## 3 Review of Sensor Requirements for A3R
### Determine Relative Position
As SWaP-C decreases with increases in technology, consideration should be given to adding an A/A radar, an IRST, or an EO/IR system to a UA. Along with providing ranging information to aircraft within the operating area, the target aspect, bearing angle, speed, and closure rates could also be calculated using the same sensors with the assistance of a fusion engine to correlate the information [66]. These sensors would also enable the UA to provide additional information during ISR-T missions while increasing the overall situation awareness available to the AVO. Use of these sensors and all active sensors on the platform will be determined by the emission restrictions associated with the mission, as outlined in previous research [17]. Tactical Air Navigation (TACAN) systems have been in service for decades for both surface navigation and A/A ranging and bear information with reports dating back to 1977 covering the accuracy A/A range and information [67]. Operating in the UHF frequency band, A/A TACAN, also known as \"yardstick\", provides a cooperative range and bearing information to a target when systems are set 63 MHz apart. Having the ability to set both X and Y channels, setting 29X on the tanker aircraft would provide a cooperative range and bearing information to a receiver aircraft that has a 92X set. The fusion of land-based TACAN information with INS/GNSS information already exists to aid in airway navigation [68]. Multiple UAs already have a X frequency band radar for Synthetic Aperture Radar mapping in support of ISR-T missions to include Predator B, Global Hawk, and Gray Eagle [66; 69]. While previous research focused on using a X-band radar to assist in collision avoidance, this type of technology demonstrates the ability of a UA to fuse radar information to assist in navigation [70]. The ideal transition of technology would be to integrate air-to-air radar information to the navigation solution to arrive within the reception range of DGPS information to continue the join of the receiver on the tanker. As noted in [17], once the receiver aircraft achieves visual of the tanker, the receiver shall cease radiation of the radar by placing the radar into standby. Additionally known as Infrared Surveillance Systems (IRSS), IRST systems have been employed by military aircraft for decades, such as the system that was installed on the F-14D Super Tomcat. In July 1984, the USN signed a full scale development contract with Grumman to upgrade the F-14A with additional avionics to include the introduction of an IRST system for long-range air-to-air detection [71]. IRST systems work by passively detecting and capturing infrared-specific radiation of target systems, and modern day IRSTs are able to track a target in three dimensions (3D) [72]. Research is already ongoing towards the integration of IRST information with INS/GNSS information installed onboard a UA, with the use case for the analysis being A3R [73]. The last sensor, the EO/IR system, will be discussed to support decreasing the range of the receiver to the tanker to arrive at the astern position. When operating multiple UAs with crossing flight paths, collision avoidance systems become a critical safety component for safe and efficient operations. In terms of perception, EO/IR systems and tracking algorithms have been researched to assist active sensors, such as sonar or radar, in providing perception for a collision avoidance system to ensure safe deconfiction [74; 75]. Understanding that these EO/IR systems provide perception for avoidance, these systems could also use the same perception information to inform a GNC on how to join and maintain formation with another aircraft [76]. Additional discussion to follow about using an EO/IR system for drogue tracking to achieve engagement of the probe tip and coupler. Fusing all the available information into a blended solution will require synchronization among all contributing sensors [29; 68; 77].