Optical GPS Using Pulsed Light for Underwater Positioning
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Solution Overview
Problem
Precise location of underwater objects is challenging due to the impenetrability of radio waves in seawater, but light waves, particularly in the blue-green spectrum, can penetrate, offering a potential alternative for positioning systems, which must contend with significant solar background noise.
Innovation Solution
A system using geosynchronous satellites to transmit pulsed light beams with synchronized atomic clocks, synchronized to activate light sources at precise times, allowing for the determination of an underwater terminal's position by calculating differences in arrival times of light pulses at the terminal, with an atomic line filter to mitigate solar background noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If radio waves are used for positioning, then the system can provide global coverage, but the signal cannot penetrate seawater
Solution Approach 1:
The patent replaces radio wave-based electromagnetic positioning with light-based optical positioning. Specifically, it uses pulsed laser beams in the blue-green spectrum (450-500 nm wavelength) that can penetrate seawater, substituting the traditional radio frequency electromagnetic waves with optical waves to achieve underwater signal transmission capability
2Reliability
If light waves are used for underwater positioning, then the signal can penetrate seawater, but solar background noise significantly reduces signal-to-noise ratio
Solution Approach 1:
The patent applies narrowband optical filtering to selectively transmit only the specific wavelength range of the laser signal (450-500 nm blue-green spectrum) while blocking other wavelengths including solar background radiation. This localized frequency selection enables the receiver to distinguish the weak optical positioning signal from the intense solar background noise
Solution Approach 2:
The system uses pulsed laser transmission with synchronized periodic timing. The satellites transmit light pulses at precisely scheduled intervals, and the underwater receiver detects these periodic pulses by correlating their arrival times with the expected periodic pattern, enabling signal detection through temporal modulation rather than continuous transmission
3Measurement precision
If atomic clocks are installed on each satellite for synchronization, then precise timing can be achieved, but system cost and complexity increase
Solution Approach 1:
The patent uses identical copies of standardized transmitter assemblies on each satellite, each containing synchronized atomic clocks. These identical units transmit light pulses with precisely correlated timing, allowing the underwater receiver to determine position by measuring arrival time differences from multiple satellites without requiring complex individual clock synchronization mechanisms
4Measurement precision
If multiple satellites transmit light pulses simultaneously, then positioning accuracy improves, but the system requires precise coordination and increases operational complexity
Solution Approach 1:
The patent pre-synchronizes the transmission timing of all satellite transmitter assemblies using a master time reference. Each satellite is programmed with predetermined transmission schedules that ensure simultaneous or precisely coordinated pulse emission, allowing the underwater receiver to calculate position based on known transmission times and measured arrival times without requiring real-time complex coordination
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
Enables accurate and cost-effective passive location of underwater terminals using light pulses, overcoming the limitations of radio wave-based systems by providing a high signal-to-noise ratio and precise positioning without the need for an atomic clock on the underwater vehicle.
Implementation Method 1
several types of optical filters exist which only transmit light over a very narrow wavelength bandwidth, such as Lyot filters and atomic line filters
Implementation Method 2
U.S. Pat. No. 5,731,585, which issued to Menders et al. on Mar. 24, 1998, for an invention entitled 'Voigt Filter,' discloses a kind of atomic line filter which can operate at the 455 nm cesium resonance wavelength
Implementation Method 3
a light source (i.e. a pulsed laser unit)
Implementation Method 4
each transmitter assembly comprises a universal clock (e.g. an atomic clock)
Implementation Method 5
light with a wavelength λ≈455 nm can penetrate seawater to a depth d in excess of 40 m
Implementation Method 6
use of light pulse arrival times at the underwater terminal to establish the underwater terminal's terrestrial location
Data Source
AI summary
In accordance with the present invention, at least three geosynchronous satellites are employed in combination, at respective known positions above a terrestrial water surface to locate an underwater terminal (vehicle). Each satellite includes a light source, and each has a controller for activating its respective light source to simultaneously transmit a light pulse, to a predetermined cell area on the terrestrial water surface, at a precisely scheduled time, t0, for receipt by the underwater terminal. A computer at the terminal then evaluates the respective light pulse arrival times, t1, 2 & 3, to determine the location of the underwater terminal.