## 6 Availability and Angle of Transmission from the Antenna's Boresight  
In this section we analyze the relationship between the availability of a signal at the receiver's position and the angle from the antenna's boresight at which the same signal was transmitted. Figure 34: PRNs available for each receiver’s antenna elevation for each altitude of the whole considered MTO. Figure 35: Histogram of the available observations for each receiver’s antenna elevation during the whole considered MTO. Figure 33: Elevation values at which the four strongest signals are available at the receiver’s antenna. This relationship is of interest since the prediction of the elevation at which the signal is transmitted, as well as the elevation at which the signal is received, combined with the knowledge of the transmitter and receiver antenna patterns, can be used to aid the acquisition process and better attribute the available tracking channels. Indeed, for a receiver with a limited number of tracking channels (i.e., 12 for the SANAG receiver), this information can be used to optimize the attribution of the tracking channels (and thus the channels to acquire as a function of time) in such a way as to only consider the tracking of transmitted signals characterized by a power level at the receiver's position strong enough to be processed by the receiver (i.e., compatible with the receiver acquisition and/or tracking sensitivity). In this study, we did not define any PRN selection strategy based on the elevation at the receiver's antenna, since the definition of the specific spacecraft's attitude dynamics over time (that depends on the specific mission) as well as of the pattern of the receiver's antenna(s) placed on the spacecraft would be needed. However, it is possible to predict the elevation at which the signal is transmitted. Then, by assuming a certain transmitters' antenna pattern, for a certain altitude (and accordingly for a certain free space loss), it is possible to determine the signal's power at the receiver's position based on the estimated free space loss, and determine whether the same signal can be processed or not. Figure 36 displays the histogram of the visible satellites (in the line of sight (LOS)) and of the available satellites (the ones from which the signal transmitted is strong enough to be acquired and tracked by the SANAG receiver) for each transmitter's antenna elevation during the whole considered MTO. We can see that all the signals transmitted at an elevation angle between 0\({}^{\circ}\) and 30\({}^{\circ}\) from the transmitter's boresight are available, i.e., strong enough to be acquired and tracked by the receiver. If the signal is transmitted at an angle above 40\({}^{\circ}\) but below 70\({}^{\circ}\), then the chance that it will be available at the receiver's position will roughly be reduced to 50%. For an angle above 70\({}^{\circ}\), less than 30% of the signals are available. Figures 37-42 show the visible satellites (in the line of sight (LOS)) and the available satellites at the receiver's position, for which the corresponding signal is transmitted at an angle between 0\({}^{\circ}\) and 30\({}^{\circ}\), 30\({}^{\circ}\) and 37\({}^{\circ}\), 37\({}^{\circ}\) and 52\({}^{\circ}\), 52\({}^{\circ}\) and 62\({}^{\circ}\), 62\({}^{\circ}\)and 75\({}^{\circ}\), and 75\({}^{\circ}\) and 90\({}^{\circ}\), respectively, from the transmitter's boresight. It can be observed that, if the signal is transmitted between 0\({}^{\circ}\) and 37\({}^{\circ}\), it is almost always available at the receiver position, from the beginning to the end of the MTO. The signals in the LOS transmitted at angles between 37\({}^{\circ}\) and 75\({}^{\circ}\) are always available up to approximately 150,000 km of altitude, then available most of the time between 150,000 km and 250,000 km.