GB2033186A - Guide beam system - Google Patents

Abstract

A guide beam system for determining the deviation of a guided missile from a line of sight employs a laser transmitter which projects a beam of IR radiation at the missile. The beam is generated by modulating the laser output, passing the beam through a pinhole diaphragm scattering device (3), and causing the scattered beam to nutate about the line of sight by means of a rotating optical wedge (5). The modulation frequency varies in synchronism with the nutation so that at any given point ( alpha FK, @FK) in the area swept by the beam a receiver carried by the missile monitors a frequency modulation characteristic of that point to derive deviation co-ordinates for corrective control of the missile. <IMAGE>

Description

SPECIFICATION Guide beam system This invention relates to a system for determining the deviation of a missile in respect of the axis of a guide beam for tracking the missile. More particularly it relates to a system in which the guide beam comprises electromagnetic rays generated by a transmitter equipped with an optical system and a modulator.

Such a system is described in German Patent Specification No. 1,448,570. However, there has not so far been a corresponding system in which a laser, specifically a gas laser, is used as the guide beam. Also, in the known system, the transmitter is complicated by the necessity to transmit not only a modulation waveform but also the phase information specific for the signal which happens to be of interest.

It is an object of the invention to circumvent the above drawbacks and to provide an improvement on the known guide beam system so that it is suitable also for a laser beam.

According to this invention there is provided a method of determining the deviation of a guided missile from a reference axis wherein a guide beam defining the axis is projected towards the missile from a location remote from the missile, the guide beam being generated by modulating a beam of electromagnetic radiation produced by a laser, passing the beam through a scattering device, and causing the scattered beam to nutate about the said axis, thereby producing a projected beam in which the radiation at any given point in a cross-section of the beam is characteristic of that point, and wherein the beam radiation is monitored by a receiver mounted in the missile, the received signal being evaluated to determine the position of the missile in the beam cross-section in terms of a set of coordinates.Such a system, which is suitable for a laser beam, works without a movable modulation disc and without the transmission of phase information, and is therefore less costly to produce. In addition, the system can be so designed that, even in very poor visibility, corresponding ranges can be achieved with a guided weapon used both during the day and at night.

In accordance with the invention, the receiver at the missile end may comprise (in the direction of the received beam): an interference filter serving to suppress background noise, an input objective, a cooled sensor which converts the intensity modulation into an alternating voltage signal, a pre-amplifier to raise the sensor output to a workable level, a frequency discriminator, an electronic unit which extracts the angular deviation values p and tp of the missile in relation to the line of sight, and also a further electronic unit which evaluates the metric deviations x(t) and y(t) of the relevant phase of flight as guidance signals for corrective control of the missile.

According to requirements in respect of positional measurement accuracy or the design of the signal processing electronics in the missile, the modulation frequency may vary with time according to a sine, sawtooth or triangular waveform.

Furthermore, it is advantageous for a simple objective to be used at the receiver end and for an IR zoom objective to be used at the transmitter end. In this respect, by stepwise variation of the focal length of the latter in the long distance range, the image plane of the beam transmitter can be adjusted stepwise to whatever is the range of the missile so as to produce a sharp image of the pinhole diaphragm in the objective plane of the receiver. The focal length of the IR zoom objective can however also be adjusted by a continuously operating distance/time control to correspond to the range of the missile and so that metric deviations from the line of sight can be associated with specific deviation angles. This alternative, with a given distance/time control, represents a reduction in electronic complexity and allows the use of a simple objective in the missile receiver.

The pinhole diaphragm preferably comprises a partially metal-coated surface-treated germanium disc which creates a lighted area of diameter d, nutate about the middle point of a periodically lighted circular area of diameter D, the two diameters having a size ratio of d 0.5 D.

The cooler for the sensor may be a thermoelectric type or a Joule-Thomson cooler. This cooler maintains the sensitive sensor at optimum working temperature.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram showing the principle of a beam transmitter; Figure 2a is a side view and a cross-section of a variable beam in accordance with the invention, with diameter d = 0.5 D; Figure 2b is a side view and a cross-section of the beam of Figure 2a, but with diameter d > 0.5 D; Figure 3 is a diagram showing the principle of the beam receiver; Figure 4 is a cross-sectional beam diagram with diameter d = 0.5 D; Figure 5a shows a sawtooth frequency modulation characteristic; Figure 5b is a cross-section at any given time of a beam having the frequency modulation characteristic of Figure 5a; Figure 5c shows the modulation deviation as a function of p/R of the frequency modulation characteristic of Figure 5a; and Figure 6 shows a triangular frequency modulation characteristic.

