Detection of air flow speed and flow direction

Abstract: A transmitter (41) is arranged to produce and focus a beam (54) on a remote focal point (55). A small proportion of resultant light (58) is back-scattered from the focal point (55) by particles or aerosols in air flowing through point (55) and returns to a receiver (42) where it is collected. A detector combines resultant light (58) and reference light (72), splits the resultant light (58) and reference light (72) equally between two arms and detects which arm leads the other in phase thereby indicating the flow direction of air through focal point (55).

Description: This invention relates to improvements in or relating to remote air detection, in particular to a remote air detector and a method of remote air detection. The measurement of air velocity and flow direction is important when controlling the motion of an aircraft in flight. Methods currently used employ a pitot tube extending from the aircraft, which enables velocity measurement in terms of the differential pressure between air in the tube and air outside the tube. This method has the disadvantage of only being able to measure velocity close to an airframe of the aircraft where the flow direction of air may be substantially effected by the airframe and is insensitive to low velocities as the differential pressure between air inside and outside the tube is proportional to the velocity of air squared. These disadvantages are particularly acute in helicopters where the forward velocity of air can be very slow or even negative and the flow direction of air, in close proximity to the airframe, can be greatly effected by the downdraft from the engine rotor. Additionally high performance aircraft in flight, for example supersonic aircraft, operate in unstable aerodynamic configurations and require measurement of airflow direction relative to the aircraft to enable motion to be stabilised. FR-A-2556841 discloses apparatus and a method for remote air detection using a detector carried on an aircraft including a transmitter to focus a light beam on a remote focal point in air, a receiver to receive resultant light from the light beam that is scattered by air at the remote focal point and a detector to determine from the resultant light the flow direction and the velocity magnitude of air at the focal point. EP-A-0355018 discloses a Doppler velocimeter using a semiconductor laser beam which is split into two beams one of which is passed to the target and returned thereby and the other of which is reflected. The returned and reflected beams are mixed and analysed to determine target velocity. US-A-4255048 discloses a direction sensitive laser velocimeter using a transmitted laser beam to illuminate a target to produce backscattered light which is divided into two parts one of which is mixed with the transmitted beam and detected by a first photodetector. The other part is phase shifted by 90° and mixed with the other of the two parts and detected by a second photodetector. The output of the first photodetector is multiplied by the output of the second photodetector to produce a signal indicative of the direction of movement of the target. GB-A-2254975 teaches the use of a single laser transmit beam and a single receive beam which is detected and used to provide an indication of wind velocity by modulating the laser beam to achieve a Doppler shift in frequency. US-A-5587785 discloses a laser Doppler velocimeter using fibre optics to direct and focus continuously changing frequency laser light onto a region to be measured. Scattered light from the region is received and a Doppler shift frequency calculated and used to calculate flow in the region. From Figure 1, it is known to provide a Doppler anemometer 10 to measure the velocity of flowing air using a laser diode 11 to produce a laser output 12 which is collimated by a collimator lens 13 into a parallel beam 14. The laser diode is orientated to produce a laser output 12 polarised in the direction of propagation of the parallel beam 14 such that the parallel beam 14 is transmitted by a polarizing beam-splitter 15 to form a beam 16. The beam 16 then passes through a quarter-wave plate 17 which converts the linearly polarised light of the beam 16 into a circularly polarised light beam 18. The light beam 18 is expanded by a telescope 19 comprising lenses 20 and 21 and then passes through a window 22 in the aircraft. The beam 18 is brought to focus on a focal point 23 at a distance from the window 22 by adjustment of lenses 20, 21. Air flowing through the focal point 23 contains particles which may be dust, volcanic ash or aerosols in the form of microscopic water droplets. Measurements performed in various parts of the world show that such particles are present everywhere in the atmosphere at a concentration sufficiently to cause a measurable amount of back-scatter from the light beam 18. Hence, a small proportion of light 24 scattered by atmospheric particles close to the focal point 23 is scattered in a direction which returns through window 22, the lenses 20, 21 of telescope 19 and quarter-wave plate 17 to form a collimated signal beam 25. When the light 24 passes through the quarter-wave plate 17 it is converted from circularly polarised light to