  The SVOM mission under development will carry various instruments, and in
particular the coded-mask telescope ECLAIRs, with a large field of view of
about 2 sr, operating in the 4--150 keV energy band, whose goal is to detect
high energy transients such as gamma-ray bursts. The trigger software onboard
ECLAIRs will search for new hard X-ray sources appearing in the sky, as well as
peculiar behaviour (e.g. strong outbursts) from known sources, in order to
repoint the satellite to perform follow-up observations with its onboard narrow
field of view instruments. The presence of known X-ray sources must be
disentangled from the appearance of new sources. This is done with the help of
an onboard source catalogue, which we present in this paper. As an input we use
catalogues of X-ray sources detected by Swift/BAT and MAXI/GSC and we study the
influence of the sources on ECLAIRs' background level and on the quality of the
sky image reconstruction process. We show that the influence of the sources
depends on the pointing direction on the sky, on the energy band and on the
exposure time. In the Galactic centre, the known sources contribution largely
dominates the cosmic X-ray background, which is, on the contrary, the main
background in sky regions empty of strong sources. We also demonstrate the need
to clean the sources contributions in order to maintain a low noise level in
the sky images and to keep the threshold applied for the detection of new
sources as low as possible, without introducing false triggers. We briefly
describe one of our cleaning methods and its challenges. Finally, we present
the overall structure of the onboard catalogue and the way it will be used to
perform the source cleaning and disentangle the detections of new sources from
outbursts of known sources.
