Yep, it seems like you get a transmit window every ~2 hours max. All in all this service is much more reasonable(cost/antenna size/transmit rate/power) than I expected. You can basically send 2kb/day from a large chunk of the world(with a view of the sky) for $5/mo. Somehow even cheaper than m2m cell plans I've encountered, though granted I haven't looked at services in the past few years.