linearly polarised light but having a direction of polarisation perpendicular to the propagation direction of the beam 16. The signal beam 25 is substantially reflected by polarizing beam-splitter 15 and is focussed by lens 26 onto the surface of a photodector 27. A small proportion of beam 16 is reflected from a plane face 28 of the quarter-wave plate 17 positioned closest to lens 20 and passes back through plate 17 to form a reference beam 29. The portion of beam 16 which is reflected from the plane face 28 of the plate 17 is converted from linearly polarised light to circularly polarised light and then back to linearly polarised light but with a change in the direction of polarisation such that it is perpendicular to the direction of propagation of the reference beam 29. The reference beam 29 is reflected by beam-splitter 15 and is focussed by lens 26 onto the photodetector 27. The orientation of the quarter-wave plate 17 is adjusted such that the reference beam 29 is accurately parallel and collinear with the signal beam 25. The reference beam 29 and the signal beam 25 form an interference pattern on the surface of the photodetector 27 and, when the plate 17 is properly adjusted, the spacing of interference fringes formed by the beams 25, 29 is substantially larger than the diameter of either beam 25, 29 so that the photodetector 27 receives a light intensity modulated at the difference frequency between the beams 25, 29 which is the Doppler frequency corresponding to the motion of particles at the focal point 23. The photodetector 27 produces an output current 30 which is passed to a signal analyser 31 which can consist of a fast Fourier transform analyser or a pulse-pair processor that identifies the Doppler frequency which provides an indication of the corresponding air velocity at focal point 23. The air velocity may be displayed on a display 32. However, the Doppler anemometer described with reference to Figure 1 is unable to discriminate between positive and negative directions of air flow, which is a particular problem in helicopters able to fly backwards as well as forwards. It is an object of the present invention to obviate or mitigate the problems associated with the prior art. According to a first aspect of the present invention there is provided apparatus for determining air flow direction and velocity of air remote from an aircraft in flight, including: a transmitter on the aircraft, for focusing and directing a primary light beam to a focal region remote from the aircraft, and for generating a secondary light beam from the primary beam; a receiver on the aircraft, for converting return light scattered by the air at the focal region into a scattered light beam; and a detector for determining the air flow direction and the velocity of the air at the focal region, including a beam-splitter characterised in that the beam-splitter is operable to split the scattered beam into first and second signal beams, to split the secondary beam into first and second reference beams and to polarize, the signal beams to differ in phase by ninety degrees, and a detection circuit for detecting the interference between the first signal beam and the first reference beam to produce a first electrical signal having a first phase, for separately detecting the interference between the second signal beam and the second reference beam to produce a second electrical signal having a second phase, for combining the first and second signals to determine the air velocity, and for processing the first and second phases to determine the air flow direction. Preferably the transmitter includes a laser source for producing the primary light beam with a linear polarization in a plane substantially perpendicular to a direction of propagation of the primary beam, and a first quarter-wave plate for converting the primary beam to have a circular polarization. The transmitter may comprise a quarter-wave plate operably arranged to divide reference light from the light beam. The detector may comprise a beam-splitter operably arranged to combine the resultant light and reference light. A beam-splitter may be operably arranged to divide the resultant light substantially equally between two arms, each arm operably arranged to produce a signal beam substantially orthogonal and in phase to the signal beam in the other arm. Each arm of the detector may further comprise a photodetector operably arranged to detect frequency shifts between its respective signal beam and reference beam and to produce a photocurrent indicative of frequency shifts. The photocurrent from each photodetector may be operably connected to a phasemeter which is operably arranged to indicate the flow direction of air at the focal point dependant on which arm of the detector leads the other arm in phase. The photocurrent from each photodetector may be operably connected to a combining circuit which forms a combined signal that is passed to a frequency analyser operably arranged to determine the velocity magnitude of air at the focal point. The transmitter and receiver may share a common telescope that comprises a pair of lenses arranged to expand and focus the light beam on a remote focal point. A scanner may cause the light beam to move across an arc ahead of the