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Objective:The objective of this activity is to carry out a measurement campaign and develop waveform pre-compensation model based on information coming from double-stars.
Targeted Improvements:To enable optical feeder uplinks by using the downlink information from telecommunication satellites to measure and pre-compensate the tip/tilt waveform distortion.
Description:Optical satellite links suffer multiple distortions when crossing the atmosphere. One of the most noticeable impairments consists of the atmosphere continuously deviating the beam of light from the intended direction in a phenomenon known as image motion. Such distortion is critical for optical uplinks as it happens close to the Earth surface where small angular deviations translate into large pointing errors in the geostationary arc. The feasibility of optical uplinks therefore depends on the ability of a transmitting ground station to correct image motion (also called tip/tilt). This may be done based on tip/tilt information present in the downlink beam of the GEO satellite.
As orbital dynamics dictate, the received beam (downlink) from a GEO and the transmit beam (uplink) towards the same GEO are separated by an angle of typically 4 arc sec (called point-ahead angle). Such separation makes both beams partially cross different parts of the atmosphere and therefore experience different distortion of the wave-front. However, there is an angle called isoplanatic angle within which the wave-front distortions are identical. Having an isoplanatic angle similar to the point-ahead angle opens the door to the possibility of pre-compensating the atmospheric distortions to be experienced by the uplink based on the ones estimated for the downlink.
The size of the isoplanatic angle depends on the atmospheric turbulence and the Zernike aberration order of the wave-front error. Low aberration orders, such as tip/tilt, produce larger isoplanatic angles and the opposite. While high aberrationorders may be measured and compensated by means of Rayleigh or sodium guided stars, tip/tilt information needs to be measured by other means.
This activity will develop the means of measuring tip/tilt information and waveform pre-compensation models for double stars taking advantage of the fact that some show a similar separation to the point-ahead angle of GEO satellites (4 arc sec) and are representative (propagation-wise) of a GEO optical link. The activity will develop the hardware and software necessary to track suchsystems during day and night at the standard optical communication wavelengths (1064 nm and 1550 nm). The activity will carry out ameasurement campaign to extract the cross-correlation of both measurements for tip/tilt and higher order Zernike aberrations (up toquad-foil) in varying elevations and atmospheric turbulence conditions.
Note: This activity is part of the ARTES CC AT Workplan 2018 and the ARTES ScyLight Rolling Workplan. Funding can come from both programmes.Procurement Policy: C(1) = Activity restricted to non-prime contractors (incl. SMEs). For additional information please go to EMITS news "Industrial Policy measures for non-primes, SMEs and RD entities in ESA programmes".