The accuracy of time delay estimates obtainable in active localization
systems is studied, focusing on the effect of ambiguities in the time
delay estimates. Such ambiguities occur when the transmitted signal h
as small relative bandwidth. Then, for signal to noise ratios below a
certain threshold, the commonly used Cramer-Rao lower bound is not rea
lizable. The study concentrates on the region of intermediate SNR valu
es, where the Cramer-Rao bound is no longer achievable, but useful inf
ormation on time delays can still be obtained from the measurement. Re
alizable bounds for the single and two echo cases are obtained by deri
ving a new form of the Barankin bound for active time delay estimation
. This form maintains the realizability property of the most general f
orm, but is of reasonable complexity. New bounds are derived for the m
ultiple echo case. Examples are presented to illustrate the dependence
of the bound on parameters such as SNR, relative bandwidth, and echo
separation.