Assessment of Cellular Signals of Opportunity for High-Altitude Aircraft Navigation Figure 1. USAF Pilots and ASPIN researchers with the C-12 aircraft. The CNR influences the precision of the navigation observables produced by a navigation receiver. It is found that up to a dozen base stations can be acquired and tracked at 23,000 ft above ground level (AGL). Furthermore, the multipath channel is analyzed at different altitudes in different regions. Multipath can cause significant biases in navigation observables, compromising the accuracy of the navigation solution. Ground reflections could be a concern for strong multipath. However, the data shows clean channels between the aircraft and the cellular base stations with a dominantly strong line-of-sight (LOS) component at all altitudes, which in turn means that the navigation observables from cellular SOPs will have high accuracy. To demonstrate the feasibility ofaircraft navigation with cellular SOPs, a sample trajectory ofthe C-12 aircraft was estimated using cellular SOPs only, yielding a three-dimensional (3D) 10.5 m position rootmean-squared error (RMSE) over a 51-km trajectory traversed