Bernard Laurent

SILEX project: the first European optical intersatellite link experiment

The SILEX concept, based on 0.8-micron diodes, is summarized, and the key features of the mission and its technical definition are described. The SILEX project is at the beginning of its development phase. The two optical payloads will be ready for launch in 1994-1995 so as to allow a flight demonstration from 1996 on, depending on the satellite launch dates, which are to be decided as a function of the operation of SPOT 3 in orbit. The SILEX general schedule is given. One of the aims of the engineering phase has been to develop a complete system test bed which has made it possible to reinforce the overall selected concept and verify the expected performances. This test bed is to be refurbished to become an essential integration and verification tool during the hardware development and integration phase. Present technological perspectives on components confirm the selection of the wavelength range and its suitability to a longer-term application: optical power and reliability for the laser diodes, and very low noise performance for the detectors.

SILEX overview after flight terminals campaign

The ESA program dedicated to Optical Communications development has allowed to qualify two terminals in 1994 and 1995. In 1995/96, the respective proto flight models have been integrated and tested, leading to in depth knowledge of the behavior of optical terminals in optomechanical, pointing and telecommunications fields for various environmental conditions. The paper describes the program and the associated major tests results at terminal level or in coupled configuration on SPOTIV spacecraft. Considerations for future application are given in order to identify SILEX feedback on MMS optical terminal product line for commercial market.

SILEX final ground testing and in-flight performance assessment

SILEX (Semi-Conductor Inter-satellite Link EXperiment) consists of one optical terminal on-board the French LEO observation satellite SPOT 4, and another on-board the European GEO telecommunication satellite ARTEMIS. While the first part of the SILEX verification plan had been oriented towards verification at equipment and subsystem levels, the final stages have mainly been devoted to terminal and system (terminals coupling effects) verification. During this final stage, a thermal vacuum test was conducted in a class 100- cleanliness environment with optical ground support equipment of outstanding performances. The obtained tests results, used to determine software compensations and verify optical and static pointing performances, have been entered into overall system simulation models to finalize flight performances budgets. In addition, systems tests were performed on each terminal with respective partner simulator to validate system simulation models and assess link performances and robustness and to verify communication bit error rate.

System test bed for demonstration of the optical space communications feasibility

A complete system test bed (STB) built for demonstrating optical space communication feasibility is described. The STB uses functional breadboards of all key equipment required for an optical terminal. Results of experiments carried out to evaluate the communication and pointing functions of the STB, both at subsystem and at terminal levels, are presented and compared to theoretical calculations.

Space applications of optical communications

Optical communication in Space is now a reality. In this paper we present the recent developments that were undertaken in Europe for this application. We first describe the different missions where optical communications are useful: link between two geostationary satellites (geo-geo), Data Relay Mission (leo-geo) and High Data Rate Satellite Constellation Network. Then we detail the different candidate laser technologies from the most straightforward technologies that have been developed for optical fiber applications (λ=1.55 µm) and 0.8 µm technology based on Silicon detector to the recent developments based on high power fiber amplifiers. In the last chapter we describe thesilex (Semi conductor Intersatellite Link Experiment) program which performs optical communication betweenspot4 Earth observation satellite (cnes) andartemis (esa). The excellent results based on 0.8 µm laser diode technology are considered to be a major milestone in optical intersatellite communicationRésuméLes systèmes de communications optiques intersatellites sont devenus une réalité. Cet article décrit les principaux travaux qui ont eu lieu dans ce domaine en Europe. La première partie expose les principales applications des liaisons optiques: Service Fixe entre deux satellites géostationnaires (geo-geo), Service Relais entre un satellite en orbite basse et le sol via un satellite géostationnaire (leo-geo) et liaisons à haut débit à l’intérieur d’une Constellation de Satellites. La deuxième partie donne un aperçu rapide des différentes technologies envisageables de la transposition des technologies développées dans le cadre des liaisons optiques par fibres (λ=1,55 µm) aux systèmes basés sur des détecteurs faible bruit au silicium et des diodes lasers de forte puissance à λ=0,8 µm. La dernière partie partie présente la missionsilex (Semi conductor Intersatellite Link Experiment) qui consiste à transmettre des données numériques du satellite d’observation de la terrespot4 (cnes) vers le satellite relaisartemis (esa) en orbite géostationnaire. La confirmation des performances en vol de ce système de communications basé sur la technologie des diodes à laser 0,8 µm est un événement majeur validant la faisabilité des liaisons optiques intersatellites.

