WO1996010729A1 - Improved boresight with single-beam triaxial measurement - Google Patents

Abstract

Optical transceiver apparatus for measuring angular misalignment between two bodies in elevation, azimuth and roll utilizes a light transmitter (40) coupled to a first body such as an aircraft fuselage reference and a light receiver (12) coupled to a second body such as an aircraft wing or pylon. The receiver has an elevation/azimuth (EL/AZ) channel and a roll channel sharing a common objective lens, wherein a beam splitter (45) splits the beam generated by the laser diode (10) between the two channels. In the EL/AZ channel lenses are used to focus light onto a position sensitive detector (PSD) (46), while in the roll channel, a Thompson beam-splitting prism (13') or other suitable optimal element divides the two planes of polarization which are then focused on light detectors (16).

Description

IMPROVED BORESIGHT WITH SINGLE-BEAM TRIAXIAL MEASUREMENT

REFERENCE TO RELATED PATENTS

This application is directed to improvements to the optical transceiver apparatus disclosed in U.S. Patent No. 5,118,185 issued 2 June 1992 to Courten, assigned to DRS/Photronics Corporation, Hauppauge, New York, the assignee hereof, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to optical transceiver apparatus, and more

particularly, to optical transceiver systems for measuring angular misalignment between two objects. The invention is particularly useful as a tool for boresighting

gun and other weapon systems on aircraft.

In aircraft such as advanced attack helicopters, gun turrets in the nose and pylons attached to short wing sections of the aircraft are provided for carrying short

range, quick response, weapon systems. System accuracy in such arrangement is

the product of many factors which vary before, during and after combat maneuvers. These factors may include wing bending, mechanical friction between moving parts,

backlash from gears, structural vibration, deflection of the airframe, recoil and the like.

A greater appreciation for the factors involved may be obtained by reference, for example, to the article "Guns of the Fighter Helicopter" by Frank Colucci, in the

publication Defence Helicopter World, August-September, 1989. Thus, it is necessary to boresight the systems for the location where they are typically aimed. This is a tedious, time consuming operation which heretofore was accomplished with ground based sighting arrangements before the helicopter left for a mission. For example, according to U.S. Army statistics, typically it takes three personnel four hours to boresight an AH-64 helicopter and thirteen hours to boresight an AH-1 helicopter. However, aircraft flex, twist and vibration in flight render the

continued accuracy of corrections or calibration doubtful particularly when the same are implemented when the helicopter is on the ground. In such ground based boresight systems, external support equipment, such as telescopes, accelerometers, inclinometer, gyroscope platforms and the like, are generally required for such boresighting. This external support equipment is employed to make either physical adjustments to the aircraft or computer bias entries depending upon the nature of the outputs provided by the measuring equipment.

In boresight such systems it is necessary to measure misalignment in azimuth, elevation and roll axes. Systems for measuring misalignment are disclosed in the following United States patents:

Patent No. Inventor

3,486,826 Colvin et al.

3,816,000 Fiedler

4,560,272 Harris

4,847,511 Takada et al.

4,939,678 Beckwith, Jr.

5,090,803 Ames et al.

Colvin discloses an optical alignment sensing apparatus for establishing the orientation of a body about three mutually orthogonal axes, in which a reference beam is focused between two pairs of detectors by two cylindrical lenses. In the apparatus, the light from light source 10 is projected by a condensing lens 11 through an aperture 12 in a screen 14. The light from source 10 is collimated by lens 17 and travels to a truncated Porro prism 16 where a portion of the light is internally reflected from two silvered orthogonal faces 18 and 20, while the remaining portion passes through the truncated surface. Porro prism 16 is mounted to a body 22 whose orientation is to be measured about three orthogonal axes. A pair of cylindrical lenses 24 and 27 focus light passing through the truncated surface of Porro prism 16 as two orthogonal line images. The line image produced by cylindrical lens 24 normally falls between a pair of photodetectors 30 and 31. The line image produced by cylindrical lens 26 normally falls between a pair of detectors 34 and 35. Additional photodetectors 30, 31 , 34 and 35 provide electrical output when illuminated in dependence upon the orientation of body 22. Thus, if body 22 rotates about a first axis, the line image focused by cylindrical lens 26 will impinge upon detector 34 or detector 35, depending upon the direction of rotation. If body 22 rotates about an

axis perpendicular to the direction of propagation of the light, the line image focused by cylindrical lens 24 will impinge upon detector 30 or detector 31 , depending upon

the direction of rotation. Rotation about the optical axis is detected by a pair of detectors 42, 43. Accordingly, the patent discloses a single beam system which is

adapted to detect misalignment about any of three orthogonal axes simultaneously. The patent also discloses the provision of an aperture through prism 16 from the

truncated area so that light may pass therethrough without interference.

