Molecular optical air data systems (MOADS)

Abstract

A compact optical instrument called Molecular Optical Air Data System (MOADS) can directly measure wind speed and direction, density, and temperature of a body of air. From these measurements, a complete set of air data products can be determined. In contrast to pitot tubes, however, MOADS can operate at high angles of attack. In the proper configuration, MOADS can continue to measure air data products at angles of attack of 90 degrees. The MOADS instrument is a flush-mount design which lends itself to low observability since there are no aircraft protrusions to generate a radar cross section. MOADS is also airframe independent, and is much less costly to calibrate, recalibrate or service due to this lack of dependence. The system uses a Fabry-Pérot interferometer to detect the (incoherent) Doppler shift from laser light backscattered by air molecules and aerosols (Rayleigh and Mie scattering). The laser used to provide the signal utilizes short wavelengths operating in the ultraviolet at 266 nm, which is invisible to the human eye and rapidly absorbed by the atmosphere. Although the system will take advantage of aerosols when they are available, a significant advantage of MOADS over similar air data system technologies is the ability to make measurements in clear air (air molecules only), without the presence of aerosols, which also provides the ability to measure density and temperature.

Description

FIELD OF THE INVENTION

[0002] This invention relates generally to air data systems and, in particular, to an optical instrument that can directly measure wind speed and direction, density, and temperature to derive a complete set of air data products.

BACKGROUND OF THE INVENTION

[0003] “Air data products,” as they are called, are determined in an aircraft using an in-flight air data system. An air data system incorporates instrumentation to collect air data products, and supplies this data directly to an aircraft's flight computer for flight control purposes. Common air data products include, but are not limited to, Mach number, true airspeed, calibrated airspeed, vertical speed, static density, static air temperature, sideslip, angle of attack, pressure altitude, and dynamic pressure.

[0004] Perhaps the oldest type of such instrumentation is the Pitot static tube. The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by converting the kinetic energy of the flow into potential energy. The conversion takes place at the stagnation point, located at the Pitot tube entrance. A pressure higher than the free-stream (i.e. dynamic) pressure results from the kinematic to potential conversion. This “static” pressure is measured by comparing it to the flow's dynamic pressure with a differential manometer.

[0005] Pitot static tubes have proven quite effective over the years; however, there are a number of characteristics that make them undesirable in some situations. For example, at high angles of attack the air data measurements provided by pitot static systems are significantly degraded. Pitot tubes also contribute significantly to an aircraft's radar cross section, since they protrude from the aircraft body. The installation and calibration of pitot static tubes must be tailored to each airframe, and airframe modifications may require recalibration of the air data system.

[0006] Optical air data system technologies are alternatives to the traditional pitot static system. In general, an optical air data system utilizes LIDAR (Light Detection and Ranging) to remotely analyze the atmosphere. LIDAR uses an active sensor that includes a laser light source, a detection system and an analysis routine to process the signal return.

[0007] There are two types of optical air data systems: coherent and direct detection (incoherent). In a coherent LIDAR, the laser light is emitted into the atmosphere, where it scatters off of the aerosols in the air, and can be analyzed to solely determine the air velocity. For these purposes, an aerosol is defined as any type of particle that is suspended in the air.

[0008] In a direct detection LIDAR, the laser energy scatters off of both aerosols in the air, as well as the air molecules themselves, and can be analyzed to determine the air velocity, density, and temperature. A schematic of a Doppler LIDAR system configuration is presented in FIG. 1. This figure illustrates how transmitted laser light, which has been scattered by the atmosphere, is gathered by a telescope and processed by the detection system.

[0009] A coherent LIDAR system utilizes long wavelength laser energy and relies upon Mie scattering, which is the scattering of light off of the aerosols suspended in the air. More particularly, Mie scattering refers to the scattering of light off of particles greater than {fraction (1/10)}th the wavelength of light. However, since coherent detection LIDAR measures the properties of aerosols, it can only measure the wind velocity.

[0010] A direct detection LIDAR system utilizes short wavelength laser energy in order to scatter the light off of both the molecules of air (Rayleigh scattering) as well as the aerosols in the air (Mie scattering). Rayleigh scattering refers to the scattering of light off of molecules of air, and particles up to {fraction (1/10)}th the wavelength of the light. Since direct detection LIDAR measures the properties of molecules, it can measure the air velocity, as well as the air density and temperature.

