If they wanted to use a geosynchronous satellite from the ISS, it would be occluded by the earth half of the time and the other half of the time they'd have to

If they wanted to use a geosynchronous satellite from the ISS, it would be occluded by the earth half of the time and the other half of the time they'd have to track it with a satellite dish over the course of the 45 minutes (out of every 1.5 hours) they have access to it. of course the same is true of ground stations... I'm not actually sure how they do it but they probably don't need as high-gain of an antenna to reach them.