Detecting clouds using polarized sunlight
Inventors
Sun, Wenbo • Videen, Gorden • Mishchenko, Michael I.
Assignees
United States Department of the Army
Publication Number
US-9964482-B2
Publication Date
2018-05-08
Expiration Date
2036-01-26
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Abstract
A novel methodology for detecting cloud particles is disclosed herein. This methodology exploits the optical glory phenomenon. According to one embodiment, a method for detecting clouds includes receiving data from a sensor which is configured to measure polarization of scattered light in a direction substantially opposite to the direction of incident light, and identifying, from the received sensor data, a cloud based on the polarization of the scattered light.
Core Innovation
A novel methodology for detecting clouds using polarized sunlight is disclosed. This methodology exploits the optical glory phenomenon by receiving data from a sensor configured to measure polarization of scattered light in a direction substantially opposite to the direction of incident light, and identifying clouds based on the polarization of the scattered light. The sensor may measure scattered light over a range of about 0-20° from exact backscattered light direction, focusing especially on about 5-10° where polarization signal is greatest.
The method identifies clouds by detecting that the p-polarization measurement of scattered light is greater than the s-polarization measurement, or by calculating polarization parameters such as the normalized Mueller matrix element P12/P11 or the angle of linear polarization (AOLP) from Stokes parameters, using them to determine cloud presence near the backscattered light direction. Sensor data may be measured at wavelengths of about 1.38 μm or 670 nm to improve detection sensitivity. The sensor can be located on a satellite orbiting Earth, passively measuring scattered sunlight, providing a cost-effective alternative to lidar systems.
Claims Coverage
The patent discloses three independent claims detailing inventive features in cloud detection methods and systems using polarization of optical glory.
Method for detecting clouds using polarized optical glory
A method that receives data from a sensor co-aligned with incident sunlight direction to measure the polarization of scattered light in a direction substantially opposite the incident sunlight, identifying clouds based on the polarization of the detected optical glory.
Method for detecting cloud particles via polarization
A method receiving data from a sensor configured to measure polarization of scattered light diverging from the exact backscattered light direction of incident sunlight to detect the optical glory, then identifying cloud particles based on this polarization.
System for detecting clouds using polarized optical glory
A system comprising a sensor co-aligned with incident sunlight configured to measure polarization of scattered light opposite the incident direction and a processor that identifies clouds based on polarization of the detected optical glory. The sensor includes at least one polarizer filtering incoming p-polarized light, a lens focusing this light, and an image detector receiving the focused light, with polarizers implemented as either a rotating polarizer or multiple angularly offset polarizers, capable of measuring polarization at wavelengths around 1.38 μm or 670 nm.
The independent claims cover methods and a system that utilize polarization measurements of scattered light, specifically exploiting the optical glory phenomenon in the backscattered direction to identify clouds and super-thin clouds, with sensor configurations specialized for polarization detection and spectral sensitivity.
Stated Advantages
The method allows detection of super-thin clouds that are undetectable using current passive satellite instruments.
Using polarization of optical glory instead of total radiance improves robustness against surface reflectance uncertainties and atmospheric variations.
Passive measurement using sunlight reduces operational costs compared to active lidar systems.
The method can image large atmospheric regions at once, enabling broad, discrete cloud detection from satellites or other platforms.
Measuring polarization at certain wavelengths (e.g., 1.38 μm) enhances sensitivity to super-thin clouds and improves aerosol and gas retrieval accuracy.
Documented Applications
Detecting super-thin cirrus and water clouds in the Earth's atmosphere to improve weather predictions and Earth's radiation energy budget calculations.
Correcting measurement errors in aerosol optical depth, sea surface temperature, and atmospheric gas retrievals affected by undetected clouds in satellite data.
Use in satellite systems, such as polar orbiting platforms, equipped with polarimetric sensors to monitor clouds passively.
Potential application in terrestrial or fluid media for detecting clouds of particles or bubbles using polarization measurements.
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