The patent to Takada et al. (4,847,511) discloses a single beam system which

measures roll, pitch and yaw errors. The system measures X and Y axis displacement of a body, measures pitch and yaw error by detecting the angle of inclination of the body, and measures roll error by detecting the displacement of the body at two different positions. Signal fluctuations due to light source variations are corrected. The system provides a separate displacement-detecting channel and angle-detecting channel.

Referring to FIG. 1 of Takada, the system utilizes the following elements to measure displacement, pitch, yaw and roll: laser light source 12, mirrors 13, 14, luminous flux magnifier 15 (including lens 15A, pinhole 15B, iris 15C and lens 15D), half mirror 16, 4-quadrant photosensor 17, polarizing beam splitter 18, quarter-wave plate 19, displacement magnifying corner cube prism 20 (elements 18, 19 and 20 forming a displacement magnifying module 21), position detecting 4-quadrant photosensor 22, second polarizing beam splitter 23, second quarter-wave plate 24, collimator lens 25 (elements 23, 24 and 25 constituting angle transfer means 26), and 4-quadrant photosensor 27.

Fiedler (United States Patent No. 3,816,000), assigned to McDonnell Douglas Corporation, discloses a three axis alignment system including a laser, beam splitter, and angle sensing detector element positioned to respond to the laser beam to produce electrical signal outputs for use in aligning equipment in roll, pitch and yaw. The single beam of coherent light generated by the laser is split into two parallel beams, the parallel beams are detected, and roll, pitch and yaw information is developed therefrom.

The patent to Beckwith, Jr. (United States Patent No. 4,939,678) discloses a method for calibration of a coordinate measuring machine, including the use of a single laser beam system having a laser, retroreflectors, beam splits, and photodiodes. The system is capable of measuring error in X, Y and Z axes for purposes of calibration.

The patent to Ames et al. (United States Patent No. 5,090,803), assigned to Lockheed Missiles & Space Company, Inc., discloses an optical coordinate transfer assembly providing angular correlation between spatially separated objects. Roll, pitch and yaw of a second directional device 12 relative to a first directional device 11 can be measured, using an optical alignment sensor 20 mounted on the first directional device 11 and a roof mirror/lens assembly 21 mounted on the second directional device 12. Plural light beams from the optical alignment sensor 20 are reflected by reflectors 35, 36 on the rooftop mirror/lens assembly 21 to linear arrays 69, 70, 78 of photodetectors, which generate electronic signals indicating angular discrepancies in three degrees of freedom.

The patent to Harris (United States Patent No. 4,560,272) discloses an optical sensor including a device for permitting an operator to monitor the angular motion of a remote body about three orthogonal axes from a single observation point. A compact passive target such as a corner cube prism is attached to the body. The target reflects light beams representative of pitch, yaw and roll measurements. The reflected light beams are optically sensed and reduced to components about the pitch, yaw an roll axes which are visually displayed through a single eyepiece. Further this patent makes particular claims for (a) a unique analyzer, i.e., not at 90° and (b) employs a passive target and double path length since transmitter an receiver are co-housed.

Boresighting systems typical of the prior art, however, have been characterized by complexity and difficulty of use.

Accordingly, it is an object of the present invention to provide an improved optical transceiver apparatus for measuring angular misalignment in elevation, azimuth and roll axes, using a single optical beam to measure such misalignment.

It is a further object of the present invention to provide an optical transceiver apparatus which utilizes a single optical path to measure roll, azimuth and elevation which is adapted for boresighting a gun system on aircraft.

Still another object of the invention is to provide such optical transceiver apparatus which utilizes only a single transmitted light beam rather than multiple light beams, and which provides a roll reference by polarizing a single collimated beam, rather than multiple sidebeams.

