In this paper, we evaluate the potential of mega-constellation-augmented GNSS (GNSS-MC) to provide fault-free high-integrity positioning in both open sky and urban areas. We derive a method to integrate dual-frequency carrier-phase ranging measurements from GNSS at medium Earth orbit (MEO), and mega-constellations at low Earth orbit (LEO) to achieve global carrier-phase positioning. From the perspective of users on earth, LEO satellites are moving much faster than GNSS at MEO. The large angular variations generated by these fast-moving LEO satellites are exploited for rapid estimation of floating valued cycle ambiguities. The addition of mega-constellations to GNSS also improves the spatial diversity of ranging sources which enables improved navigation performance in areas where visible GNSS satellites are too few to obtain a position fix, such as in dense cities and urban canyons. This research helps identify the scope of applications potentially enabled by leveraging communication mega-constellations in safety-critical land navigation applications.