Referring to Figure 1, the proposed transmitter has a CO2 CW laser 1 which emits a sharply focused monochrome laser beam. This beam is frequency modulated in the electro-optical modulator 2, the modulation being so chosen that the maximum frequency deviation Afmax = fmax - fmin amounts to a few kHz while the centre frequency is also in the kHz range. The modulation frequency variation follows a periodic time law and may be of, for example, sine, sawtooth or triangular form.

Modulated in the above described manner, the laser beam strikes the pinhole diaphragm 3. The latter consists of a plane germanium disc which, with the exception of a circular central zone, is coated with metal.

Also, its surface is treated so that the emergent laser beam is diffusely scattered. Since the pinhole diaphragm is disposed in the image plane of the IR zoom objective 4 which is optically downstream of it, a circular lighted area is depicted in the relevant missile plane. The rotary wedge 5 which follows in the direction of beam passage and which rotates for example at 30 to 50 Hz causes the illuminated area to nutate about the central point of a circular area of diameter D, as shown in Figure 2, the diameter d of the beam being chosen in relation to the diameter D of the periodically illuminated field such that with d 3 0.5 D, uninterrupted illumination occurs. For d > 0.5 D according to Figure 2b, the middle zone of the field to be illuminated is constantly illuminated so that also the missile 7 in this area constantly receives a control signal.This results in optimum deviation measurement accuracy in the area around the line of sight 21. The diameter D can be varied by construction of the zoom system according to a given time law, so that the beam can be adapted to the range of the missile with the missile remaining within the beam cross-section of diameter D. The focal length can be switched stepwise so that in the long distance range the image plane of the beam transmitter 1 can be stepwisely adjusted to the relevant distance of the missile so that a sharp image of the pinhole diaphragm 3 in the objective plane of the receiver 9 to 15 is possible.However, the zoom focal length can also, by means of a distance/time control, be continuously adjusted to the transmitter/missile distance after the missile 7 has been fired so that definitive metric deviations from the line of sight can be associated with specific angles of deviation, independently of the transmitter/missile distance as the missile travels downrange.

The receiver 9 to 15 associated with the transmitter 1 to 5 is disposed in the tail or side of the missile and looks in the direction of the transmitter. The receiver according to Figure 3 includes a simple objective 8, and a narrow band interference filter 9 located in front of or behind the objective 8 for suppression of background noise and interference. A sensor 10, sensitive to radiation at 10.6jim, is cooled if necessary by thermo-electric or Joule-Thomson cooler 11.

The sensor converts the intensity variations of the FM signal of the laser beam into an electrical alternating current voltage signal which is amplified in the pre-amplifier 12 and fed to the frequency discriminator 13. In the electronic unit 14, the angular deviation values (p and p of the missile in relation to the line of sight or the desired flight path 21 derived from the centre frequency fm and frequency A f = f2 - f, are depicted and via co-ordinates conversion and distance/time law in the electronics unit 15, the metric deviations x(t) and y(t) in the relevant flight phase are evaluated as guide signals for corrective control of the missile 7.

In the above-described case of the variation of the objective focal length via the distance/time law, it is possible via p and cp, by co-ordinates conversion, directly to determine the metric deviations x(t) and u(t).

The principle of obtaining the deviation signal will now be explained with reference to Figure 4. The circle 16 of diameter D = 2R characterises the cross-section of that area in the missile plane which is periodically irradiated by the transmitter. The cross-section of the beam itself has in the case illustrated a diameter d = 0.5 D and describes a nutation movement about the beam centre 17, the central axis 18 of the beam rotating at a constant angular velocity co on the dotted circle 19. The laser beam is intensity modulated over its entire cross-section with a sine oscillation of frequency f = f (a), the values a = 0 and a = 360O being associated arbitrarily with the horizontal line 20.

The missile 7 with deviation of co-ordinates P FK and a FK in respect of the beam centre point 17 will for the first time receive a guidance signal when the beam centre 18 assumes the position a1 on the circle 19 and when the intensity modulation has the frequency fi = f(a1) and will receive signals from the guide beam until such time as, in the position a2, at frequency 6 = f(a2), signal transmission ceases.At a FK, the beam cross-section has the frequency fFK = f(a). It will be readily realised that as aFK becomes smaller, the frequency sweep A f = f2 - f1 becomes greater and vice versa, so that for a suitable choice of frequency f = f(t), it is possible to determine from the frequency sweep the radial deviation PFK, and if one knows the frequency fFK, also simultaneously the angle q,. Thus the position of the missile 7 with respect to the sighting line is clearly established by the receiver.

