The structure of the Si-terminated 3C-SiC(001)-c(4×2) surface reconstruction is determined using synchrotron-radiation-based x-ray photoelectron diffraction from the Si 2p and C 1s core levels. Only the alternating up-and-down dimer (AUDD) model reproduces satisfactorily the experimental results. The refinement of the AUDD model leads to a height difference of (0.4 ± 0.1) Å between the up and down Si-Si dimers. Also, the top and bottom dimers have alternating bond lengths at (2.5 ± 0.2) Å and (2.2 ± 0.2) Å, respectively. These results are in excellent agreement with ab initio density-functional calculations, which also further support the high sensitivity of this reconstruction on lateral strain and on the presence of defects. Finally, beyond well-established synchrotron-radiation-based core-level photoemission spectroscopy, an assignment is made on the structural origin of each Si 2p surface and subsurface shifted component, based on their different photoelectron diffraction patterns.