A mixed USBL / LBL acoustic navigation system has been experimentally tested in the framework of the CommsNet 13 sea trial, organized and led by CMRE in September 2013. The system set-up included a “Typhoon” AUV, equipped with a USBL/acoustic modem transducer, and a set of acoustic modems anchored at the bottom. The navigation procedure tested consisted in first geolocalizing the seabottom modems through the AUV USBL, with the AUV at the sea surface in GPS contact. Subsequently, the AUV navigates underwater using the USBL to track its relative position with respect to the moored modems and, exploiting the knowledge about their location, eventually determining its absolute position. The procedure has been tested with the AUV using the USBL communication within a network transmission protocol, i.e., at a slower transmission rate with respect to the fastest achievable with the given equipment. The AUV is equipped with a low-cost inertial measurement unit for dead reckoning navigation between two subsequent acoustic fixes. The experimental results obtained show that this navigation modality allows to bound the navigation error even in presence of low rate acoustic updates of the position.