This manuscript reviews recent progress in optical frequency references and optical communication systems and discusses their utilizations in global satellite navigation systems and satellite geodesy. Lasers stabilized with optical cavities or spectroscopy of molecular iodine are analyzed, and a hybrid architecture is proposed to combine both forms of stabilization with the aim of achieving a target frequency stability of 10 -15 [s/s] over a wide range of sampling intervals.The synchronization between two optical frequency references in real-time is realized by means of time and frequency transfer on optical carriers. The technologies enabling coherent optical links are reviewed, and the development of an optical communication system for synchronization, ranging and data communication in space is described. An infrastructure exploiting the capabilities of both optical technologies for the realization of a modernized constellation of navigation satellites emitting highly synchronized signals is reviewed. Such infrastructure, named Kepler system, improves satellite navigation in terms intra-system synchronization, orbit determination accuracy, as well as system monitoring and integrity. The potential impact on geodetic key parameters is addressed.

In this paper, a packet-level forward error correction (FEC) coding technique and pre-distortion adaptive optics (AO) technology are applied to a digital transmission scheme for optical feeder links in a geostationary Earth orbit (GEO) satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s highthroughput satellite (HTS) applications. The performance of the digital transmission scheme is evaluated in the forward and return links. The turbulent atmospheric optical channel is modeled for different optical ground station (OGS) altitudes. It is shown that fade mitigation techniques such as packet-level FEC coding and pre-distortion AO in the forward link, as well as large-aperture OGS telescope in the return link, are essential to close the link budget of a Terabit/s satellite communication system. Prepared using satauth.cls [Version: 2010/05/13 v2.00] 2 S. DIMITROV ET AL.Index-of-refraction turbulence (IRT) [3] is known to degrade the achievable data rates of FSO communication links [4]. For uplinks above 30 • elevation, the IRT falls within the weak turbulence regime [5], where the probability density function (PDF) of the irradiance at the receiver plane can be modeled by a log-normal distribution [6]. Moreover, the correlated fading events represent a lowpass process defined by the Greenwood frequency [7]. Regarding site selection and deployment, an OGS at sea level may suffer from strong influence of the turbulence, since the atmosphere is denser at lower altitudes. Astronomical sites are generally located on mountain tops, but these locations are usually isolated, and Terabit/s infrastructure needs to be developed. In this paper, 300 m, 900 m and 1500 m are chosen as examples of possible OGS altitudes.Adaptive optics (AO) is a technology used for correction of the incoming optical wavefront phase which is distorted by the turbulent atmosphere [8]. In ground-to-GEO FSO links, the scale size of the phase distortions is much larger than the feasible satellite telescope aperture size [3], and therefore, the spatial phase at the satellite receiving aperture is rather flat. Nevertheless, in an uplink scenario the traveling wavefront is initially perturbed by the atmosphere, and then it propagates through free space up to the GEO satellite. Thus, at the satellite plane the received wavefront profile differs from that of a pure diffracted laser beam. These distortions can be mitigated by applying pre-distortion AO (PAO) techniques to the outgoing optical wave at the ground station, using a beacon reference that senses the distortions introduced by the turbulent atmosphere.The simplest PAO correction method is tilt compensation of the first three spatial modes of the wavefront phase information [9]. It uses pointing by tracking, where a downlink beacon from the satellite is used a...

Optical feeder links will become the extension of the terrestrial fiber communications towards space, increasing data throughput in satellite communications by overcoming the spectrum limitations of classical RF-links. The geostationary telecommunication satellite Alphasat and the satellites forming the EDRS-system will become the next generation for high-speed data-relay services. The ESA satellite ARTEMIS, precursor for geostationary orbit (GEO) optical terminals, is still a privileged experiment platform to characterize the turbulent channel and investigate the challenges of free-space optical communication to GEO. In this framework, two measurement campaigns were conducted with the scope of verifying the benefits of transmitter diversity in the uplink. To evaluate this mitigation technique, intensity measurements were carried out at both ends of the link. The scintillation parameter is calculated and compared to theory and, additionally, the Fried Parameter is estimated by using a focus camera to monitor the turbulence strength.In this paper, we present the results of two measurement campaigns, carried out during October 2012 and April 2013. The main scope of both campaigns was to analyze the transmitter diversity mitigation effect on the uplink scintillation. Experiments were also carried out to improve the tracking performance, reducing the receiver aperture. These measurement campaigns were possible thanks to the collaboration This paper is focused on characterization of the delay line that is deployed to create an uncorrelated second laser beam out of one common source. It is organized as follows: section II introduces the main theoretical background, section III describes the measurement setup, section IV presents the results and section V discusses the main conclusions.

To overcome data rate limitations of RF communication links with satellites, TNO and DLR envision optical free-space communication feeder links for next generation high throughput satellites. This paper provides a feasibility assessment of such links and the technology needed. The main results of the link budget and the turbulence modeling of terabit/s optical links are presented. Based on these parameters, requirements and status of the link-subsystems are discussed, and a roadmap is presented, aimed at achieving terabit per second optical feeder links.

The sensitivity characteristics of optical receiver frontends for high-speed data communications depend on modulation format, detector type, and specific operational constraints. A general mathematical model of the receiver sensitivity that fits to analytical as well as measured data is required to compare different receiver implementations and assess the reliability of data links under varying received power as common in free-space optical communication links. In this paper, a new approach based on Q-factor modeling is presented, compared with analytical receiver models, and applied to a multitude of exemplary receiver implementations. A methodology is introduced to generally apply the model to ideal or practical binary optical receiver frontends.