SUMMARY OF THE INVENTION

The foregoing objects are attained by the present invention, which provides optical transceiver apparatus for measuring angular misalignment between two bodies in elevation, azimuth and roll utilizing a single optical path to measure roll, azimuth and elevation, and which provides a roll reference by polarizing a single collimated beam, rather than multiple sidebeams.

In accordance with the present invention, the optical transceiver apparatus

includes a light transmitter coupled to a first body accurately fixed on the fuselag with reference to the aircraft dataline and a light receiver coupled to a second bod such as an aircraft wing or pylon. The receiver includes first and second analyzer characterized by first and second planes of polarization which differ with respect to each other.

If the plane of the transmitted polarized light rotates with respect to the plane

of polarization 17, 18, Figure 2 (as would occur if the wing or pylon rotates with respect to the fuselage), then the amount of light transmitted through one of the analyzers will increase while the amount of light transmitted through the other one will decrease. If the plane of polarization of the transmitted polarized light is rotated in the opposite direction, the converse will occur. The optical transceiver apparatus includes circuitry for measuring these differences to provide a signal representative of angular misalignment between the two bodies in the roll axis.

In one aspect of the invention the apparatus includes a single transmitter having a laser diode or other light source for producing a collimated beam of linearly polarized light, and a receiver having an elevation/azimuth (EL/AZ) channel and a roll channel sharing a common objective lens, wherein a beam splitter splits the beam generated by the laser diode between the two channels. The beam splitter may be

comprised of a perforated mirror or other suitable optical elements. In the ELVAZ channel lenses are used to focus light onto a position sensitive detector (PSD), while

in the roll channel, a polarizing Thompson beam-splitting prism or other suitable optical elements divide the roll channel into two beams of differently polarized light which are then focused on light detectors.

The invention will next be described in connection with certain illustrated

embodiments; however, it should be clear to those skilled in the art that various

modifications, additions and subtractions can be made without departing from the

spirit or scope of the claims. uco HlPTION OF DRAWINGS

For a fuller understanding of the invention, reference should be ma to the following detailed description and the accompanying drawings, in which:

FIG. 1 is a schematic diagram for roll only illustrating an embodime of optical transceiver apparatus in accordance with the present invention;

FIG. 2 is a schematic diagram for roll only illustrating first and second polarization components utilized in the apparatus of FIG. 1 ;

FIG. 3 is a schematic representation of a second embodiment of the prese

invention which utilizes a non-polarizing cube-type beam splitter and two separat analyzers;

FIG. 4 illustrates optical element details of an embodiment of the present invention;

FIG. 5 illustrates the effect of a cube-type beam splitter on the rotation of a light bundle as utilized in the embodiment shown in FIG. 3;

FIG. 6 is an illustration of a collimator pair utilized for the detection of azimuth and elevation;

FIG. 7 is an illustration of a collimator pair for azimuth and elevation

detecting wherein one collimator includes a 90 degree bend;

FIG. 8 is a ray trace diagram of the collimator pair shown in FIG. 7 when t

collimators are substantially parallel but laterally misaligned;

FIG. 9 is a schematic diagram of a circuit for measuring the output of a du

axis position-sensitive detector;

FIG. 10 is a schematic diagram of a circuit for generating roll detector output signals;

FIG. 11 is a schematic diagram of a circuit for generating the output of the elevation/azimuth (ELVAZ) detector;

FIG. 12 depicts a circuit for processing the output of the roll, elevation and azimuth channels;

FIG. 13 depicts control/display unit circuitry for the roll, elevation and azimuth channels; and

FIG. 14 is a schematic diagram of an optical transmitter utilized in the embodiment shown in FIG. 4.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Overview: FIG. 1 shows optical transceiver apparatus for detecting angular roll misalignment between two bodies in accordance with the present invention. The optical transceiver apparatus includes a transmitter 10 which generates a beam of polarized light 11. The apparatus further includes a light receiver 12 which contains first and second analyzers 13, 14, respectively and light intensity detectors 15, 16 positioned behind the analyzers which may be photovoltaic detectors of conventional design.

