GB1585054A

GB1585054A - Laser rangefinder

Info

Publication number: GB1585054A
Application number: GB644075A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1975-02-14
Filing date: 1975-02-14
Publication date: 1981-02-25
Also published as: DE2605798A1

Description

(54) LASER RANGEFINDER (71) I, Secretary of State for Defence, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to laser rangefinders.

Prior art laser rangefinders which operate with a pulsed laser time the interval between transmission of a pulse and receipt of that pulse reflected from a target.

Rangefinders are known in the radar field which operate using both pulsed and continuous wave (CW) microwave radiation. To a certain extent there exists a similarity between pulsed microwave and pulsed laser rangefinder systems, but for CW rangefinders the frequency operation of a laser source e.g. 3 x 1013Hz, gives rise to many problems not present in microwave source, e.g. 3 x 109Hz. One form of C.W. rangefinder is described and claimed in U.K. Patent Application No. 6439/75 (Serial No. 1585053).

Ideally a rangefinder should not present a health hazard to its users, i.e. operators or 'friendly' targets. Unfortunately most of the known pulsed lasers operate in the 0.6-1.1 ijm wavelength and this wavelength can cause eye damage. Fortunately eye damage can be minimised by using lasers operating in the 10.6 llm region, e.g. a CO2 laser also pulsed.

Another advantage of operating in the 10.6 llm region is that atmospheric attenuation of the laser light, due to moisture droplets, etc., is much less than for lasers operating in say the 0.6-1.1 m wavelength region.

According to this invention in a laser rangefinder radiation from a continuous wave laser is mixed with a pseudo random binary amplitude coded signal and transmitted to a target, radiation reflected back from the target is detected and mixed with a proportion of the laser output to extract the coded signal, the coded signal reflected from the target is then cross correlated with the transmitted coded signal to determine the time delay between them and hence range. Additionally the frequency of the reflected radiation may be compared with the transmitted radiation to determine any change in frequency, i.e. a Doppler signal, and hence target velocity.

According to this invention a laser rangefinder preferably comprises a continuous wave laser, a pseudo random binary code signal generator, a timing pulse generator, for initiating generation of a binary code an optical modulator for encoding radiation from the laser with the code signal, focussing surfaces for directing the transmission of radiation from the optical modulator onto a target, focussing surfaces for receiving radiation reflected from a target and directing it onto a detector capable of giving an output representative of radiation received, reflecting surfaces for directing a proportion of the laser output onto the detector for mixing with the received reflected radiation, circuitry for cross-correlating the output -of the detector with the transmitted code to provide a range pulse whose time delay from a transmitted coded signal represents target range. Preferably the rangefinder includes an integrator for integrating successive returns to improve range accuracy.

The modulated signal generator may be a pseudo random binary code generator, whose output is mixed with a carrier frequency signal generator. The correlator may be a surface acoustic wave correlation filter or a digital cross correlator.

A binary code is herein defined as a two amplitude level code. Preferably one level is at zero so that a code may be represented by binary ones and zeros, e.g. ON and OFF states of a switch.

The invention will now be described by way of example only with reference to the drawing accompanying the Provisional Specification which is a schematic diagram of one form of the invention.

As shown a rangefinder comprises a CO2 CW laser 1 which emits 10.6 llm wavelength radiation (frequency wO) through a first reflecting and transmitting mirror 2 into an optical modulator 3. This modulator 3 comprises a suitably orientated Germanium crystal 4 having flat polished end faces with a Lithium Niobate acoustic transducer 6 on one side face and an acoustic absorber 5 on the other side face. A pseudo random binary code surface acoustic wave (SAW) generator 7 has an output connected to the acoustic transducer 6. This code generator has an output of binary ones and zeroes on a carrier frequency, fl (=wl/2z), of about 50 MHz, the code duration M being 1,024 code samples each of duration 50 nanosecs.

