The multi-band signal fusion method is a practical and effective technique to improve the range resolution of the radar. This paper proposes a multi-stage approach based on the Generalized Likelihood Ratio (GLR) test for estimating the target range profile using sparse subband measurement data. The proposed subband fusion algorithm estimates, and compensates for the incoherent factors between the subbands, in the first step. In the second step, by combining the data of subbands, the model parameters and the number of scattering centers are estimated based on the Geometrical Theory of Diffraction (GTD) model. Finally, based on the initial values of the model parameters and the incoherent factors obtained in the preceding steps, the GTD model parameters, and the incoherence parameters are re-estimated simultaneously based on the GLR test to improve the parameters’ accuracy. In comparison to the conventional methods, where the incoherence parameters and the model parameters are independently estimated, the accuracy of the estimation of the parameters in the proposed method has improved with the simultaneous estimation of these two types of parameters. Also, the proposed method does not require prior information about the number of scattering centers. The validity and performance of this algorithm have been investigated with analytical and simulated data.