In an aircraft boresighting application the transmitter might be mounted on the fuselage, while the receiver might be mounted on a wing or pylon. Further

discussion of the application of optical transceiver apparatus to aircraft boresighting is provided in U.S. Patent No. 5,118,185 issued 2 June 1992 to Courten, assigned to DRS/Photronics Corporation, Hauppauge, New York, the assignee hereof, the disclosure of which is incorporated herein by reference. It will be apparent to those skilled in the art that the present invention is not limited to aircraft boresighting applications but that the invention can also be adapted to

measurement of misalignment between objects in ground-based applications, assembly lines, armored vehicles, etc.

As shown in FIG. 2, the analyzers 13, 14 are characterized by polarization

planes 17, 18 respectively. Planes 17 and 18 are offset from each other by 90°. Thus, if polarized fight enters the receiver with a plane of polarization that is midway between the planes 17 and 18, then each analyzer will transmit substantially equal amounts of light.

The amount of light which passes through the analyzers will be at a level between the maximum transmission possible, which occurs when the plane of polarization of the received light is aligned with the plane of the polarization of the analyzer, and the minimum transmission possible, which occurs when the plane of polarization of the received light is offset from the plane of polarization of the analyzer by 90°.

If the plane of the transmitted polarized light rotates with respect to the planes of polarization 17, 18 (as would occur if the wing or pylon rotates with respect to the fuselage), then the amount of light transmitted through one of the analyzers will increase while the amount of light transmitted through the other one will decrease. If the plane of polarization of the transmitted polarized light is rotated in the opposite direction, the converse will occur. The optical transceiver apparatus includes circuitry described hereinafter for measuring these differences to provide a signal representative of angular misalignment between the two bodies in the roll axis.

FIG. 3 depicts another embodiment of apparatus in accordance with the invention for detecting angular misalignment between first and second bodies. The apparatus shown in FIG. 3 includes elements 34 for generating a beam of collimated light, which beam is then transmitted through a polarizer 33. The polarizer 33 filters out all but one plane of polarization. The transmitted light bundle then passes through the decollimating lens 32 which focuses the light bundle down to a small spot of light. The now-converging light is transmitted through a non-polarizing beam splitter cube 31. A first portion of the beam passes straight through the beam splitting cube without any change in its state of polarization. The second portion of the beam is diverted 90° (in the upward direction in the diagram) with no change in that state of polarization.

A ray bundle passing through the beam splitter tube behaves as depicted in FIG. 5. If the ray bundle entering the beam splitter has a clockwise circular component, the bundle which passes straight through the beam splitter will also have a clockwise circular component. The ray bundle which is diverted 90°, however, will have a counterclockwise circular component. Once again, the system includes analyzers 13 and 14 as well as light intensity detectors 15 and 16. It will be apparent that by employing an appropriate configuration of analyzers 13 and 14, the system will operate in accordance with the same principles illustrated in FIGS. 1 and 2. In particular, if the source of polarized light is rotated in one direction (as will occur if the wing or pylon twists in a given direction), analyzer 13 will increase its transmission of light and analyzer 14 will decrease its transmission of light. Conversely, if the source of polarized light rotates in the opposite direction, analyzer 14 will increase its transmission of light and analyzer 13 will decrease its transmission of light.

Details of the optical components utilized in an embodiment of the invention are shown in FIG. 4, wherein optical measurement channels for elevation, azimuth

(EL/AZ) and roll axes all utilize a single optical axis. The illustrated system includes a laser diode 40, a polarizer 41 , a beam expander 42, air-spaced doublet collimator 43

and Brewster plate polarizer 44.

The laser diode, compared to other light sources, has several advantages. First, it is relatively efficient at converting electrical energy to light energy. In

addition, while most of the light emitted by the laser diode falls into one plane of polarization, other light sources typically produce randomly polarized light. This disadvantage of other sources would automatically reduce the efficiency of the system up to fifty percent. Although the beam emitted by the laser diode is very highly polarized, some impurities exist due to internal reflections within the diode structure. Accordingly, polarizing element 41 is placed after the laser diode to filter out these impurities.

