## 3 Review of Sensor Requirements for A3R
### Determine Relative Position  
The tanker and receiver aircraft must have an accurate assessment of the relative position throughout refueling. To calculate the relative position, the receiver aircraft could employ a blended solution between the aircraft's inertial navigation system (INS), Global Navigation Satellite System (GNSS), and the CV system [29]. In 2001, the NASA Dryden Flight Research Center used an instrumentation system called the Formation Flight Instrumentation System (FFIS), built by the University of California, Los Angeles (UCLA). The FFIS fused GPS and INS data and demonstrated relative position estimates down to the centimeter level during a flight test between two F-18 hornets [60]. The A3R demonstrations conducted by DARPA and NASA and the USN also demonstrated the capability of an INS/GPS/CV-blended solution for the A3R mission in 2005 and 2006 [37] and 2015 [38], as mentioned in Section 2.4. Modern day INS are dead reckoning systems that utilize the initial velocity and position information, as well as information from three-ring laser gyro (RLG) systems mounted on orthogonal axes [61]. After initialization of the system, no additional reference information is required, but the system will drift over time. This drift can be addressed through a multitude of additional sensors, including GNSS information (obtained through a receiver in the aircraft) and signals of opportunity (SOPs). The ability to integrate GNSS information through a receiver attached to a UA is a critical component of a UA's guidance, navigation, and control (GNC) system, since it is necessary to address INS drift and since accurate location information is critical for all aspects of uncrewed flights. The space segment of GNSS is comprised of 24 satellites in medium Earth orbit (MEO), the user segment consists of a receiver and ground module, and the control segment for monitoring proper functionality of the constellation. GNSS provides the UA position and time information when in LOS of four or more satellites [61]. Additional GNSS systems have come online recently to compete with GPS and the Global Navigation Satellite System (GLONASS) to include the European Galileo and Chinese Bei Dou [62]. With well over 100 satellites available, once all services reach full operational capability (FOC), UA will benefit from increased Precision Navigation Timing (PNT) capabilities. GNSS signals (L5, L2, L3, and L1) operate in the L Band of the spectrum, and therefore, the UA must be equipped with a aperture capable of receiving the L Band signal [49]. The operational environment is an important consideration when determining the optimal sensors for relative position. GNSS jammers may threaten the ability of an INS/GNSS system onboard the UA [63]. In a contested environment, the UA may only have INS solutions available, but as the UA exits the contested environment and transitions to a more permissive environment, the UA could begin to blend INS with GNSS or SOPs, given the proper radio receivers were installed on the platform, to increase the overall accuracy of the solution. As A3R advances, the tanker and receiver may assess the operational environment and delay execution of refueling until the conditions are more favorable. Currently in the research phase, SOPs include signals not normally associated with Precision Navigation Timing (PNT) and include AM/FM radio, cellular, digital television,and low Earth orbit (LEO) satellites [64]. UA-mounted receivers' capability of receiving frequencies associated with those sources (MF, VHF, UHF, K, and L Bands) would be required to refine the spatiotemporal signal landscape while localizing the receiver in both space and time [65]. Having solved for their own ship position, determining the relative position at range comes with the inclusions of different airborne sensors to include A/A TACAN, the A/A radar system, an infrared search and track (IRST) system, or an electro-optical/infrared (EO/IR) system to a UA.