Ramon Mata Calvo

Transmitter diversity verification on ARTEMIS geostationary satellite

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.

Terabit-throughput GEO satellite optical feeder link testbed

The paper presents an experimental testbed, which will be used for the demonstration of more than 1 Tbps free-space optical (FSO) transmission in a scenario similar to that of GEO uplink. The chosen terrestrial scenario corresponds to worst-case conditions of a GEO uplink scenario according to the atmospheric channel characteristics and the geometry of the scenario. The testbed consists of a DWDM-based FSO link over 26km distance with less than 2° elevation. The communications system consists of several DWDM transmitters, which are multiplexed, amplified and transmitted in a single FSO channel. The transmitter as well as the receiver will be pointed and tracked by a high-precision fine-pointing mechanism. The state of the atmospheric transmission media will be monitored in realtime, allowing us to deepen our understanding of the optical satellite uplink transmission.

Optical transmission schemes for GEO feeder links

A novel transmission scheme for the forward link of a broadband satellite system that aims to avoid the need for feeder-link RF spectrum is described1. The hybrid scheme uses one or more optical uplinks from a hub station to geostationary satellite, plus one or more Ka band downlinks. The approach aims to retain the advantages of strong channel coding with high spectral efficiency, as used in recent high-speed digital video broadcast satellites, yet minimise on-board processing requirements. Promising simulation results are presented for a range of channel models, including channel measurements made with ESAs Artemis satellite.

Optical feeder link program and first adaptive optics test results

TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links for the next generation of Very High Throughput Satellites. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used. TNO and DLR are developing breadboards to validate Terabit/s communication links using an AO system. In this paper the breadboard activities and first results of the sub-systems will be presented. Performance of these subsystems will be evaluated for viability of terabit/s optical feeder links.

Speckle-based sequential optimization of adaptive receivers in downlink laser communications

Free-space optical communications (FSOC) are rapidly becoming a key technology for terrestrial, aerial, and space communication, mainly because of its very high throughput capacity. To achieve multi-gigabit laser downstream, an efficient single-mode fiber coupling is required. However, atmospheric turbulence remains one of FSOC’s main limitations. The turbulence affects the communications performance by inducing wavefront distortions that develop into coupled power fluctuations. In regimes of very strong turbulence, the use of traditional adaptive optics systems is limited due to strong scintillation and higher number of phase singularities. These limitations could be solved by relying on systems based on the stochastic iterative maximization of the coupled power. The drawback of such systems is that a high number of iterations are required for signal optimization. We address this problem and propose a different iterative method that compensates the distorted pupil phasefront by operating directly on the focal plane. The technique works by iteratively updating the phases of individual speckles to maximize the received power coupled into a single-mode fiber. We show numerically and experimentally that the method can improve the quality of the received signal with reduced bandwidth utilization.

LEO-ground scintillation measurements with the optical ground station Oberpfaffenhofen and SOTA/OPALS space terminals

The optical satellite-ground channel is turbulent and causes scintillation of the power received by a ground based telescope. Measurements are important to quantify the effect and evaluate common theory. A telescope with 40 cm primary mirror is used to measure the signals from the OPALS terminal on the International Space Station and the SOTA terminal on the SOCRATES satellite. The measurement instrument is a pupil camera from which images are recorded and intensity scintillation index, power scintillation index, probability density function of intensity and intensity correlation width are derived. A preliminary analysis of measurements from three satellite passed is performed, presented and discussed. The intensity scintillation index ranges from ~0.25 to ~0.03 within elevations of 26 to 66 deg. Power scintillation index varies from ~0.08 to ~0.006 and correlation width of intensity between ~11 and ~3 cm. The measurements can be used to estimate the fluctuation dynamics to be expected for a future operational ground receiver. The measurements are compared to model calculations based on the HV5/7-profile. Good agreement is observed to some part in the intensity scintillation index. Agreement is less for the power scintillation index and intensity correlation width. The reason seems to be a reduction of aperture averaging in some sections of the measurements due to increased speckle size. Finally, topics for future work are identified to improve the measurement analysis and deeper investigate the origin of the observed behavior.