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. Such a limited availability is mainly due to the spill-over of the beams irradiated from satellites above the other side of the Earth and to secondary lobes (Fig. 1). Due to the distance and/or due to the originally radiated power the received power in such conditions will be indeed low but still exploitable. In fact, a limited service capability will be available to lunar mission with specific characteristics for each possible mission phase