[0011] However, since coherent LIDAR approaches rely solely on Mie scattering, they cannot make measurements in clean air where there are no aerosols present. In addition, coherent approaches utilize long wavelength light, which is not absorbed by the atmosphere, presenting additional issues with long-range detection, and increased eye safety hazards.

SUMMARY OF THE INVENTION

[0012] This invention, termed the Molecular Optical Air Data System (MOADS), is a compact, direct detection LIDAR optical instrument that can directly measure wind speed and direction, density, and temperature of a body of air. From these measurements, a comprehensive set of air data products can be determined.

[0013] In some respects, MOADS is a replacement for pitot static tubes. In contrast to pitot tubes, however, MOADS can operate at high angles of attack. In the proper configuration, MOADS can continue to measure air data products at angles of attack of 90 degrees. The MOADS instrument is a flush-mount design which lends itself to low observability since there are no aircraft protrusions to generate a radar cross section. MOADS is also airframe independent, and is much less costly to calibrate, recalibrate or service due to this lack of dependence.

[0014] In terms of apparatus, MOADS uses a Fabry-Pérot interferometer to detect the (incoherent) Doppler shift from laser light backscattered by air molecules and aerosols (Rayleigh and Mie scattering). The laser used to provide the signal utilizes short wavelengths operating in the ultraviolet at 266 nm, which is invisible to the human eye and rapidly absorbed by the atmosphere.

[0015] Although the system will take advantage of aerosols when they are available, a significant advantage of MOADS over similar air data system technologies is the ability to make measurements in clear air (air molecules only), without the presence of aerosols.

[0016] The advantages of the MOADS instrument include the following:

[0017] Low observability

[0018] Operates at high angles of attack (in the proper configuration, MOADS can continue to measure air data products at angles of attack of 90 degrees)

[0019] Operates in clear air (aerosols are not required)

[0020] Airframe independent

[0021] Less costly to calibrate or recalibrate

[0022] Accurate for highly maneuverable aircraft as well as hoverable aircraft

[0023] Reduced Life Cycle Cost

[0024] Possible application to detection of wind shear, wake vortex, clear air turbulence, and engine unstart conditions

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a schematic illustrating a Doppler LIDAR concept;

[0026] FIG. 2 is a diagram of an etalon optical system according to the invention;

[0027] FIG. 3A shows normal Etalon fringes;

[0028] FIG. 3B depicts an etalon illuminated with four fiber input channels;

[0029] FIG. 4 shows how the four channels of fringes are collapsed to four lines in the shape of a cross pattern on an opto-electric detector;

[0030] FIG. 5 is a MOADS opto-mechanical system diagram;

[0031] FIG. 6 is a fringe analysis diagram;

[0032] FIG. 7 shows MOADS measured air data products; and

[0033] FIG. 8 shows MOADS derived air data products.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] The MOADS (Molecular Optical Air Data System) direct detection LIDAR system described herein is unique in that it can operate in clear air, using only molecular backscatter (i.e., no aerosols present). The MOADS system takes advantage of aerosols when present, but does not rely upon the presence of aerosols. The signal generated by MOADS is used to directly measure velocity, true airspeed, vertical speed, angle of attack, angle of sideslip, static density and static temperature. From these data products the following can be directly calculated: calibrated airspeed, Mach number, static pressure, total pressure, dynamic pressure, pressure altitude, air density ratio, total temperature, angle of attack, pressure differential, and angle of sideslip pressure differential.

[0035] In the preferred embodiment, a beam of ultraviolet laser light at a wavelength of 266 nm is emitted in three directions from a surface-mounted aperture in the aircraft, and to detect the return from scattering of this beam by atmospheric molecules and aerosols. The initial wavelength of 266 nm was selected for its stealth, eye safety and molecular scattering properties. There is very little natural background light due to absorption of most natural 266 nm light by ozone and molecular oxygen. However, if longer wavelength light were to be used, a simple gating system would eliminate most of the stray light. Ultraviolet (UV) light at 266 nm is readily absorbed by glass and plastic, such as aircraft wind screens.

