Optical Satellite Downlinks to Optical Ground Stations and High-Altitude Platforms

Abstract

Earth-observation (EO) satellite missions using highresolution optical or radar sensors are producing an immense amount of data which needs to be send down to earth. The fraction of satellite operational time in future missions is therefore clearly limited by the downlink-capability. The current X-Band architecture is facing its technological limitations in terms of data rate while causing increased demand on antennasizes and transmit power. This bottleneck can be overcome by direct optical downlinks from EO-satellites to the ground with multi-gigabit data rates. According optical satellite terminals will be extremely small and lightweight and will require few transmit power, but one drawback is the link blockage by thick clouds. This can be overcome either by ground station diversity and careful site selection or by using optical terminals onboard high altitude platforms which serve as relays-stations for the satellite. Here we present feasibility and expected performance of these two optical scenarios and propose according space and ground station architectures.

System requirements for optical HAP-satellite links

“…To maximize the communication time with a LEO satellite, Ref. [4] proposes links down to the cloud ceiling (i.e., α = -2.7° with H HAP = 20 km). To make such links reliable, some challenges must be surmounted.…”

“…Large amount of data are generated on many LEO satellites and must be brought down to the Earth surface. The use of HAP as Earth-observation data relay has been investigated recently by several authors [1]- [4]. Data can also be generated by sensors onboard a HAP or may result from the reception by the HAP of several RF signals from the ground.…”

Feasibility assessment of optical technologies for reliable high capacity feeder links

“…The diversity gain is maximised when the site separation of the ground stations is large enough to reach statistical independence of cloud coverage. Link availabilities derived from Earth observation data suggest that with a four station network in the south of Europe link availabilities between 98% and 99% can be achieved [6,7]. These estimates cannot, however, account for small scale local weather effects.…”

Optical data downlinks from Earth observation platforms

“…Ground station (GS) locations near the earth's poles allow frequent contacts but operations of these GSs is aggravated. However, sensor operational time is mostly limited by the downlink capacity [4]. Therefore, higher downlink data rates directly allow increased sensor usage.…”

“…Clouds also can cause fading or transient link blockages in cases A to C while the aircraft platform usually operates below cloud layers. Compact cloud layers can even totally prevent the optical downlink in cases A to C, a situation that has to be overcome either by ground station diversity [4] or through a lower-rate RF backup link.…”