Juan Antonio Fernández Rubio

Scintillation and beam-wander analysis in an optical ground station-satellite uplink

In an optical communication link between an optical ground station and a geostationary satellite the main problems appear in the uplink and are due to beam wander and to scintillation. Reliable methods for modeling both effects simultaneously are needed to provide an accurate tool with which the robustness of the communication channel can be tested. Numerical tools, especially the split-step method (also referred to as the fast-Fourier-transform beam propagation method), have demonstrated their ability to deal with problems of optical propagation during atmospheric turbulence. However, obtaining statistically significant results with this technique is computationally intensive. We present an analytical-numerical hybrid technique that provides good information on the variance in optical irradiance with an important saving of time and computational resources.

Atmospheric-turbulence-induced power-fade statistics for a multiaperture optical receiver

To estimate the probability distributions of power fades, we consider two basic types of disturbance in electromagnetic wave propagation through atmospheric turbulence: wave-front intensity fluctuations and wave-front distortion. We assess the reduction in the cumulative probability of losses caused by these two effects through spatial diversity by using a multiaperture receiver configuration. Degradations in receiver performance are determined with fractal techniques used to simulate the turbulence-induced wave-front phase distortion, and a log normal model is assumed for the collected power fluctuations.

Temporal statistics of the beam-wander contribution to scintillation in ground-to-satellite optical links : an analytical approach

The beam-wander contribution to the scintillation in a ground-to-satellite free-space optical link is one of major importance. An analytical model, based on the duality between beam wander and angle-of-arrival fluctuations, is proposed for the temporal statistics. The expression of the probability density function of the log-amplitude fluctuations is first obtained. Then, the expressions of the spatial and temporal autocovariances are also obtained. We present plots of the beam-wander contribution to the log-amplitude variance, as a function of the transmitter aperture size and the turbulence accumulated in the propagation path. We also present the angular fluctuation and log-amplitude scintillation spectrum plots for some selected cases.

Ground to space optical communication characterization

Since the European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, ESA and the Instituto de Astrofisica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The experiments aimed at characterizing statically and dynamically the performance of the optical downlinks and uplinks in different atmospheric turbulence conditions, together with the development and testing of appropriate theoretical models to predict the link performance. An overview of the OGS and additional facilities on top the IAC Teide Observatory is given, as well as a summary of the statistical results on propagation and communication for different experimental configurations, including different number of transmitting subapertures and divergence in the uplink. Key parameters, like scintillation and fade and surge statistics, are correlated with theoretical predictions and an analysis of the far field pattern is included. The results of the deep space uplink experiments between the OGS and ESA satellite SMART-1 are also presented. Finally ESA free space optical communication programs are summarised, including optical payloads on board different satellites.

Modeling of power fluctuations induced by refractive turbulence in a multiple-beam ground-to-satellite optical uplink

As a support for the experimental activities related to the operation of bi-directional optical links between the European Space Agency (ESA) ARTEMIS geostationary satellite and the Optical Ground Station (OGS) in Teide Observatory (Tenerife island, Canary Islands, Spain), carried out by ESA and Instituto de Astrofisica de Canarias (IAC), calculations on the behavior of a multi-beam ground-to-satellite link have been performed. The goal is to assess the impact of refractive turbulence on the uplink (deemed to be more critical than the downlink because of beam-wander effects) and the mitigation effect on the power fluctuations in the satellite receiver achievable with such a space diversity approach, that involves several mutually time-incoherent beams in the uplink. Results from the multiple-beam uplink model and comparison with experiments are presented.

Propagation statistics of ground-satellite optical links with different turbulence conditions

The European Space Agency (ESA) geostationary data-relay satellite ARTEMIS started its operation in February 2003, after reaching its final position in the geostationary orbit. Since then, ESA and the Instituto de Astrofísica de Canarias (IAC) have carried out routinely bidirectional optical link experiments between ARTEMIS and the Optical Ground Station (OGS), installed in the Teide Observatory of the IAC in the Canary Islands, Spain. The main purpose of such experimental campaigns is to characterise and model the optical links performance from the propagation and communication points of view, under different atmospheric turbulence conditions. The statistical results presented in this paper cover the uplink and downlink performance, including scintillation, fade and surge statistics, intensity distributions and spectral analysis. The effect of using different number of transmitted beams and different divergences is also considered. Additionally, the results are correlated with the atmospheric turbulence conditions, in terms of profiles of the index of refraction structure constant, isoplanatic angle, seeing and wind profiles, measured in most of the cases simultaneously with the laser communication experiments

