Exploration in underwater navigation using acoustic beacons

Underwater navigation is a challenging problem that requires accurate and reliable methods to determine the position and orientation of underwater vehicles or objects. Traditional satellite-based navigation systems, such as GPS, are ineffective underwater due to the attenuation and scattering of radio signals in water. Therefore, alternative techniques have been developed to enable underwater positioning, navigation, and timing (PNT) for various applications, such as ocean exploration, environmental monitoring, and defense.

One of the most common techniques for underwater navigation is acoustic positioning, which uses sound waves or acoustic signals to measure the distance and direction between a transmitter and a receiver. Acoustic positioning systems can be classified into two types: long baseline (LBL) and ultra-short baseline (USBL). LBL systems use multiple fixed transmitters, called acoustic beacons, that are deployed on the seafloor or on buoys, and a single receiver on the underwater vehicle. The vehicle determines its position by measuring the time of arrival of the signals from at least three beacons. USBL systems use a single transmitter on the vehicle and a single receiver on a surface vessel or a buoy. The receiver measures the time difference of arrival and the angle of arrival of the signals from the vehicle, and computes its position relative to the receiver.

Both LBL and USBL systems have advantages and disadvantages. LBL systems offer high accuracy and stability, but require a large number of beacons and prior knowledge of their locations. USBL systems are easy to deploy and operate, but suffer from low accuracy and drift due to the effects of water currents, waves, and noise. Moreover, both systems rely on continuous acoustic communication, which consumes power and bandwidth, and may interfere with other acoustic devices or marine life.

To overcome these limitations, researchers have proposed novel algorithms that combine acoustic positioning with inertial navigation, which uses sensors such as accelerometers and gyroscopes to estimate the motion of the vehicle. Inertial navigation systems (INS) can provide high-frequency and low-latency position updates, but they are prone to cumulative errors over time. By integrating INS with sparse and intermittent acoustic signals from one or two beacons, the researchers were able to correct the inertial errors and improve the positioning accuracy and robustness.

The researchers developed two algorithms for underwater inertial error rectification: RMAN and VLBL. RMAN, inspired by matching navigation without the need for reference maps, uses a single beacon to estimate the relative position increment (RPI) between two acoustic measurements, and compares it with the RPI from INS to correct the inertial error. VLBL, which adjusts for errors in RPI, uses two beacons to form a virtual long baseline, and computes the position of the vehicle using geometric constraints. Both algorithms exploit the minimal acoustic beacon interactions to amend the inertial navigation inaccuracies.

The researchers tested their algorithms through extensive simulations and field experiments, and demonstrated a substantial improvement in positioning precision. The results showed that the algorithms reduced the inertial error by over 90% with single beacon and more than 98% with double beacon configurations, respectively. The algorithms also outperformed existing methods, such as extended Kalman filter (EKF) and geophysical matching aided navigation (GMAN), in terms of accuracy and stability.

The research not only addresses the persistent challenge of underwater navigation, but also opens new avenues for oceanic exploration, environmental monitoring, and defense applications by providing a more reliable and efficient means of underwater positioning. Dr. Fangneng Li, the lead researcher, said, “Our techniques offer a paradigm shift in underwater navigation, providing over 90% reduction in inertial error with single and double beacon configurations, respectively.”