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A moving platform roll sensor system comprises an ellipsometric detector capable of detecting a polarized beam within the detector's line-of-sight, and measuring the beam's polarization state, such that the polarization state indicates the rotational orientation of the moving platform with respect t

A moving platform roll sensor system comprises an ellipsometric detector capable of detecting a polarized beam within the detector's line-of-sight, and measuring the beam's polarization state, such that the polarization state indicates the rotational orientation of the moving platform with respect to a predefined coordinate system. The ellipsometric detector comprises a venetian blind component through which the polarized beam passes, arranged such that the intensity of the exiting beam varies with its incident angle with respect to the moving platform, a polarizing beamsplitter which splits the exiting beam into components having orthogonal circular polarizations, the relative intensities of which vary with the relative polarization vector of the beam, and first and second detectors which receive the first and second orthogonal circular components and generate respective outputs that vary with the intensities of their received components. The beamsplitter preferably comprises a quarter wave plate and a polarization grating.

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1. A moving platform roll sensor system, comprising: a moving platform, said platform comprising an ellipsometric detector capable of detecting a polarized beam of electromagnetic radiation when said ellipsometric detector is within the line-of-sight of said polarized beam, and of measuring said pol

1. A moving platform roll sensor system, comprising: a moving platform, said platform comprising an ellipsometric detector capable of detecting a polarized beam of electromagnetic radiation when said ellipsometric detector is within the line-of-sight of said polarized beam, and of measuring said polarized beam's polarization state such that said polarization state indicates the rotational orientation of said moving platform with respect to a predefined coordinate system;said ellipsometric detector comprising: a polarizing beamsplitter which receives said detected beam and splits said beam into first and second components having orthogonal circular polarizations, the relative intensities of which vary with the relative polarization vector of said detected beam;first and second detectors arranged to receive said first and second components having orthogonal circular polarizations, respectively, and to generate respective outputs D1 and D2 that vary with the intensities of their received components; andcircuitry arranged to receive D1 and D2 and to calculate the rotational orientation θ of said moving platform with respect to said predefined coordinate system, with θ given by: θ=cos-1⁢D⁢⁢1D⁢⁢1+D⁢⁢2. 2. The system of claim 1, wherein said beam is a linearly polarized laser beam. 3. The system of claim 1, wherein said polarizing beamsplitter comprises a quarter wave plate and a polarization grating. 4. The system of claim 3, wherein said polarization grating comprises a holographically-treated liquid crystalline material. 5. The system of claim 1, further comprising a venetian blind component through which said detected polarized beam passes, said component arranged such that the intensity of the beam after it passes through the component varies with the incident angle of said detected beam with respect to said moving platform. 6. The system of claim 5, wherein said ellipsometric detector further comprises a protective window through which said detected polarized beam passes prior to reaching said venetian blind component. 7. The system of claim 6, further comprising a coating applied to said protective window to filter out wavelengths outside the spectral range of said polarized beam's electromagnetic radiation. 8. The system of claim 5, wherein the polar angle-dependent transmission characteristic of said venetian blind component is arranged such that orientational ambiguity in the ellipsometer is resolved. 9. The system of claim 5, wherein said venetian blind component comprises first and second intensity gratings spatially separated from each other. 10. The system of claim 9, wherein said first and second intensity gratings are located on opposite sides of a substrate. 11. The system of claim 9, wherein said first and second intensity gratings have respective duty cycles and share a common period. 12. The system of claim 11, wherein said second intensity grating is offset with respect to said first intensity grating. 13. The system of claim 9, wherein said at least one of first and second intensity gratings comprises a reflective element to block light and an absorptive element to attenuate internally reflected light. 