The GRBM detectors can also be used as an All Sky Monitor. Given their large area they can be able to detect transient hard X-ray sources in the sky at any time. However, since the GRBM experiment has no imaging capabilities, steady sources can only be monitored by using the Earth Occultation Technique, as well as currently done by BATSE. When a bright hard X-ray source is obscured by the Earth during a satellite orbit, its contribution to the detector count rate disappears, and it will be visible again when the source will rise from behind the Earth. Due to the repetitive cycle of the satellite orbit, this situation will occur once per orbit as long as the same satellite pointing is maintained, producing a count rate "step" at any rise/set of the source. One can therefore take advantage of summing several "steps" due to the source in subsequent orbits and therefore reduce the statistical error on the measurement. The occultation profile is smoothed by the Earth atmosphere and in particular it depends on the time needed to the source to cross the atmospheric profile, that is by the inclination of the line of sight to the source with respect to the orbital plane. In Fig. 4.5 we show the occultation step due to the Crab Nebula in the GRBM band with LS1, obtained with the thresholds configuration given above and with the source on-axis. Over-imposed to the detector time profile is the result of a modeling of the atmospheric absorption. The inferred net Crab count rates are reported in Tab. 4.1
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