  A Guide Tube Calibration System (GTCS) has been designed for the Jiangmen
Underground Neutrino Observatory (JUNO), in order to measure the detector
energy response near the outer radius of the active volume. Recently, a
prototype system has been constructed and tested, and the calibration algorithm
has also been studied to evaluate the risk when the simulation tuning and the
error control fail. In this paper, we first report its construction and the
performance tests in the lab. Then the influence on the global energy
measurement caused by the simulation bias of GTCS is discussed, in order to
make sure the algorithm is qualified.
