker, the receiver aircraft decreases airspeed to begin to increase the distance between the receiver and tanker. Upon reaching the limits of the hose reel, the probe tip will disconnect from the coupler and the receiver returns to aster position prior to transitioning to right echelon, when cleared by the tanker, for final administrative procedures. Phase 5. UA Receiver Proceeds on Mission. 7. Final separation to enable independent flight. Once cleared by the tanker, safely maneuver to increase separation between the two aircraft such that the UA can transition away from relative position keeping to continue the next mission task. The relative location of the receiver and tanker begins at maximum separation in the bottom right of Figure 8 during Phase 1, task 1. As the receiver comes the five phases, the receiver moves clockwise through Figure 8 until proceeding onto the next mission task. A process map of A3R activities is shown in Figure 9.  
and zones that must be accounted for during the design and certification of a UA tasked with conducting A3R (Figure 10). Figure 10: An example of an ODD for a UA tasked with A3R. Figure 9: Process map of the A3R tasks. In early deployments and during system development, it is common to limit the ODD to evaluate new technologies.