C-Ray is an amphibious robot that is capable of swimming in water and crawling on land using its undulating fins, enabling operations in a wide range of environments. The robot can be modeled as a hybrid dynamical system whose dynamics and propulsion change when the robot transitions between water and land. Most importantly, the direction of wave travel in the robot's fins is reversed between its swimming and crawling locomotion styles. To operate autonomously, C-Ray requires both accurate identification of when transitions between water and land occur and robust state estimation in littoral environments where the transition dynamics are highly discontinuous and transient. This paper presents a hybrid observer for estimating continuous states and identifying state-driven mode switches for C-Ray, enabling autonomous water/land-transitions. The proposed observer is a combination of the multiplicative extended Kalman filter (MEKF) and the salted Kalman filter, a newly proposed Kalman filter for mapping state uncertainty during hybrid transitions. We also propose an altitude and sea floor geometry observer and incorporate this directly into the MEKF. The performance is evaluated in simulations.

We present the Perseus autonomous underwater vehicle (AUV) - an A-sizedaaA-size stands for the standard sonobuoy [2] form factor, with a maximum diameter of 124 mm and a length of around 0.9 m, ensuring the ability to launch from standard sonobuoy launchers onboard a wide array of fixed wing and rotary wing air crafts, surface ships and submarines [3] micro AUV, outfitted with a low-cost passive inverted ultra-short baseline (piUSBL) acoustic reception system, which allows it to acoustically track-and-trail a leader vehicle that carries an acoustic transmission source. With a long-term goal of linear convoying of multiple A-sized AUVs, in this work, we used an unmanned surface vehicle (USV) towing an acoustic source, as a proxy for a lead AUV, demonstrating that the Perseus AUV is able to successfully track-and-trail a leader vehicle. The AUV was also outfitted with a tuna-inspired morphing fin mechanism that allowed the vehicle to achieve good directional stability as well as good maneuverability; properties that are useful for linear convoying AUVs, but are presently difficult to achieve because they impose contradictory requirements. We demonstrated this system with real-world, in-water experiments in the Charles river, Massachusetts, USA.