The beam expanding Barlow lens 42 provides the transmitter of the illustrated system with a large working aperture and short physical length. This is done to provide a uniform optical couple between the receiver and transmitter over a wide range of lateral shift between the two units. The second polarizing element, Brewster plate 44, inclined at Brewster's angle eliminates impurities in the polarization caused by the optical elements themselves.

While the receiver depicted in FIG. 3 utilizes a combination of a non- polarizing cube-type beam splitter followed by two polarizing elements, the receiver 12 of FIG. 4 utilizes a single component in the form of a Thompson polarizing beam splitter 13'. The Thompson beam splitter 13', in accordance with well-characterized principles of construction thereof, provides two orthogonally polarized beams of light separated by an included angle of 45°. Polarization for this type of beam splitter is wavelength independent and has a high level of purity, providing an excellent extinction ratio. The recollimation lens 47 located in front of the Thompson beam splitter 13' in FIG. 4 assures that the light passing through the beam splitter is a parallel ray bundle. This avoids or minimizes localized confusion of the polarization caused by the internal structures of the beam splitter.

As previously indicated, the system shown in FIG. 4 provides a roll detection channel along with an elevation and azimuth (EL/AZ) measuring channel. Beam splitter 45 directs a portion of the beam received from decollimating lens 48 down to the EL/AZ detector shown generally by reference numeral 46. To preserve the purity of the polarization of the light as it goes through beam splitter 45, a perforated mirror type beam splitter is utilized. This consists of a plain metal mirror having an aperture through it. The central part of the converging ray bundle 49 passes through the hole in the center of the perforated mirror to the Thompson beam splitter 13'. The peripheral portion of the light bundle intersects with the mirror and is bent 90° to the EL/AZ detector 46. Although the perforated mirror-type beam splitter depicted in the drawing degrades image resolution (i.e., spatial resolution) of the reflected energy, azimuth and elevation accuracy is not affected because of the detectors sense the centroid of the bundle of peripheral rays which are reflected by beam splitter 45. The light which passes straight through the perforated mirror type beam splitter maintains a very high degree of polarization purity. This latter aspect is essential to efficient operation of the roll axis measurement channel. Azimuth and Elevation Detection System: The components of the EL/AZ detector 46 of FIG. 4 operate in accordance with the principals discussed hereinafter with respect to FIG. 6. Mirror 51 redirects the light reflected by beam splitter 45 to field lens 52 which focuses it upon position sensitive detector 53. It will be appreciated that by combining the EL/AZ detector channel and the roll axis detector channel as shown in FIG. 4, the optical transceiver apparatus of the invention can utilize a single optical path to measure angular misalignment in elevation, azimuth and roll axes. Referring now to FIG. 6, collimators 70, 71 are each provided with an objective lens 73, 74, respectively. Collimator 70 has a point source of light at the center of its field. Collimator 71 is provided with a viewing area 75 with crosshairs at the center of its field of view. As shown in FIG. 6, collimator 70 projects a virtual image of a point source to infinity. Collimator 71 decollimates the ray bundle and refocuses it into a real image as a point of light. If the optical axis of both

collimators are parallel to each other, the point of light will fall in the center of the field. However, if the optical axis of one collimator is tilted or rotated with respect to the other as shown in FIG. 6, the point of light moves off center as shown in viewing area 75.

Additionally, as shown in FIG. 8, if the optical axis of both collimators remain substantially parallel to each other but are laterally displaced from each other, the image in collimator 71 remains at the intersection of the crosshairs in visual field 75. This is because with collimated light, the rays are focused at infinity, and thus each ray is parallel to each other ray. When the bundle is intercepted by a decollimating lens, all rays are bent back to a common point. If the optical axis of

both collimators are parallel to each other, the rays of each are also parallel to

each other and the individual rays converge at the focal plane at the optical axis.

However, some of the energy of the light bundle is lost, as shown.

A variation of the FIG 8 configuration is depicted in FIG. 7. In the embodiment shown in FIG. 7 the optical path of the receiver includes a 90° bend. In most cases this bend is employed to render the system more compact or to

adapt its shape for a particular purpose. In the present case it is implemented to facilitate the inclusion of elevation, azimuth and roll in a single optical path. Thus, in the embodiment depicted in FIG. 7 the first surface mirror 61 bends the ray bundle 90°. Upon any misalignment in the plane of the paper and perpendicular to the optical axis, the image will move in and out. While the function of the collimator pair is not changed, the sense of direction will not be the same as that for the in-line system of FIG. 6. This difference is readily corrected as a matter of conventional optical practice.

