  The Cylindrical GEM-Inner Tracker (CGEM-IT) is the upgrade of the internal
tracking system of the BESIII experiment. It consists of three layers of
cylindrically-shaped triple GEMs, with important innovations with respect to
the existing GEM detectors, in order to achieve the best performance with the
lowest material budget. It will be the first cylindrical GEM running with
analog readout inside a 1T magnetic field. The simultaneous measurement of both
the deposited charge and the signal time will permit to use a combination of
two algorithms to evaluate the spatial position of the charged tracks inside
the CGEM-IT: the charge centroid and the micro time projection chamber modes.
They are complementary and can cope with the asymmetry of the electron
avalanche when running in magnetic field and with non-orthogonal incident
tracks. To evaluate the behavior under different working settings, both planar
chambers and the first cylindrical prototype have been tested during various
test beams at CERN with 150 GeV/c muons and pions. This paper reports the
results obtained with the two reconstruction methods and a comparison between
the planar and cylindrical chambers.
