WO2015170070A2

WO2015170070A2 - Sensor system and method

Info

Publication number: WO2015170070A2
Application number: PCT/GB2015/000135
Authority: WO
Inventors: Jeremy COPLEY; Iain Clark; James Henderson; Robert Lamb; Philip Hiskett
Original assignee: Selex Es Ltd
Priority date: 2014-05-06
Filing date: 2015-05-06
Publication date: 2015-11-12
Also published as: WO2015170070A3; GB201507771D0; GB201407987D0

Abstract

A tracking system for tracking a target is described. The tracking system comprises a radiation source capable of generating high data rate optical pulse patterns at low pulse energies. A method of tracking a target is also described, the method comprising the step of transmitting a high data rate optical pulse pattern at a low pulse energy from a transmitter; illuminating the target with said pulse pattern; and confirming that the target is illuminated by the pulse associated with the transmitter, thereby confirming the target. The use of the system and method as optical communication means is also disclosed.

Description

SENSOR SYSTEM AND METHOD

The invention relates to sensor systems and methods. More specifically but not exclusively, it relates to a system and method of optically tracking a target.

Current guidance systems for moving platforms, use a remote laser transmitter system to illuminate a target with an optical code that is known to the platform. The use of a shared optical code enables a second platform to recognise the correct code and therefore verify the target location. The optical receiver on the platform typically utilises a quadrature cell analogue detector to provide feedback information to "steer" the device towards the illuminated target location.

The established codes are currently based on very low frequencies (~20Hz) and the laser sources used for such an application generally have high pulse energies (>1mJ per pulse). The established wavelength for these lasers is traditionally 1064nm.

The current technology has been in use now for over 40 years. In that time significant advancements have taken place in the fields of single photon detection; telecommunication technology and amplified fibre lasers.

The pulse energy of the laser sources in existing optical tracking systems is often high, >1mJ per pulse, usually because the sensitivity of the analogue sensor in the optical receiver of the platform is often limited. The high average powers of these laser sources are problematic for the personnel using the transmitter on the ground for reasons of covertness and eye safety. There are many military applications for an optical tracking system and covertness is a significant factor since the laser transmitter may need to be positioned in an area close to enemy combatants.

The operation of these existing systems at low repetition rates limits the data update rate at the sensor in the platform.

These high energy / low repetition rate laser sources are often heavy, typically a few kilogrammes for a portable unit and require significant battery power to operate. According to the invention there is provided a tracking system for tracking a target, the system comprising a radiation source capable of generating high data rate optical pulse patterns at low pulse energies.

According to the invention there is further provided a method of tracking a target comprising the step of transmitting a high data rate optical pulse pattern at a low pulse energy from a transmitter; illuminating the target with said pulse pattern; and confirming that the target is illuminated by the pulse associated with the transmitter, thereby confirming the target.

The problems associated with the current technologies which are addressed by the use of semiconductor and fibre laser sources; single photon sensitive receivers and pattern recognition techniques are: reduction of cost; improved eye safety; lower size, weight and power (SWaP); improved reliability; improved covertness for the laser transmitter; and an increase in the working distance (or range) between the receiver and target location.

The invention will now be described with reference to the accompanying diagrammatic drawing in which:

Figure 1 is a schematic diagram of the optical tracking system in accordance with one form of the invention.

As can be seen in Figure 1 , a remote laser transmitter transmits an encoded data pattern to a target location. A quadrature cell sensor with single photon sensitivity located on the platform detects the weak optical returns that are scattered by the target object. The returns at the sensor are time-stamped and used to form a received pattern; the cross-correlation technique determines if the received code matches the transmitter code to confirm the correct target has been identified. The relative data rate at the four quadrants of the sensor enables the platform to steer itself to the target. The clocks in the transmitter and receiver shall be selected such that the de-phasing between the two clocks is small with respect to the relative movement of the transmitter and receiver.

This invention utilizes a more sensitive detection technology than existing optical tracking systems, i.e. single photon detection. An ultra-weak return pulse energy incident at the receiver aperture could be a consequence of (1) the use of a low power laser source to illuminate the Lambertian target surface and/or (2) a large separation between the illuminated Lambertian surface and the receiver and/or (3) a target surface of low reflectivity. A widely used laser source that produces low energy pulses at high output rates is the semiconductor laser. Semiconductor laser sources are widely commercially available and are generally low cost and modulation of the optical output of these sources at high data rates has been extensively demonstrated for decades. These sources are widely used in industrial applications such as telecommunications where long term reliability of components is necessary and the efficient replacement of optical modules is readily achievable. Optical amplifiers for semiconductor laser sources are also readily available.

In one form of the invention a high data rate optical pulse pattern at low pulse energies replaces the much slower rate-high pulse energy encoded data pattern of the existing system. As with the high energy laser system pattern recognition techniques are fundamental in the new system where the pattern formed from the detected optical returns from an illuminated target surface are cross-correlated with the encoded data pattern transmitted by the laser source. In an optical tracking system this encoded data is sensed by the receiver on the moving platform incorporating a single photon sensitive quadrature cell. The use of an encoded optical signal and the cross-correlation process enables the sensor in the optical tracking system to confirm that a surface is illuminated by the specific code associated with the transmitter and therefore confirm the target. See Figure 1.

