Figure 11

Secure key production during one overpass for two orbital scenarios: (a) for a direct overpass, and (b) for an overpass at a horizontal distance of 500 km. The origin of the time axes corresponds to the distance of closest approach (550 km, respectively 743 km). In both figures, the bell curves show the secure key rate on the left y-axis. For reasons of clarity, they are shown for only one \(\mbox{DCR} = 250~\mbox{cps}\). The secure key length is given on the right y-axis for two different values of the dark count rate: 250 cps (solid curves) and 1000 cps (dashed curves). The lower (blue) curves are for the case of a source that generates \(10^{8}~\mbox{pairs/s}\) and \(\tau = 1~\mbox{ns}\) (\(\mu = 0.1\)), while the uppoer (red) curves are for a pair production of \(3\cdot10^{8}~\mbox{s}^{-1}\) and \(\tau = 200~\mbox{ps}\) (\(\mu = 0.06\)). All other parameters are as in Table 3 and the link attenuations are calculated for atmospheric conditions characterized by \(r _{0} = 20~\mbox{cm}\). In the conservative case of \(\tau = 1~\mbox{ns}\) and \(R_{\text{pair}} = 10^{8}~\mbox{s}^{-1}\) the key length would equal zero bit for the distant pass (b) in case the DCR exceeds 735 cps per detector. The curve shown is therefore for \(\mbox{DCR} = 500~\mbox{cps}\), instead of 1000 cps