  Gamma-ray bursts (GRB) sign energetic explosions in the Universe, occurring
at cosmological distances. Multi-wavelength observations of GRB allow to study
their properties and to use them as cosmological tools. In 2012 the space borne
gamma-ray telescope ECLAIRs is expected to provide accurate GRB localizations
on the sky in near real-time, necessary for ground-based follow-up
observations. Led by CEA Saclay, France, the project is currently in its
technical design phase. ECLAIRs is optimized to detect highly red-shifted GRB
thanks to a 4 keV low energy threshold. A coded mask telescope with a 1024 cm^2
detection plane of 80x80 CdTe pixels permanently observes a 2 sr sky field. The
on-board trigger detects GRB using count-rate increase monitors on multiple
time-scales and cyclic images. It computes sky images in the 4-50 keV energy
range by de-convolving detector plane images with the mask pattern and
localizes newly detected sources with <10 arcmin accuracy. While individual GRB
photons are available hours later, GRB alerts are transmitted over a VHF
network within seconds to ground, in particular to robotic follow-up
telescopes, which refine GRB localizations to the level needed by large
spectroscopic telescopes. This paper describes the ECLAIRs concept, with
emphasis on the GRB triggering scheme.
