Patent Application
20250123400
The present disclosure relates to air data measurements, and more particularly to laser based air data systems for aircraft.
There is an industry wide push for dissimilar air data sensing (compared to traditional pneumatics probes). The Laser Air Data System (LADS) is a flush mount Light Detection and Ranging (LIDAR) system for measuring air data parameters such as True Air Speed (TAS) and provides dissimilar failure modes from pneumatic systems such as bird strike, volcanic ash, and ice ingestion. The detection method utilizes Rayleigh-Brillouin and Mie backscattering to measure air data parameters and can function in high Mie to no particulate loaded environments. The conventional techniques have been considered satisfactory for their intended purpose. However, there is an ever present need for improved systems and methods for LIDAR based air data measurements for aircraft and the like. This disclosure provides a solution for this need.
A method includes emitting scanned reference laser light from a first tunable laser into a first atomic vapor filter to output filtered scanned reference laser light and receiving the filtered reference laser light with a first detector. The method includes emitting scanned outgoing signal laser light from a second tunable laser into an atmospheric space, receiving a return of the scanned signal laser light from Doppler shifted scattering from the atmospheric space into a second atomic vapor filter to output Doppler shifted filtered scanned signal laser light, and receiving the Doppler shifted filtered scanned signal laser light with a second detector. The method includes controlling the center wavelength of the scanned outgoing signal laser light of the second tunable laser to continuously maintain the Doppler shifted filtered scanned signal light on the center of an absorption well related to the second atomic vapor filter as the air velocity changes in the atmospheric space.
Emitting scanned reference laser light from a first tunable laser can include cycling the reference light over a limited range of reference wavelengths centered on a reference absorption well related to the first atomic vapor filter. The method can include calculating air speed data based on the difference between the center wavelength of the scanned outgoing signal laser light and the center wavelength of the scanned reference laser light. The method can include outputting the air speed data to a consuming system of an aircraft.
Controlling scanned outgoing signal laser light wavelength of the second tunable laser can include using engine or other aircraft state data to control the limited scan wavelength range to initially center the scan wavelength range on the absorption well. Controlling scanned outgoing signal laser light wavelength of the second tunable laser can include initializing the limited scan wavelength range to center the limited scan wavelength range of the Doppler shifted filtered scanned signal light on the second atomic vapor cell absorption well at zero airspeed, and wherein the aircraft state is zero speed and grounded. Controlling scanned outgoing signal laser light wavelength of the second tunable laser can include, for wavelength cycles after initialization, iteratively stepping the limited scan wavelength range up or down for each wavelength cycle to maintain the Doppler shifted filtered scanned signal light at the center of the second atomic vapor cell absorption well. The method can include combining the reference laser light with the outgoing laser light to output combined laser light to a frequency detector, detecting a beat note in the combined laser light, and converting the beat note to air speed data.
The method can include multiple outgoing channels. This can include emitting respective outgoing laser light from at least one additional tunable laser for each respective one of one or more additional outgoing light channels, receiving respective returns of the respective outgoing laser light from the atmospheric space into a respective additional atomic vapor filter to output respective filtered scan light for each additional outgoing light channel, receiving the respective filtered scan laser light with a respective additional detector for each additional outgoing light channel, and controlling respective center wavelength of each respective outgoing laser light of the at least one additional tunable laser to continuously remain centered on a respective absorption well related to the second atomic vapor filter as the Doppler shift changes due to changes in air velocity in the atmospheric space.
A system includes a first tunable laser operatively connected to be controlled by a first laser control. A first atomic vapor filter is operatively connected to receive laser light from the first tunable laser and to pass filtered light to a first detector that is operatively connected to the first laser control for control of the first tunable laser. A plurality of transmit channels each including: a second tunable laser operatively connected to be controlled by a second laser control, a second atomic vapor filter operatively connected to receive returned laser light from the second tunable laser and to pass filtered return light to a second detector that is operatively connected to the second laser control for control of the second tunable laser, and a controller operatively connected to the first laser control and to the second laser control of each of the plurality of transmit channels. The controller is configured to perform methods as disclosed herein to determine air data from the difference in center wavelengths between the first tunable laser and the second tunable laser.