detector. The scanner may comprise a plane mirror operably connected to an actuator that is operated by a control signal generated by a control means. The scanner may comprise a lens through which the light beam is arranged to pass, the lens being arranged to focus the light beam on the remote focal point and operably connected to an actuator that is operated by a drive signal generated by the control means. The detector may be carried by an aircraft. According to a second aspect of the present invention a method of determining air flow direction and velocity of air remote from an aircraft in flight, comprising the steps of: focusing and directing a primary light beam to a focal region remote from the aircraft, and generating a secondary light beam from the primary beam; converting return light scattered by the air at the focal region into a scattered light beam; and determining the air flow direction and the velocity of the air at the focal region, characterised by splitting the scattered beam into first arid second signal beams, and splitting the secondary beam into first and second reference beams, polarizing the signal beams to differ in phase by ninety degrees, and detecting the interference between the first signal beam and the first reference beam to produce a first electrical signal having a first phase, separately detecting the interference between the second signal beam and the second reference beam to produce a second electrical signal having a second phase, combining the first and second signals to determine the air velocity, and processing the first and second phases to determine the air flow direction. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a Doppler anemometer according to the prior art; Figure 2 is a schematic diagram of a remote air detector according to the present invention; and Figure 3 shows a portion of apparatus for moving a beam emitted by a transmitter across an area ahead of an aircraft. In Figure 2, a remote air detector 40 according to the present invention comprises a transmitter 41 and a receiver 42. The transmitter 41 comprises a laser source 43 that produces an elliptical cone of laser radiation 44 which is collimated by a collimator lens 45 into a parallel beam 46. An example of a suitable laser source 43 is a single-frequency laser diode which can have a distributed feedback, single-stripe indium gallium arsenide-phosphide laser operated to have an output power level of approximately fifty milliwatts at a wavelength of approximately one thousand, five hundred and fifty nanometres. A laser source 43 of this type operating at this power level will produce laser radiation 44 having a total spectral width of approximately one megahertz and therefore have a coherence length of approximately three hundred metres, which is greater than the maximum range at which the detector 40 is to be used. The laser source 43 is orientated in such a way that the parallel beam 46 is linearly polarised in a plane substantially perpendicular to the direction of propagation of the parallel beam 46. Most of the power in the beam 46 is reflected by a polarizing beam-splitter 47 to form a reflected beam 48 which then passes through a quarter-wave plate 49 that converts the reflective beam 48 from a linearly polarised beam to a circularly polarised beam 50. The circularly polarised beam 50 then passes through a telescope 51 comprising lenses 52 and 53 which are operably arranged to form and project an expanded light beam 54 having a diameter typically between ten and a hundred millimetres. The lenses 52, 53 are arranged to focus the expanded beam 54 on a remote focal point 55 in atmosphere 56 at a range of between one and one hundred and fifty metres from the lens 53. The light beam 54 can be arranged to pass through a plane parallel glass window 57 placed between lens 53 of the telescope 51 and the focal point 55 to allow the beam 54 to pass out of the body of an aircraft, not shown. A small proportion of resultant light 58 is back-scattered from the focal point 55 by particles or aerosols in air flowing through point 55 in a direction which returns to the receiver 42 and further to a detector, through window 57. The light 58 is focussed by lenses 52, 53 into a substantially parallel beam 59 travelling in a direction opposite to that of the circularly polarised beam 50. The parallel beam 59 passes through the quarter-wave plate 49 where it is substantially converted from a circularly polarised beam to a linearly polarised beam 60 which passes substantially unattenuated through polarizing beam-splitter 47. The linearly polarised beam 60 passes through a quarter-wave plate 61 which converts the beam 60 into a circularly polarised beam 62 that then passes through a non-polarizing beam-splitter 63 that has a low reflectivity of approximately one to ten percent to form a beam 64. The beam 64 then passes through a polarizing beam-splitter 65 which divides the beam 64 into two substantially equal signal beams 66 and 67. One signal beam 66 is linearly polarised and passes to a first arm and the other signal beam 67 is linearly polarised but in a direction orthogonal to that of beam 66 and passes to a second arm. Owing to the circular polarisation of beam 64, signal beams 66 and 67 are of approximately