WO2015170068A1

WO2015170068A1 - Sensor system and method

Info

Publication number: WO2015170068A1
Application number: PCT/GB2015/000133
Authority: WO
Inventors: Jeremy COPLEY; James Henderson; 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: GB201507772D0; GB201407990D0

Abstract

A system for passive range-finding, the system comprising a transceiver and at least one passive receiver, the transceiver comprising a radiation source, in which the transceiver and passive receiver are not co-located and the location of each with respect to each other is not necessarily known. The technique uses a remote transceiver incorporating a laser source and one, or more, remote passive receiver(s). The transceiver and passive receiver are not co-located and the location of each with respect to each other is not necessary for any party to know. Both parties exploit GNSS technology, not to establish position, but to establish a common timing reference to which they can synchronise events and phase-lock their system clocks. The clocks use the one pulse per second (1PPS) signal from the GNSS to phase-lock to each other. The 1 PPS is also used as a universal timing reference by all parties to which to time the transmission of the laser pulses (for the transceiver) or to time the detection events (for the transceiver and the passive receivers).

Description

SENSOR SYSTEM AND METHOD

The invention relates to sensor systems and methods. More specifically, but not exclusively it relates to Bi-static and passive time-of-flight measurements at single photon sensitivities using remote Global Navigation Satellite System- disciplined clocks.

A typical active approach to determine range information to a target object is to transmit discrete laser pulses to the target and detect the returning optical pulse. The time delay between the transmission of each pulse and the corresponding return pulse yields the return time-of-flight and therefore the range. In certain applications, the covertness of a time-of-flight measurement system is a key requirement. However, the use of a laser source by the system potentially reveals its location. An example of this may be a system located in hostile territory in a military application where the use of laser source may reveal the location of the system to enemy combatants. There are several problems with existing technology. Firstly, in known target marking systems, such as target marking systems which are used in conjunction with receivers without single photon sensitive detectors, these use high pulse energy and low repetition rate laser sources. The repetition rate is typically a few 0's of Hz and the laser pulse energy is≥1mJ. These lasers are generally not eye-safe. These lasers are not covert and are therefore not ideal for use in certain applications.

Secondly, known systems use a pre-determined set of codes to encode their optical patterns. The output from these traditional laser sources are often encoded using established military codes. Single photon time-of-flight measurements systems have used encoded laser sources to provide absolute range information however the laser sources in these systems are often ultrahigh speed (≥1GHz) semiconductor laser sources. The use of these low pulse energy laser sources is possible because of the ultra-high sensitivity of the single photon detectors in the receiver systems. Finally, the existing techniques for target-marking do not enable the passive receiver (i.e. a receiver without a laser source) to further utilize the encoded optical pattern, being used to mark the target, to determine the time-of-flight and therefore their range to the target.

In known time-of-flight measurement systems, the system has to incorporate a laser source to determine its range to a target. If time-of-flight information is required from multiple locations a laser source will be required in each location which adds to the size weight and power of each system.

This invention describes a measurement technique that includes aspects of target-marking but uses a novel clock synchronisation and encoding scheme such that one (or more) secondary passive receivers (i.e. receivers without laser sources of their own) can determine their range to the target being illuminated.

This invention further describes a technique where high clock rate codes are used. These codes are not required to be a conventional established code, the only criterion is that the laser source and receivers share the code in advance of deployment. The flexibility of the choice of code also aids covertness. According to the invention there is provided a system for passive range-finding, the system comprising a transceiver and at least one passive receiver, the transceiver comprising a radiation source in which the transceiver and passive receiver are not co-located and the location of each with respect to each other is not necessarily known. According to the invention there is also provided a passive receiver system, the system being capable of determining its range to an object or target illuminated by a separate remote laser transmitter.

According to the invention there is further provided a method of determining a range to a target comprising the steps of illuminating a target with radiation; detecting returning radiation from the target at a number of remotely located receivers; and correlating the returned data signals across the receivers to establish relative positions of transceiver and targets.

