Complex sentence: Some of these corrections apply to any location, the corrections to the clocks and ephemeris data for instance, but the billows cover only a certain portion of the sky so a correction measured at any one ground station is only really useful for receivers located nearby.
Split into simpler sentences: Some of these corrections apply to any location-the corrections to the clocks and ephemeris data for instance. In contrast, the billows cover only a certain portion of the sky, so a correction measured at any one ground station is only really useful for receivers located nearby.