DETECTING CLOUDS USING POLARIZED SUNLIGHT

• US 20160216198A1
• Filed: 01/26/2016
• Published: 07/28/2016
• Est. Priority Date: 01/26/2015
• Status: Active Grant

• Current Assignee: The United States Of America As Represented By The Secretary Of The Army
• Original Assignee: US Army Research Laboratory (Government of the United States of America)
• Inventors: Sun, Wenbo, Videen, Gordon, Mishchenko, Michael I.
• Granted Patent: US 9,964,482 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01N 2021/4792 Polarisation of scatter light
• CPC Class Codes: G01N 21/21 Polarisation-affecting properties G01N21/19 takes precedence
• CPC Class Codes: G01N 21/538 for determining atmospheric attenuation and visibility
• CPC Class Codes: G01N 2201/0616 Ambient light is used
• CPC Class Codes: G01W 1/02 Instruments for indicating weather conditions by measuring two or more variables, e.g. humidity, pressure, temperature, cloud cover or wind speed G01W1/10 takes precedence

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.

Claims

1. A method for detecting clouds, the method comprising:

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.

2. The method of claim 1, wherein the sensor is configured to measure scattered light over a range of about 0-20°

from the exact backscattered light direction.

3. The method of claim 2, the sensor is configured to measure scattered light over a range of about 5-10°

from the exact backscattered light direction.

4. The method of claim 1, wherein the sensor is configured to detect the s-polarization and p-polarization intensities of the measured scattered light.

5. The method of claim 4, wherein a cloud is identified if the detected p-polarization measurement is greater than the s-polarization measurement of the scattered light direction.

6. The method of claim 1, wherein the sensor and/or image processor associated therewith is configured to provide the Stokes Parameters Q, U and I of polarization.

7. The method of claim 6, further comprising:

calculating the degree of polarization normalized Mueller matrix element P12/P11 from the Stokes Parameters of polarization as follows;

P12/P11=(I_S−

I_P)/(I_S+I_P),

where I_S is the intensity of the s-polarized component of the light and I_P is the intensity of the p-polarized component of the light.

8. The method of claim 7, wherein a cloud is identified if the calculated P12/P11 is negative near the backscattered light direction.

9. The method of claim 6, further comprising:

calculating the angle of linear polarization (AOLP) from Stokes Parameters of polarization as follows;

10. The method of claim 9, wherein a cloud is identified if the calculated AOLP≦

about 60°

or ≧

120°

.

11. The method of claim 1, wherein the sensor data is measured at a wavelength of about 1.38 μ

m.

12. The method of claim 1, wherein the sensor data is measured at a wavelength of about 670 nm.

13. The method of claim 1, wherein the sensor is located on a satellite orbiting the Earth.

14. The method of claim 13, further comprising:

transmitting the sensor data from the satellite to a ground station, and/or receiving the transmitted sensor data at the ground station.

15. The method of claim 1, wherein only scattered sunlight is measured by the sensor.

16. The method of claim 1, further comprising:

identifying, from the received sensor data, a super-thin cloud based on the polarization of the scattered light.

17. A method for detecting cloud particles, the method comprising;

receiving data from a sensor which is configured to measure polarization of an optical glory associated with a cloud; and

identifying, from the received sensor data, cloud particles based on the polarization of the optical glory.

18. A system for detecting clouds, the system comprising:

a sensor configured to measure polarization of scattered light in a direction substantially opposite to the direction of incident light; and

a processor configured to identify, from the measured sensor data, a cloud based on the polarization of the scattered light.

19. The system of claim 18, wherein the sensor comprises:

at least one polarizer which filters incoming light having p-polarization, a lens which focuses the filtered light, and an image detector which receives the focused light.

20. The system of claim 19, wherein the at least one polarizer comprises (i) a rotating polarizer or (ii) three polarizers which are offset in angular orientation.