A method for estimation of relative coordinates between at least two parts of a system. The method generally includes two main steps: measuring a set of relevant coordinates for each part of the system; and processing each measurement result to estimate the relative coordinates between at least two parts of the system. The system includes a main body and at least one sub assembly. A first sensor A is configured to measure a movement rate of a sub assembly. A second sensor B is configured to measure a movement rate of a main body. A third absolute measurement sensor C is configured to estimate an integrator offset and a drift rate due to the first sensor A bias and due to the second sensor B bias.