Instruments based on collimation techniques have been used to visually measure angles or angular deflection for many years. Such techniques are generally effective but are subject to human interpretation. The advent of the photovoltaic position sensing detector (PSD) has improved the ability to measure angular relationships by limiting the requirement of human interpretation.

In one embodiment of the present invention, a dual axis position sensing detector is placed directly in the plane of the cross-hairs and the point of light is focused directly on the active surface of the detector. Position sensing detectors are available in two basic formats. Single axis detectors work in a single line only, and can measure either left-right or up-down. The measured translation must be

parallel to and on the sensitive axis of the detector. Dual axis detectors, in contrast, have two axis of sensitivity, oriented at right angles to each other. Since

this type of detector can be obtained in configurations where the two axis are totally independent of each other, elevation and azimuth can be sensed as

individual values.

The general mode of operation for position sensing detectors involves

directing a small light source onto the active surface of the detector. This light can

be in the form of a laser beam or a projected source such as a lamp filament or LED. Either the detector or the light source can move.

As an example, the dual axis detector 103 depicted in Figure 9 behaves as follows: meter 101 , connected between the elevation terminals E1 and E2, will deflect to the right as a point of light moves up the elevation arrow and to the left as the point of light moves down the arrow. Correspondingly, meter 102 deflects right as a spot moves to the right, and left as the spot moves left. As the spot moves about the surface of the detector, a unique combination of meter readings for 101 and 102, sign and magnitude, will define the quadrant and distance from center at which the point of light is located. If the spot is at the center of the detector 103, both meters will be centered. While the foregoing is illustrative of the functioning of position sensing detectors, most practical applications require the use of electronic differential amplification to increase the sensitivity and accuracy of the system.

Dual-axis detectors are commercially available in three different varieties. The first variety is the spot type of detector, utilizing arrays of multiple photovoltaic detectors on a single substrate. The advantages of spot detectors are their relatively low cost and high sensitivity when used in nulling systems. One disadvantage of spot detectors is that if the illumination spot is directed entirely

into one segment, true sense of location is lost since the detector will only then indicate that the spot is within that particular segment. In addition, spot detectors

have very limited vector accuracy without the aid of complicated electronics.

A more recently developed detector, referred to as the lateral-effect diode detector, is also available in both single and dual axis models. There are no lines of demarcation and the sensing effect is progressive as long as the illumination spot does not migrate off the detector surface. This detector type is highly cost- effective if true vector information is required, provides reasonable linearity, and requires only simple electronics for dual axis sensing. A disadvantage of lateral- effect diode detectors is that their linearity is not perfect and degrades further when the illumination spot moves off-axis. In addition, the mechanical center of this type of detector is not necessarily its optical center.

The most recently developed position detecting sensor is the super-linear

device, which is manufactured by using advanced ion implantation technology. These sensors are available in dual axis configurations. The advantages of super- linear devices are that they have very high resolution and a very high degree of linearity over the entire active surface. However, these detectors are costly and require complex electrical and bias systems.

Receiver Electronics: FIGS. 10-12 depict details of electronic components utilized in conjunction with the embodiment of the invention shown in FIG. 4. Specifically, in the optical transceiver system shown in FIG. 4, each measurement axis utilizes two transimpedance amplifiers, one differential amplifier, one summing

amplifier and appropriate detectors and bias network components.

The detectors for the roll channel are two photovoltaic units which are

preferably very well matched, in order to provide optimum accuracy. The elevation and azimuth detector is a single unit having separate outputs.

FIG. 10 shows is the bias network for the roll channel. Output from photovoltaic sensors 114 and 115 are provided at points 111 and 112 respectively.

Bias is provided at 113.

FIG. 11 shows a bias network for the EL/AZ detector, which is a single unit having separate outputs. Reference numerals 121 and 122 represent the elevation outputs. Numerals 123 and 124 represent the azimuth outputs, while numeral 125 represents common.