Small optical-terminal designs with a softmount interface

Disturbances to the line-of-sight induced by satellite microvibrations pose a major design challenge for optical terminals. The conventional approach is to hard-mount the terminal to the satellite so that the fine pointing control system has to cope with disturbance spectra extending above a hundred Hz. The need for a wideband tracking sensor and fine pointing mechanism and fast control electronics results in a substantial mass, power and cost overhead. An alternative approach which offers considerable advantages has been pursued by MMS. This is to attenuate the high frequency components of the microvibrations by introducing an anti-vibration interface between the terminal and the satellite. A set of elastomeric elements with associated launch locking devices performs this filtering function. MMS has employed this technology in a number of small optical terminal designs including the SOUT and the SOTT intended for LEO-GEO and GEO-GEO intersatellite links respectively. This paper provides an overview of the softmount approach including the benefits, hardware implementations and design constraints on the terminal. The SOUT and SOTT terminals are used to illustrate implementations for both fixed and gimballed terminals and pertinent measurements on the SOUT breadboard model are reported. Finally, a program to verify softmount performance experimentally for a terminal configuration dedicated to future earth observation missions is described. This successful program has led MMS to baseline this type of interface for future missions when the terminals are mounted on highly perturbed satellites.

Multi/Demultiplexer And Spectral Isolator For Optical Inter-Satellites Communications

The multi/demultiplexers use filtering, polarizing and phase retarding means to combine and split 4 optical channels, spectrally close to each other (821, 827, 833, 839 nm) with an high isolation ratio. The spectral isolator using similar means shows 3 paths : transmitting, communication and stracking. In nominal mode it transmits the MUX channels, receives the 870 nm communication channel and divides it into a given apportionment ratio to provide the tracking function. In calibration mode it connects the transmit path (MUX) to the tracking path. The performances of these equip ments require to quality optical components and technologies (coatings, polishing, assembling...). Part of these activities have been funded by CNES contract.

Inter-satellite optical communications: from SILEX to next generation systems

The continuous growth in data rate demand, the importance of real time commanding and real time access to the information for diverse civilian and military applications as well as the in-orbit demonstration of optical communication have led to boost the interest of such systems for future applications. After a presentation of the different fields of application and their associated performances requirements, this paper presents the possible optical link candidates. Then, the architecture, the design and the performances of new optical terminal generations, which profits from SILEX experience and the use of new technologies such as SiC and APS, are detailed. This new optimised generation, highly simplified with respect to SILEX terminals and dimensioned to offer higher data rate, presents attractive mass, volume and power characteristics compatible with a simple accommodation on the host vehicle.

Optical communications terminals for multimedia applications

Constellations of satellites will perform a key role in the provision of multi-media services both on a continental and global scale. Such constellations are likely to involve satellites in GEO, LEO and MEO orbits -- either in a single orbital system such as GEO or LEO, or in combinations e.g. MEO and GEO. In some of these systems ISLs are fundamental to their operation while others derive significant additional benefit from the inclusion of ISLs. MMS is developing terminal designs to meet these various anticipated requirements under both MMS and ESA funded programmes. This paper outlines some of the European multimedia initiatives that MMS is involved in and discusses the technology choices for the ISLs. In particular, the terminal requirements for both long and short range GEO to GEO links are presented. Two critical technologies which MMS is developing for these applications are described -- namely, silicon carbide telescopes and acquisition and rear-tracking (ARTS) sensors. These are important building blocks for robust and cost-effective optical terminals for multimedia applications.