This paper presents an enhanced ${alpha}-{eta}$ tracking filter whose ${alpha}-{eta}$ gains are updated by an approximation method at every scan to account for the transition of measurement dependent observation error variance in two-dimensional Cartesian coordinates. The approximate ${alpha}-{eta}$ gains are calculated from the amount of the change in the tracking index and the partial derivatives of the ${alpha}-{eta}$ gains with respect to a nominal tracking index. It is shown via simulation that the proposed tracker provides improved performance compared to the conventional ${alpha}-{eta}$ tracking filter.