Instrument configuration changes since AO4

To cope with rising instrumental background rates in the Solar Minimum, the standard telemetry allocation to the different instruments has been slightly changed and for IBIS and SPI the onboard configuration was adapted to cope with higher event rates.

With respect to IBIS it was decided to reduce the number of indirect mode packets produced with a minimal impact on science yield. This is accomplished by incrementing the integration time of the Spectral Timing packets of PICsIT by a factor of four from 3.90625 msec to 15.625 msec, while doubling the number of energy channels from 4 to 8. Increasing the PICsIT integration time means a loss in sensitivity for fast pulsars, but doubling the number of energy channels helps in GRB studies. The boundaries of the 8 Spectral Timing channels are set to:

• Channel 1: 208-260 keV,
• Channel 2: 260-312 keV,
• Channel 3: 312-364 keV,
• Channel 4: 364-468 keV,
• Channel 5: 468-572 keV,
• Channel 6: 572-780 keV,
• Channel 7: 780-1196 keV,
• Channel 8: 1196-2600 keV.

Another change is to switch off the DPE (Data Processing Electronics) of the dormant JEM-X2 unit and to the re-allocate one telemetry packet currently used to transmit housekeeping data of this unit to the SPI science telemetry.

Nevertheless, the TM bandwidth allocated to SPI was again fully saturating after the 8th annealing, carried out in June 2006. In order to recover some telemetry bandwidth, with reduced impact to the science, the SPI Multiple Event Reduction Function was enabled after radiation belt entry of revolution 453 (1st July 2006).

The new default telemetry allocation in packets per 8 sec is now IBIS: 128, SPI: 105, JEM-X1: 8, JEM-X2: 0 instead of the previous IBIS: 129, SPI: 103, JEM-X1: 8, JEM-X2: 1