The signal processing electronics in the optical head for all three channels is substantially identical. An exemplary circuit is shown in FIG. 12. As shown therein, the two transimpedance amplifiers 131 and 132 convert the current output of the detector into voltage. The output of both amplifiers is fed into two mathematical elements, differential amplifier 133 and a summing amplifier 134. The differential amplifier subtracts the signal from the transimpedance amplifier 132 from that of amplifier 131. If the signal from 132 is greater than from 131, then the output of 133 will be negative. Conversely, if the signal from 132 is less than the signal from 131, then the output will be positive. If the signals from 131 and 132 are equal, then the output will be zero and have no sign. This circuit thus performs two mathematical functions and provides the direction of the vector. In particular, a positive output means that elevation is "up", while a negative output means elevation is "down". A "no output" signal indicates that the elevation is centered.

The summing amplifier adds the signals from 131 and 132. Since both 131 and 132 have positive values, the result will be positive.

Although the functions are the same, the roll channel requires special attention. The transimpedance amplifiers for the roll channel must be extremely well matched. They must also have the lowest noise figure possible, and thus the optimal configuration places the amplifiers as close as possible to the detectors.

Control display Unit (CPU) Electronics: FIG. 13 depicts exemplary control/display unit circuitry for one of the three measurement channels. In the embodiment illustrated in FIG. 4 the signal processing electronics for all three channels are substantially identical, and consist of two DC restorer switches, one analog divider circuit, one sample and hold circuit, and one readout and scaling circuit, all of which are constructed in accordance with known engineering practice utilizing conventional components. In addition, common clock circuit and power supply are provided for all three channels.

As shown in FIG. 13, the signal input to the control/display unit electronics is via a resistor network and DC blocking capacitor 141. The sum and difference inputs are substantially identical in format, and simply connect to different functions on the conventional analog divider 161. The DC restorer switches 142 are open only when the laser diode is active. They close when the diode is not emitting. This effectively shorts out any dark signal noise to a DC level of zero volts.

The processed difference signal is fed into the numerator of the analog divider, while the summation signal is fed into the denominator of the analog divider. The intent of this circuit is to compensate for variations in the light output of the laser diode. The function of this circuit may be summarized by the following formula: voltage 131 minus voltage 132, divided by the sum of voltage 131 plus

voltage 132.

The output of the entire circuit is proportional to the output for the transimpedance amplifiers in a given ratio - for example, voltage 131 equals one

volt while voltage 132 equals two volts. Thus, if the laser illumination doubles, so will the output voltages, such that voltage 131 equals two volts and voltage 132 equals 4 volts. The outputs from the detectors will track the laser input. Thus, the output of the equation compensates for changes in the drive illumination. However, it will not compensate for errors in linearity in the amplifiers or for errors in tracking due to optical problems.

The analog device selected must be able to give both negative and positive outputs. Additionally, the circuit must never be confronted with a situation where it divides by zero, since if this happens the output of the amplifier will automatically attempt to go to infinity.

Again referring to FIG. 13, the output of the analog divider is fed to the sample- and-hold circuit. As noted below, the sample-and-hold is triggered to receive

signals only when the clock pulse is HIGH. This function is mutually exclusive of that of the DC restorer switches. Accordingly, there a pulse invertor 162 is provided between the input of the sample and hold and that of the dc restorer.

Buffers are provided between the output devices and the remainder of the electronics. These buffers serve to match impedances and isolate the output

devices from each other. Each output device requires its own calibration control for individual and cross-calibration. The particular output device shown is a

centered zero analog meter 163. Other options would be a digital display to an analog-to-digital converter (ADC) to interface directly with the computer. The clock

143 synchronizes the system, controlling the laser, the DC restorer switches and the sample and hold circuit. The table set forth below shows how the system is

synchronized. CLOCK HIGH LOW

Laser ON OFF

Sample and Hold SAMPLE HOLD

DC Restorer OPEN CLOSED

Thus, when the clock is HIGH the laser is emitting. Signal passes through the differential amplifier, summing amplifier, analog divider and into the sample and hold. When the clock is LOW the laser is OFF. Therefore, the signal is blocked from the analog divider. The sample and hold is holding the DC level of the display unit until another cycle occurs.