Ground-to-satellite bidirectional laser links for validation of atmospheric turbulence model

The European Space Agency (ESA) has launched the geostationary data-relay satellite ARTEMIS with one of its payloads being a laser communication terminal (LCT). The LCT is used within the semiconductor-laser intersatellite link experiment (SILEX) to receive Earth observation data transmitted from a similar LCT onboard the SPOT-4 satellite. To support SILEX, ESA has also reached an agreement with the Instituto de Astrofisica de Canarias (JAC) to build the Optical Ground Station (OGS), in the Teide Observatory ofthe IAC. ARTEMIS and the OGS are an ideal and unique test-bed to study and characterise laser beam propagation through atmospheric turbulence. Theoretical models of laser beam propagation through atmospheric turbulence have been reviewed and developed, to predict the performance of the optical links from the propagation and communication point of view. Special effort has been invested in modelling the uplink effects. Optical link experiments have been planned in detail, to gather the necessary data required to be statistically representative, to compare the results with theoretical predictions and to validate and adjust the theoretical models. This comparison will pave the way towards the implementation of deep-space laser communication links. The first results ofthese experiments, presenting the theoretical models, analysing separately downlink and uplink measurements, and comparing the data with the theoretical predictions, are presented.

Performance of a multiple-aperture optical system

Atmospheric turbulence causes intensity and phase disturbances on the wavefront of electromagnetic waves propagation through it that can seriously degrade the reliability of free- space optical communication links. This paper deals with the estimation of the statistics for power fades resulting from the combined effects of distortion of the receiving system instantaneous point-spread function and from the fluctuations of the collected power arising from wavefront intensity fluctuations. Fractal techniques are employed to simulate the turbulence-induced point-spread function distortions, while a log-normal model is assumed for the collected-power fluctuations. The reduction in the cumulative probability of losses due to these two effects through spatial diversity using a multiaperture receiver configuration is assessed.

Propagation experiments in the near infrared along a 150-km path and from stars in the Canarian archipelago

Within the framework of the European Space Agency (ESA) SILEX project, aimed at experimentally demonstrating the feasibility of inter-satellite optical communications links, an Optical Ground Station (OGS) has been built by ESA in the premises of the Instituto de Astrofisica de Canarias (IAC, Institute of Astrophysics of the Canary Islands) Observatory of Teide, in the Tenerife island. The OGS is designed to test the optical communications payload on board the ESAs Artemis satellite and to perform ground-satellite optical communications experiments. As part of the OGS design study, an assessment of the impact of the atmosphere on the ground- satellite links was carried out. This assessment included experimental characterizations of the atmospheric effects through both measurements from stars in positions close to the Artemis one in bands comprising the SILEX wavelengths, using the IACs Mons telescope in the Observatory of Teide, and measurements on a horizontal link with a transmitter near the IACs Roque de los Muchachos Observatory in La Palma island, based on a laser diode similar to those to be used in SILEX, and a receiver in the Teide Observatory, almost 150 km apart, in the Tenerife island. The 830-nm wavelength horizontal measurements allowed checking the variations in the behavior of the atmospheric turbulence through the diurnal cycle. Besides the information relevant to assess the OGS performance, the horizontal-propagation experiments allowed to gather a considerable amount of propagation data on a very long path, most of it 2400 m above the sea.

Cancellation of external and multiple access interference in cdma systems using antenna arrays

This paper deals with the problem of multisensor-multiuser detection in CDMA systems in the presence of interferences external to the system. The paper presents a new method able to estimate the spatial signature of all active users projected onto the subspace orthogonal to the external interference. With this information a specific beamformer can be designed for each user in order to combat the external and multiple access interference. The method performs in frequency non-selective multipath scenarios as they arise from some CDMA satellite and indoor communications systems. The estimation process does not require any training signal nor any a priori spatial information. It exploits the temporal structure of CDMA signals and extract the required information directly from the received signals. In addition, the method is independent of the nature of the external interference and it can cope with narrow-band and wide-band interfering signals.This paper deals with the problem of multisensor-multiuser detection in CDMA systems in the presence of interferences external to the system. The paper presents a new method able to estimate the spatial signature of all active users projected onto the subspace orthogonal to the external interference. With this information a specific beamformer can be designed for each user in order to combat the external and multiple access interference. The method performs in frequency non-selective multipath scenarios as they arise from some CDMA satellite and indoor communications systems. The estimation process does not require any training signal nor any a priori spatial information. It exploits the temporal structure of CDMA signals and extract the required information directly from the received signals. In addition, the method is independent of the nature of the external interference and it can cope with narrow-band and wide-band interfering signals.