# Beyond Mitigation: New Metrics for Space Sustainability Assessment +
## 4 Satellite Constellation Comparison: Starlink vs. OneWeb  
In this section, we first compare two of the main mega constellations already deployed, namely Starlink and OneWeb using the defined SSI. For the purpose of the comparison, we only consider the two terms of the SSI related to the debris and to the performance requirements. Table 1 shows the parameters that have been used for both constellations. Moreover, for both constellations, the value of the addressability \(\eta_{addr}\) is assumed to be 30%, the value of the service load \(\eta_{serv}=50\%\) and the value of the overbooking factor \(\xi_{div}=50\)[11]. For both constellations, we assume the bandwidth \(B=100\) Mbps. The flux \(F\) measured in [1/m\({}^{2}\)/yr] is derived from the ESA's MASTER8 tool [10]. The tool requires specifying the the radius of the impactor debris in a collision, which has been assumed to equal 0.005 m as we have decided to consider all debris with diameter \(>1\) cm. Moreover, in the calculation, we have considered that Starlink has an advanced CA mechanism while Oneweb a simple one.  
Let us note that in the calculation of \(\epsilon_{debris}\), we have given more weight to the low probability of collision and hence we have chosen the following weights: \(u_{1}=0.55\), \(u_{2}=15\), \(u_{3}=0.15\), \(u_{4}=0.15\). From Table 2 it is possible to conclude that Oneweb is slightly more sustainable than Starlink. However, it is important to better understand this conclusion. From the comparison of the single terms, we can conclude that Starlink is oversized with respect to the current number of subscribers (it could serve more users) and the considered coverage area. Moreover, Starlink has a slighter higher probability of increasing  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Starlink** & **Oneweb** \\ \hline No. of Satellites & 6416 & 650 \\ \hline \(h\) & 550 km & 120 km \\ \hline \(\epsilon_{UT}\) & 25\({}^{\circ}\) & 40\({}^{\circ}\) \\ \hline \(F\) & \(0.3342\times 10^{-3}\) & \(0.1195\times 10^{-3}\) \\ \hline \(r_{sat}\) & 2.39 m & 0.93 m \\ \hline \(T\) & 5 yrs & 5 yrs \\ \hline \(\phi_{1}\) & 55 & 85 \\ \hline \(\phi_{2}\) & \(-55\) & \(-85\) \\ \hline \(T_{max}\) & 19.7 Gbps & 9.97 Gbps \\ \hline \(N_{sub}\) & 2.2 mil & 0.2 mil \\ \hline \hline \end{tabular}
\end{table}
Table 1: Constellation-specific parameters.  
the debris. In order to better analyze those results, we have made the comparison between the two constellations without considering the added technology needed for collision avoidance and debris retrivial. From the results shown in Table 3 it is possible to conclude that for Starlink the advanced collision avoidance system plays a key role in making the constellations more space sustainable. The SSI of Oneweb is only slightly decreased. Finally, to outline the importance of the altitude of the satellites, we have compared two constellations characterized by the same parameters such as the same number of satellites N = 640, the same radius of the satellite, the same number of subscribers (0.2 mil), and the same maximum capacity per satellite (9.97 Gbps) but different altitudes of 550 km and 1200 km. Results are shown in Table 4. Results clearly show the importance of the choice of the altitude of the constellation when space sustainability is considered as a requirement.