High-resolution X-ray diffraction measurements have been performed on (In,Ga,AI)As/GaAs quantum-dot superlattice structures. We are able to observe up to 31 satellite peaks from 6 periods of a quantum-dot superlattice. Theoretical simulations of the rocking curves based on the Takagi-Taupin equations of dynamical diffraction theory agree well with the experimental data. It is found, however, that to obtain agreement between theory and experiment, the height of the quantum dot, including the wetting layer under the dot, must be used in the simulations; furthermore, the indium composition is apparently reduced from its nominal value. Our results appear to support the notion that for the (In,Ga,Al)As materials system, the dots play an important role in contributing to the diffraction signature from the superlattice structure.