  The Space multi-band Variable Object Monitor (SVOM) is a proposed Chinese
astronomical satellite, dedicated to the detection, localization and
measurement of the gamma-ray bursts (GRBs) on the cosmological scale. An
efficient algorithm is developed for the purpose of onboard star extraction
from the CCD images obtained with the Visible Telescope (VT) on board SVOM. The
CCD pixel coordinates of the reference stars will be used to refine the
astronomical position of the satellite, which will facilitate trigger rapid
ground-based follow-up obsevations of the GRBs. In this algorithm, the image is
divided into a number of grid cells and the "global" pixel-value maximum within
each cell is taken as the first-guess position of a "bright" star. The correct
center postion of a star is then computed using a simple iterative method.
Applying two additional strategies, i.e., scanning the image only by even (or
odd) lines or in a black-white chess board mode, are proposed to further reduce
the time to extract the stars. To examine the efficiency of the above
aglorithms, we applied them to the experimental images obtained with a
ground-based telescope. We find that the accuracy of the astronomical
positioning achieved by our method is comparable to that derived by using the
conventional star-extraction method, while the former needs CPU time about 25
times less than the latter. This will significantly improve the performance of
the SVOM VT mission.