The relative movement of the platform and the laser transmitter will cause the cross-correlation peak to stretch across multiple clock periods. To obtain the strongest correlation, consecutive small sections of the received data are used in the cross-correlation process such that there is negligible movement (comparable to a single clock period) of the transmitter and receiver during the time associated with the small section of received data. The data rate has to be sufficient so that in this small time period- dictated by the relative velocity of the transmitter and receiver- there are sufficient bits in the received pattern to yield a strong correlation. Also, to obtain a strong correlation between the measured received patterns and the transmitted pattern it is necessary that there is minimal de- phasing between the clock in the transmitter and the clock in the receiver over the time interval corresponding to the small section of the received pattern being used in the cross-correlation process. This can be achieved by the use of stabilised crystal oscillators in the transmitter and receiver. In general the effect on the correlation peak of the de-phasing of two high quality clocks is significantly less than the effect of the high relative velocity of the transmitter and receiver and therefore it is expected that stabilised crystal oscillators will be viable clocks for this application.

The system will use a quadrature single photon detector which may be a Geiger mode or Linear Mode Photon Counting (LMPC) device. The detection rate in each quadrant will determine the required steering of the platform. Since a Geiger mode device is non-photon number resolving and has a limited dynamic range, an active attenuation system would typically be required to attenuate the incident return pulses so that the single photon detector is not saturated as the energy of these return pulses increases as the platform approached the illuminated surface. A LMPC device is photon number resolving and therefore the rate of photon detection in each quadrant is measureable even at higher photon numbers.

It may be desirable to use two quadrature cell sensors in the optical receiver- one single photon sensitive and the other analogue (i.e. a linear device but not a single photon sensitive device). Therefore at large platform to illuminated surface separations the single photon detector is used and as the platform approaches the illuminated surface the system can switch to using the analogue quadrature cell.

In the transition period between the traditional low sensitivity optical tracking systems to this newer scheme it is proposed that the intermediate systems incorporate both analogue and single photon detection technologies. This is conceivable since the additional size, weight and power requirements of a system with a secondary semiconductor laser sources is sufficiently low to enable both technologies to co-exist in a system.

It is also possible to use this system as a laser communication system. In the optical tracking configuration the entire pattern is known to both the transmitter and receiver whereas an optically encoded message is unknown to the receiver. To enable optical communications a short header code, which must be known to both transmitter and receiver, is inserted into the message code at intervals along the message. The length of the inserted header code has to be sufficient for the receiver to successfully cross-correlate the header code with the received pattern and the spacing of the headers is determined by the rate of compression of the pattern as the transmitter to platform separation decreases and by any clock de- phasing. Since single photon detection is a sparse measurement technique the message must be transmitted many times by the transmitter until the receiver has fully determined the message.

In summary switching to a significantly more sensitive sensor technology would enable:

1) The use of established laser sources with lower pulse energies at high data rates. These sources have been shown to be reliable and readily modulated. and/or

2) Enable the platform to detect the optical signal at longer distances from the target

This invention combines three techniques:, high-speed optical modulation of semiconductor laser sources; single photon detection; and optical pattern recognition.

It will be appreciated that in this way, the problems associated with the current technologies which are addressed by the introduction of semiconductor laser sources; single photon sensitive receivers and pattern recognition techniques are: reduction of cost; improved eye safety; lower SWaP; improved reliability; improved covertness for the laser transmitter; and increase in the working distance between the receiver and target location.

Furthermore, the target surface is marked with a known optical code- this feature allows the receiver to clearly identify surface under test by presence of correlation peak. The probability of false positive object identification is negligible. Therefore the platform could potentially be deployed without a direct line-of-sight at the time of launch but the target surface will be acquired and confirmed by the cross- correlation process when the illuminated target surface is within the field-of-view of the optical tracking system sensor.

Claims

1. A tracking system for tracking a target, the system comprising a radiation source capable of generating high data rate optical pulse patterns at low pulse energies.

2. A tracking system according to claim 1 further comprising one or more quadrature single photon detectors.

3. A tracking system according to claim 2 in which the quadrature single photon detector is a Geiger mode or Linear Mode Photon Counting (LMPC) device.

4. A tracking system according to any preceding claim further comprising an active attenuation system to attenuate incident return pulses such that the single photon detector is not saturated.

5. A tracking system according to any preceding claim in which the detection rate in each quadrant determines the required steering of the platform.

6. A tracking system according to any one of claims 2 to 5 one single photon sensitive and the other analogue.

7. A tracking system according to claim 6 in which the analogue device may be a a linear device such that at large target to illuminated surface separations the single photon detector is used and as the platform approaches the illuminated surface the system switches to the analogue quadrature cell.

8. A laser communication system for communication between a receiver on a platform and a transmitter, the system comprising a short header code for transmission to both the transmitter and the receiver, the code being inserted in to a message code at intervals along the message, the length of the inserted header code being sufficient for the receiver to successfully cross-correlate the header code with the received pattern, the spacing of the header codes being determined by the rate of compression of the pattern as the transmitter to platform separation decreases and by any clock de-phasing.

9. A laser communication system according to claim 8 in which the message is transmitted a plurality of times by the transmitter until the receiver has fully determined the message.

10. A method of tracking a target comprising the step of transmitting a high data rate optical pulse pattern at a low pulse energy from a transmitter; illuminating the target with said pulse pattern; and confirming that the target is illuminated by the pulse associated with the transmitter, thereby confirming the target.