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To overcome the shortcomings of the traditional measurement error calibration methods for spaceflight telemetry, tracking and command (TT&C) systems, an online error calibration method based on low Earth orbit satellite-to-ground double-differential GPS (LEO-ground DDGPS) is proposed in this study. A fixed-interval smoother combined with a pair of forward and backward adaptive robust Kalman filters (ARKFs) is adopted to solve the LEO-ground baseline, and the ant colony optimization (ACO) algorithm is used to deal with the ambiguity resolution problem. The precise baseline solution of DDGPS is then used as a comparative reference to calibrate the systematic errors in the TT&C measurements, in which the parameters of the range error model are solved by a batch least squares algorithm. To validate the performance of the new online error calibration method, a hardware-in-the-loop simulation platform is constructed with independently developed spaceboe dual-frequency GPS receivers and a Spirent GPS signal generator. The simulation results show that with the fixed-interval smoother, a baseline estimation accuracy (RMS, single axis) of better than 10 cm is achieved. Using this DDGPS solution as the reference, the systematic error of the TT&C ranging system is effectively calibrated, and the residual systematic error is less than 5 cm.