I. Introduction
Localization of autonomous underwater vehicles (AUVs) has always been a challenging problem due to rapid attenuation of radio-frequency and global positioning system (GPS) signals [1], [2]. Inspired by the cooperative idea in [3] , an acoustic range measurement based cooperative localization scheme has been proposed to solve this problem [4]. A few surface craft or AUVs that serve as leaders are equipped with high-accuracy navigation systems, and they aid the remaining AUVs with low-accuracy dead-reckoning systems by transmitting the relative range measurements through acoustic modems so that the localization errors are bounded [5], [6].