Centering and observability in attitude-independent magnetometer-bias determination

The TWOSTEP algorithm is examined for the case where the centered portion of the negative-log-likelihood function provides incomplete observability of the magnetometer-bias vector. In those cases where the full negative-log-likelihood function provides a complete estimate, the TWOSTEP algorithm can be modified to provide an estimate of all three components of the magnetometer bias vector. However, the procedure leads to a discrete degeneracy of the estimate which can be resolved only by explicit evaluation of the negative-log-likelihood.