Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2000-03-23
2001-11-20
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C356S027000, C356S028000, C356S029000, C356S327000
Reexamination Certificate
active
06320651
ABSTRACT:
This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 199 13 049.3, filed on Mar. 23, 1999, the entire disclosure of which is incorporated herein by reference.
INCORPORATION BY REFERENCE
The disclosure of U.S. Pat. No. 3,528,741 (Benson et al.), issued on Sep. 15, 1970; U.S. Pat. No. 4,988,190 (Miles), issued on Jan. 29, 1991; U.S. Pat. No. 5,029,999 (Kremer et al.), issued on Jul. 9, 1991; U.S. Pat. No. 5,088,815 (Garnier et al.), issued on Feb. 18, 1992; and U.S. Pat. No. 5,267,010 (Kremer et al.), issued on Nov. 30, 1993 is incorporated herein by reference
FIELD OF THE INVENTION
The invention relates to a method and apparatus for determining a velocity from a Doppler frequency shift of a relocated or backscattered laser signal.
BACKGROUND INFORMATION
The Doppler lidar method is the classic method for determining velocities by measuring backscattered or reflected laser light. The wind lidar method, for example, detects a laser signal that is backscattered by air molecules or aerosols. If the air molecules or aerosols are moving under the influence of wind, the backscattered or reflected laser signal is frequency-shifted as a function of the wind velocity. The resulting frequency difference or so-called Doppler frequency between the frequency of the emitted or reflected laser signal or radiation and the backscattered signal or radiation is measured and provides a direct measure of the line-of-sight (LOS) velocity of the wind, also known as radial wind velocity.
In the conventional Doppler lidar method the Doppler frequency is measured with the aid of a local oscillator (LO) for example in the form of an auxiliary laser and the LO signal is heterodyned onto the received signal. The resulting beat frequency corresponds to &Dgr;&lgr; or Doppler frequency, which is the basis for determining the wind velocity.
A useful beat signal with this type of coherent detection, however, is generated only when a required coherence condition between the received signal and the LO signal is satisfied, i.e. when a constant phase relationship exists between the two signals. The coherence condition might not be satisfied due to various reasons. For example, it can be disturbed by depolarization or by polarization mismatch of both the emitted and the received signals. Further, the coherence condition may not be satisfied due to wave-front disturbances resulting from a poor quality of the optical components, misalignment (wave-front tilt), or misfocussing (wave-front bending). Interference effects in the backscattered signal by so-called “speckles” can also prevent satisfying the coherence condition. If the coherence condition is not satisfied, the usable signal recedes or diminishes rapidly or even disappears completely. A further disadvantage of the conventional Doppler lidar method is that, the shorter the wavelength, the more difficult it is to satisfy the coherence condition. This is disadvantageous because the backscattering in the atmosphere increases as the wavelength decreases.
Due to the difficulties of satisfying the coherence condition when using short wavelength signals, attempts have been made to replace the coherent detection method with a non-coherent method. One method that has been suggested is the so-called “edge” technique which uses a narrow band optical filter that is so constructed that the original wavelength of the laser lies precisely on the edge of the filter transmission curve. The filter then transmits precisely 50% of the received signal. If the laser frequency of the received beam or signal shifts in one direction, transmission increases; if the laser frequency shifts in the other direction, transmission decreases. The increase or decrease of the transmission indicates the direction and the magnitude of the Doppler frequency shift of the backscattered laser beam, relative to the emitted original laser beam. The wind velocity can then be determined based on the frequency shift.
The “edge” technique, however, runs into great difficulties when put into practice. For example: the filter band width must be extremely narrow in order to provide the desired velocity resolution of approximately 1 m/s in the visible or near infrared range. Moreover, relatively expensive equipment is required in order to precisely tune the filter edge and to provide a consistently stable match of the filter to the laser wavelength. Further, the evaluation and calibration of the absolute value of the filter transmission entail significant difficulties. Yet another drawback of the “edge” technique is that it can be used only with a stationary measuring platform. As soon as the measuring platform moves, the frequency of the backscattered signal shifts relative to or about the velocity of the measuring platform and, under unfavorable conditions, the backscattered signal falls outside the filter transmission curve. Thus, if the measuring platform moves, it is critical that its velocity be known very precisely and that a method be applied that will exactly tune or track the filter curve to the measuring platform velocity. For this reason, the “edge” technique according to the current state of the art appears to be impractical for use in aircraft or satellites.
OBJECTS OF THE INVENTION
In view of the above it is the aim of the invention to achieve the following objects singly or in combination:
to provide a method and apparatus for measuring velocity that will eliminate the disadvantages of coherent detection methods and of the “edge” filter technology;
to evaluate the polarization status of the returned laser radiation as a basis for the Doppler shift and respective velocity measurements;
to avoid the dependency of the velocity measurements on the satisfaction of the above discussed coherence conditions;
to avoid the effects of signal dissipation due to absorption and destructive interference which are troublesome in conventional optical filter techniques;
to substantially increase the measuring range and resolution for measuring velocities especially wind velocities; and
to use a moving platform for mounting the Doppler measuring equipment without the need for any tracking or follow-up adjustments.
SUMMARY OF THE INVENTION
The foregoing objects have been achieved according to the invention by providing a method for determining the Doppler shift, for the purpose of a velocity measurement, of a reflected or backscattered laser signal from a change in the polarization state of the backscattered or received signal. According to the present method, the change in the polarization state of the received signal, following its passage through a polarization dispersive element, is determined from an intensity shift of two beams of radiation including for example infrared radiation, that are obtained by splitting the received signal and processing the two respective split beam signals in a computer for obtaining the corresponding velocity information based on the Doppler shift.
According to the method of the invention, the backscattered laser beam is first guided through a polarization-dispersive element, e.g. through a birefringent crystal or through a medium that rotates the polarization, and the resulting signals are then analyzed with respect to the polarization state. The deviation of the polarization from the expected polarization value of the original wavelength &lgr;
0
of the emitted beam or radiation provides a measure for the frequency deviation of the received signal from the emitted signal and thus for the Doppler shift caused by the LOS velocity of the respective reflecting and/or scattering medium. In a practical embodiment the emitted signal SS with the original wavelength &lgr;
0
is directly passed through the reception channel to function as a reference signal for ascertaining the polarization deviation between the polarization of the backscattered signal ES and the polarization of the reference signal by comparing these polarizations in a computer. The reference signal signifies or represents reflection from a stationary object, while the sign
Manhart Sigmund
Schmidt Elke
Andrea Brian
Daimler-Chrysler AG
Fasse W. F.
Fasse W. G.
Tarcza Thomas H.
LandOfFree
Method and apparatus for determining a velocity does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for determining a velocity, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for determining a velocity will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2571044