Analyses are performed on the potential of using radio-frequency signals from massive LEO satellite constellations. This work aids in the creation of a navigation system independent of current GNSS. A tightly-coupled carrier Doppler shift/INS filter is developed to determine the feasibility of using signals of opportunity from LEO satellites for navigation purposes. This portion of the work makes two major contributions to the field of satellite-based radio-navigation systems. The first contribution is an analysis that shows GNSS-like position accuracy is possible using only INS measurements and carrier Doppler shift from LEO communication constellations. The second contribution is that INS quality, signal availability, and constellation design can significantly impact the navigation accuracy of a carrier Doppler shift/INS Kalman filter. An analysis of the costs and benefits of using model replacement over a Markov model in the dynamic propagation step of a tightly-coupled carrier Doppler shift/INS Kalman filter is performed in the next part of this work. This portion of the work makes contributions to the field of satellite-based radio-navigation systems. The main contribution is an analysis that shows Gauss-Markov models can be used instead of model replacement without increasing navigation error. Next, a DOP analysis is developed for systems using pseudorange and carrier Doppler shift measurements in point-solution batch filters that do not rely on INS data or dynamic propagation. This section's contributions to the field of satellite-based radio-navigation systems include a combined pseudorange and carrier Doppler shift DOP analysis using a novel DOP metric and an example of how to use the DOP analysis to identify the constellation characteristics, such as alternating ascending and descending nodes, that the OneWeb constellation could change to increase navigation accuracy. / Doctor of Philosophy / This dissertation presents research on using large communication satellite constellations as an independent backup to GPS. Simulated data are used to study the feasibility and navigation accuracy of such a system. Also investigated are different implementations of the algorithms used to navigate. Finally, a general analysis is developed to quickly approximate the navigation accuracy of a system that uses multiple measurement types.