  We present a method for determining the background of Fermi GBM GRBs using
the satellite positional information and a physical model. Since the polynomial
fitting method typically used for GRBs is generally only indicative of the
background over relatively short timescales, this method is particularly useful
in the cases of long GRBs or those which have Autonomous Repoint Request (ARR)
and a background with much variability on short timescales. We give a Direction
Dependent Background Fitting (DDBF) method for separating the motion effects
from the real data and calculate the duration (T90 and T50, as well as
confidence intervals) of the nine example bursts, from which two resulted an
ARR. We also summarize the features of our method and compare it qualitatively
with the official GBM Catalogue. Our background filtering method uses a model
based on the physical information of the satellite position. Therefore, it has
many advantages compared to previous methods. It can fit long background
intervals, remove all the features caused by the rocking behaviour of the
satellite, and search for long emissions or not-triggered events. Furthermore,
many part of the fitting have now been automatised, and the method have been
shown to work for both Sky Survey mode and ARR mode data. Future work will
provide a burst catalogue with DDBF.
