  The problem on the Cramer-Rao Lower Bounds (CRLBs) for the joint time delay
and Doppler stretch estimation of an extended target is considered in this
paper. The integral representations of the CRLBs for both the time delay and
the Doppler stretch are derived. To facilitate computation and analysis, series
representations and approximations of the CRLBs are introduced. According to
these series representations, the impact of several waveform parameters on the
estimation accuracy is investigated, which reveals that the CRLB of the Doppler
stretch is inversely proportional to the effective time-bandwidth product of
the waveform. This conclusion generalizes a previous result in the narrowband
case. The popular wideband ambiguity function (WBAF) based delay-Doppler
estimator is evaluated and compared with the CRLBs through numerical
experiments. Our results indicate that the WBAF estimator, originally derived
from a single scatterer model, is not suitable for the parameter estimation of
an extended target.
