Here is a short introduction on
Recursive online estimation of parameters of
time-varying AR processes [ps file]. The notes are very heuristic,
summarising what time-varying AR processes are and how one can estimate
the parameters of a time-varying AR process
using an recursive online algorithm.
For more information on applications
see the group of Prof. Witte at the
The Institute of Medical Statistics and Computer Science,
University of Jena,
where they have modelled EEG data with time-varing vector AR processes and have
used a recursive online algorithm to estimate the time-varying parameters.