By utilizing this arrangement, the system is rendered immune to DC radiation infiltrating into the optical system, barring detector saturation, and the system is relatively insensitive to pulsed radiation unless such radiation is synchronized to the clock. Thus, an optical pulse may cause the meter to "jump" but will not have a continuing effect. The control/display unit also contains power conversion and regulation devices.

Transmitter Assembly: The transmitter assembly depicted in FIG. 14

includes laser 158 and modulator 154. The transmitter receives power at modulator input 151 and a series of clock pulses (153) at modulator input 152.

Modulator 154 supplies power supply to the laser 158 via driver 157 and error amplifier 156 which receives a referencing signal at input 160. Feedback loop

compensator 159 keeps the laser at a nominally constant output while protecting it from thermal overload.

It will thus be seen that the invention attains the objects set forth above,

among those made apparent from the preceding description. In particular, the invention provides optical transceiver apparatus adapted for use in boresighting apparatus which can be used on aircraft during flight to correct for misalignment

caused by dynamic airframe deflection, or on static installations such as preflight checking stations or on assembly lines.

Having described particular embodiments of the invention with reference to the accompanying drawings, it will be appreciated that the present invention is not

limited to the precise embodiments or applications described, and that various changes and modifications may be effected therein by one of ordinary skill in the art without departing from the scope or spirit of the invention as defined in the appended claims.

It is accordingly intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative rather than in a limiting sense.

Having described the invention, what is claimed as new and secured by

Letters Patent is:

Claims

1. Optical transceiver apparatus for detecting angular misalignment between two bodies, comprising: an optical transmitter which transmits a beam of polarized light as a reference, said optical transmitter being mounted on one of said bodies, and an optical receiver which receives said optical beam from said optical transmitter, said optical receiver being mounted on another of said bodies, said optical receiver including an elevation/azimuth detector responsive to elevation and azimuth changes of said two bodies from a nominal or null position in response to said optical beam, and a roll angle detector responsive to angular twist between said bodies from a nominal or null position in response to polarization components generated from said optical beam, said roll angle detector including first and second polarization analyzers for receiving said beam of polarized light, said first and second analyzers having first and

second characteristic planes of polarization, respectively, with a predetermined angular relationship there between.

2. Apparatus according to claim 1 wherein said first and second analyzers have planes of polarization which are substantially perpendicular with respect to one another.

3. Apparatus according to claim 2 further comprising first and second light intensity detectors in proximity with said first and second analyzers, respectively, for detecting the intensity of light passing through said first and second analyzers respectively.

4. Apparatus according to claim 1 further comprising a first beam splitter for splitting said source of polarized light into at least two separate beams of polarized light with a first of said two separate beams of

polarized light being directed at said first analyzer and with a second of said two separate beams of polarized light being directed at said second analyzer, said first analyzer having a plane of polarization which forms a first angle with respect to the plane of polarization of said first separate beam of polarized light, said second analyzer having a plane of polarization which forms a second angle with respect to the plane of polarization of said second separate beam of

polarized light, and wherein said first and second analyzers are oriented so that when the plane of

polarization of said source of polarized light is rotated in a first direction, the intensity of light passing through one said analyzer increases and the intensity of light passing through the other said analyzer decreases.

5. Apparatus according to claim 4 further comprising first and second

light intensity detectors for detecting the intensity of light which has passed through said first and second analyzers respectively.

6. Apparatus according to claim 5 wherein said beam splitter and said first and second analyzers are comprised of a unitary polarizing beam splitter.

7. Apparatus according to claim 6 wherein said beam splitter provides two substantially orthogonally polarized beams.

8. Apparatus according to claim 7 wherein said polarizing beam splitt provides said substantially orthogonally polarized beams separated by an includ angle of approximately 45°.

9. Apparatus according to claim 5, further comprising a second beam splitter for splitting light received from said light source int a beam directed toward an azimuth and elevation detector and a beam directed

towards said first beam splitter.

10. Apparatus according to claim 9 wherein said light source provides light which is both collimated and polarized, an said receiver further comprises a decollimator which decollimates said polarized and collimated light received from said light source and transmits the decollimat

light to said azimuth and elevation detector.

11. Apparatus according to claim 9 wherein

said second beam splitter is a perforated mirror-type beam splitter.