  Slow fluctuations of Overhauser fields are an important source for
decoherence in spin qubits hosted in III-V semiconductor quantum dots. Focusing
on the effect of the field gradient on double-dot singlet-triplet qubits, we
present two adaptive Bayesian schemes to estimate the magnitude of the gradient
by a series of free induction decay experiments. We concentrate on reducing the
computational overhead, with a real-time implementation of the schemes in mind.
We show how it is possible to achieve a significant improvement of estimation
accuracy compared to more traditional estimation methods. We include an
analysis of the effects of dephasing and the drift of the gradient itself.
