Understanding the vertical distribution of atmospheric optical turbulence is essential for the global-scale implementation of free-space optical communications (FSOC). Maintaining communications with satellites in low-Earth orbit (LEO) requires tracking over changing elevation angles. Decreasing elevation angles in optical communication links due to a satellite’s orbit attributes to significant signal losses due to increased propagation lengths and strong turbulence. Here we present the variance in atmospheric optical turbulence measurements in the form of scintillation index and Fried parameter measured on the Island of La Palma. These measurements are taken between elevation angles of 90° and 0° with reference measurements being taken concurrently at zenith to remove temporal variations. The results are compared with the existing theory.
Atmospheric optical turbulence causes signal loses in laser propagation. Here we present vertical measurements of optical turbulence taken in London’s financial district. Additionally, we demonstrate a method of modelling atmospheric states in simulation from the measured data. From this we derive the predicted system performance of an optical downlink from a satellite in low Earth orbit (LEO) to ground in the atmospheric conditions observed on the night. We also present the improvements in performance with the addition of adaptive optics at the receiver end.
We present the 24-hour Shack-Hartmann Image Motion Monitor (24hSHIMM), the first truly continuous, 24-hour optical turbulence monitor. Atmospheric optical turbulence is a significant limitation for free-space optical communications and other technologies. Knowledge of the turbulence conditions allows for the selection of favourable sites for optical ground stations. It also enhances operations though providing data for assimilation into turbulence forecasting models and real-time monitoring of conditions. The 24hSHIMM uses a Shack-Hartmann wavefront sensor to measure a low-resolution vertical optical turbulence profile, from which the coherence length, angle and Rytov variance are calculated. Additionally a vertical wind speed profile from meteorological forecast data is used to calculate the coherence time. Due to its portability, the instrument can operate in a wide variety of locations, even urban, to provide continuous information about the atmospheric turbulence. To demonstrate this, we show parameters recorded at the astronomical observatory in La Palma for a continuous 36-hour period. With its wide array of capabilities, the 24hSHIMM offers strong support for future research in free-space optics.
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