The laser radiation mixes in the modulator 3 with the coded carrier so that the modulator 3 output I(t) can be expressed as IOA(t)ei(Wo + w1)t where A(t) is the code at time t (either a one or zero) and 10 is the laser intensity. From the modulator 3 the signal I(t) is focussed by a lens 8 onto transmitting mirrors 9, 10 and thence onto a target (not shown). Radiation scattered by the target is focussed by receiving mirrors 11, 12 onto a second reflecting/ transmitting mirror 13 and thence into an infra red detector 14. Additionally a proportion of the laser 1 radiation is reflected by the first reflecting mirror 2, through the second mirror 13 into the detector 14 to mix with the target signal. The detector 14 may be a liquid nitrogen cooled CdHgTe photodiode or a PbSnTe photodiode.Output from the detector 14, which contains the delayed coded signal, is passed through a first envelope detector 15, which removes the return carrier signal at frequency wl + wD, where WD is any Doppler shift superimposed by target motion, to a mixer 16 to be mixed with the original carrier frequency w1 from a SAW oscillator 17. From the mixer 16 the signal passes through a SAW decoder 18, a second envelope detector 19, and a range integrator 20.

The range integrator 20 may use L.S.I. shift registers giving a 16 bit 50 nanosec resolution (corresponding to a range resolution of 25 feet) 1024 channel integrator using either one or four bit inputs. The integrator is connected 21 to the code generator 7 to transmit timing or trigger pulses.

A signal received from a stationary target will fall into the same range bin on successive returns since the returns will occur at the same position in time after the store receives its timing pulse from the pulse generator. These stores add successive returns, thus integrating the target return and noise. It is observed that target signals rapidly add whereas noise signals tend to average out to zero.

Output from the infra red detector 14 is also taken in series through an AND gate 22, a second mixer 23, a SAW chirp filter 24, an envelope detector 25 and velocity integrator 26.

An output is taken from the code generator 7 to a SAW variable delay 27 whose output is fed into the AND gate 22. This SAW delay 27 may also receive an input from the range integrator 20. A SAW sweep generator 28 has an input from the timing pulse connection 21 and an output into the mixer 23.

The various SAW devices may be conventional SAW devices. These are piezo electric, e.g. quartz, substrates with interdigital transducers spaced on the substrate. By suitable arrangement of transducer finger electrode spacing and length various operations may be performed on an input signal. For example a short impulse unit can be converted into a binary amplitude (e.g. a one level or zero level amplitude) code carried on a carrier frequency wl, or the pulse can be converted to a long output pulse whose frequency varies with time e.g. a chirp output. These devices work in reverse, e.g. when fed with a long suitably coded pulse the device output is a short pulse i.e. a decoder. Unless the correct code is received by the device no decoded pulse output is obtained. Similarly a swept frequency, chirp, input can be converted to a short pulse.

The pseudo random binary amplitude code used has the propety that the envelope of the code takes the form of a telegraph function whose value can change every 50 nanoseconds.

For pulse compression to be possible the autocorrelation of the code must have the form

where b = c = M/4 for pseudo-random binary amplitude coding. The particular form of code used is the maximal length M - sequence binary amplitude code formed by predetermined feedback conditions in a shift register of length L where 2L - 1 = M.

When this code is generated by the SAW code generator 7 this envelope is superimposed on the carrier at frequency w1.

In operation to obtain target range information the transmitting mirrors 9, 10 are aimed at the target. Radiation reflected by the target is received by the receiving mirrors 11, 12 and mixed on the infra red detectors 14 with a proportion of laser radiation. Output from the detector 14, a delayed version of the transmitted code, possibly shifted in frequency by target movement, is converted to base band operation by removing its carrier frequency in the first envelope detector 15. This coded signal is then mixed with a frequency wl in the mixer 16 and passed through the SAW decoder 18 which is the reverse of the SAW code generator and therefore correlates the received signal with the transmitted code.The SAW decoder 18 output is then envelope detected to produce a sharp pulse whose time delay from a timing pulse (generated by a separate timing circuit or by a clock within the range integrator or SAW generator 7) represents target range.

