Exploring the Architecture Trade Space of NextGen Global Navigation Satellite Systems

This paper studies the trade-offs between two metrics that play a key role in the design of Global Navigation Satellite System (GNSS)architectures: Dilution Of Precision (DOP)and cost. DOP is a multiplicative factor that quantifies the satellite-user geometric diversity and sets the limit on the achievable User Navigation Error (UNE). We focus on tradeoffs between DOP and cost driven by satellite constellation architecture decisions - e.g., orbit altitude, number of satellites. For simplicity, we restrict the analysis to global constellations resulting in a worst-site Geometric DOP (GDOP)value less than 6. This GDOP value is considered a threshold above which the resulting system would add little value for most GNSS users. The cost metric is subdivided in flight unit production costs based on the satellite dry mass and the learning factor experienced when producing N identical flight units, as well as the launch costs. Satellite dry mass is derived from the estimated payload power required to close the navigation signal link budget at the target received signal power of -155dBW. User navigation error is inferred with the computed GDOP values and a typical GPS pseudorange measurement error budget for dual frequency signal processing. We identified orbit altitude regions of interest that can be used in the design of future GNSS and potentially outperform the current global constellation architectures. In particular, results show that several constellations in “low” Medium Earth Orbits (MEO), - approximately half the orbit altitude of current GNSS constellations- are on the UNE-Cost Pareto front. When compared with the current GPS and GALILEO architectures, these alternatives consist of larger constellations of smaller satellites and have the potential to achieve both better performance and lower costs. Our analysis indicates that Low Earth Orbit (LEO)constellations are significantly more expensive to deploy due to the larger number of satellites required. Finally, we assess the impact of operational costs, launch cost reduction, different end-of-life disposal strategies and transmit power on the overall results.