[0036] Although the invention is described herein with respect to an airframe-mounted unit, it must be appreciated that the invention can measure air data products on a variety of platforms, not limited to an aircraft proper. Other possibilities include, but are not limited to, smart-guided weapons, stationary weather stations (most notably on icy mountain tops), and possibly wind-propelled boats. In addition, although the current prototype uses ultraviolet (UV) laser light, the system can operate on large range of wavelengths spanning from the visible down to the ultraviolet. The UV light provides additional stealth characteristics for the system since the light is quickly absorbed by the atmosphere, and not easily detected from long-range distances. However, MOADS can also operate in other wavelength regions, such as longer UV wavelengths or even visible wavelengths. The laser source can be either pulsed or continuous wave (CW).

[0037] Nor is the invention limited in terms of the number of channels, nor the geometry of the channels in relation to each other. Although the system described herein is configured with three channels, spaced 120 degrees apart from each other, other angles may be used to calculate a wind vector. In addition, although three channels are necessary to calculate a wind vector, the system may have extra redundant channels, or dual channels to measure wind in a particular plane, or single channels to measure the wind along a specific light of sight.

[0038] An aspect that sets MOADS apart from other direct detection optical air data systems is the efficient use of the etalon. In the preferred embodiment, a single Fabry-Pérot etalon is used for four channels of light: three lines of sight, plus a reference channel. FIG. 2 is a diagram of the etalon optical system used by the invention. A fiber optic illuminates a portion of the etalon, producing the image of a partial ring pattern shown in FIG. 3A. Note that, for the purposes of clarity, FIG. 2 traces only the path associated with one of the fibers; in the actual system implementation, four fiber inputs are used. FIG. 3B depicts an etalon illuminated with four fiber input channels.

[0039] The reference channel is injected directly from the laser source. All three lines of sight are compared to the reference channel, to produce an instrument that is self-calibrating by design. If laser wavelength drift is not accounted for in the data, then errors will arise when making a measurement of the Doppler shift (wavelength shift) of the signal return. In the MOADS instrument, however, the laser wavelength drift is automatically calibrated out of the data since each measurement is compared to the reference light source.

[0040] A quad circle-to-line interferometer optic (CLIO) is preferably used to collapse the four channels of circular fringes down to four linear patterns, forming the shape of a cross. Although use of the CLIO is not strictly necessary to the invention, its use does provide additional signal throughput. The resultant cross pattern is much easier to detect using standard linear-based detectors, such as linear arrays or CCDs. The CLIO, described in U.S. Pat. No. 4,983,033, the entire content of which is incorporated herein by reference, employs an internally conical reflector for converting the circular fringe information into linear information whereby the reflected light can be received by conventional linear array detectors, such as charge coupled device which are used in spectroscopic analysis.

[0041] The azimuthal angle of the detected circular fringe pattern is reduced with the use of a tele-kaleidoscope having a predetermined arrangement of mirrors. Electromagnetic energy which is issued from the interferometer is propagated substantially along the conical axis of the cone of which the reflector forms a segment, and is reflected and focused substantially onto a line in the vicinity of the conical axis, where the linear detector is situated. In a preferred embodiment, the apex of the cone is situated where the conical axis intersects the focal plane of the circular fringe pattern. FIG. 4 shows how the four channels of fringes are collapsed to four lines in the shape of a cross pattern on an opto-electric detector.

[0042] Important hardware aspects of the MOADS system are shown in FIG. 5. The optical head is responsible for directing the outgoing laser beam, as well as collecting the backscattered signal utilizing three separate telescopes. The optical head can be custom configured, but shown here at the center of the optical head, the laser is divided into three beams with a beam splitter optic, and then directed into three lines of sight, each spaced 120 degrees from each other and 30 degrees off of the center axis. The signal is then collected by an array of three telescopes built into the head.

[0043] It should be noted that scattering is preferably only detected in the regions where the field of view of the detecting telescope and the laser beam overlap. This creates a near-field region where there is no return, which is a desirable feature since the airstream near the aircraft is turbulent. The far-field measurement is not as contaminated by the aircraft's wake. The signal from the detecting lens system is collected by a fiber optic that transfers the returned photons to the interferometer/detection system.

[0044] The system uses the geometry of the laser beam and the telescope observation system to define the range from the sensor, rather than employing timing as is done with most LIDAR systems. This is analogous to the situation encountered by passive sensing space flight instruments, where the return signal is integrated along the line of sight. Such an approach simplifies the system, but range gating could easily be introduced if desired.

