On-Board Trigger Conditions

In order to detect any sudden increase in the units' count rates due to the occurrence of a GRB, an on-board trigger condition is implemented. In this case, the count rates used to this aim refer to the energy band defined between the LLT and the ULT. A moving average count rate is continuously calculated on a Long Integration Time (LIT), that can be set in the range 8-128 s (5 steps corresponding to the middle powers of 2). This mean rate is compared to another one computed on a Short Integration Time (SIT), adjustable in the range 7.8125 ms (also corresponding to the time resolution) and 4 s (10 steps, according to a geometric progression). Whenever the latter mean rate exceeds the former by $n\sigma$ (where $\sigma$ is the poissonian standard deviation and $n$ can be equal to 4, 8 or 16), the On-Board Trigger Condition (OBTC) of a single LS is matched. When this OBTC is simultaneously matched in at least two LSs, the GRBM logic is triggered, letting the High Time Resolution (HTR) Mode start ([PDS S/W User Manual, 1995], p. 155).