## 2 Background
### Conceptual Framework for Air-to-Air Refueling  
This section describes the conceptual framework for air-to-air refueling; it is important to have a strong conceptual understanding of the process to understand the sensors required for automation. To begin air-to-air refueling operations, aircraft must maneuver to achieve a visual formation between the receiver aircraft or section of aircraft and the tanker. Even though the ATP-3.3.4.2(D), the current NATO Standard manual for air-to-air refueling at time of publication, outlines seven rendezvous (RV) procedures, the same general safety procedures apply to all RV [6; 17]. The NATO refueling procedures vary depending on factors such as the available communications (e.g., whether ground-based radar is available), the purpose (e.g., combat or not), whether the aircraft are directed by a radar control station (ground based, seaborne or airborne), and the participants (e.g., whether the tanker and aircraft are from the same base). Example procedures include:  
1. A heading-based procedure that utilizes air-to-air equipment on both tanker and receiver. 2. A heading-based procedure that allows an airborne intercept radar to control the procedure upon radar contact. 3. A procedure in which the receiving aircraft maintains a specified track and the tanker maintains a reciprocal track at a predetermined offset. In all cases, all aircraft, unless specifically directed, use the standard altimeter setting of 29.92 inches, which is verbally confirmed during the initial RV call. While joining, the receiver aircraft should maintain a minimum of 1000 ft. of vertical separation stepped down from the tanker until a visual join can be commenced, usually inside of 1 nautical mile (NM). To control closure, both the tanker and receiver aircraft should fly the speeds prescribed in their respective flight manuals or National Standards Related Document (SRDs). For probe-and-drogue configured tankers, receivers will normally, unless specifically briefed, join in left echelon outside of any aircraft already in position (Figure 3). This allows for sequencing of receivers and monitoring of receivers by the tanker aircrew. Once cleared, the receiver aircraft will transition to the astern position located approximately 10 to 15 feet aft of the drogue (Figure 4). Of note, if coordinated prior to the mission or established in Standard Operating Procedures, the receiver may be cleared directly to astern, removing the necessity for the receiver to establish in left echelon. Figure 3: Visual depiction of left echelon formation (tanker at top and two receiving aircraft at bottom). Relative to the position of the tanker aircraft's fuselage or wing, the drogue will nominally flow in the airstream 50 to 90 feet aft of the tanking platform (Figure 5) [18]. Therefore, an astern position can be thought of as a position 60 to 100 feet aft of the wing of the tanker aircraft for the receiver aircraft to prepare for refueling. To conduct probe-and-drogue refueling, the receiver pilot must visually identify the drogue within the domain to maneuver the receiver aircraft's probe tip into the coupler of the drogue (Figure 6). Figure 4: Astern position aft of a centerline drogue (tanker at top left; receiving aircraft at top right). Figure 5: Fuel transfer zone example [18]. Figure 6: United States Naval Academy probe-and-drogue test equipment. During air-to-air refueling, the human pilot must perform object detection and localization within the pilot's field of view (FOV). After identifying the drogue in the asteron position and determining the location relative to the probe tip installed on the receiver aircraft, the pilot must then reduce the separation between the coupler and probe tip by increasing closure. The nominal closure rates range from approximately 1 to 3 knots of closure. During the approach, the pilot must continually input small flight path adjustments via the control stick to account for the drogue movement in free air.