Marcos Reyes Garcia-Talavera

Long-term statistics of laser beam propagation in an optical ground-to-geostationary satellite communications link

A ground-to-space laser communications experiment was conducted to verify the optical interfaces between a laser communications terminal in an optical ground station and an optical payload onboard a geostationary satellite 38 000 km away. The end-to-end optical characteristics such as intensity, sensitivity, wavelength, polarization, and the modulation scheme of optical signals as well as acquisition sequences of the terminals were tested under fairly good atmospheric conditions. The downlinks bit error rate was on the order of 10/sup -10/ in spite of atmospheric turbulence. Atmospheric turbulence-induced signal fading increased the uplink bit error rate, the best value of which was 2.5 /spl times/10/sup -5/ because the turbulent layer near the earth surface affects the uplink signal more than it does the downlink one. The far-field optical antenna patterns were measured through the ground-to-satellite laser links. The long-term statistics of the optical signal data is in good agreement with the calculated joint probability density function due to atmospheric turbulence and pointing jitter error effects, which means the stationary stochastic process can be applied to not only the static link analysis but also the dynamic link performance of the optical communications link. The equivalent broadened optical beam pattern should be used for the fading analysis even though the atmospheric coherence length is larger than the antenna diameter or the optical beam diameter of the transmitter. From these results, a more accurate dynamic link design of the optical communications link can be performed that would be useful for system designers, especially for designers of commercial systems.

Analysis of the preliminary optical links between ARTEMIS and the Optical Ground Station

In the frame of the SILEX project, the European Space Agency (ESA) has put into orbit two Laser Communication Terminals, to establish an experimental free space optical communication link between a GEO satellite (ARTEMIS) and a LEO satellite (SPOT IV), to relay earth observation data. In order to perform In Orbit Testing (IOT) of these, and other, optical communications systems, ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreement for building the Optical Ground Station (OGS), in the Teide Observatory of the IAC. With ARTEMIS placed in a circular parking orbit at about 31000 kilometres, its optical payload has been preliminary tested with the OGS. First results and analysis are presented on the space-to-ground bi-directional link, including pointing acquisition and tracking performance, Bit-Error Rate (BER) and transmitted beam divergence effects related with atmospheric models and predictions. Future plans include deeper optical bi-directional communication tests of OGS, not only with ARTEMIS but also with OSCAR-40 (downlink) and SMART-1 (up-link) satellites, in order to do a full characterisation of the performances of laser beam propagation through atmospheric turbulence and a comparison with theoretical predictions.

Preliminary results of the in-orbit test of ARTEMIS with the Optical Ground Station

ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreemenet for building the Optical Ground Station (OGS), in the IAC Teide Observatory, in order to perform In Orbit Testing (IOT) of Optical Data Relay payloads onboard communication satellites, the first being ARTEMIS. During its recent launch, ARTEMIS was put into a degraded orbit due to a malfunction on the launchers upper stage. ESA rapidly adopted a recovery strategy aimed to take the satellite to its nominal geostationary position. After completion of the first manoeuvres, ARTEMIS was successfully positioned in a circular parking orbit, at about 31,000 kilometers, and turned into full operation. In this orbit, its optical payload has been tested with the OGS, before establishing the link with SPOT IV. New tracking algorithms were developed at OGS control system in order to correct for ARTEMIS new orbit. The OGS has established a bi-directional link to ARTEMIS, behaving, seen from ARTEMIS, as a LEO terminal. Preliminary results are presented on the space-to- ground bi-directional link, including pointing acquisition and tracking (PAT) performance, received beam characterization and BER measurements.

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.

Design and performance of the ESA Optical Ground Station

The European Space Agency (ESA) has undertaken the development of Optical Data Relay payloads, aimed at establishing free space optical communication links between satellites. The first of such systems put into orbit is the SILEX project, in which an experimental link between a GEO satellite (ARTEMIS) and a LEO satellite (SPOT IV) will be used to relay earth observation data. In order to perform In Orbit Testing (IOT) of these and future optical communications systems, ESA and the Instituto de Astrofisica de Canarias (IAC) reached an agreement for the building of the Optical Ground Station (OGS) in the IAC Teide Observatory, which consists basically of a 1-meter telescope and the suitable instrumentation for establishing and testing bi-directional optical links with satellites. The presence of the atmosphere in the data path posses particular problems, with an impact on the instrumentation design. The transmission, reception and measurement functions, along with the overall control of the instruments, are performed at OGS by the Focal Plane Control Electronics (FPCE). The design and performance of this instrumentation is presented, emphasizing the Pointing, Acquisition and Tracking, the Tuneable Laser and the Master Control.

