The most robust estimators minimizing Wahba's loss function are Davenport's q method and the Singular Value Decomposition method. The q method, which computes the optimal quaternion as the eigenvector of a symmetric 44 matrix with the largest eigenvalue, is somewhat faster. The fastest algorithms, the QUaternion ESTimator (QUEST) and the EStimators of the Optimal Quaternion (ESOQ and ESOQ2), are less robust since they solve the characteristic polynomial equation for the maximum eigenvalue. This is only an issue for measurements with widely differing accuracies, so these estimators are well suited to star trackers that track multiple stars with comparable accuracies.
