Characterisation of the optical channel GEO to ground: using five years of data from Alphasat TDP1 and T-AOGS for investigation of different conditions

Abstract

An important step towards reliable optical communication between ground and space is the characterization and understanding of atmospheric conditions -even when being at one place and performing links into one direction, but at different times. Within this paper the analysis of five years of optical links from a laser terminal on a geostationary satellite (TDP1-LCT on Alphasat) to ground (T-AOGS at Tenerife) is shown. The data, in total 40.2 h of reliable measurements extracted from 516 performed links, are spread over several times of the year and almost all times of day. For one of the main parameters describing the atmosphere (the Fried Parameter r0) a clear characteristic over the day is validated. For the up-link the influence of different transmitted beam divergence angles on the link budget in terms of mean power and fade statistic is shown. A short outlook on upcoming changes within the TDP-1 software and the T-AOGS functionality is given.

Pre-distortion adaptive optics: experimental results from bi-directional tracking links between DLR’s optical ground station and Alphasat’s TDP-1 terminal

“…It is also worth noting that TDP-1 has been used extensively over the past decade for technology and scientific demonstrations that involve establishing FSOC tracking links via a Transportable Adaptive Optical Ground Station (T-AOGS). 20,22,23 However, these prior links would be established by transmitting approximately 50 W in a smallest beam diameter of 35 mm from the T-AOGS. The results presented in this work demonstrate that a larger beam can be pre-distorted and then transmitted with only ~1-2 W of power from the telescope aperture and still have the same effect, thus providing an extremely power-efficient method of establishing bi-directional FSOC links with Alphasat in GEO.…”

“…The uplink laser beam then inherits the appropriate pointahead angle (of approximately 18 μrad) via the point-ahead angle mirror, and is ‶pre-distorted″ as it propagates via the DM and FSM on its way to the telescope. As the LCT-135 on Alphasat was designed for coherent optical communication links at 1064 nm, 20 the downlink and uplink laser beams were isolated by polarization for these experiments. This was achieved by first merging the orthogonally polarized downlink and uplink laser beam paths via a polarization beam splitter and then placing a quarter-wave plate prior to the final three mirrors of the telescope in the uplink light path.…”

“…It is a communication terminal designed for coherent optical communication at 1064 nm and offers binary phase shift keying at user data rates of 1.8 Gbps or 600 Mbps. 20 The TDP-1 also consists of a Ka band antenna that can relay either communication data or telemetry (at 25 kHz) to ground, where those data are recorded. Thus, in addition to communication, the LCT-135 can be used as a measurement device to quantify the received light during tracking or communication links.…”

“…The dynamic range that can be achieved with the heterodyne detection mode is 22 dB, with the minimum required received power being -84 dBW. 20 The uplink laser beam for this work was amplitude modulated at 1 MHz to enable effective tracking by the LCT-135.…”

Status on laser communication activities at Tesat-Spacecom

“…With these options, e.g. analyses of the atmospheric/optical path characteristics can be performed for a variety of applications [10]. For more information on the implementation of the demonstrator, refer to [5].…”