  With the growing interest in satellite networks, satellite-terrestrial
integrated networks (STINs) have gained significant attention because of their
potential benefits. However, due to the lack of a tractable network model for
the STIN architecture, analytical studies allowing one to investigate the
performance of such networks are not yet available. In this work, we propose a
unified network model that jointly captures satellite and terrestrial networks
into one analytical framework. Our key idea is based on Poisson point processes
distributed on concentric spheres, assigning a random height to each point as a
mark. This allows one to consider each point as a source of desired signal or a
source of interference while ensuring visibility to the typical user. Thanks to
this model, we derive the probability of coverage of STINs as a function of
major system parameters, chiefly path-loss exponent, satellites and terrestrial
base stations' height distributions and density, transmit power and biasing
factors. Leveraging the analysis, we concretely explore two benefits that STINs
provide: i) coverage extension in remote rural areas and ii) data offloading in
dense urban areas.
