As the EarthProbe TOMS instrument orbits the Earth, it scans back and forth across the surface, perpendicular to the orbit track. A scanning mirror comes to rest in each of 35 positions long enough to take each measurement, then moves on to the next position. The portion of the Earth seen by the instrument while the mirror is at rest is the instrument's field of view (FOV). At scan position 18, the instrument is looking to the nadir (straight down). At scan positions 1 and 35, the instrument is looking off to the sides, and the field of view is much larger at these positions than at position 18.

The overpass files contain the TOMS data for the instrument's FOV that is best matched to a particular site on the Earth. There is a maximum of one record for each day. Sites that are at high latitudes will have no records during the polar night portion of the year. Also, prior to the boost of EarthProbe, because of the lower altitude, the instrument's scanning did not cover the entire sunlit surface of the Earth in 24 hours; during this period, for low latitude sites, there are many days when no instrumental FOV is close enough to a particular site to make a match. Finally, there are occasionally brief periods when the instrument is not collecting data, or when communication difficulties have resulted in data drops, and there will be "missing" overpass records in these cases, too.

The data compiled in the overpass files are selected from the TOMS Level-2 data. That is, each quantity is derived from a single FOV. This differs from the TOMS Level-3 data, which are averages of the Level-2 data, tailored to a regular grid of latitude-longitude quadrilaterals covering the Earth's surface. Though it is possible to construct a time series from the Level-3 data for any site on the Earth, the overpass data will generally more accurately represent the conditions at that site.