OBT-UT Conversion

Furthermore, an OBT-UT conversion file yields a set of OBT-UT couples, about ten per orbit, measured at each passage of the satellite over the Ground Station: this file is needed to transform all the events, tagged with the on-board time, into Universal Time. The OBT recycles every 65536 ($2^{16}$) s and UT is expressed in Seconds of Day (SOD).

The BeppoSAX on-board clock consists of a quartz oscillator, whose frequency little depends on the temperature; this fact is responsible for a little drift, giving origin to a systematical timing error of $\sim$1 s every $10^5$ s. This potential source of error makes the synchronization of the on-board clock with the Ground Station time at every passage necessary.

In order to account for this, the S/W maintaining the GRBM archive and developed by the author, performs the OBT-UT conversion with a least square linear fit between all the couples belonging to the same OP. In fig.  an example of this linear fitting procedure is shown; below the coefficients that best fit the OBT-UT relationship with their 1-$\sigma$ uncertainties are reported:

$$UT = a \cdot OBT + b$$

$$a = 1 - (3.697 \pm 0.002)\times 10^{-7}$$

$$b = (19371.93399 \pm 0.00002)~\mbox{s}$$

The indetermination introduced by this method in estimating the UT is a few $10^{-4}$ s in the worst cases, for the longest OPs ($10^5$ s, see fig. ).

Figure: Linear fit of the OBT-UT relationship in the case of the OP 11729 (July, 31 - August, 1, 2001). In the upper panel, the conversion couples concerning the overall OP, the best linear fit and the temporal residuals are plotted : in the case of FOT data, a unique linear fit is performed on data taken from several contiguous orbits. In the lower panel, the zoom on a single orbit data from the same OP is shown: in this figure, the fit is the same as before, but when dealing with RAW data, the OBT-UT conversion is performed one orbit at a time.

For each OP, the time boundaries of every Observation are converted into universal times; then, when there are HTR data belonging to some events that triggered the on-board logic, their correspondent trigger times are converted into UT, as well.