I've heard that US spy satellites used to eavesdrop on soviet telephone calls transmitted between line-of-sight ground-based microwave relay towers. The relay

I've heard that US spy satellites used to eavesdrop on soviet telephone calls transmitted between line-of-sight ground-based microwave relay towers. The relay towers used directional microwave antenna aimed at each other, but satellites in space could pick up those signals. I can't find a direct reference to it on wikipedia, but I suspect this is what the Vortex satellites were doing: https://en.wikipedia.org/wiki/Vortex_(satellite) https://en.wikipedia.org/wiki/Vortex_(satellite)