Simple relationships can be arrived at if, as shown in Figure 5A, the frequency variation is a linear function of a between the frequencies fmin = f(O ) and fmax = f(3600), a = 0 = 360 being a point of discontinuity. For a ratio d = 0.5D, the relationship between radial deviation p/R and frequency A f is given by the standardised curve A in Figure 5c. For a ratio d = 0.6D, curve B shows the relationship. In both cases, the frequency fFK aFK is determined as fFK = 0.5 (f2 - f) and thus the positions are clearly fixed.

A further form of variation of the modulation frequency with a(t) is the triangular modulation shown in Figure 6 in which once again p = (p (Af) and ambiguity of the centre frequency fm is avoided by the fact that between 0 and 180 the frequency rises, then drops between 180" and 360". Taking this law into account, it is possible again to obtain the missile deviation values, the dependency between p/r and Af being shown in curve C in Figure 5c.

Claims (11)

1. A method of determining the deviation of a guided missile from a reference axis wherein a guide beam defining the axis is projected towards the missile from a location remote from the missile, the guide beam being generated by modulating a beam of electromagnetic radiation produced by a laser, passing the beam through a scattering device, and causing the scattered beam to nutate about the said axis, thereby producing a projected beam in which the radiation at any given point in a cross-section of the beam is characteristic of that point, and wherein the beam radiation is monitored by a receiver mounted in the missile, the received signal being evaluated to determine the position of the missile in the beam cross-section in terms of a set of co-ordinates.

2. A system for determining the deviation of a missile with reference to the axis of a guide beam adapted to track the missile, the guide beam comprising electro-magnetic rays generated by a transmitter equipped with an optical system and a modulator, the projected beam having a radiation energy which in respect of its cross-section is typical for any point in the cross-section, the relative distribution of the radiation energy being the same in all cross-sections and, measurable in the form of a pulsating current of specific wave form by a receiver located in the missile, serving for co-ordinate-related evaluation, wherein the guide beam is generated by a CO2 CW laser, modulated in intensity in the same phase by means of an electro-optical modulator over the entire cross-section, diffusely scattered in the outer radiation region by a coated pinhole diaphragm situated in the image plane of the objective and, in the central axial region, is nutationally imaged at a reciever in the plane of the missile by means of a rotating wedge.

3. A system according to claim 2 wherein the receiver, following the signal path, comprises an interference filter which serves to suppress background noise, an input objective, a sensor which converts the intensity modulation into an electrical alternating voltage signal, a cooler enclosing the sensor, a pre-amplifier to raise the alternating signal to a workable level, a frequency discriminator, an electronic unit to extract angular deviation values cp and d of the missile in relation to the reference axis, and a further electronic system to evaluate the metric deviations (x)t and y(t) of the relevant phase of flight as guidance signals for corrective control of the missile.

4. A system according to claim 2 or claim 3 wheren the modulation frequency varies in time according to a sine, sawtooth or triangular characteristic.

5. A system according to any of claims 2 to 4 wherein a simple objective is used on the receiver side while on the transmitter side, an IR zoom objective is used.

6. A system according to claim 5 wherein, by stepwise focal length variation of the IR zoom objective, the image plane of the beam transmitter is adaptable stepwise to the range of the missile to produce a sharp image of the pinhole diaphragm in the objective plane of the receiver.

7. A system according to claim 5 wherein the focal length of the IR zoom objective is, in response to a pre-set distance/time control, continuously adaptable to the instantaneous range of the missile thereby to allow defined metric deviations from the reference axis to be associated with specific deviation angles regardless of the range.

8. A system according to any of claims 2 to 7 wherein the pinhole diaphragm comprises a metal-coated surface-treated germanium plate.

9. A system according to any of claims 2 to 8 wherein the cross-sectional area of light of diameter d created by the pinhole diaphragm nutates about the central point of a periodically lighted circular area of diameter D, the two diameters having a size ratio such that d 0.5D.

10. A system according to any of claims 2 to 9 wherein a thermoeiectric or Joule-Thomson cooler is used as the cooler.

11. A missile guidance system substantially as herein described with reference to the drawings.