The invention will now be described with reference to the attached diagrammatic drawings in which: Figure 1 is a schematic diagram of a target tracking system in accordance with one form of the invention.

As will be described in more detail below, this invention describes a technique which enables a passive receiver system to determine its range to an object or target illuminated by a separate remote laser transmitter. This single transmitter can be used as an illumination source for one or more additional passive receivers such that each of the passive receivers can determine the absolute range to the target under illumination. This single laser source acts as a target illuminator and the receivers are all passive. This invention also allows the passive time-of-flight receiver system(s) to determine the location of the transmitter (if the transmitter allows them to). This technique is applicable to both single photon and analogue detection schemes. The advantages of using this technique with a single photon detector-based receiver instead of an analogue detector-based receiver are discussed in this invention. In one form of the invention, as shown in Figure 1 , the technique for passive range-finding uses a remote transceiver incorporating a laser source and one, or more, remote passive receiver(s). The transceiver and passive receiver are not co-located and the location of each with respect to each other is not necessary for any party to know. Both parties exploit GNSS technology, not to establish position, but to establish a common timing reference to which they can synchronise events and phase-lock their system clocks. The clocks use the one pulse per second (1 PPS) signal from the GNSS to phase-lock to each other. The 1 PPS is also used as a universal timing reference by all parties to which to time the transmission of the laser pulses (for the transceiver) or to time the detection events (for the transceiver and the passive receivers).

Figure 1 shows a schematic of the passive range-finding system. System A is the transceiver which incorporates a weak semiconductor laser source and a single photon detection system. System B is one (possibly of many) passive receivers. The transmitter (System A) initially determines the time-of-flight between it and the target which is typically uncooperative. Once its range to the illuminated surface is known the transceiver simultaneously transmits two encoded patterns to the surface. The timing of one encoded pattern is unmodified such that the first bit of the pattern is transmitted from the laser synchronous to the 1 pulse per second (1 PPS) of the Global Navigation Satellite System (GNSS). The other encoded pattern is simultaneously transmitted but the bits in the pattern are shifted by a number of clock periods equivalent to the 1-way time-of-flight from the transmitter to the test surface such that the first bit in the encoded pattern interacts with the test surface synchronous to the 1 PPS. The illuminated surface now acts as a beacon which emits the encoded pattern starting at the 1PPS signal from the GNSS. Each receiver measures the time-of- arrival of the detection events at their detector with respect to the 1 PPS from the GNSS and generates a received pattern. The cross correlation technique using the unmodified code reveals to each passive receiver the total time-of-flight between it and the transceiver (Path a + Path β) and the use of the time shifted pattern reveals the time-of-flight from the receiver to the test surface (Path β only). The clocks in the transmitter and passive receiver(s) are phase-locked using the 1 PPS signal from the GNSS.

The transceiver which incorporates a laser and a receiver uses an encoded data pattern to determine its absolute range to a target surface. The range being determined by the correlation between the received pattern and the transmitted pattern. The transceiver implements one of two schemes below to simultaneously reveal to all the passive receivers both the total time of flight from the transceiver to that particular passive receiver and the time-of-flight from the particular passive receiver to the illuminated surface.

Once the transceiver has determined its range to the illuminated surface it may either:-

1. Simultaneously transmit two independent encoded patterns to the surface. The timing of one encoded pattern is unmodified such that the first bit of the pattern is transmitted from the laser synchronous to the 1 PPS of the GNSS. The other encoded pattern is simultaneously transmitted but the bits in the pattern are shifted by a number of clock periods equivalent to the 1-way time-of-flight from the transmitter to the surface such that the first bit in the encoded pattern interacts with the surface synchronous to the 1 PPS. The illuminated surface now acts as a beacon which emits the encoded pattern synchronised to the 1 PPS signal from the GNSS. Each receiver measures the time-of-arrival of the detection events at their detector with respect to the 1 PPS from the GNSS and generates a received pattern. The cross correlation technique using the unmodified code reveals to each passive receiver the total time-of-flight between it and the transceiver and the time shifted code reveals the time-of-flight from the receiver to the surface being illuminated.