equal amplitudes but differ in phase by ninety degrees. A small proportion of parallel beam 46, for example two percent of the power of beam 46, is transmitted through polarizing beam-splitter 47 to form a beam 68. The proportion of parallel beam 46 used to form beam 68 can be adjusted by altering the orientation of the laser source 43 with respect to the polarizing beam-splitter 47. Beam 68 is reflected by a prism 69, passes through a half-wave plate 70, is reflected by a prism 71 to the non-polarizing beam-splitter 63 where it is reflected to form reference light 72. Light 72 is made parallel and collinear with beam 64 by adjustment of prisms 69 and 71. The half-wave plate 70 rotates the plane of polarisation of beam 68 by forty-five degrees, so that reference light 72 contains components of substantially equal intensity polarised in the same direction as signal beams 66 and 67. Light 72 passes through polarizing beam-splitter 65 to form reference beams 73 and 74 having substantially equal intensity and phase. Reference beam 73 interferes with signal beam 66 forming interference fringes which are focussed by lens 75 onto photodetector 76, Similarly, reference beam 74 interferes with signal beam 67 forming interference fringes which are focussed by lens 77 onto photodetector 78. Photodetectors 76, 78 can be indium gallium arsenide photodiodes which are sensitive to laser wavelengths in the region of one thousand five hundred and fifty nanometers. The frequencies of signal beams 66, 67 are shifted with respect to their reference beams 73, 74 by the Doppler effect caused by motion of scattering particles in the atmosphere 56 at focal point 55 such that photocurrents 79, 80 produced respectively by the photodetectors 76, 78 are modulated at the Doppler frequency. Owing to the ninety degree phase shift between signal beams 66, 67, the photocurrents 79, 80 have a phase difference of ninety degrees. The direction of circular polarisation introduced by the quarter-wave plate 61 and the direction of the forty-five degree rotated polarisation introduced by half-wave plate 70 are adjusted so that the phase of the alternating photocurrent 80 leads the phase of photocurrent 79 when the velocity of scattering particles is positive with respect to the direction of the outgoing light beam 54 and lags in phase when the velocity of scattering particles is reversed. The direction of air flow through focal point 55 can be established by passing the photocurrents 79, 80 into a phasemeter 81 and from there to a phase indicator 82. An output from indicator 82 of plus ninety degrees indicates flow in one direction while an output of minus ninety degrees indicates flow in the opposite direction. The magnitude of air velocity through focal point 55 can be found by adding the photocurrents 79, 80 in a combining circuit 83 to produce a combined signal 84 which is passed to a frequency analyser 85. The output from frequency analyser 85 provides a measure of the Doppler frequency and hence the air velocity. The magnitude of signal beams 66, 67 are usually very low due to the low level of atmospheric back-scatter at a wavelength of one thousand five hundred and fifty nanometres. Long integration times are therefore required in the frequency analyser 85 to obtain an accurate measurement of air velocity. This can be remedied by raising the power of the transmitted light beam 54, for example by using a laser source 43 having an erbium-doped optical fibre amplifier or a tapered-waveguide semiconductor optical amplifier. The output from such an amplifier may need to be shaped into a circular beam suitable for coupling with the optical system described with reference to Figure 2. From Figure 3, the light beam 54, explained with reference to Figure 2, can be made to scan an area ahead of an aircraft. A scanner 230 comprises a plane mirror 231 which can be tilted in response to a control signal 232 applied to an actuator 233 operably connected to the mirror 231. The control signal 232 can be generated by a control means, now shown. Light 234 emerging from a final lens 235 of a telescope, not shown, but which has been described with reference to Figure 2, is reflected by mirror 231 to form reflected beam 236. The reflected beam 236 passes out of an aircraft, not shown, through a suitable plane parallel glass window 237 and is focussed on a remote focal point 238 at a distance comparable with the maximum distance at which objects in the path of the aircraft are to be detected. An object 239 in the path of beam 236 scatters a proportion of incident light which returns through window 237, is reflected by scanning mirror 231 and passes through lens 235. A signal beam is detected in accordance with the procedures described with reference to Figure 2 giving the range and velocity of the object 239 relative to the aircraft carrying the detector. The angular orientation of scanning mirror 231 is then changed by a small amount, causing reflected beam 236 to be projected in a new direction. The range of any object in the path of new beam 236 is determined as before. Mirror 231 is moved through a sequence of discrete angles by the signal 232 applied to the actuator 233 and the range of any objects in the path of beam 236 is recorded in each angular