GB2515000A - Seeker system - Google Patents

Abstract

A semi-active laser (SAL) seeker system is disclosed. Radiation 8 emitted by a laser designator 2 is incident on a target 10, which reflects the light. The emitted radiation is polarised by a series of optical components which may comprise a polarizer 4 and a quarter wave plate 6. Circular polarisation may advantageously be used as it is rare in nature and does not constrain the rotational orientation of the designator or the seeker. Specular reflection of circularly polarised light changes the handedness of the polarisation. The invention relies on ensuring that the light 12 reflected from target 10 has a strong known polarisation. The seeker includes optical components which may comprise a quarter wave plate 14 and a polarizer 16 acting so that the detector 18 receives light of a given polarisation. The use of polarised light improves the signal to noise ratio by 3dB and allows the range to the target 10 to be increased by approximately 41%. A simple PIN diode detector may be used.

Description

Seeker Systems The invention relates to seeker systems. More specifically, but not exclusively it relates to semi active laser (SAL) seeker systems.

Semi-active laser detectors are a mature technology. In the simplest instances they use simple PIN photodiodes to detect the laser pulses originating at a designator and reflecting from a target. They must reliably detect the pulse energy above both background illumination and any noise in the detector; and they must also measure accurately the timing of the pulse to detect so that this matches the correct code emitting from a designator. Increased timing accuracy can also be exploited using techniques such as last pulse logic to avoid the seeker being seduced by reflections between the designator and the target e.g. from battlefield smoke. They must provide a signal, that gives the position of the laser spot relative to some origin (normally the boresight of the camera), to the guidance system. Known sensors use four PIN diode elements in a quadrant arrangement to provide the fast response and the position. A newer sensor is disclosed, and summarised below, that uses a single semiconductor element.

In known detectors, one of the factors that govern whether the seeker can detect the pulse above background illumination (clutter) is the range to target. An increase in the maximum operating range can be achieved either by limiting the clutter e.g. by using a bandpass optical filter centred on the laser wavelength; by reducing the collection time of the sensor, e.g. for semiconductors reducing the integration time; or by increasing the laser pulse energy (this may be unattractive for reasons of safety or power/size/weight increase).

However, even if these approaches are used, a further affordable increase in range is desirable.

The problem addressed by the present invention is the requirement to increase the operating range of a platform by improving the signal to clutter noise ratio in the detector. If the atmospheric absorption at the signal wavelength is low, the signal loss with range, for a diffuse reflection, can be taken to vary as 1/r2 where r is the range. Thus an increase in the signal to noise ratio of 3dB gives a i2 improvement of the range or approximately 41%.

In accordance with one form of the invention there is provided a seeker system comprising a radiation source emitting radiation towards a target and a detector detecting radiation reflected by the target, the seeker system further comprising optical elements disposed between the source and the target, said elements acting so as to polarise the radiation emitted toward the target, and further optical elements disposed between the target and the detector, said further elements acting so as to detect only the radiation reflected by the target of a given polarisation, thereby improving the signal to noise ratio of the seeker system.

Advantageously, the present invention uses the polarisation of laser pulses to further suppress clutter that can provide range increases of up to 41% over known detectors. It is a further advantage of the invention that a seeker in the form of the present invention will not be seduced by secondary reflections from other objects or light scattered by the atmosphere between the designator and the target.

Polarisation can be used to distinguish these false reflections from the correct reflection from the target. Additionally, the system is able to provide a seeker system capable of operation in misty or foggy atmospheres.

The invention will now be described with reference to the following diagrammatic drawings in which: Figure 1 is a schematic diagram of the seeker system comprising a designator 1 including a radiation emitter 2 and the seeker 3 including a detector 18 as two separate units within the whole seeker system.

The seeker system comprises a designator 1 and a seeker 3. The designator 1 comprises a radiation emitter 2 such as, for example, a laser. The laser 2 emits radiation towards a target to be detected 10. In the present embodiment, the radiation may be pulses or laser light but other wavelength of radiation may be used with a suitable emitter replacing the laser 2 of the present embodiment.

The radiation emitted by the laser 2 is transmitted through a series of simple optical elements 4, 6 that in the present invention comprise a polariser 4 and a quarter waveplate 6. The radiation emitted by the designator 3 in the present invention is therefore polarised and is transmitted 8 towards the target 10. The radiation is reflected by the target 10 and as such the nature of the polarisation is affected, depending on the nature of the polarisation of the transmitted radiation 8. The radiation reflected 12 is detected by the seeker 3. The radiation 12 is transmitted through a quarter waveplate 14 and a polariser 16 to the detector 18. It will be appreciated that the waveplates 6, 14 and the polarisers 4, 16 are matched to take account of the change in polarisation of the radiation caused by reflection by the target 10.

As can be seen from the attached diagram, one form of the present invention uses a single analogue PIN sensor, such as one found in the prior art cited above, together with a single set of polarising optics. As will be described below, the optics required will vary but the principle behind the invention remains the same for each embodiment.

Light scattered inelastically by the atmosphere between the designator 1 and the target 10 will be unpolarised and hence its energy will be reduced at the designator 1 reducing the risk that the seeker 3 will be seduced to follow this light rather than its direct path to the target 10. Elastic scattering such as Rayleigh or Mie scattering will result in a reduced amount of circular polarisation depending on the geometry and scattering time constants e.g. if the observer, the scatter point and the source are at degrees the scattered light will appear to be linearly polarised. This polarisation has the added advantage that seduction is avoided.

