Abstract:
An orientation tracking system for a moving platform includes a transmitter which generates an beam having a known polarization with respect to a predefined coordinate system. The moving platform includes an ellipsometric detector capable of detecting the polarized beam when within the line-of-sight of the transmitter, and measuring its polarization state. The polarization state indicates the rotational orientation of the moving platform with respect to the predefined coordinate system. The beam could also be used to convey guidance commands to the platform.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to techniques for determining the rotational orientation of a platform moving in space. 
         [0003]    2. Description of the Related Art 
         [0004]    It is often necessary to know the rotational orientation of a moving body or platform along with its spatial location. For example, it may be necessary to know the orientation of a moving projectile such as a missile in order to provide the missile with appropriate guidance data. 
         [0005]    Several techniques are used to provide rotational orientation data of this sort. For example, it may be possible to determine the orientation of a moving platform by means of a radar system. However, such systems tend to be large, costly, consume large amounts of power and are easy to detect. Another approach is to affix accelerometers, gyroscopes, magnetometers, etc. to the platform; however, these devices tend to be expensive, bulky and complex. 
         [0006]    It may also be possible to determine the rotational orientation of a moving platform by imaging it as it moves. However, this is likely to be difficult if conditions are turbulent or otherwise less than ideal and may be impossible if the projectile is small and rapidly spinning. 
         [0007]    Another approach, described in co-pending U.S. patent application Ser. No. 13/531,918, employs an optical link between a transmitter and the moving platform to determine the rotational orientation of the platform. The transmitter preferably directs a linearly polarized laser beam towards the moving platform, and an ellipsometric detector capable of detecting the polarized beam when within the line-of-sight of the transmitter and measuring its polarization state is mounted to the moving platform. The polarization angle indicates the rotational orientation of the moving platform with respect to the predefined coordinate system. However, an optical link of this sort may suffer from problems due to certain environmental conditions. For example, an infrared laser does not efficiently penetrate fog, clouds, rain or dust. 
       SUMMARY OF THE INVENTION 
       [0008]    A moving platform roll angle determination system is presented which addresses several of the problems noted above, providing a robust means of determining the rotational orientation of a moving platform through all weather conditions. 
         [0009]    The present moving platform roll angle measurement system includes a transmitter which includes a transmitting antenna that generates at least one radio frequency (RF) signal, with the transmitted RF signals having known but different polarizations with respect to a predefined coordinate system. The system also includes at least one moving platform, each of which includes a receiving antenna capable of receiving the polarized RF signals. The transmitter and receiving antenna are arranged such that the roll angle of a moving platform can be determined based on the received RF signals. 
         [0010]    In one embodiment, the transmitter generates two orthogonal linearly polarized RF signals, and the receiving antenna is a linearly polarized antenna which receives the two orthogonal linearly polarized RF signals. The roll angle is then determined based on the ratio of the power amplitudes of the received orthogonal linearly polarized RF signals. 
         [0011]    In another embodiment, the transmitter generates one linearly polarized RF signal and the receiving antenna is a dual-polarized antenna which receives the linearly polarized RF signal and produces first and second output signals which represent the power amplitudes of the received linearly polarized RF signal at first and second orthogonal polarizations, respectively. The roll angle is then determined based on the ratio of the power amplitudes of the first and second output signals. The transmitter may be arranged to encode information into the RF signals by modulating one or both of the transmitted orthogonal linearly polarized RF signals; the information may include, for example, guidance commands. 
         [0012]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following drawings, descriptions, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1 a    is a block diagram of an orientation measurement system in accordance with the present invention. 
           [0014]      FIG. 1 b    is a diagram illustrating the components making up the rotational orientation value θ. 
           [0015]      FIG. 2A  is a block diagram of another embodiment of an orientation measurement system in accordance with the present invention. 
           [0016]      FIG. 2B  is a diagram of a horn antenna as might be used in an orientation measurement system in accordance with the present invention. 
           [0017]      FIG. 2C  is a graph which illustrates two orthogonal linearly polarized RF signals which have unequal frequencies. 
           [0018]      FIG. 3  is a block diagram of another embodiment of an orientation measurement system in accordance with the present invention. 
           [0019]      FIG. 4  is a block diagram of another embodiment of an orientation measurement system in accordance with the present invention. 
           [0020]      FIG. 5  is a plot illustrating how roll angle θ will vary over time as the moving platform rotates. 
