XMM-Newton Science Analysis System


oal (oal-3.139) [xmmsas_20230412_1735-21.0.0]

SAS_OBT_MET_FIT

The conversion of Spacecraft-Onboard-Time (OBT) to Mission-Elapsed-Time (MET) tags makes use of a polynomial to which the data in the ODF Time Correlation File has been fitted. This is controlled through five parameters, viz: method (see below), deg (maximum degree of the polynomial), rms (Root-Mean-Square abandoning criterion for the fit), frac (fraction of OBT-MET data points that are allowed to be removed from the fitting procedure to fulfill the RMS criterion), and igngshov (see below). Default values are (0, 4, 5.e-5, .9, 1).

method can either be given as 0 or 1. A value of 0 means that the OBT-MET fit uses ready-made time correlation data that have been corrected for signal propagation delays using predicted orbit data. time. A value of 1 forces this calculation to be re-done with more accurate reconstructed orbit data. This value is ingored if TCX data is used, since the Reconstructed Time Correlation data is already corrected.

The value assigned to deg chooses one of two possible options for the interpolating function:

$\mbox{{\em deg}}>0$	: the maximum degree of the computed fit polynomial. Please note: the actual value after the fitting process can be smaller if the RMS criterion can be achieved with a smaller degree.
$\mbox{{\em deg}}<0$	: the time correlation data are not fitted by a single global function but by piecewise-linear segments through the instantaneous data values.

igngshov can either be given as 0 or 1. A value of 1 means that all groundstation handovers in the time correlation data are ignored, i.e., all data from all stations are considered in the construction of the fit polynomial. igngshov=0 on the other hand will trigger that all time correlation data after a handover are not taken into account, i.e. the fit polynomial is constructed just on the basis of the data from the first ground station alone. In the past there have been cases where the ground times of different stations varied by an amount large enough to make the OBT-MET fit fail. If this happens the igngshov provides a way to work around the problem such that processing of the ODF in question can be performed. Please note that this can only be a temporary measure and that the anomaly should be brought to the attention of the SOC. The default value of igngshov is 1.

The actual values of method, deg, rms, frac, and igngshov shall be written to corresponding attributes TFIT_RPD, TFIT_DEG, TFIT_RMS, TFIT_PFR, TFIT_IGH in the event list being processed.

Caution: Please alter the above parameters via SAS_OBT_MET_FIT only when you exactly know what you are doing. Out-of-range conditions are caught but improper values can lead to inaccurate event arrival times.