ESA GNC Conference Papers Repository

On December 5th, 2006, a laser link was for the first time established between an optical terminal on-board an aircraft flying at 9,000 m and the SILEX terminal on the Artemis geostationary satellite. This world first event was the result of the 3-year demonstration program LOLA performed by EADS Astrium for the arms procurement agency of the French Ministry of Defence (DGA) to investigate the feasibility of high data rate optical communications through the atmosphere. This first link was followed by a 6-month flight test campaign totalizing more than 50 successful communication sessions over 20 flights. The campaign allowed assessing the operational link performances (bit error rate before and after decoding, availability) in a variety of flight conditions (altitude & weather) and to correlate the model of optical propagation in the atmosphere. The test campaign was concluded by real-time data transmission offering HD video quality between the aircraft and the DGA stand at the Paris Air Show. One of the major challenges of optical communications is the so-called PAT (Pointing, Acquisition & Tracking) function, in charge of co-operative acquisition of the two-way link and then of directing the narrow laser beam (3.5 ?rad divergence) in order to illuminate the other terminal distant by about 40,000 km. LOLA builds on the PAT strategy developed for SILEX, whose performances and robustness have been validated since 2001 by more than 1600 LEO-GEO links between Spot4 and Artemis with an availability close to 100%. Nevertheless, significant design innovations have been introduced to benefit from technology improvements since Silex design in the early 90’, but also to face the new challenges imposed by communications through the atmosphere with an airborne terminal:<br> · As apposed to spacecraft where stable and accurate pointing is guarantied by the AOCS, the a priori pointing reference necessary for initial acquisition is not available on an aircraft.<br> · The dynamic environment onboard the aircraft is much severe than in a spacecraft. Vibrations generated by engines, aircraft movements due to piloting and atmospheric turbulences cause broad-band dynamic disturbances with a level of several mrad.<br> · The propagation through the atmosphere combined with the aircraft velocity results in a dynamic scintillation and deviation of the laser signal with a spectrum extending up to several hundreds of Hz.<br> The test campaign proved that these points are well addressed by the LOLA PAT design, since measured performances were better than predicted and most importantly, the tracking function was found very robust to dynamic signal fading due to the fast scintillation introduced by the atmosphere. Moreover, flights test demonstrated a highly desirable asset of the design, i.e. the optical link remains locked when communication performance is degraded by atmospheric propagation, allowing immediate recovery of nominal data transmission when propagation improves.<br> The paper will first describe the technical innovations introduced in the LOLA terminal to address these new challenges as well as the simplification of the terminal to pave the way for future space terminals. The main body of the discussion will then be focused on the synthesis of the PAT performances measured during the flight test campaign:<br> • A priori pointing accuracy<br> • Acquisition duration<br> • Tracking accuracy and its sensitivity to control bandwidth, varied from 300 to 700 Hz during the flights<br> • Robustness to signal fading due to scintillation in the atmosphere<br> A concluding part shows how the advanced techniques & technology validated in the LOLA programme can be applied to other fine pointing applications.