2. Simultaneously transmit two copies of a single 'shared' encoded pattern to the surface. The timing of one copy of the pattern is unmodified such that the first bit of the pattern is transmitted synchronous to the 1PPS of the GNSS receiver. The other copy of the same encoded pattern is simultaneously transmitted but the bits in the pattern are shifted by a number of clock periods equivalent to the 1- way time-of-flight from the transmitter to the test surface such that the first bit in the encoded pattern interacts with the test surface synchronous to the 1 PPS. The illuminated surface now acts as a beacon which emits the encoded pattern synchronised to the 1PPS signal from the GNSS. Each receiver measures the time-of-arrival of the detection events at their detector with respect to the 1 PPS from the GNSS and generates a received pattern. The cross-correlation technique reveals to each passive receiver the total time-of-flight between it and the transceiver and the time-of-flight between it and the illuminated surface- i.e. there will be two observed correlation peaks. This is analogous to a time-of-flight measurement system (which actually uses a laser) measuring the returns from two separates target objects where a correlation peak will occur for each surface where the laser is returned. This technique enables only one encoded pattern to be shared.

If the two codes are sparse- i.e. the ratio of '1' bits (laser pulses in that clock period) to '0' bits (laser does NOT pulse in that clock period) is low then the two codes (whether the two codes are independent or two copies of the same code and one is shifted) shall be OR'd together and the combined pattern used to drive a single laser source. If the two codes are pseudorandom then OR'ing the patterns together may cause issues. To enable the two codes to be successfully de-multiplexed by each receiver the two data patterns need to be distinguishable. A practical approach is to use two lasers of slightly differing wavelengths. This is not ideal since it requires two lasers to be used by the transceiver and generally two detectors to be used by each receiver. If multiplexing of the shifted code and the unmodified code is non-trivial then the codes can be transmitted sequentially using a single laser source. For example the transceiver first transmits an unmodified code such that the receiver(s) determine the time-of-flight between them and the transceiver and secondly the transceiver transmits the time shifted code and the receiver(s) determine the time-of-flight between them and the illuminated surface. This technique although takes twice the time to transmit the codes mitigates the need for multiple laser sources and detectors.

If the transceiver for some reason does not want to reveal its location to the passive receivers then it can first determine its range to the target using a code that is unknown to the receivers so that the receivers cannot observe a cross- correlation. The transceiver then time shifts a code that is shared with the passive receiver such that the first bit in this shared code is coincident with the target with the 1 PPS from the GNSS system. Sending only the single modified shared code to the target causes the passive receivers to only obtain a single cross-correlation peak corresponding to the time-of-flight from them to the target. The passive receiver has no information regarding the time-of-flight between the transceiver and the target.

It has been shown that a transceiver was able to successfully mark target locations so that the passive single photon detector-based receiver could successfully identify the marked target object(s); that the passive receiver could also determine its distance to the marked target and also determine the total time-of-flight to the transceiver. The transceiver and receiver sub-systems each contained a clock which was disciplined by the 1 PPS signal from a GNSS receiver and were separated by a non-trivial distance. Both methods of encoding were used using sparse patterns and a single laser source.

It has been shown in literature that single photon sensitive time-of-flight measurement systems would have, at least, two clear advantages over analogue detector-based systems: (1) enables the use of low pulse energy (~picojoules per pulse) laser sources (e.g. semiconductor lasers) and (2) enables the distance between the illuminated object and receiver to be increased [1 , 2]. The use of lower pulse energy laser sources can significantly increase the covertness of a system particularly those systems which use encoded data sources, i.e. a randomly encoded data pattern. Lower pulse energy laser sources also reduce the laser safety risks in both military and non-military applications. The use of such laser sources also generally reduces the size, weight and power requirements of the system. By increasing the distance between the target and receiver the covertness of the receiver is increased because the distance between the hidden receiver and the enemy combatants is increased. In civilian applications a longer range capability extends the applications for which the system can be used.