14. The system of claim 5, wherein said venetian blind component and said polarizing beamsplitter are formed as a monolithic structure, said structure comprising: a single substrate having an input side and an exit side;first and second intensity gratings fabricated on said input and exit sides, respectively, to form said venetian blind component;a quarter wave plate fabricated or placed on said input side or said exit side of said substrate;a polarization grating fabricated or placed on said input side or said exit side of said substrate, said quarter wave plate and said polarization grating arranged such that an incoming beam impinges on said quarter wave plate before impinging on said polarization grating. 15. The system of claim 14, wherein said quarter wave plate is fabricated or placed on said input side of said substrate and said polarization grating is fabricated or placed on said exit side of said substrate. 16. The system of claim 1, wherein said ellipsometric detector further comprises a lens through which said detected polarized beam passes and which focuses said first and second components onto said first and second detectors. 17. The system of claim 16, wherein said lens has an f-number of F/3.0 or less. 18. The system of claim 3, wherein the polarization grating of said polarizing beamsplitter is arranged such that >90% of the light exiting said beamsplitter goes to +1 order or −1 order, with <10% of said light being zero order. 19. The system of claim 1, wherein said moving platform further comprises a retroreflector arranged to reflect said polarized beam, said system further comprising a detector, an array of detectors, or a camera arranged to receive said reflected beam. 20. The system of claim 1, wherein the output D1 of said first detector is proportional to cos2 θ and the output D2 of said second detector is proportional to sin2 θ. 21. The system of claim 1, wherein said first and second detectors are sized to detect incoming polarized beams having an angle of incidence of +/−α degrees, where α is from 2-30 degrees. 22. The system of claim 1, wherein said moving platform has an associated center axis and said ellipsometric detector is arranged such that it is weight- and rotation-balanced around said center axis. 23. The system of claim 1, further comprising a means of dithering the polarization state of said polarized beam such that the polarization of said polarized beam is occasionally or periodically rotated. 24. The system of claim 1, wherein said polarized beam is generated by an transmitter and a free space link is established between said transmitter and said ellipsometric detector when said ellipsometric detector is within the line-of-sight of said polarized beam. 25. The system of claim 24, further comprising a phase-locked-loop (PLL) circuit coupled to said ellipsometric detector and arranged to track said rotational orientation and thereby mitigate the degradation in the accuracy of said rotational orientation determination that might otherwise occur when said link is disrupted. 26. The system of claim 1, wherein said polarized beam is generated by a transmitter, said transmitter further arranged to encode guidance commands into said beam by pulsing said beam, said moving platform arranged to detect and decode said pulses and thereby detect said guidance commands. 27. The system of claim 26, wherein said moving platform is arranged to vary its spatial orientation in response to said guidance commands. 28. A moving platform guidance system, comprising: a transmitter which generates a pulsed beam having a known polarization with respect to a predefined coordinate system;a moving platform, said platform comprising an ellipsometric detector capable of detecting a polarized beam when said ellipsometric detector is within the line-of-sight of said polarized beam, and of measuring said polarized beam's polarization state such that said polarization state indicates the rotational orientation of said moving platform with respect to a predefined coordinate system;said ellipsometric detector comprising: a venetian blind component through which said detected polarized beam passes, said component arranged such that the intensity of the beam after it passes through the component varies with the incident angle of said detected beam with respect to the said moving platform;a polarizing beamsplitter which receives said detected beam after it passes through said venetian blind component and splits said beam into first and second components having orthogonal circular polarizations, the relative intensities of which vary with the relative polarization vector of said detected beam, said polarizing beamsplitter including a polarization grating arranged to create angular separation between said orthogonal circular polarizations;first and second detectors arranged to receive said first and second components having orthogonal circular polarizations, respectively, and to generate respective outputs D1 and D2 that vary with the intensities of their received components;a retroreflector arranged to reflect said polarized beam; andcircuitry arranged to receive D1 and D2 and to calculate the rotational orientation θ of said moving platform with respect to said predefined coordinate system, with θ given by: θ=cos-1⁢D⁢⁢1D⁢⁢1+D⁢⁢2; and a detector, an array of detectors, or a camera arranged to receive said reflected beam.