position. Signal 232, for example, can be programmed to cause the focal point 238 of beam 236 to execute a circle or a figure of eight, subtending an angle of, for example six degrees, at mirror 231, with a central axis aligned with the direction of forward motion of the aircraft. The range of any object such as a pylon, overhead wire or building in the path of beam 236 can then be detected, enabling a warning signal to be presented to a pilot of the aircraft. The scanner 230 can be operated in rapid sequence by attaching lens 235 to an actuator 240 which rapidly moves lens 235 in an axial direction by a known amount in response to a drive signal 241 thereby focussing the light beam at different ranges from the lens 235. The control means can also be arranged to generate the signal 241 to move lens 235 to a first position, which can be a position which causes the focal point 238 of beam 236 to be approximately twenty metres from the aircraft. The control means also generates a signal 232 to move beam 236 in a first direction. The air velocity at the focal point can then be processed as previously described, the control means then generates a further signal 232 to move beam 236 to a second position substantially different to the first position and the air velocity is computed again. The control means then generates a third signal 232 to move beam 236 in a third direction substantially different to the first and second directions and a third measurement of air velocity is calculated. The three measurements of air velocity in three known beam directions, which can take approximately three hundred millisecond to complete, are used by a suitably arranged processor, not shown, to calculate the direction of air velocity vector relative to the aircraft axis by applying conventional vector transformations. The control means then generates a signal 241 to move lens 235 to a second position at which the focal of beam 236 is at a maximum distance from the aircraft. The control means then sends a signal 232 causing beam 236 to rapidly move to a sequence of positions required for obstacle warning, taking a measurement of range to any obstacle in the path of the beam 236 in each position. Each range measurement can require less than one millisecond to complete. After a time of approximately seven hundred milliseconds, the control means generates a signal 241 to move lens 235 back to the first position and the measurement cycle is repeated. In this way repeated measurements of air velocity and the range and position of objects in the path of the aircraft are continuously made available with an information update time of approximately one second. The lens 235 can be the final lens of a telescope used to project the beam, as previously described with reference to Figure 2 or can be a separate lens 235 appropriately positioned after the telescope.

Claims: Apparatus (40) for determining air flow direction and velocity of air remote from an aircraft in flight, including: a transmitter (41) on the aircraft, for focusing and directing a primary light beam (46) to a focal region (55) remote from the aircraft, and for generating a secondary light beam (68) from the primary beam; a receiver (42) on the aircraft, for converting return light (56) scattered by the air at the focal region (55) into a scattered light beam (58); and a detector for determining the air flow direction and the velocity of the air at the focal region (55) including a beam-splitter (65) characterised in that the beam- splitter (65) is operable to split the scattered beam into first and second signal beams (66, 67), to split the secondary beam (72) into first and second reference beams (73, 74), and to polarize, the signal beams (66, 67) to differ in phase by ninety degrees, and a detection circuit (76, 78, 81, 83, 85) for detecting the interference between the first signal beam (66) and the first reference beam (73) to produce a first electrical signal having a first phase, for separately detecting the interference between the second signal beam (67) and the second reference beam (74) to produce a second electrical signal having a second phase, for combining the first and second signals (79, 80) to determine the air velocity, and for processing the first and second phases (79, 80) to determine the air flow direction. Apparatus as claimed in claim 1, wherein the transmitter (41) includes a laser source (43) for producing the primary light beam (46) with a linear polarization in a plane substantially perpendicular to a direction of propagation of the primary beam, and a first quarter-wave plate (49) for converting the primary beam (46) to have a circular polarization. Apparatus as claimed in claim 2, wherein the transmitter (41) further includes a telescope (51) for expanding and directing the primary beam (46) through a window (57) on the aircraft. Apparatus as claimed in claim 3, wherein the receiver (42) is operative for receiving the return light entering through the window (57) and passing through the telescope (51) and the first quarter-wave plate (49), and for receiving the return light through a second quarter-wave plate (61) to convert the scattered beam (60) to have a circular polarization. Apparatus as claimed in claim 4, wherein the beam-splitter (65) is