Complex sentence: Since 1998 through the end of 2003, the survey covered 550 square degrees with sensitivity of 22.5, i.e. estimated 50% of objects of this magnitude have been found.
Split into simpler sentences: Since 1998 through the end of 2003, the survey covered 550 square degrees with sensitivity of 22.5. I.e., an estimated 50% of objects of this magnitude have been found.