In operation to obtain radial velocity of the target output from the infra red detector 14 is passed through the AND gate 22 and mixed with a signal with a linearly varying frequency w2(t) from the sweep generator 28. During those code samples when a return signal from the target is observed the output from the mixer 23 will be at a frequency w2(t) + wl + wD, where wD is the Doppler frequency, and for those code samples where no return is observed the mixer output has frequency w2(t). These outputs pass through the chirp filter 24 whose response is centred at frequency w1.Output of the chirp filter 24 is a signal of delay proportional to wD which is then envelope detected to give a sharp pulse and fed into the velocity integrator, which is similar in design to the range integrator but would only require 100 channels, where it slots into an appropriate bin corresponding to the delay of a particular velocity value.

The SAW variable delay enables the velocity of a particular target from numerous targets to be obtained by range gating the targets. The return from a particular range can be suppressed by gating the received signal in gate 22 by the relevant inverted code from the generator 7 delayed in gate 27. Actual delay may be automatically selected from targets identified in the range integrator 20. Thus the velocity of a specified target (i.e. a target within a range bin) can be identified and later its value determined by observing which component is removed from the total frequency spectrum of all targets when all returns from a selected range are removed. In particular reflections from within the rangefinder which would show up as zero range signals in the range integrator may be range gated out.

Another technique for removing these zero range returns which may be used in addition or instead is to include quarter wave plates so that the transmitted and received radiation is circularly polarised. A half wave plate may be necessary to orientate received radiation into the infra red detector for mixing with the desired proportion a laser radiation. Reflections in the rangefinder representing zero range will have an incorrect sense of polarisation and will not mix in the infra red detector with target signals.

Various components are shown in the drawing to be surface acoustic wave devices, e.g.

oscillator 17, these may be replaced by standard non surface wave devices.

WHAT I CLAIM IS: 1. A laser rangefinder in which radiation from a continuous wave laser is mixed with a pseudo random binary amplitude coded signal and transmitted to a target, radiation reflected back from the target is detected and mixed with a proportion of the laser output to extract the coded signal, the coded signal reflected from the target is then cross-correlated with the transmitted coded signal to determine the time delay between them and hence range.

2. A laser rangefinder as claimed in claim 1 and comprising a continuous wave laser, a pseudo random binary code signal generator, a timing pulse generator for initiating generation of the binary code an optical modulator for encoding radiation from the laser with the code signal, focussing surfaces for directing the transmission of radiation from the optical modulator onto a target, focussing surfaces for receiving radiation reflected from a target and directing it onto a detector capable of giving an output representative of radiation received, reflecting surfaces for directing a proportion of the laser output onto the detector for mixing with the received reflected radiation, circuitry for cross-correlating the output of the detector with the transmitted code to provide a range pulse whose time delay from a transmitted coded signal represents target range.

3. A laser rangefinder as claimed in claim 2 wherein the code signal generator generates a carrier signal amplitude modulated by the pseudo random binary code.

4. A laser rangefinder as claimed in claim 2 wherein the circuitry for cross-correlating the output of the detector includes a decoder capable of giving a short output pulse after receiving a signal having the same code as the signal from the code generator.

5. A laser rangefinder as claimed in claim 3 and including an envelope detector for removing the code signal generator carrier frequency from the detector output, an oscillator and mixer for adding to the output of the envelope detector a carrier frequency at the code generator carrier frequency, and means for applying this mixer output to the decoder.

