Beamrider Missile Guidance Method

Abstract

In the improved beamrider guidance an optical beam is electronically modued and transmitted toward a target. A missile is launched into the beam path and is caused to automatically seek the center of the beam as it flies toward the target. The transmitted beam is spatially encoded to provide guidance signals which the missile automatically responds to in seeking the center of the beam. The beamrider missile does not require tracking or correctional guidance from an external source as it generates its correctional commands internally from the beams's spatial encoding. The missile is immune to optical countermeasures, being within the narrow corridor of the beam and responsive only to the encoded intelligence in the beam for seeking or maintaining flight along the beam center.

Description

BACKGROUND OF THE INVENTION

In guidance systems for directing a missile toward a target the system must be capable of avoiding jamming signals which may be deliberate or spurious in nature. Optical command guidance systems are highly accurate and have reduced missile susceptibility to jamming on counter-measure efforts. However, the missile beacon and beacon tracker of the command guidance systems are still highly susceptible to jamming.

An optical beamrider guidance system is disclosed in U.S. Pat. No. 3,398,918 to Girault wherein a missile is guided from a launching site to a target. A laser source and optical means for radiating fan shaped beams are at the launch site. The four fan shaped beams are independently modulated and are projected toward a target, forming four optical walls of a corridor for guiding the projectile. The projectile ricochets or bounces within the corridor until it reaches the target. Alternative to the optical wall method of guidance, a proportional guidance system provides means whereby two beams sweep the guiding volume in directions perpendicular to each other for directing the projectile.

SUMMARY OF THE INVENTION

In the improved beamrider missile guidance the gunner or observer at a launch site visually locates and identifies a target. A visual line-of-sight to the target is established through a telescopic sight. An optical transmitter, boresighted to the telescope, directs optical energy toward the target. The transmitted optical energy is spatially coded, allowing onboard missile sensors to respond to missile deviations from the gunner's line-of-sight. Deviation or error information generated in the missile allows automatic generation of correctional commands for returning the missile to the line-of-sight. The beamrider system provides countermeasures hardening over existing systems by utilizing the extreme accuracy of optical command guidance while eliminating the susceptible missile tracker and beacon from the system. The beamrider system has a re-usable transmitter which provides a myriad selection of electronic signal modulation. The accuracy with which a missile seeks the center of a beam, while moving toward a target, is determined by the particular spatial encoding of the transmitted beam and the resulting beam resolution .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of the beamrider guidance system.

FIG. 2 is a diagrammatic view of a preferred embodiment of the beamrider transmitter encoding method.

FIGS. 3A, 3B and 3C disclose coded beam patterns A, B, and C that can be projected toward a target.

FIG. 4 is a graph of relative intensity across the beam from a single source such as FIG. 3C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like numbers represent like parts in each of the several figures, FIG. 1 discloses an optical beamrider system. From a target tracking station 10 an optical beam 12 is directed toward a target 14 by a beam transmitter or optical source 16 which is boresighted to follow the telescopic tracking of the target by an observer or gunner. A missile 18 is launched from the target tracking station into the beam path toward the target. An optical receiver or receivers 48 on the aft end of the missile respond to the spatially encoded light beam for directing the missile toward the center of the beam or null axis representing the line-of-sight axis between the target tracking station and the target.

FIG. 2 discloses the method of optical transmission toward the target wherein the transmitter 16 comprises a group of symmetrically arranged diode lasers 31, 32, 33 and 34. A bank of electronic modulators 22 provide respective output frequencies f1, f2, f3 and f4 for respectively modulating diode lasers 31, 32, 33 and 34. Optical output energy from the diode lasers is coupled through respective light pipes 41, 42, 43 and 44, forming the coded beam pattern at a common junction 24 for projecting the beam. The beam pattern 26 projected from light pipe terminal 24 is coupled to a nutator 28. Nutation of the beam during target tracking allows vertical and horizontal position information to be obtained onboard the missile for generation of guidance commands. Beam nutation as seen by a missile receiver is equivalent to moving the receiver in a small circle within a stationary beam, as shown in FIGS. 3A, 3B, and 3C. The output optical energy from nutator 28 is directed to beam forming optics 29 for transmitting the beam toward the target. The missile, launched into the beam path, has an optical energy detector 48A on the aft end thereof for detecting the modulated frequencies spatially coded into the formed beam and activating correctional guidance control systems.