The use of encoded data streams and cross correlation of the detected signal to the transmitted signal to determine range unambiguously has been shown for both analogue and photon-counting based systems [1-4]. Consider the ideal case for covertness for the hidden time-of-flight measurement system, whether it is using single photon sensitive or analogue detectors, where the system does not directly use a laser source. The receiver would instead utilise a second laser source positioned in a completely separate location from the receiver to mark a target object for the hidden receiver to observe. The output from the laser source shall be encoded such that the receiver with apriori knowledge of the code can identify the optical code and therefore is able to positively identify the surface / object being illuminated. The receiver however cannot determine its range to the illuminated surface by using the encoded data stream being used to mark the target because there is no common timing reference shared by the transmitter and receiver. Without this shared reference time the receiver cannot determine the latency between the transmitted and detected pattern because the time the pattern was transmitted is unknown to the receiver.

It will be appreciated that the receiver (with no laser source) could in addition to identifying the code, actually utilise the encoded data to passively determine its range to the target. In a military application the receiver may be located in a hostile environment and the transmitter is located in a safer position. This technique applies to both analogue and single photon detection schemes.

Furthermore, there is also the possibility that multiple systems are required to determine their range to a common object. There are numerous examples of this requirement in applications where several platforms are converging on a common target but also in civil engineering where multiple receivers are required to determine range accurately to a common location on a structure, for example a dam, a bridge or a building under construction. These systems would require a laser source for each receiver which is expensive and increases the size, weight and power of the instruments. The technique described in this invention describes how multiple receivers can passively determine their range to the marked surface.

Moreover, the technique described in this invention is applicable to both single photon and analogue detector-based receivers. For a receiver with an analogue detector the laser source may need to be high power to ensure that the receiver can detect the signal scattered from a potentially un-cooperative surface successfully. The use of high pulse energy optical data patterns may not be desirable for reasons of laser safety; covertness of the transmitter and limitations on transmitter size, weight and power requirements. However the use of single photon sensitive detectors in the receiver system typically enable the use of laser sources that can produce very low pulse energy encoded optical data patterns. These low pulse energy patterns would significantly reduce the risk of the transmitter system being detected by a sensor on the surface or by an independent observer viewing the surface, especially an independent observer without apriori knowledge of the optical code that is being used. A typical laser source used in single-photon detector-based systems is the semiconductor laser. These lasers are capable of being directly and indirectly modulated at ultra-high clock rates and therefore the scope of encoding schemes is greatly increased. The optical pulse output from these sources can also be readily amplified using a semiconductor optical amplifier or a doped fiber amplifier for example. The passive time-of-flight measurement technique described in this invention is applicable to analogue and single photon sensitive detector-based receivers. The use of single photon sensitive detectors in the passive receivers will typically increase the maximum separation between the receiver and the illuminated target. The advantages of using a single photon detector-based receiver and the subsequent advantages of using alternative laser sources such as semiconductor laser sources are discussed in this invention.

The technique of cross-correlating a measured signal with a transmitted signal to find the latency between the two signal patterns has been used for decades in radar applications and optical applications such as telecommunications. For example a bit error rate tester can readily measure latency between a transmitter and received telecommunications data pattern. Analogue detection schemes are described in the art where the time-of-flight of an optical data pattern was determined by cross-correlating the received data pattern with the transmitted pattern.

This established technique has now also been exploited by time-of-flight measurement systems incorporating single photon sensitive detectors.

The research described in the prior art concentrated on time-of-flight measurements using pattern recognition but did not include research on the use of phase-locked remote clocks, GNSS disciplining of remote clocks or passive range-finding. Whilst the research in in the art did make use of differential GNSS to determine the distance between two locations, this data was only used to validate their range data. The GNSS system used in the art was not used to phase-lock two remote clock and the transmitter and receiver system in that research shared a common clock.

