  The next generation of Extremely Large Telescope (24 to 39m diameter) will
suffer from the so-called "pupil fragmentation" problem. Due to their pupil
shape complexity (segmentation, large spiders ...), some differential pistons
may appear between some isolated part of the full pupil during the
observations. Although classical AO system will be able to correct for
turbulence effects, they will be blind to this specific telescope induced
perturbations. Hence, such differential piston, a.k.a petal modes, will prevent
to reach the diffraction limit of the telescope and ultimately will represent
the main limitation of AO-assisted observation with an ELT. In this work we
analyse the spatial structure of these petal modes and how it affects the
ability of a Pyramid Wavefront sensor to sense them. Then we propose a
variation around the classical Pyramid concept for increasing the WFS
sensitivity to this particular modes. Nevertheless, We show that one single WFS
can not accurately and simultaneously measure turbulence and petal modes. We
propose a double path wavefront sensor scheme to solve this problem. We show
that such a scheme,associated to a spatial filtering of residual turbulence in
the second WFS path dedicated to petal mode sensing, allows to fully measure
and correct for both turbulence and fragmentation effects and will eventually
restore the full capability and spatial resolution of the future ELT.
