The DoD (DARPA I think) is working on a two-phase test aircraft which will have a one, and then five year endurance (yes, years). It will have articulating wings like a seagull to allow the solar panels a more direct incidence to the sun at certain times of day, season, and latitude.
The benefit of these things will be low cost surviellance and communication relays in a fixed position, at a tiny fraction of the cost of a satellite. Satellites can only remain in an apparently fixed position over the equator, and only at a high altitude and low angle above the horizon in many latitudes.
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