Characterisation and mitigation of atmospheric turbulence is critical for free space optical communication that relies on adaptive optics such as high bit rate coherent modulation or quantum key distribution. Turbulence profiling, i.e. measuring turbulence at different altitudes, provides more detail than typical seeing monitors and supports sophisticated AO and the possibility to forecast conditions. We present the implementation of a Ring-Image Next Generation Scintillation Sensor (RINGSS) instrument that profiles turbulence with a novel approach of defocused ring images introduced by A. Tokovinin (2021) 1 . RINGSS is exceptionally low-cost, small, and fully automated, requiring significantly simpler equipment than previous turbulence profilers. We have demonstrated preliminary results that demonstrate the capability of this instrument for measurements of seeing and a low resolution turbulence profile. Future work is outlined that includes cross-calibration with a Stereo-SCIDAR instrument recently commissioned on the ANU 2.3m telescope at Siding Spring Observatory and plans for deployment at prospective optical ground station sites for an Australia-New Zealand optical network.
The Australian National University (ANU) Optical Communications Ground Station (OCGS) is currently under development at Mt. Stromlo Observatory in Canberra, Australia. The OCGS will be compatible with a range of wavelengths, coding schemes, and techniques to cover satellites in Low Earth Orbit to Lunar and deep-space, and provide a platform for quantum communication from satellites. We have conducted a feasibility study and preliminary design review for the development of an instrument to support the CCSDS high photon efficiency (HPE) standard so the OCGS can support future lunar missions featuring optical communication terminals. The development of lunar communication capabilities in Australia offers site diversity and increased visibility, allowing for improved optical link availability during missions.We present the preliminary design for the transmitter and receiver which will integrate on the 70 cm telescope in the OCGS. A lab prototype of the transmitter has been built to demonstrate the generation of a pulse position modulation (PPM) waveform which is compatible with the CCSDS high photon efficiency (HPE) standard. The transmitter is made up of four 15 cm apertures which is mounted by a piggyback to the telescope. Each can operate as an independent channel with fine steering control through a fast steering mirror. The apertures are separated by characteristic atmospheric turbulence length r 0 to minimise fading at the spacecraft.The receiver is installed at the Nasmyth port of the 70 cm telescope. The receiver features a fast steering mirror to maximise coupling into a multimode fibre. The signal is split with a photonic lantern and sent to several superconducting nanowire single photon detectors (SNSPD).
Network capacity and reliability for free space optical communication (FSOC) is strongly driven by ground station availability, which is dominated by local cloud cover causing an outage. Here, we combine remote sensing data and novel methods to provide a generalized framework for assessing and optimizing optical ground station networks. This work is guided by an example network of eight Australian and New Zealand optical communication ground stations that span approximately 60° in longitude and 20° in latitude. Utilizing time-dependent cloud cover data from five satellites, we present a detailed analysis that determines the network availability and diversity, which showed that the Australasian region is well-suited for an optical network with a 69% average site availability and low spatial cloud cover correlations. Employing methods from computational neuroscience, we provide a Monte Carlo method for sampling the joint probability distribution of site availabilities for an arbitrarily sized and point-wise correlated network of ground stations. Furthermore, we develop a general heuristic for site selection under availability and correlation optimizations and combine it with orbital propagation simulations to compare the data capacity between optimized networks and the example network. We show that the example network may be capable of providing tens of terabits per day to a low Earth orbit satellite and up to 99.97% reliability to geostationary satellites. We therefore used the Australasian region to demonstrate, to the best of our knowledge, novel, generalized tools for assessing and optimizing FSOC ground station networks, as well as the suitability of the region for hosting such a network.
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