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The initial step of the analysis is represented by the definition of the GNSS capabilities and receiver requirements in the environment of interest. The service volume concept has been introduced to this aim, and recent studies pointed out the interest to define and include GNSS performance above the altitude of the constellation itself [1], [2]. It is well known that signals, even if primarily broadcast from GNSS sources in order to serve Earth surface and proximity (i.e. with a beam directed nadir), are available at a limited extent also above the constellation.