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The majority of Earth Observation satellites, whether for meteorology or land analysis, downlink their data in the X-Band to either government-run or privately-owned ground stations. In Europe, the company Tesat has developed laser terminals that have been successfully tested in orbit, from one EO satellite to one geostationary communications satellite, with TerraSAR-X and recently with Sentinel-1A. Apart from the obvious - all types of lasers interact with the atmosphere (absorption, attenuation and scattering) - and the pointing accuracy required, what would be the main technological challenges in using lasers for direct downlinks to the ground?

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  • $\begingroup$ Atmospheric attenuation is already a very good reason to avoid LEO-ground laser links: you need to have a cloudless sky and you need to be high above the horizon (>30°) for it to work, which severly restricts your access time. Note that the planned European Data Relay System is advertised for LEO-GEO laser links and for UAV-GEO links. The UAVs envisionned are the large ones, which fly above the cloud cover. $\endgroup$ – gosnold Dec 16 '14 at 15:14
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The technical challenges aren't the main barrier; several demonstration missions have demonstrated laser downlink to the ground.

The main obstacles are:

  • Operational: Clouds necessitate ground stations on mountain tops and in deserts, which generally means expensive + poor backhaul for the data as well as making it difficult to achieve 99% uptime without many spare stations all over the world
  • Regulatory: All currently planned systems rely on a powerful laser uplink "beacon" for the satellite to lock onto; that can be a hazard for aircraft and potentially to other satellites, requiring permission to be obtained days in advance of each comm session from the Laser Clearing House
  • Necessity: X-band works pretty well!
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