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A dual channel inflight ice detection system to detect ice accretion on an aircraft surface by illuminating an ice collecting surface mounted on an aircraft with linear polarized light. The backscattered light is acquired in two light conductors one with polarization sensitivity aligned to the trans

A dual channel inflight ice detection system to detect ice accretion on an aircraft surface by illuminating an ice collecting surface mounted on an aircraft with linear polarized light. The backscattered light is acquired in two light conductors one with polarization sensitivity aligned to the transmitted light and the second with polarization sensitivity orthogonal to the first. The presence of ice is determined by the change in the ratio of light intensities in the two light conductors.

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We claim: 1. A dual channel method of detecting, inflight, accretion of ice on the surface of an aircraft, the method comprising: (a) Illuminating an ice collecting surface fixed in a housing mounted on an aircraft with a light source of single linear polarization; (b) Acquiring backscattered light

We claim: 1. A dual channel method of detecting, inflight, accretion of ice on the surface of an aircraft, the method comprising: (a) Illuminating an ice collecting surface fixed in a housing mounted on an aircraft with a light source of single linear polarization; (b) Acquiring backscattered light from said ice collecting surface using a pair of polarization sensitive light conductors when the polarization sensitivity of the first receiving light conductor is aligned to the polarization of the light source and the polarization sensitivity of the second receiving light conductor is aligned orthogonal to the polarization of the light source; (c) Coupling each receiving light conductor to a light detector to determine the backscattered light intensity for each polarization state; (d) Processing the detected light intensities in each light detector to determine ratio between the light intensity for each polarization state; (e) Processing the change in said ratio to indicate ice accretion on the ice collecting surface; (f) Activating a heat source adjacent to said housing upon indication of ice accretion; (g) Continued operating of heat source until a change in detected light intensities indicates the removal of ice on the ice collecting surface; (h) Monitoring the detected light intensities in each light detector and processing the change in detected light intensities until re-accretion of ice is detected on the ice collecting surface; (i) Processing time for re-accretion of ice to indicate the ice accretion severity. 2. A method as claimed in claim 1 wherein said method is repeated continuously whilst the aircraft is inflight. 3. A method as claimed in claim 1 wherein said step (e) includes the step of: Activation of said aircraft inflight ice protection and pilot warning systems. 4. A method as claimed in claim 1 wherein said polarization sensitivity of said light source is determined by orientation of a polarizing filter placed at the end of a light conductor. 5. A method as claimed in claim 1 wherein said polarization sensitivity of receiving light conductors is determined by orientation of a polarizing filter placed on the end of each conductor. 6. A method as claimed in claim 1 wherein said ice collecting surface is an optically transparent material. 7. A dual channel method of detecting, inflight, accretion of ice on the surface of an aircraft, the method comprising: (a) Illuminating an ice collecting surface fixed in a housing mounted on an aircraft with a light source of single linear polarization; (b) Acquiring backscattered light from said ice collecting surface using a pair of polarization sensitive light conductors when the polarization sensitivity of the first receiving light conductor is aligned to the polarization of the light source and the polarization sensitivity of the second receiving light conductor is aligned orthogonal to the polarization of the light source; (c) Coupling each receiving light conductor to a light detector to determine the backscattered light intensity for each polarization state; (d) Processing the detected light intensities in each light detector to determine ratio between the light intensity for each polarization state; (e) Processing the change in said ratio to indicate ice accretion on the ice collecting surface; (f) Activating a heat source adjacent to said housing upon indication of ice accretion; (g) Continued operating of heat source until a change in detected light intensities indicates the removal of ice on the ice collecting surface; (h) Monitoring the detected light intensities in each light detector and processing the change in detected light intensities until re-accretion of ice is detected on the ice collecting surface; (i) Processing time for ice removal to indicate the ice accretion severity. 8. A method as claimed in claim 7 wherein said method is repeated continuously whilst the aircraft is inflight. 9. A method as claimed in claim 7 wherein said step (e) includes the step of: Activation of said aircraft inflight ice protection and pilot warning systems. 10. A method as claimed in claim 7 wherein said polarization sensitivity of said light source is determined by orientation of a polarizing filter placed at the end of a light conductor. 11. A method as claimed in claim 7 wherein said polarization sensitivity of receiving light conductors is determined by orientation of a polarizing filter placed on the end of each conductor. 12. A method as claimed in claim 7 wherein said ice collecting surface is an optically transparent material. 13. An inflight ice accretion detection system as claimed in claim 7 wherein said ice collecting surface is an optically transparent material. 14. An inflight ice accretion detection system as claimed in claim 7 wherein said housing is mounted in a sensing tube. 15. An inflight ice accretion detection system as claimed in claim 7 wherein said housing is mounted such that the ice collecting surface is flush to an aircraft surface. 16. An inflight ice accretion detection system as claimed in claim 7 wherein said light source transmission means incorporates an optical fiber. 17. An inflight ice accretion detection system as claimed in claim 7 wherein said receiving light conductors are optical fiber. 18. An inflight ice accretion detection system as claimed in claim 7 wherein polarization sensitivity is established using polarizing filters. 19. An inflight ice accretion detection system as claimed in claim 7 wherein said light detectors act as wavelength filters in being non-sensitive to ambient light. 20. A dual channel system to detect inflight ice accretion on the surface of an aircraft, said system comprising: ice collecting surface fixed in a housing mounted on an aircraft; transmission means to transmit a light source of single linear polarization to said ice collecting surface; receiving means to acquire backscattered light from said ice collecting surface using one receiving light conductor with polarization sensitivity aligned to the polarization of the light source and a second receiving light conductor with polarization sensitivity aligned orthogonal to the polarization of the light source; detection means to detect light intensities in each of said receiving light conductors; processing means to determine the ratio of detected light intensities; processing means to determine the change in the ratio as indication of ice accretion on said ice collecting surface; a heat source adjacent to said housing that is initially activated upon indication of ice accretion and then maintained active until a change in detected light intensities indicates the removal of ice on the ice collecting surface; and control means to repeat process while inflight, determine ice accretion severity and activate the ice protection and pilot warning systems.