The beam pattern of FIG. 3A is obtained from a solid state laser diode array wherein the separate beams generated by individual lasers are brought together to form a single composite beam. Beam pattern 26a is comprised of beam quadrants f1, f2, f3 and f4, representative of the modulated frequency superimposed on respective optical beam segments. To provide very sharp edges to the quadrants, and relatively uniform intensity over a given quadrant, the light pipes are placed between the diode arrays and nutator 28. Thus, the light pipes function as optical integrators for each quadrant and may consist of glass or quartz sections. The integrator is sufficiently long to permit total internal reflections to smooth out dark and bright areas of the array, thereby providing uniform intensity over a given quadrant. For the particular quadrant formation shown, vertical position error is the relative duration of f1 + f2 compared to f3 + f4. Similarly, horizontal error is given by the relative duration of f1 + f4 compared to f2 + f3. The small nutation circle within pattern 26a represents the missile position within the beam at a given time during flight. As shown, there is no error in the vertical deviation, hence, no vertical correction is required. For the horizontal direction, however, the missile will sense an error since the relative duration of energy received from f1 + f4 is greater. Obviously, the beam axes can be rotated in space. Encoding of signal processing requirements would change mechanically with a different system arrangement but the function would be identical.

For a 4-line beam as shown in FIG. 3B the information desired from a nutated quadrant beam is contained in the crossing times from one quadrant to the next. Thus, four coded lines are transmitted and the quadrant areas left blank. In response to detector 48A the missile electronics senses the relative time between axis crossings and processes the signals in the same manner as in the quadrant configuration. The quadrant or line shape of the beam pattern is determined by the optical integrators used. A linear system may also be embodied when a reference sine or cosine voltage is sampled at each crossing in a 4-line beam. However, the reference voltages must be phase locked to the nutation frequency for this embodiment, which makes the relative duration processing method more suitable for the beamrider. An advantage of the 4-line beam over the quadrant beam is the significant reduction in transmitter power required. For the same intensity of transmitted energy and effective beam size, the required power is reduced by the ratio of the active array areas. This is approximately a factor of 10 for a line aspect ratio of 10:1, for which linear arrays of laser diodes are well suited.

The amplitude coded beam shown in FIG. 4 utilizes a single frequency fixed (not nutating) beam with a high intensity center portion, and a reduction in intensity with distance from the center. A single diode source provides the simple beam. During flight the missile is required to roll at least one-half the rate that correctional commands are required. Two detectors 48B are placed on the aft end of the missile, preferably on the wings and as widely separated as possible. Deviation of the missile, away from a position where the signal level is equal on both detectors, results in a strong imbalance of signal energy due to the high intensity center of the beam. This indicates to the missile guidance that correction is needed. If the missile is directed toward the detector receiving the greatest signal during each one-half turn the average command will be towards the beam center. Since the fall-off of beam intensity is controllable, appropriate sensing logic on the missile allows linear error correction to be achieved. If it is desired the tracking rate or other known bias can also be transmitted to the missile from the launch site via the coded beam. If it is desired not to roll the missile, four detectors 48B can be placed on the aft of the missile, forming a quadrant plane normal to the missile longitudinal axis. Deviation of the missile from positions of equal intensity will indicate and initiate correctional guidance.

The amplitude coded beam of FIG. 4 can be nutated as shown in FIG. 3C. Processing is identical on the missile to that of the non-nutated beam except that only one optical detector is required for this mode and the angle of rotation of nutation wedge 28 is transmitted as a timing signal on the beam for detection by the missile. The missile guidance then responds to a signal proportional to the variation in signal amplitude received during each nutation cycle. The guidance response is in a direction determined by the received nutation angle at the time of received maximum intensity.

The basic guidance information of a beamrider missile is angular, measured at the transmitter, between the boresight axis and missile position. Thus, for long range targets, the linear miss distance increases with range. For instance if the nominal guidance accuracy is 1/3 milliradian at 5 miles, the miss distance is 8.8 feet. Such accuracy is acceptable for ground targets such as tanks, vehicles, and hardened positions, since they are normally engaged at much shorter ranges. For the longer target engagement ranges a dual mode operation is desirable wherein the missile first rides the beam to the vicinity of the target, then a homing seeker on the missile activates providing homing guidance for the terminal phase of the flight. Depending on the missile-target range at which it is desired to initate terminal homing, the homing seeker can be rudimentary, with the reflected energy from the target providing the homing beam path.

