A single FRF represents the largest unit of Astro-D data and is one or more orbit's worth of data which is transmitted to the ground when the satellite passes overhead at a ground station (ten times a day). Each pass usually generates two sorts of file: ``stored data", which are collected and stored while the satellite is out of contact; and ``real-time data", which are collected and relayed to the ground instantaneously as the satellite passes overhead. Real-time data have a higher bit rate, usually manifested in higher time resolution or larger fields of view. A contiguous pair of stored and real passes from the same orbit--or, indeed, from several orbits--can be analyzed together.
ISAS will add a FITS header to each FRF in order to ease the identification and tracking of files in the processing system.