  A cylindrical GEM tracker is under construction in order to replace and
improve the inner tracking system of the BESIII experiment. Tests with planar
chamber prototypes were carried out on the H4 beam line of SPS (CERN) with
muons of 150 GeV/c momentum, to evaluate the efficiency and resolution under
different working conditions. The obtained efficiency was in the 96 - 98%
range. Two complementary algorithms for the position determination were
developed: the charge centroid and the micro-TPC methods. With the former,
resolutions <100 micron and <200 micron were achieved without and with magnetic
field, respectively. The micro-TPC improved these results. By the end of 2016,
the first cylindrical prototype was tested on the same beam line. It showed
optimal stability under different settings. The comparison of its performance
with respect to the planar chambers is ongoing. Here, the results of the planar
prototype tests will be addressed.
