Figure 9

Link Loss \(\Lambda_{L}\) and secure key rates \(R^{S}_{\mathit {DSP}}\), \(R^{S}_{\mathit {E}91}\) as functions of zenith angle φ. All curves are shown for 0° incliniation w.r.t. the OGS. Similar curves can be calculated for different inclination angles. Top: Link transmission \(\Lambda_{L}(\varphi )\) of a 500 km orbit for different Fried parameters \(r_{0}\). E91 is more tolerant w.r.t. high link losses because of its immunity to photon number splitting attacks, it can therefore always be performed during the full quantum connection. A minimum elevation from horizon of 30° is required (i.e. \(\vert \varphi \vert \leq 60^{\circ}\)). Middle: Secure key rates for DSP using optimal starting points for those values of \(r_{0}\) that are too small to allow communication throughout the orbit (marked by pluses). Other contributions to Λ are being accounted for in these points. The temporal integral over these curves, i.e. the total key acquired during one pass, amounts to 6.8 kbit (\(r_{0}=5\mbox{ cm}\)), 33.9 kbit (10 cm), 95.2 kbit (20 cm) and 137.4 kbit (30 cm) respectively. Bottom: Secure key rates of E91 for different \(r_{0}\). The total key acquired during one pass amounts to 2.6 kbit (\(r_{0}=5\mbox{ cm}\)), 11.0 kbit (10 cm), 29.4 kbit (20 cm) and 42.0 kbit (30 cm) respectively.