Kassas et al. Figure 2. Hardware setup with which the C-12 aircraft was equipped. corresponding CNRs. The hardware and software setup are shown in Figures 2 and 3, respectively. FLIGHT REGIONS AND AIRCRAFT MANEUVERS The campaign took place in three regions as follows. i) RegionA: A rural region in Edwards AFB, California. ii) RegionB: A semiurban region in Palmdale, California. iii) Region C: An urban region in Riverside, California. Different maneuvers were planned over the three regions to test several aspects of aircraft navigation with cellular SOPs. Figure 4 shows the regions in which the experiments were performed. More than 70 3G base transceiver stations (BTSs) and 4G eNodeBs were mapped throughout the experiment via the method described in [40]. The mapped towers were cross-checked via Google Earth and online databases and are shown in Figure 4. This article investigates the potential ofcellular SOPs for navigation; therefore, mapping the SOPs will not be discussed. The different maneuvers performed by the aircraft are described next. Two main types of maneuvers were performed in each region. The first was a teardrop-like pattern while climbing/ descending. The patterns have a focal point that is aligned with geographic points of interest (see Figure 4). The measurements used to characterize the CNR and multipath were taken exactly above the geographic point ofinterest to maintain the horizontal distance between the aircraft and the cellular base stations. The second was a grid-like pattern with many turns and straight segments. Such patterns were used as stress-test for the navigation receivers to assess their ability to track cellular synchronization signals in a robust and accurate fashion as well as navigation solution evaluation. The two types ofmaneuvers are shown in Figure 5. GROUND-TO-AIR CHANNEL CHARACTERIZATION This section characterizes the ground-to-air radio channel by analyzing the CNR and multipath at different altitudes and horizontal distances in Regions A, B, and C. CNR RATIO CHARACTERIZATION The CNR influences the precision of the pseudorange navigation observable. The pseudorange is obtained by correlating the received cellular signal with known replicas of Figure 3. Software setup used for cellular SOP signal collection. OCTOBER 2022 IEEE A&E SYSTEMS MAGAZINE 7