Optical high-capacity satellite downlinks via high-altitude platform relays

Earth-observation (EO) satellite missions produce a large amount of data using high-resolution optical or radar sensors. During the last decades the amount of data has steadily increased due to improved sensor technologies with increased temporal resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps mitigate the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability through the atmosphere due to cloud blockage above the ground station. Apart from diversity concepts with several ground stations or satellite networks, a stratospheric High Altitude Platform (HAP) could act as a relay station to forward the optical communication beam over the last 20km through the atmosphere to the ground station, where short-range, high data-rate microwave systems are feasible. This paper will discuss the capabilities of HAP and GEO relay stations to increase the downlink capacities of LEO satellites. Environmental aspects for the deployment of HAP relays and regulatory/technology issues for a microwave downlink on the last 20km to the ground will be discussed.

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