From the above, it will be appreciated that the present invention as described with reference to examples only, combines five techniques:-

1. Synchronisation of remote clocks

2. The use of the 1 pulse per second signal from the GNSS system as a "universal time reference"

3. Determination of absolute time-of-flight using pattern recognition techniques 4. Time-of-flight measurements with single photon detectors and Class 1 laser sources. The use of lower energy laser sources, such as semiconductor laser sources, typically extends the scope of encoding schemes achievable with the laser sources, for example direct or indirect modulation of a semiconductor laser output at GHz clock rates is readily achievable.

5. The utilisation of a novel encoding scheme where two codes are used to reveal the time-of-flight from passive receiver to both the marked surface and the transceiver. By determining the range from several marked surfaces, the position of the transceiver can be determined by the passive receiver. In this way, the present system enables multiple passive receivers to be used simultaneously. None of the passive receivers are required to know the location of the laser transmitter or each other.

The use of single photon sensitive detectors enables the passive receiver to be located a long distance from the illuminated target object. The absence of any laser source at the passive receiver and the reduced need for communication between the two parties significantly increases the covertness of the passive receiver(s). The use of single photon detector technology often lowers the pulse energy requirements of the laser source which will also increases the covertness of the transceiver. The advantages of this invention are: low power laser required because of single photon sensitivity of the detector in the transceiver and passive receiver(s); low cost laser sources; eye safety; reliability; low size, weight and power (SWaP); covertness; and increased flexibility of encoding schemes can be exploited.

Additionally, novel encoding schemes enable the passive receiver to determine the total absolute time-of-flight to transceiver and the absolute time-of-flight to the target object simultaneously. The difference in these two time-of-flight values also reveals to the passive receiver the time-of-flight from the target object to the transmitter therefore by determining the time-of-flight to multiple test objects the passive receiver can triangulate the position of the transmitter. Furthermore, the target surface is marked with a known optical code. This feature allows the passive receiver to clearly identify the surface by the presence of a correlation peak. The probability of a false positive identification of the target object is negligible. This functionality is particularly useful for military applications where minimal communication between two parties is desirable and the positive correlation can confirm the target.

Additionally, very low data acquisitions times are achievable. Data transmission times and measurement times as low as 1 ms have been demonstrated by the authors for Lambertian surfaces at distance up to 1km from the transceiver for a system using a Class 1 laser source at a wavelength of 850nm. This also potentially increases the covertness of the transceiver system since the lower the time for which the optical data is being transmitted the more covert the transceiver.

Finally, hardware implementation of encoded optical data streams from semiconductor lasers has been demonstrated at GHz clock rates on simple FPGA demonstrator boards and compact semiconductor laser sources. Encoded data patterns are readily generated in software or hardware. The size, weight and power of the optical and optomechanical system are therefore conceivably low which is important for portable systems.

Claims

1. A system for passive range-finding, the system comprising a transceiver and at least one passive receiver, the transceiver comprising a radiation source, in which the transceiver and passive receiver are not co-located and the location of each with respect to each other is not necessarily known.

2. A system according to claim 1 in which the radiation source may be used as an illumination source for one or more additional passive receivers such that each of the passive receivers can determine the absolute range to a target under illumination.

3. A system according to claim 1 or 2 in which the single radiation source acts as a target illuminator and the receivers are all passive.

4. A system according to any preceding claim in which the or each receivers are single photon or analogue detection receivers.

5. A system according to any preceding claim in which the passive receiver comprises means for synchronising the receiver and or the or each transmitter.

6. A system according to claim 5 in which the means for synchronising comprises target-marking, clock synchronisation and an encoding scheme such that one or more secondary passive receivers can determine their range to the target being illuminated.

7. A system according to any preceding claim in which high clock rate codes are used whereby the radiation source and the or each receivers share the code in advance of deployment.

8. A passive receiver system, the system being capable of determining its range to an object or target illuminated by a separate remote laser transmitter.

9. A method of determining a range to a target comprising the steps of illuminating a target with radiation; detecting returning radiation from the target at a number of remotely located receivers; and correlating the returned data signals across the receivers to establish relative positions of transceiver and target.

10.