In order to correct ionosphere error on the GNSS signals, this paper proposes an algorithmto estimate the ionosphere corrections on the basis of the observability analysis. The distributed Kalman filter consisting of some parallel sub-filters is utilized and the ionosphere corrections are obtained from weighting the results of these sub-filters. However, the performance of some sub-filters is degraded by the sub-filters with poor observability. To evaluate the observability of these sub-filters, the Ionosphere observable factor (IOF) is defined and calculated by the eigenvalue decomposition. Moreover, to mitigate the adverse effect from the poor observability and boost the accuracy of estimates, an observability-based weighting strategy is designed and adopted. The simulation result demonstrates that the a significant improvement on the accuracy is gained by the observability-based weighting strategy.