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B.4 HRI Timing Problems

    

The space-craft clock onboard the ROSAT satellite has undergone several resets during the first two years of the mission. There has been two reasons for a space-clock reset:

1. The satellite is lost or enters safe mode.
2. Annual reset due to an overflow in the on-board counter.

Until recently the processing of these data has introduced an fractional second offset in times assigned to the photons. Below is a list of the clock resets and the fractional seconds by which the times are offset.

1991 January 26

Satellite Gliche. HRI data taken after this time have event times with an absolute offset of 33/64 seconds.

1991 June 25

Annual overflow. HRI data taken after this time have event times with an absolute offset of 58/64 seconds.

1992 February 12

Satellite Gliche. The reset of 63/64 second caused the severe problems in the HRI software. Data taken after this time can have backward times of less than 1 second in duration. Timing analyses for which timescales of a few seconds or less are important may be affected and should be deferred until reprocessed data become available. Fixed in Release SASS5_8 (HRI_4_5).

1992 July 17

Annual Overflow. Negative fractional reset of -1/64 second. The event times have an absolute offset of -1.0 second. This absolute offset should not inhibit users from searching for periodic behavior. Fixed in SASS HRI_4_5_2.

If an observation interval (OBI) processed previous to HRI 4_5 is merged with one run post HRI_4_5, the times in the early OBIs will be offset by a constant fractional second, and the new OBIs will be correct to the fraction. All data run with a software version previous to HRI_4_5 and since the first reset will also be offset by the fractional seconds of the reset. This value will be constant throughout the sequence unless OBIs that were merged were taken at different resets.

 
• HRI Timing Summary