OPTEL-D: an optical communication system for the deep space

We report the design of OPTEL-D which is an optical communication system for direct data transfer from deep space to earth. OPTEL-D extends the link distances of current optical communication systems to the deep space. The design covers the electrical, thermal, mechanical and optical challenges that such a link distance imposes to both the ground and the space segment. The detailed conceptual system design is presented together with its trade-offs and performance analyses. The space terminal is based on a modular approach which groups the fiber-optical, the opto-mechanical, and the electrical functions into separated modules for independent accommodation. The transmitter beam is provided by an amplitude modulated MOPA system operating at 1.5um. The opto-mechanical unit accomplishes beam forming and steering of the transmit beam as well as reception of the uplink channel. An electrical unit comprises power-conditioning, data-handling, and terminal control through its interfaces to the spacecraft. It is concluded that the OPTEL-D space terminal mass and power are 34 kg and 116 Watts. Depending on the ground station configuration, a data rate of several Mbps is reached over distances of >75 Mio kilometers. Critical assumptions on the receiver sensitivity were verified by tests and the results will be presented. The design of OPTEL-D was derived in collaboration with the European Space Agency to serve the Asteroid Impact Mission. Due to its modular approach using commercially available technology, it can be realized in a competitive schedule and cost frame providing flexible yet powerful data downlinks to most deep space missions.

The Gran Telescopio Canarias laser guide star AO system: error budget and expected performance

The Natural Guide Star Adaptive Optics system for the Gran Telescopio Canarias (GTC) is in its integration phase, and meanwhile the Laser Guide Star update, which will follow two years later, has recently passed its Preliminary Design Phase. This LGS Facility will feature a TOPTICA Na laser, and it will open up the scientific possibilities of GTC enlarging the sky coverage of the AO system and allowing to study at high resolution more scientific targets. A trade-off study was undertaken to decide, among other details, the launching position of the laser and the feasibility of a further upgrade to an MCAO system vs technical complexity, cost and maintenance. As part of this study we have analysed the performance of the GTCAO LGS system to ensure that it will fulfil the specifications in all the different scenarios. Complete end-to-end (E2E) simulations have been performed using the versatile Durham AO Simulation Platform (DASP), including not only real atmospheric profiles from Observatorio del Roque de los Muchachos but also the measured windshake spectrum of the secondary mirror of GTC, the different control loops (TT, DM, focus), the laser uplink jitter and launching telescope divergence, the segmented primary mirror and its cophasing residual errors, the rotating pupil etc... In this contribution we present a detailed error budget of the system and the results of the E2E simulations that show the impact that such a system will have on the science done with GTC.

EMCCD in-situ periodic characterization in Shack-Hartmann wavefront sensor for GTCAO

The Gran Telescopio Canarias Adaptive Optics (GTCAO) will measure the wavefront with a Shack-Hartmann sensor. This wavefront sensor (WFS) is based on the CCD220, an electron-multiplying CCD (EMCCD) that achieves sub-electron readout noise, increasing the signal to noise ratio when weak natural guide stars (NGS) are used as reference. GTCAO will start its operation in telescope with NGS, using only one wavefront sensor, and later it will incorporate a Laser Guide Star (LGS) and consequently a second WFS, also based on an EMCCD. Both EMCCDs and a third one used as spare, have been characterized and compared including the system gain, electron- multiplication gain, readout noise vs gain, excess noise and linearity. The EM gain calibration is important to keep all EMCCD channels in the linear regime and the camera manufacturer carries it out, but it is reported that the multiplication gain may suffer ageing and degradation even if the camera is not in use. This suggests the need to monitor this ageing. In this paper it is proposed and tested a procedure for predictive maintenance that re-characterize the system gain, electron- multiplication gain and linearity periodically in order to predict the eventual ageing of the EMCCD multiplying registers. This procedure can be carried out quickly while the detector is installed in the WFS and in operational status. In order to provide the required illumination, the GTCAO calibration system is used.

Servo control simulations and preliminary laboratory results for GTC adaptive optics with NGS

The Gran Telescopio Canarias Adaptive Optics (GTCAO) is a single-conjugated post-focal system with a Shack Hartmann wavefront sensor, and one Deformable Mirror (DM) conjugated to the pupil. The optical design for tip-tilt correction includes two different mirrors, DM and the telescope M2, being M2 also used for off-loading the DM to avoid reaching its stroke limits. This optical configuration is open to different control strategies that have been simulated with Matlab. Later it has also been simulated using Durham Adaptive optics Real-time Controller (DARC) and its AO simulator, DASP. Finally some preliminary laboratory results are presented.

GTCAO real time AO closed loop software implementation and initial computer performance analysis

The Gran Telescopio Canarias Adaptive Optics (GTCAO) is a single-conjugated post-focal system with a Shack Hartmann wavefront sensor working at visible wavelength and one Deformable Mirror (DM) conjugated to the pupil. GTCAO does not include a fast tip-tilt mirror in its optical bench so it relies on the telescope secondary mirror (M2) to correct low frequency tip-tilt and offload the DM. This paper describes specific details of the software implementation of the mirror control for GTCAO, analyses its computational needs, presents the series of tests performed on the newly designed AO closed loop, and summarises software optimizations and operating system configurations set in order to optimise computer performance in the available hardware architecture