Underwater Navigation Algorithm Using Acoustic Travel Time
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Solution Overview
Problem
Current underwater navigation systems for vehicles lack the ability to accurately determine geographic position without prior location information or ocean sound speed data, limiting their effectiveness for long-range navigation and increasing the risk of detection.
Innovation Solution
The implementation of a Cold Start Algorithm (CSA) that uses iterative linearized least squares estimation to determine absolute coordinates and ocean sound speed from acoustic signals received by underwater vehicles, allowing for position calculation without initial location or sound speed information, utilizing a single hydrophone and onboard processor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If underwater vehicles use GPS or surface reference points for navigation, then positioning accuracy is improved, but the ability to operate covertly and continuously underwater deteriorates
Solution Approach 1:
The patent introduces acoustic signals from coastal acoustic tomography (CAT) buoy arrays as an intermediary medium for positioning. Instead of directly using GPS satellites or surface references, the system uses acoustic wave propagation through water to convey position information, enabling underwater vehicles to determine their location acoustically while remaining submerged and covert.
2Duration of action of moving object
If underwater vehicles use inertial sensors for dead reckoning, then short-term navigation capability is improved, but error accumulation over time deteriorates
Solution Approach 1:
The patent implements feedback by periodically updating inertial navigation estimates with acoustic position measurements from CAT buoys. The system uses acoustic signals to provide corrective feedback on accumulated inertial errors, resetting the position estimate and sound speed profile without requiring the vehicle to surface or break covert operations.
3Measurement precision
If underwater vehicles surface to receive GPS signals, then position fixation is improved, but detection risk and mission continuity deteriorate
Solution Approach 1:
The patent replaces the mechanical/electromagnetic GPS positioning system with an acoustic positioning system. Acoustic waves can penetrate seawater and provide positioning information to submerged vehicles, eliminating the need to surface for GPS signal reception and thereby removing the detection risk associated with surfacing operations.
4Measurement precision
If high-end navigation sensors like navigation grade IMU and DVL are used, then navigation performance is improved, but cost and power consumption deteriorate
Solution Approach 1:
The patent makes the CAT buoy array serve multiple functions: it acts as both a positioning reference and a sound speed profile reference. This multi-functionality eliminates the need for separate, expensive sound speed sensors and reduces the navigation system's power consumption while maintaining high positioning accuracy through acoustic measurements.
5Adaptability or versatility
If acoustic signals from CAT buoys are used for positioning, then covert long-range underwater navigation capability is improved, but dependence on external acoustic infrastructure deteriorates
Solution Approach 1:
The patent uses preliminary sound speed profile measurements obtained during the vehicle's first acoustic contact with the CAT array. These preliminary measurements are stored and used to correct acoustic travel time calculations for subsequent positioning operations, enabling the system to function autonomously without requiring continuous external infrastructure support.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The CSA achieves a mean position error of 57 meters with a standard deviation of 31 meters, significantly improving upon existing state-of-the-art navigation systems by enabling accurate underwater vehicle positioning over large ocean areas without the need for GPS or surface reference points.
Implementation Method 1
receiving, at a receiver hydrophone located on the underwater vehicle, acoustic signals from a predetermined number of acoustic sources at known locations
Implementation Method 2
determining, by an onboard processor, an end of the Coda (EOC) travel time for each of the received acoustic signals from each of the sources
Data Source
AI summary
An underwater navigation algorithm (UNA) uses acoustic signals transmitted in the ocean from sources at known positions to compute a position underwater for a vehicle that requires no initial a priori position or any ocean sound speed information or any initial GPS position that would require surfacing. The UNA consists of two parts, (1) the Cold Start Algorithm (CSA) and (2) the CSA with Modeling (CSAM). The underwater vehicle needs to be equipped with only a single hydrophone acoustic receiver and an onboard processor. The CSA requires only measuring the travel time of the end of the arrival coda (EOC) from each of the sources to compute a position. The CSAM is a post CSA procedure to calculate a higher accuracy position using the CSA position. CSAM utilizes the CSA position and a 4D sound speed field derived from an ocean 4D General Circulation Model (GCM) constrained using Ocean Acoustic Tomography (OAT) in the ocean area of operation and further includes (1) computing a modeled result with an acoustic propagation modeling code that is compared with the received acoustic data using a known procedure called “bulk shifting” and/or (2) a new proposed procedure that uses calculated group speeds from the 4D sound speed field, to provide a higher accuracy estimate of the receiver position.