Each transmit channel can include: an amplifier operatively connected to amplify the laser light from the second tunable laser to a transceiver. The transceiver can be operatively connected to receive the amplified laser light, to transmit the amplified laser light out into an atmospheric space, and to receive Doppler shifted return light from the atmospheric space. For each transmit channel, the system can include a first optical splitter operatively connected to split light from the first tunable laser, a second optical splitter operatively connected to split light from the second tunable laser, a light combiner operatively connected to receive and combine light split from the first optical splitter and light split from the second optical splitter, and a frequency detector configured to detect a beat note in combined light from the light combiner. The controller can be configured to determine air speed based on Doppler shift apparent in the beat note detected in the frequency detector.
These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings.
So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein:
Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an embodiment of a system in accordance with the disclosure is shown in
The system 100 includes a first tunable laser 102 operatively connected to be controlled by a first laser control 104. A first atomic vapor filter 106 is operatively connected to receive laser light from the first tunable laser 102 and to pass filtered light to a first detector 108 that is operatively connected to the first laser control 104 for control of the first tunable laser 102, including wavelength control. A plurality of transmit channels 110 are included. In
Each channel 110 includes a second tunable laser 112 operatively connected to be controlled by a second laser control 114. An amplifier 120 is operatively connected to amplify the laser light from the second tunable laser to a transceiver 122. Those skilled in the art will readily appreciate that an amplifier can be used to increase the magnitude of the output light, the system 100 can work with a tunable laser of sufficient magnitude where an amplifier is not needed without departing from the scope of this disclosure. The transceiver 122 is operatively connected to receive the amplified laser light from the amplifier 120, to transmit the amplified laser light 124 out into an atmospheric space 126, and to receive Doppler shifted return light 128 from the atmospheric space 126. A second atomic vapor filter 116 is operatively connected to receive Doppler shifted return light 128 from the transceiver 122 and to output filtered Doppler shifted return light to a second detector 118 that is operatively connected to the second laser control 114 for control of the second tunable laser 112, including wavelength control.
A controller 130, e.g. a main controller, is operatively connected to the first laser control 104 and to the second laser control of 114 each of the plurality of transmit channels 110. The controller 114 is configured to, for each of the plurality of transmit channels 110, control wavelength of the outgoing laser light 124 of the second tunable laser 112 to continuously remain within a limited scan wavelength range centered on an absorption well related to the atomic vapor filter 116, even when the absorption well shifts wavelength due to changes in air velocity in the atmospheric space 126.
The scan range chosen for the tunable lasers should be large enough to be able to identify the center of the absorption well, but not so large as to reduce the accuracy of the ability to lock onto the center of that absorption well. Controller 104 controls the wavelength tuning of laser 102 in its entirety to ensure that the laser wavelength sweep is exactly centered on the absorption well of the vapor cell 106. This can be done controlling the current applied to the laser, the laser cavity temperature, or in the case of an external cavity laser diode (ECLD), the voltage applied to a piezo-electric actuator controlling the position of a tuning mirror. Controller 114 controls the wavelength tuning of laser 112 in its entirety to ensure the outgoing signal laser wavelength sweep of laser 112 is adjusted to ensure that the Doppler shifted filtered scanned signal laser light 128 is also exactly centered on the absorption well of the second vapor cell 116. Because of that Doppler shift from the relative motion of the atmosphere to the laser system (in our case on an airplane), the center of the wavelength sweep for the second tunable laser will be offset from the center of the wavelength sweep for the reference laser by an amount exactly twice the Doppler shift for that relative velocity. Main controller 130 can monitor the drive signals for the two lasers and use the difference in drive signals to determine that center wavelength difference and therefore the Doppler shift. The vapor cell absorption wells are centered at fixed, known wavelengths and are identical to one another. By locking both the wavelength sweeps of the reference laser light and the Doppler shifted return light on those identical absorption lines, the center wavelengths for both lasers are be shifted with respect to one another by twice the Doppler shift.
