While searching for bursts the BAT performs an all-sky hard X-ray survey and monitor for hard X-ray transients. The BAT accumulates detector-plane maps every five minutes. These are included in the normal spacecraft telemetry stream. Sky images are searched to detect and position sources. The sensitivity of the survey is about 1 mCrab in the 15-150 keV band for two years. For regions where there are perpetually numerous strong sources in the BAT's FoV (i.e. the Galactic Plane), the limiting sensitivity is approximately 3 mCrab.
For on-board transient detection 5-minute detector-plane count-rate maps (5 minutes is the nominal exposure; in some cases the exposure may be greater or less) are accumulated in 80 energy bandpasses. Sources found in these images are compared against an on-board catalog of sources. Those sources either not listed in the catalog or showing large variability are deemed to be transients. Sources that do not match anything in the on-board catalog are considered to be GRBs. A subclass of long smooth GRBs that are not detected by the burst trigger algorithm may be detected with this process. Data on all of the hard X-ray transients that are detected will be distributed to the astrophysical community through the GCN, just like the bursts are.