           [0021]      FIG. 6  is a diagram illustrating the use of beamsteering to resolve up/down ambiguity. 
           [0022]      FIG. 7  is a diagram illustrating the use of radiometry to resolve up/down ambiguity. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The basic principles of a roll angle measurement system for a moving platform per the present invention are illustrated in  FIG. 1 a   . The system  10  includes a transmitter  12  which includes a transmitting antenna which generates at least one radio frequency (RF) signal  14 , with the transmitted RF signals having known but different polarizations  16  with respect to a predefined coordinate system. The moving platform, here a projectile  18 , includes a receiving antenna  20  capable of receiving the at least one polarized RF signal; receiving antenna  20  would typically be coupled to a receiver (not shown). Transmitter  12  and receiving antenna  20  are arranged such that the roll angle θ of moving platform  18  can be determined based on the received RF signals. 
         [0024]    This determination of roll angle may be accomplished in a number of ways. For example, transmitter  12  can be arranged to generate two orthogonal linearly polarized RF signals. Receiving antenna  20  is a linearly polarized antenna which receives the two orthogonal linearly polarized RF signals, with the roll angle θ determined based on the ratio of the power amplitudes (P 1 , P 2 ) of the received orthogonal linearly polarized RF signals. This is illustrated in  FIG. 1   b.    
         [0025]    Alternatively, transmitter  12  can be arranged to generate one linearly polarized RF signal. Receiving antenna  20  is a dual-polarized antenna which receives the linearly polarized RF signal and produces first and second output signals (P 1 , P 2 ) which represent the power amplitudes of the received linearly polarized RF signal at first and second orthogonal polarizations, respectively. Roll angle θ can then be determined based on the ratio of the power amplitudes of P 1  and P 2 . 
         [0026]    By establishing a communications link between transmitter  12  and moving platform  18  using RF signals, problems that can adversely affect an optical communications link, such as signal attenuation due to environmental conditions like fog, clouds, rain or dust, are avoided. In this way, an all-weather, day-night communications link which enables roll angle to be determined can be established. The wavelengths of the RF signals are preferably long enough to avoid attenuation due to weather conditions and any disadvantageous atmospheric absorption lines. Frequencies in the range of 30-300 GHz are acceptable, with frequencies in the range of 100-250 GHz preferred. The transmitter  12  and receiver (not shown) can be implemented in numerous ways; a heterodyne-based architecture is preferred. 
         [0027]    A simplified diagram of one possible embodiment is shown in  FIG. 2A . Here, transmitter  12  is preferably a dual-polarized transmitting antenna which generates two linearly polarized RF signals  30 ,  32 , with the two RF signals having orthogonal polarizations such as polarizations  34 ,  36  shown. The moving platform&#39;s receiving antenna  38  is a linearly polarized antenna which receives orthogonal linearly polarized RF signals  30 ,  32 ; one possible embodiment (shown in  FIG. 2B ) is a horn antenna  40 , typically rectangular in shape, which includes a flared horn  42  that receives the RF signals and a waveguide  44 . The two orthogonal linearly polarized RF signals are preferably of equal power and contain identical data, but have frequencies which are preferably unequal. This is illustrated in  FIG. 2C , with RF signal  30  having a y-axis polarization and a frequency f 1 , and RF signal  32  having a z-axis polarization and a frequency f 2 . Both the transmitting and receiving antennas have associated bandwidths (which may be unequal). The two frequencies should be selected so that both are within the bandwidths of the transmitting and receiving antennas. 
         [0028]    The roll angle θ is then determined based on the trigonometric ratio of the power amplitudes P 1 , P 2  of the orthogonal linearly polarized RF signals. Roll angle θ may be calculated with, for example, an on-board electronics module  39 . 
         [0029]    The present system can be arranged such that information can be encoded into the transmitted RF signals, by modulating one or both of transmitted signals. Such information might include guidance commands. Some moving platforms are capable of altering their direction when moving using flaps or other devices which can be actuated in response to guidance commands. The present system can be arranged such that the transmitter encodes such guidance commands into the RF signals, by modulating one or both of transmitted orthogonal linearly polarized RF signals  30 ,  32 . The modulation can be any of a number of types, including amplitude modulation, frequency modulation, or phase modulation. The transmitter preferably modulates the power amplitudes of one or both of the transmitted orthogonal linearly polarized RF signals. The modulated signals are then received by the moving platform&#39;s receiving antenna and the moving platform is preferably arranged to decode the guidance commands by determining the sum of the power amplitudes of the received orthogonal linearly polarized RF signals. 
