Have these optical satellite ground station locations been chosen for clear skies?
Yes, mean cloud cover is routinely measured from satellites.  Like all satellite data (and in fact all measurements), it does have an uncertainty, but for the purpose of this question the satellite product of mean cloud coverage is good enough.  Personally, I would hesitate to trust the data at very high latitudes with frequent cover of snow or ice, because those are hard to distinguish from clouds with any form of passive down-looking remote sensing.

The figure below is released by ESA and derived from the MERIS and AATSR instruments on the former ENVISAT satellite.  Although the caption does not specify, I suspect that it uses visible channels and that it therefore only relates to daytime cloudiness. It would be interesting to see how it compares to 24-hour cloud cover!



Full image caption and credit:


  Global annual mean cloud cover derived from three years (2007–09) of Envisat data. The map shows areas with little to no cloud coverage (blue) as well as areas that are almost always cloudy (red). Clear skies are immediately visible over deserts (such as the Sahara, Namib and over the Atacama). Regions with constant high-pressure systems are detectable, such as the Pacific. On the other hand, the North Atlantic Ocean in the mid-latitudes is mostly covered with clouds, where storms develop. The UK is often cloudy, whereas the Mediterranean shows sunny skies.
  Data from both the MERIS and AATSR instruments on Envisat were used.
  
  Credits: ESA/Cloud–CCI


(On behalf of ESA and the Cloud-CCI I would like to apoligise for the poor choice of colourmap, but this figure predates the time at which many of us were educated on its poor suitability)