Either photoemissive diodes or photodiode lasers can be used as optical sources for the beamrider transmitter. These devices offer a direct power conversion from electrical to modulated optical power with typical efficiencies from 10 to 25 percent. They are capable of high frequency modulation, which eliminates wake modulation of background (including sun interference) as a noise source. They are easily arrayed to provide any desired beam configuration, and simple optics can project the array geometry eliminating the need of beamsplitters and mechanical modulators, since each quadrant or any portion of the array may be independently modulated electronically.

Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention should be limited only by the claims appended hereto.

Claims

We claim:

1. In a missile guidance system wherein a target is tracked by visual line-of-sight, a method of directing a missile toward a target and comprising the steps of: visually tracking a target from a target tracking station; directing an amplitude coded beam of optical energy, having a high intensity center portion and a reduced intensity outer portion, from said target tracking station along the line-of-sight to the target; launching a missile into the path of said optical beam toward said target; encoding said optical energy by electronically modulating a single solid state, photodiode source for providing an optical frequency reference; and generating correctional guidance within the missile in response to said optically encoded energy for maintaining missile trajectory along the line-of-sight.

2. A method of directing a missile toward a target as set forth in claim 1 and further comprising the steps of: simultaneously detecting first and second intensity levels of said optical energy by said missile for directing said missile toward the high intensity center portion of the beam.

3. A method as set forth in claim 2 and further comprising the steps of: nutating said amplitude coded beam, transmitting a nutator position timing signal in said amplitude coded beam, and detecting by said missile the variation in signal amplitude during nutation and the timing signal for directing said missile toward the high intensity center portion of the beam.

4. In a missile guidance system wherein a target is tracked by visual line-of-sight a method of directing a missile toward a target and comprising the steps of: electronically modulating a plurality of solid state, photodiode sources with respective individual frequencies for providing individually encoded optical beams; forming said individually encoded beams into a single beam of adjacent segments having a common central point by transmitting the output of said plurality of diode sources through an optical integrator and imaging the output face of the optical integrator to form the projected beam; visually tracking a target from a target tracking station; directing said beam of optical energy from said target tracking station along the line-of-sight to the target; launching a missile into the path of said optical beam; and generating correctional guidance within the missile in response to said optically encoded energy for maintaining missile trajectory along the line-of-sight.

5. A method of directing a missile toward a target as set forth in claim 4 and further comprising the steps of electronically modulating first, second, third, and fourth laser diodes with respective first, second, third, and fourth frequencies for providing said encoded optical energy path to said target; and forming said single segmented beams into quadrants of uniform intensity.

6. In a missile guidance system wherein a target is tracked by visual line-of-sight a method of directing a missile toward a target and comprising the steps of: electronically modulating an array of photodiodes with a plurality of individual frequencies for providing individual modulated optical beams; shaping said beams with respective light pipes into a plurality of optical lines; forming said optical lines into a plural line beam having a common, central, line terminal point with optically void zones adjacent said optical lines; visually tracking a target from a target tracking station; directing said plural line beam of optical energy from said target tracking station along the line-of-sight to the target; launching a missile into the path of said optical beam; and generating correctional guidance within the missile in response to said optically encoded energy for maintaining missile trajectory along the line-of-sight.

7. A method of directing a missile toward a target as set forth in claim 6 and further comprising the steps of: modulating first, second, third, and fourth photodiodes with respective first, second, third, and fourth frequencies for providing an orthogonal optical axes energy path as said plural line beam.

8. A beamrider missile guidance system comprising: apparatus for transmitting an optical beam toward a target; a missile for traversing the path of said optical beam, an optical detector on said missile responsive to said optical beam energy for directing the missile trajectory within said beam and wherein said apparatus for transmitting an optical beam includes a plurality of photodiodes arranged in an array, a plurality of electronic modulators coupled to respective of said diodes for individually modulating said diodes, an optical integrator responsive to the output optical energy of said diode array for directing said energy as distinct adjacent sections of a composite beam, and a nutator for nutating the optical output beam.

9. A missile guidance system as set forth in claim 8 wherein said optical integrator is a plurality of light pipes for shaping individual beam segments into a pattern segment, said photodiodes are laser diodes, and said composite beam is a quadrant beam.