## 3 Review of Sensor Requirements for A3R
### Object Identification to Support Probe Placement  
The purpose of a CV system installed on a UA is to detect, classify, and localize objects within the domain. A key aspect of the CV system is that it must be trained on data collected and labeled prior to the system being able to detect, classify, and locate objects within the domain. While organizations have begun to develop frameworks to provide assurance of machine learning algorithms, such as neural networks [91], that could be used for detection, classification, and localization, additional research is still ongoing for completing T&E for a LES [92], such as the CV system that would be used during this portion of A3R.  
After detecting objects within the domain, the CV system, trained on data collected during the development of the system, can begin to classify the object detected and provide the information to the GNC. The resulting information is then fused into the UA's GNC to maneuver the probe tip of the UA into the coupler portion of the drogue attached to the tanking aircraft. In many cases, their capabilities have been tested through modeling [93; 94]; scaled physical experiments [95]; and field experiments that utilize robots, ground vehicles, and other equipment to demonstrate the core technologies involved in A3R at a much lower cost.  
A variety of methods have been attempted to detect the required objects within the domain, mainly the drogue, coupler, and probe tip. Some of the most common methods include color information, visible markers, or IR markers [96]. The use of markers or beacons to locate the drogue is considered an active vision system (AVS), whereas passive vision systems (PVS) require no additional hardware [22]. The cost to modify and maintain the entire NATO inventory of tanker aircraft to enable AVS would require extensive coordination and commitment from all participating NATO countries. A primary benefit of a PVS is that it requires no additional hardware on the drogue or the tanker aircraft. The primary detractor of a PVS is degraded performance in low visibility or low lighting domains. The limitations associated with low visibility or low lighting conditions may be overcome with the addition of a probe illumination source, such as the lighting system installed on the F-35 family of aircraft [97].  
Less commonly, acoustic methods have been suggested for object identification, as well as for air navigation applications [98]. Although not fully developed, acoustic sensors have a strong potential and may offer significant advantages, especially in a multi-sensor navigation system [98]. Although there has been no published research directly related to acoustic methods for the aerial refueling of UA, there has been research published documenting stable and efficient performances for communications for uncrewed underwater vehicles and uncrewed surface vehicles [99].  
While transitioning from astem to contact, using DGPS to form a corridor of autonomy (COA) as described in [95]; this COA will bound the position of the UA by allowing for autonomy only when located within the COA. The COA can be thought of as a geofence built from information provided, and the DGPS ensures the safe separation between the tanker and receiver aircraft. If the UA reaches a limit of the COA derived by DGPS, the UA will retrograde to a safer outcome, which could be defined by the center of the COA or may require returning to the asterm position.  
When considering the camera system installed on the UA, many options exist, depending on the expected domain of operation. Using an EO system coupled with an illumination source capable of illuminating objects within the domain from the asterm position represents one of the many different combinations available for satisfying the perception requirement of A3R. Enabling a UA to exercise higher levels of autonomy by utilizing a CV system within the COA enables completion of the positional transition from asterm to contact and currently represents the best method available to complete this portion of A3R.