  Recently, a relative transfer function (RTF)-vector-based method has been
proposed to estimate the direction of arrival (DOA) of a target speaker for a
binaural hearing aid setup, assuming the availability of external microphones.
This method exploits the external microphones to estimate the RTF vector
corresponding to the binaural hearing aid and constructs a one-dimensional
spatial spectrum by comparing the estimated RTF vector against a database of
anechoic prototype RTF vectors for several directions. In this paper we assume
the availability of a calibrated array of external microphones, which is
characterized by a second database of anechoic prototype RTF vectors. We
propose a method, where the external microphones are not only exploited to
estimate the RTF vector corresponding to the binaural hearing aid but also
assist in estimating the DOA of the target speaker. Based on the estimated RTF
vector for all microphones and both prototype databases, a two-dimensional
spatial spectrum is constructed from which the DOA is estimated. Experimental
results for a reverberant environment with diffuse-like noise show that
assisted DOA estimation outperforms DOA estimation where the prototype database
characterizing the array of external microphones is not used.