[0045] The signal conditioner employs eight bandpass filter mirrors centered around 266 nm which filter out the background light. The filter box performance exhibits high out-of-band rejection, as well as low in-band attenuation. The signal delivered to the etalon consists of virtually pure 266 nm wavelength light.

[0046] The interferometer is made-up of a Fabry-Pérot etalon, as well as some steering optics to focus the image onto the detector. The signal received in any one of the three interaction regions is processed through the Fabry-Pérot interferometer that acts as a comparator to determine the wind speed, temperature, and air density. The techniques for determining these quantities were fully developed for the satellite instruments that have been flown since the early 1980s. Basic system calibration is maintained by performing simultaneous observations of the emitted laser beam with every observation of the three interaction regions.

[0047] The circular fringes generated by the Fabry-Pérot etalon are transformed into a linear cross-pattern and then imaged onto a charge-coupled device (CCD) detector. The CLIO device referenced above greatly improves the efficiency of the signal detection process. The CCD camera is low-light sensitive, and provides low noise image readout.

[0048] MOADS Data Analysis

[0049] Wind velocity, density, and temperature are directly calculated from the Fabry-Pérot etalon fringe data. All other air data products are derived from these three basic measurements, and the knowledge of the instruments geometry. FIG. 6 is a fringe analysis diagram.

[0050] Velocity Measurement

[0051] The relative wind velocity is determined by the difference between the centroids of the atmospheric fringes versus the laser reference fringes. The fringe position is based directly on wavelength. Therefore the difference in wavelength between the signal and reference fringes is a direct measure of the molecular/aerosol Doppler shift.

[0052] Density Measurement

[0053] The air density is determined by the integral of the molecular component of the atmospheric fringes. Air density is related to the molecular density, not aerosol density, therefore the Rayleigh backscatter must be separated from the Mie backscatter. The denser the air is, the more molecules are present to scatter light back to the detector. The density measurement is a measurement of the total number of photons received.

[0054] Temperature Measurement

[0055] The absolute temperature is determined by the width of the molecular component of the atmospheric fringes. The air temperature is related to the vibrational mode of the molecular content. As the molecules exhibit a faster vibrational state, the backscatter bandwidth is widened, producing wider fringes. The absolute temperature is directly related to this signal bandwidth.

[0056] Calculation of the Air Data Products

[0057] FIGS. 7 and 8 illustrate the transformation of instrument line-of-sight data to measured and derived air data products.

Claims

1. An optical air data system, comprising: a laser outputting one or more beams of light; an optical head for directing the beams of light into the atmosphere, and to collect the light backscattered by molecules or aerosols present in the atmosphere; an interferometer operative to generate an atmospheric fringe pattern associated with the light backscattered along multiple channels; a sensor to detect the incoherent Doppler shift in the fringe patterns and output corresponding electrical signals; and a data processor to determine a set of air data products based upon the electrical signals.

2. The optical air data system of claim 1, wherein the interferometer is based upon a Fabry-Pérot etalon.

3. The optical air data system of claim 1, wherein the air products include wind speed, temperature, and air density.

4. The optical air data system of claim 1, further including a circle-to-line interferometer optic to transform the fringe patterns from circular pattern to a linear cross-pattern.

5. The optical air data system of claim 1, further including a reference beam from the laser into the interferometer to calibrate the optical channels.

6. The optical air data system of claim 1, further including telescopes associated with the collection of the backscattered light with respect to each optical channel.

7. The optical air data system of claim 1, wherein the backscattered light is only detected in the regions of overlap with the outgoing beams.

8. The optical air data system of claim 1, wherein the laser beams are in the ultraviolet region of the spectrum.

9. The optical air data system of claim 1, wherein the laser beams are in the visible region of the spectrum.

10. The optical air data system of claim 1, wherein the interferometer is operative to detect Rayleigh and Mie backscattering.

11. The optical air data system of claim 1, further including a reference fringe pattern derived directly from the laser; and wherein the data processor is operative to determine relative wind velocity by comparing the atmospheric fringe pattern to the reference fringe pattern.

12. The optical air data system of claim 1, wherein the data processor is operative to determine air density in accordance with the integral of the molecular component of the atmospheric fringes.

13. The optical air data system of claim 1, wherein the data processor is operative to determine absolute temperature in accordance with the width of the molecular component of the atmospheric fringes.