Specular reflection of circularly polarised light results in a change of the handedness' of the polarisation. Thus right circularly light reflected at the target 10 will become left polarised and it is this polarisation that should be sought by the seeker 3, if the designator 1 is right circularly polarised. A secondary reflection of the light reflected by the target 10, e.g. by the sea between the target 10 and the seeker 3, will return the polarisation to right handedness and this will be eliminated by the optics in the seeker 3 thus improving the seeker's ability to avoid being seduced by secondary reflections. A tertiary reflection will be seen by the seeker 3, but by this time the energy should be significantly reduced. In general the seeker 3 should see' the odd reflections (first, third, fifth etc) but be blind to the even ones (second, fourth, sixth etc). Thus the use of polarisation mitigates the problem of secondary reflection.

If a strong polarisation signature is expected but its type is not known a priori, e.g., circular or linear, and if linear the orientation of the polarisation, some adaptation may be possible in the optics e.g. rotating the plane of polarisation of the polariser for linear polarisation.

In this way, the integration characteristic sensing time for position detection may be improved and this in turn may improve the signal to clutter ratio. Additionally, complexity, cost, size and weight; are also reduced.

The invention relies on ensuring that the reflection from the target 10 has a strong known polarisation. This may be either linear or circular, and may be natural e.g. legacy laser designators have a linear polarisation and, provided this is oriented in a known direction e.g. vertically, the reflected polarisation may be known.

The present invention includes the potential to control this actively i.e. by putting elements 4, 6 on the designator 1, or passively i.e. by taking advantage of the polarised nature of the light emitting from the laser. The receiving elements 14, 16 can be matched to the reflected polarisation radiation 12 and allow for the use of the single element rather than the switchable sets envisaged in prior art.

If linear polarisation is used rotating it to a particular orientation may be beneficial e.g. sky light is linearly polarised tangential to a circle centred on the sun -orienting the polarisation to be normal to this may provide benefits in suppressing sky light reflection. This can be achieved by placing simple optical elements in front of the designation laser.

It can be seen that placing simple optical components, such as polarisers 4 and quarter waveplates 6 in front of the designator laser 2 allows conversion of the radiation emitted by the laser 2 to circular polarisation. Circular polarisation is particularly attractive as it is rare that it occurs in nature. It is also beneficial as it will not constrain the rotational orientation of the seeker 3 or the designator 1 with respect to each other.

The use of linear polarisation has a disadvantage in the polariser 16 in the seeker 3 must be aligned with the plane of polarisation of the detected light 12, which may vary. Thus it is believed one of the major advantages of this technique may be realised by the use of circular polarisation. By controlling the state of polarisation of the pulse at the seeker 3, single fixed optical polarising element 16 in front of the detector 18, again reducing cost, size and weight, and providing the improved signal to clutter ratio from the first pulse.

The invention includes simple optical elements 14, 16 in front of the detector 18 within the seeker 3 to match the polarisation of the reflected laser pulse 12. For example to detect circular polarisation a quarter waveplate 14 and a polariser 16 may be used.

Since the background illumination will have a low degree of polarisation, these will allow the seeker 3 to capture all of the pulse energy 12, (less a small amount due to insertion loss of the components), while rejecting half of the background, (less a small amount due to insertion loss of the components).

Thus by including some simple low cost optical elements 14, 16 into the seeker 3 (and possibly but not necessarily the designator 1) up to 41% increased range can be achieved.

It will be appreciated that to detect circular polarisation, a quarter waveplate 14 and a polariser 16 may be used. However, the optical components may vary depending on the polarisation to be detected. For example linear polarisers such as polaroids may be used for linear polarisation.

Furthermore, it will be appreciated that the invention need not be limited to the embodiment having a single PIN detector. Any suitable detector such as a quadrant detector may be used.

Figure 1 is a schematic representation of a Semi Active Laser application for the present invention where the transmitting portion of the system is remote from the receiving detector. However, it will be appreciated that it is possible for the transmitting and receiving portions of the system to be co-located at a single position or on a single platform. This would be particularly advantageous in an imaging system such as a Burst Illumination Lidar (BIL) system.

Claims (7)

1. CLAIMS1. A seeker system comprising a radiation source 2 emitting radiation towards a target 10 and a detector 18 detecting radiation reflected 12 by the target 10, the seeker system further comprising optical elements 4, 6 disposed between the source 2 and the target 10, said elements 4, 6 acting so as to polarise the radiation 8 emitted toward the target 10, and further optical elements 14, 16 disposed between the target 10 and the detector 18, said further elements 14, 16 acting so as to detect only the radiation reflected 12 by the target 10 of a given polarisation, thereby improving the signal to noise ratio of the seeker system.
2. 2. A seeker system according to claim 1 in which the optical elements comprise a polariser 4 and a quarter waveplate 6.
3. 3. A seeker system according to claim 1 or 2 in which the further optical elements comprise a polariser 16 and a quarter waveplate 14.
4. 4. A seeker system according to any preceding claim in which the radiation emitted is polarised by the optical elements 4, 6 so as to be of a left handed circular polarisation nature, the radiation 12 being reflected by the target 10 being polarised in a right handed circular polarisation, the further optical elements 14, 16 being selected so as to be capable of transmitting right handed circular polarisation reflections to the detector 18 and thereby enabling improved detection of the target 10.
5. 5. A seeker system according to any preceding claim in which the radiation source 2 and the detector 18 are co-located in a single device or on a single platform.
6. 6. A seeker system according to any preceding claim in which the emitter 2 is a laser, emitting pulses of radiation towards the target 10.
7. 7. A seeker system as hereinbefore described with reference to the accompanying diagrammatic drawing.