# _photonics_
## 1 Introduction
Even under the cloud-free line of sight conditions, optical satellite links are mainly affected by atmospheric turbulence, absorption and scattering, high background solar noise, and pointing losses [16; 21; 22; 23]. To cope with the above channel-induced limitations, highly efficient photon detection systems combined with large aperture telescopes and narrow-band filters can be used among others for OGSs [24]. This approach can be easily combined with QKD sender stations featuring narrow beam divergences. Following the above methodology to cope with the challenges of optical-satellite-based links, numerous satellite-to-ground QKD experiments have been successfully demonstrated. Low-Earth-orbit (LEO) satellite-to-ground QKD links have been demonstrated to reach distances up to 1200 km and key rates up to kbps [25; 26]. Moreover, both satellite uplink and downlink QKD feasibility has been thoroughly examined under daylight and nighttime conditions [24; 27]. Another 53 km FSO link that exhibits high loss has been realized, examining the feasibility of QKD in daylight aiming towards inter-satellite QKD communication [28]. Finally, [PERSON] et al. have summarized the progress in satellite QKD in [29]. One step beyond the successful demonstration of QKD over optical-satellite-based links is that infrastructure initiatives are targeting the integration of these space-based QKD links with the existing terrestrial fiber segments. In this context, European Quantum Communication Infrastructure (EuroQCI)--the quantum communication infrastructure initiative in Europe--aims to integrate QKD-based systems into conventional communication infrastructures [30]. In this path, the observatories have been selected as the suitable gateways for interconnecting QKD satellite sender stations with terrestrial fiber-based segments fibers [30]. We aimed to contribute to this implementation path by presenting a thorough feasibility analysis for Greek QCI [31]. The results of our study went further from a feasibility analysis in the physical layer and the investigation of satellite-to-ground link availability. QKD-based security architectures were introduced to maximize the availability of quantum key resources. More specifically, our research focused on the feasibility analysis of an LEO satellite-to-ground prepare and measured the decoy-state BB84 QKD link over a turbulent atmospheric channel under daytime and nighttime conditions. For QKD transmitter stations, we considered a satellite constellation consisting of 10 LEO satellites flying at an orbital height of about 600 km. We assumed that each satellite can communicate with up to three OGSs at the same time. These ground stations are hosted in astronomical observatories located across Greece, and they were selected to support three different segments of Greek QCI. In order to model the atmospheric link, we took into account the position, height, and receiver's telescope diameter of each OGS. By examining a period of one year, we reported secure key rates (SKRs) up to kbps and total distilled key bits per ground station up to Gbits. Finally, we described how the trusted satellites could share keys with the OGS in order to establish inter-OGS communication, and we examined if the distilled key rates can meet the demands for AES 256-bit key refresh time necessary to keep the attack success probability as low as possible. The paper is structured as follows. Section 2 briefly describes the BB84 QKD protocol and its system architecture in the proposed satellite QKD network of ground stations located at Greek territory. Section 3 describes the methodology for the modeling of the atmospheric downlink channel, by taking into account the OGSs' positions and altitudes. Finally, Section 4 provides the results of this study, and Sections 5 and 6 discuss and conclude this work, respectively.