The center wavelength of an absorption well is fixed due to the properties of the atomic vapor filter 106/116. What changes, e.g. as an aircraft moves through the atmospheric space 126, is the Doppler shifted filtered scanned signal laser light that is collected from atmospheric space.
So, to keep the Doppler shifted filtered scanned signal laser light centered on that fixed absorption well, the controller 114 must shift the center of the outgoing signal laser light from the tunable laser 112 so that the Doppler shift moves that light to the center of the absorption well. As the relative velocity of the air changes, the Doppler shift amount changes, and the center wavelength of the tunable laser light 112 necessary to keep the Doppler shifted return light centered on the absorption well needs to be adjusted.
The method includes controlling wavelength, e.g. using laser control 114, of the outgoing signal laser light of the second tunable laser 112 of each channel 110 such that the Doppler shifted filtered scanned signal laser light collected from atmospheric space remains continuously within a limited scan wavelength range 132 (labeled in
The filtered reference laser light is collected by the first detector 108 and the information is provided to a laser controller 104. The laser controller adjusts the master laser 102 scan range to wavelengths limited to wavelengths within the absorption well. Similarly, each transmit channel detector 118 provides Doppler shifted filtered signal laser light received from atmospheric space information to their associated laser controller 114. This information is used to control the channel laser 112 wavelength scan range to ensure Doppler shifted filtered signal laser light collected from the atmosphere is limited to wavelengths within the absorption well with the center wavelength of the scan at the bottom of the absorption well. Since the light received by the detectors 108,118 include only wavelengths within the absorption wells, all of the information collected is useful in determining the Doppler shift of the signal laser light scattered from atmospheric space. This results in reduced noise in the velocity determination, considerably improved relative to systems scanning the full wavelength spectrum, e.g. comprehensively scanning across a fuller spectrum potentially encompassing multiple absorption wells and all the data between the wells. For the same detection time, a system which limits the wavelength scan range to include only wavelengths inside the absorption wells will collect more useful information as all the data collected is useful in determining the Doppler shift that occurs in the scattering of the signal laser light transmitted into atmospheric space. This ultimately results in a better velocity measurement with reduced output noise.
The controller 114 can control outgoing laser light wavelength of the second tunable lasers 112 to include the usage of engine or other aircraft state data 138 to aid in the control of the limited scan wavelength range 132 (labeled in
With reference now to
Systems and methods as disclosed herein provide potential benefits over traditional systems and methods as follows. By controlling the channel laser(s) scan wavelengths such that the Doppler shifted light collected from the atmosphere is centered in the absorption well, a significant increase in useful signal collected per unit time is achieved compared to traditional methods. Existing methods also use an atomic vapor filter as a wavelength reference which can be used to measure the Doppler shift between a reference source of light and light collected from the atmosphere. These existing methods use a single laser for the entire system, which is scanned for a wide enough wavelength range such that the reference absorptive well and the absorptive well from the Doppler shifted light collected from the atmosphere are present within the scan. These methods may also include a scan range wide enough to measure two wells from the atomic vapor filter to provide an absolute frequency reference is the separation between the centers of the two absorptive wells. This wavelength scan includes time spent collecting light that is not in the absorptive well and provide no information regarding the Doppler shift. By controlling the channel laser(s) with an offset which shifts the return light to the center of the well, the entire scan time is spent collecting wavelengths of light within an absorption well. The laser control characteristics used to keep the channel laser(s) centered in the well can be used to determine Doppler shift relative to the reference laser for generating air data. Alternatively, the light from the reference can be combined with a portion of the light prior to being transmitted to the atmosphere and the frequency difference can be measured directly. This provides an advantage over approaches which use multiple atomic vapor filter absorption wells to determine an absolute frequency reference within a wavelength scan.
The methods and systems of the present disclosure, as described above and shown in the drawings, provide for enhance signal to noise ratio in Rayleigh-Brillouin and Mie backscattering air data measurements such as for use aboard aircraft. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.