         [0030]    Another possible embodiment  48  of the present system is shown in  FIG. 3 . Here, the transmitter  50  generates one RF signal  52  which is linearly polarized  54 , and the receiving antenna  56  is a dual-polarized antenna which receives the linearly polarized RF signal and produces first and second output signals P 1 , P 2  which represent the power amplitudes of the received linearly polarized RF signal at first and second orthogonal polarizations, respectively. The roll angle is determined based on the ratio of the power amplitudes of said first and second output signals. Roll angle θ may be calculated with, for example, an on-board electronics module  58 . Dual-polarized receiving antenna  56  is suitably a patch antenna, which is typically inexpensive and mechanically robust; dual frequency designs with narrow frequency bands can also be easily realized with a patch antenna. 
         [0031]    Dual-polarized receiving antenna  56  might also be a horn antenna having a square or round shape, coupled to a diplexer so that both polarizations can be output. Horn antennas typically provide a high degree of polarization discrimination, and can be machined into the moving platform. 
         [0032]    The receiving antenna should be structurally strong, as it would typically be located at the back of the moving platform and thus subject to significant forces when the platform is launched. The moving platform is often spinning around a longitudinal axis as it moves; as such, the antenna is preferably rotationally symmetric around the longitudinal axis to avoid precession. The antennas are preferably inexpensive to manufacture in quantity, sized appropriately for the moving platform, and narrow band to avoid jamming. 
         [0033]    As with the embodiment described above, the embodiment shown in  FIG. 3  can be arranged such that transmitter  50  encodes information such as guidance commands into RF signal  52 , by modulating the linearly polarized RF signals by a modulation means (amplitude modulation, frequency modulation, phase modulation, etc.). The transmitter  50  preferably modulates the power amplitude of transmitted linearly polarized RF signal  52 . The modulated signal is then received by the moving platform&#39;s receiving antenna  56 , with the moving platform preferably arranged to decode the guidance commands by determining the sum of power amplitudes P 1  and P 2 . 
         [0034]    One advantage with an RF signal-based system as described herein is that, due to the broad coverage range inherent in RF signals, one transmitter can be arranged to generate RF signals such that the roll angle of multiple moving platforms can be determined simultaneously. This might be accomplished by assigning unique frequencies to each platform and then transmitting linearly polarized signals as described above on each of the assigned frequencies. 
         [0035]    The moving platform might be any device that is designed to be propelled through space. For example, the moving platform may be a steerable projectile such as a bullet. 
         [0036]    One problem that might be encountered is that the P 1  and P 2  values will be the same whether the roll angle is X, or X+180°. This ‘up/down’ ambiguity is referred to as degeneracy in roll position. The present system may be arranged to overcome this degeneracy by forcing a flight path deviation. By tracking the platform&#39;s path after the deviation is effected, the up/down ambiguity can be resolved. Once the ambiguity has been resolved, the correct orientation can always be determined unambiguously by tracking the roll angle. 
         [0037]    To force a flight path deviation, the moving platform would typically be arranged to be able to vary its trajectory in response to guidance commands encoded into the transmitted RF signals. Such a platform would typically include a control device which affects the path of the platform when actuated, and which rotates with the platform. For example, as shown in  FIG. 4 , movable platform  18 , with a receiving antenna  60  mounted at the rear, might include a flap  62  which, when actuated, causes the platform&#39;s trajectory to trend higher or lower depending on the flap&#39;s orientation at the time the flap was actuated. Thus, the transmitter is arranged to send guidance commands which actuate a control device such as flap  62 , thereby affecting the platform&#39;s trajectory. The system is then arranged to detect the change in trajectory and to thereby determine the position of the control device at the time of the actuation. This process need only be performed once per flight, preferably near the start of each flight. 
         [0038]    For example, for an embodiment such as that shown in  FIG. 2A , where transmitter  12  generates two orthogonal linearly polarized RF signals  30 , 32  that are received by linearly polarized receiving antenna  38 , the roll angle θ is given by tan −1 (P v /P h ) where P v  is the power amplitude of the received RF signal in the vertical direction and P h  is the power amplitude of the received RF signal in the horizontal direction. As the moving platform rotates, the roll angle θ will vary over time as shown in  FIG. 5 . Once the up/down orientation has been determined as described above, the system can be arranged to track the up/down orientation by, for example, counting the changes in slope of angle θ. 
