Sixth-generation (6G) communication systems are expected to support direct-to-device (D2D) connectivity, enabling standard user equipment (UE) to seamlessly transition to non-terrestrial network (NTN), particularly satellite communication mode, when operating beyond terrestrial network (TN) coverage. This D2D concept does not require hardware modifications to conventional UEs and eliminates the need for dedicated satellite ground terminals. D2D-capable UEs can be mounted on both manned and unmanned aircraft, however, they are especially well-suited for low-altitude unmanned aircraft due to their compact form factor, lightweight design, energy efficiency, and TN-NTN roaming capabilities. D2D can also enable beyond-visual-line-of-sight operation by providing NTN support for Communications, Navigation, and Surveillance (CNS) services during TN outages or congestion. This paper investigates the capabilities and limitations of D2D connectivity for low-altitude unmanned aircraft operating in urban environments. We analyze the variation in line-of-sight probability for both TN and NTN links as a function of aircraft altitude. We further compute path loss and received signal strength while accounting for a representative TN deployment with down-tilted antennas. The results show that the TN and NTN links complement each other, significantly improving the availability of the CNS service at low altitudes. These findings provide insights to support the design and optimization of future 6G-enabled integrated CNS services.

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