The European Space Agency has initiated a number of exploratory projects in order to analyze potential improvements on a GNSS attained from inter-satellite ranging (ISR) and inter-satellite communication links. Such links can improve the overall system capabilities in several ways. Major improvements concerning the orbit and clock prediction accuracy and the reduction of the ground infrastructure are anticipated. Aside of this, in general high priority aspects, precise ranging information gained from ISR can also be used to monitor both the in case of microwaves neutral atmosphere as well as the ionosphere at different altitudes. This proposed activity is a scientific investigation on how the observed microwave signal delay can be utilized to support troposphere tomography on the one hand and provide valuable information about ionospheric variability on the other. In this context we also investigate how the number of this rare transit events can be increased by optimized orbit injection of spare satellites.