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?
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!