6. A laser rangefinder as claimed in claims 2, 3 or 4 wherein the timing pulse generator is arranged to emit a series of pulses so that a series of binary coded signals are transmittd to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and mixed on the infra red detectors 14 with a proportion of laser radiation. Output from the detector 14, a delayed version of the transmitted code, possibly shifted in frequency by target movement, is converted to base band operation by removing its carrier frequency in the first envelope detector 15. This coded signal is then mixed with a frequency wl in the mixer 16 and passed through the SAW decoder 18 which is the reverse of the SAW code generator and therefore correlates the received signal with the transmitted code. The SAW decoder 18 output is then envelope detected to produce a sharp pulse whose time delay from a timing pulse (generated by a separate timing circuit or by a clock within the range integrator or SAW generator 7) represents target range. In operation to obtain radial velocity of the target output from the infra red detector 14 is passed through the AND gate 22 and mixed with a signal with a linearly varying frequency w2(t) from the sweep generator 28. During those code samples when a return signal from the target is observed the output from the mixer 23 will be at a frequency w2(t) + wl + wD, where wD is the Doppler frequency, and for those code samples where no return is observed the mixer output has frequency w2(t). These outputs pass through the chirp filter 24 whose response is centred at frequency w1.Output of the chirp filter 24 is a signal of delay proportional to wD which is then envelope detected to give a sharp pulse and fed into the velocity integrator, which is similar in design to the range integrator but would only require 100 channels, where it slots into an appropriate bin corresponding to the delay of a particular velocity value. The SAW variable delay enables the velocity of a particular target from numerous targets to be obtained by range gating the targets. The return from a particular range can be suppressed by gating the received signal in gate 22 by the relevant inverted code from the generator 7 delayed in gate 27. Actual delay may be automatically selected from targets identified in the range integrator 20. Thus the velocity of a specified target (i.e. a target within a range bin) can be identified and later its value determined by observing which component is removed from the total frequency spectrum of all targets when all returns from a selected range are removed. In particular reflections from within the rangefinder which would show up as zero range signals in the range integrator may be range gated out. Another technique for removing these zero range returns which may be used in addition or instead is to include quarter wave plates so that the transmitted and received radiation is circularly polarised. A half wave plate may be necessary to orientate received radiation into the infra red detector for mixing with the desired proportion a laser radiation. Reflections in the rangefinder representing zero range will have an incorrect sense of polarisation and will not mix in the infra red detector with target signals. Various components are shown in the drawing to be surface acoustic wave devices, e.g. oscillator 17, these may be replaced by standard non surface wave devices. WHAT I CLAIM IS:

1. A laser rangefinder in which radiation from a continuous wave laser is mixed with a pseudo random binary amplitude coded signal and transmitted to a target, radiation reflected back from the target is detected and mixed with a proportion of the laser output to extract the coded signal, the coded signal reflected from the target is then cross-correlated with the transmitted coded signal to determine the time delay between them and hence range.

the target, and wherein the resulting range value associated with each transmitted coded signal is integrated in a range integrator.

7. A laser rangefinder as claimed in any one of claims 2 to 6 and further including a frequency sweep generator and mixer for adding to an output of the detector a swept frequency signal, and a chirp filter having a response centred around the code generator carrier frequency and whose output is a short pulse time delayed from the transmitted code by an amount representing target velocity.

8. A laser rangefinder as claimed in claim 7 and further including an integrator for integrating successive returns indicating target velocity.

9. A laser rangefinder as claimed in claim 8 and including range gating means comprising a variable delay for receiving a coded signal from the code generator and combining the coded signal with an output from the detector in a gate whereby signals from the detector representing a particular target range may be prevented from entering the chirp filter and velocity integrator.

10. A laser rangefinder as claimed in any one of claims 3 to 9 wherein the code generator and decoder are surface acoustic wave devices.

11. A laser rangefinder as claimed in any one of claims 7 to 10 wherein the sweep generator is a surface acoustic wave device.

12. A laser rangefinder as claimed in any one of claims 7 to 11 wherein the chirp filter is a surface acoustic wave device.

13. A laser rangefinder as claimed in any one of claims 2 to 12 wherein the optical modulator is a Germanium crystal carrying an acoustic transducer.

14. A laser rangefinder as claimed in any one of claims 2 to 13 wherein the detector material is a cadmium, mercury, telluride compound.

15. A laser rangefinder as claimed in any one of claims 2 to 13 wherein the detector material is a lead, tin, telluride compound.

16. A laser rangefinder constructed arranged and adapted to operate substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.