Method for measuring airspeed by optical air data sensor
The object of the present invention is to provide an air data sensor that does not require an external input of a reference velocity as a Doppler LIDAR, has a function of autonomously determining the absolute airspeed, and has no position error. The optical air data sensor in accordance with the present invention is an optical air data sensor, mounted on an aircraft, for emitting a laser light as a transmission light into atmosphere, and then receiving a laser scattered light produced by scattering of the laser light by aerosol present in the atmosphere as a reception light, thereby to measure an airspeed and a wind velocity of airflow in a distant region on the basis of a Doppler shift amount between the transmission light and the reception light, wherein a true airspeed is autonomously measured, without setting a reference velocity, by successively sweeping a frequency offset corresponding to a reference velocity for providing an offset to a measurement frequency, and performing this sweeping within a frequency range in which the Doppler shift amount is measured.
1. A method for measuring airspeed by an optical air data sensor mounted on an aircraft, said method comprising emitting a laser light as a transmission light into atmosphere, and then receiving a laser scattered light produced by scattering of the laser light by aerosol present in the atmosphere as a reception light, and measuring an airspeed and a wind velocity of airflow in a distant region on the basis of a Doppler shift amount between the transmission light and the reception light,
wherein, in the range ranging from 30 m/s to 250 m/s, a true airspeed is autonomously measured, without setting a reference velocity, by successively sweeping a frequency offset corresponding to a reference velocity for providing an offset to a measurement frequency, and performing this sweeping within a frequency range in which the Doppler shift amount is measured.
2. The method for measuring airspeed according to claim 1 , wherein the Doppler shift amount measurement range is limited by taking a true airspeed measurement value of the immediately preceding cycle as a reference velocity.
3. The method for measuring airspeed according to claim 1 , wherein an angle of attack or a sideslip angle of the aircraft is calculated by performing scanning of the transmission direction of the laser beam and two-dimensionally or three-dimensionally expanding an airspeed vector.
4. The method for measuring airspeed according to claim 2 , wherein an angle of attack or a sideslip angle of the aircraft is calculated by performing scanning of the transmission direction of the laser beam and two-dimensionally or three-dimensionally expanding an airspeed vector.
5. The method for measuring airspeed according to claim 3 , wherein turbulence measurements are conducted by considering a difference between an immediately preceding average airflow velocity vector and the newest airflow velocity vector as a turbulence component.
6. The method for measuring airspeed according to claim 4 , wherein turbulence measurements are conducted by considering a difference between an immediately preceding average airflow velocity vector and the newest airflow velocity vector as a turbulence component.
7. The method for measuring airspeed according to claim 1 , wherein an equivalent airspeed is calculated by multiplying a true airspeed by √(ρ/ρ0) in measuring the true airspeed, where ρ is an air density at a flight altitude and ρ0 is an air density at a sea level altitude.
8. The method for measuring airspeed according to claim 2 , wherein an equivalent airspeed is calculated by multiplying a true airspeed by √(ρ/ρ0) in measuring the true airspeed, where ρ is an air density at a flight altitude and ρ0 is an air density at a sea level altitude.
9. The method for measuring airspeed according to claim 1 , wherein a sound velocity a (m/s) is determined from the following equation by using an outside air temperature t (° C.) at a flight altitude, and a Mach number is calculated as a ratio of a true airspeed to the sound velocity a
a= 340.4×√{(273 +t )/288}.
10. The method for measuring airspeed according to claim 2 , wherein a sound velocity a (m/s) is determined from the following equation by using an outside air temperature t (° C.) at a flight altitude, and a Mach number is calculated as a ratio of a true airspeed to the sound velocity a
a= 340.4×√{(273 +t )/288}.
11. The method for measuring airspeed according to claim 2 , wherein the Doppler shift amount measurement range is limited by taking a measurement value determined by another airspeed measurement means instead of the true airspeed measurement value of the immediately preceding cycle as a reference velocity.
12. The method for measuring airspeed according to claim 3 , wherein the Doppler shift amount measurement range is limited by taking a measurement value determined by another airspeed measurement means instead of the true airspeed measurement value of the immediately preceding cycle as a reference velocity.
13. The method for measuring airspeed according to claim 5 , wherein the Doppler shift amount measurement range is limited by taking a measurement value determined by another airspeed measurement means instead of the true airspeed measurement value of the immediately preceding cycle as a reference velocity.
14. The method for measuring airspeed according to claim 7 , wherein the Doppler shift amount measurement range is limited by taking a measurement value determined by another airspeed measurement means instead of the true airspeed measurement value of the immediately preceding cycle as a reference velocity.
15. The method for measuring airspeed according to claim 9 , wherein the Doppler shift amount measurement range is limited by taking a measurement value determined by another airspeed measurement means instead of the true airspeed measurement value of the immediately preceding cycle as a reference velocity.