         [0039]    Similarly, for an embodiment such as that shown in  FIG. 3 , where transmitter  50  generates one linearly polarized RF signal  52  that is received by dual-polarized receiving antenna  56 , the roll angle θ is given by tan −1 (P v /P h ) where P v  is the power amplitude of the received RF signal in the vertical direction and P h  is the power amplitude of the received RF signal in the horizontal direction. Once the up/down orientation has been determined as described above, the system can be arranged to track the up/down orientation by, for example, counting the changes in slope of angle θ. Alternatively, the up/down orientation might be tracked by counting the number of times that the roll angle crosses a predetermined value, such as 45°. 
         [0040]    It may also be possible to track the up/down orientation by monitoring the summed powers (P 1 +P 2 ), which will be modulated by the precession of the platform in response to the forced flight path deviation. 
         [0041]    Up/down ambiguity might also be resolved by means of ‘beamsteering’, which requires that the receiving antenna be capable of having a directional reception pattern or ‘beam’. One example of an antenna having this capability is a patch antenna. Here, degeneracy is overcome by breaking the symmetry of the beam about the receiving antenna&#39;s axis of rotation. For example, as shown in  FIG. 5 , transmitter  70  generates a beam which may have an associated directional pattern  72 , and the receiving antenna on moving platform  74  has an associated directional reception pattern  76 . To overcome degeneracy, pattern  76  is directed off-axis to the platform&#39;s axis of rotation  78 . 
         [0042]    If the system is arranged as shown in  FIG. 2A , with a dual-polarized transmitting antenna generating two linearly polarized RF signals having different frequencies, the receiving antenna must be capable of receiving either two frequencies, two polarizations, or both. If the system is arranged as shown in  FIG. 3 , with a transmitting antenna generating one RF signal which is linearly polarized, the receiving antenna must be dual-polarized. For this latter case, the beam for each polarization must be off-axis (not rotationally symmetric). Up/down ambiguity resolution increases as the degree of rotational symmetry decreases; however, too little rotational symmetry can compromise the reception of guidance commands. 
         [0043]    When so arranged, the receiving antenna beam for one or both frequencies/polarizations is pointed off-axis. The power modulation at the frequency of rotation is then monitored to determine ‘up’ versus ‘down’. The received signal power will be a sine wave with a period equal to the rotation of the platform; however, the amplitude of the sine wave decreases as the platform travels away from the transmitter. This would result in an amplitude profile sin(x)/(R 2 ), where R is the distance between the platform and transmitter. This can be approximated by a damped sine wave. 
         [0044]    Up/down ambiguity might also be resolved using radiometry. The black body radiation of the sky is shifted with respect to that of the ground; this shift can be detected as the moving platform rotates to determine ‘up’ versus ‘down’. As illustrated in  FIG. 6 , this technique requires the addition of an additional antenna having a directional reception pattern  80  which extends normal to the axis of rotation  82  of the moving platform  84 . The antenna, preferably broadband, is coupled to a power detector (radiometer) (not shown), which filters out any control signals that may be received and effectively detects the temperature difference between sky and ground; the power detector&#39;s output would typically be sinusoidal as the platform rotates, with an amplitude that varies with the detected temperature. 
         [0045]    The embodiment shown in  FIG. 2A  is preferred, as it reduces the RF circuit component count needed on the moving platform; some of the RF circuit complexity is instead moved to the transmitter on the ground. Simplifying the moving platform&#39;s electronics in this way potentially offers significant cost savings. 
         [0046]    The present system provides a number of benefits. Establishing an RF link between the transmitter and a moving platform as described herein provides a connection that is robust, even in an extremely turbulent environment. This is further aided by the use of polarization as a means to determine orientation, as polarization is less affected by atmospheric turbulence and scattering. The RF signals are subject to negligible refractive index gradients, and can be made to produce a uniform transmitted field intensity, while their source is difficult to detect. The distance between the transmitter and moving platform can be extended as needed, by simply increasing the power of the transmitted RF signals. Furthermore, due to the availability of high power sources, the transmitter can have a relatively low gain. 
         [0047]    The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.