operative for splitting the scattered beam of circular polarization into the first and second signal beams (66, 67) of equal amplitudes. Apparatus as claimed in claim 5, wherein the transmitter (41) includes an optical splitter (47) for forming the secondary beam (68) from the primary beam, and a half-wave plate (70) through which the secondary beam passes. Apparatus as claimed in claim 1, wherein the detection circuit includes a first photodetector (76) for detecting interference between the first signal beam (66) and the first reference beam (73) to produce the first electrical signal as a first current signal, and a second photodetector (78) for detecting interference between the second signal beam (67) and the second reference beam (74) to produce the second electrical signal as a second current signal. Apparatus as claimed in claim 7, wherein the detection circuit includes means (81) for comparing the first and second phases of the first and second current signals to determine one direction of the air flow when one of the phases leads the other of the phases, and to determine an opposite direction of the air flow when said one phase lags said other phase. Apparatus as claimed in claim 8, wherein the comparing means includes a phasemeter (81) and a phase indicator (82). Apparatus as claimed in claim 7, wherein the detection circuit includes means (83) for combining the first and second current signals (79, 80) to produce a combined signal, and a frequency analyzer (85) for measuring a frequency characteristic of the combined signal. Apparatus as claimed in claim 1, wherein the transmitter (41) and the receiver (42) have a common scanner (230) for scanning an area ahead of the aircraft, the scanner (230) including a generally planar mirror (231) and a controller (233) for moving the mirror to a plurality of angular positions. Apparatus as claimed in claim 11, wherein the scanner (230) includes a movable focusing lens (235), and a drive (240) for moving the lens along a direction of propagation of the primary beam to focus the primary beam at different distances relative to the focusing lens (235). Apparatus as claimed in claim 12, wherein the detection circuit is operative for determining the air flow direction at each of the distances. Apparatus as claimed in claim 12, wherein the focusing lens is a component of a telescope (235) for expanding and directing the primary beam through a window (237) on the aircraft. A method of determining air flow direction and velocity of air remote from an aircraft in flight, comprising the steps of: focusing and directing a primary light beam (46) to a focal region (55) remote from the aircraft, and generating a secondary light beam (48) from the primary beam (46); converting return light (56) scattered by the air at the focal region (55) into a scattered light beam; and determining the air flow direction and the velocity of the air at the focal region (55), characterised by splitting the scattered beam (58) into first and second signal beams (66, 67), splitting the secondary beam (48) into first and second reference beams (73, 74), polarizing the signal beams (66, 67) to differ in phase by ninety degrees, and detecting the interference between the first signal beam (66) and the first reference beam (73) to produce a first electrical signal having a first phase, separately detecting the interference between the second signal beam (67) and the second reference beam (74) to produce a second electrical signal having a second phase, combining the first and second signals (79, 80) to determine the air velocity, and processing the first and second phases to determine the air flow direction. The method of claim 15, wherein the determining step includes detecting interference between the first signal beam (66) and the first reference beam (73) to produce the first electrical signal (79) as a first current signal, and detecting interference between the second signal beam (67) and the second reference beam (74) to produce the second electrical signal (80) as a second current signal. The method of claim 16, wherein the determining step includes comparing the first and second phases of the first and second current signals to determine one direction of the air flow when one of the phases leads the other of the phases, and to determine an opposite direction of the air flow when said one phase lags said other phase. The method of claim 16, wherein the determining step includes combining the first and second current signals to produce a combined signal (84), and analyzing a frequency characteristic of the combined signal (84). The method of claim 15 and further comprising the step of scanning an area ahead of the aircraft by moving a generally planar mirror (231) to a plurality of angular positions. The method of claim 19, wherein the scanning step includes moving a movable focusing lens (235) along a direction of propagation of the primary beam to focus the primary beam at different distances relative to the focusing lens (235).

Inventors: LANGDON ROGER MARTIN, WATTS PHILIP MICHAEL, LANGDON, ROGER MARTIN, WATTS, PHILIP MICHAEL
Applicants: BAE SYS ELECTRONICS LTD, BAE SYSTEMS ELECTRONICS LTD.
Application year: 1999
Main classification: G01P5
IPC codes: B64D43, G01P13, G01P5, G01S17