DE19837256C1 - Method to determine wind speed with digitalized analog test signals; applies Doppler frequency displacement irradiated from earth satellites to identify wind speeds from the proper motion of particles moved by gust of wind - Google Patents

Abstract

An analog test signal is digitalized by alias-spot scanning to detect Doppler frequency displacement in laser light irradiation reflected from particles and arising from an object in motion such as an earth satellite. The test signal also detects Doppler frequency shifts in a digital calculation based on the laser light irradiation. Higher frequency displacement or cyclic fold-over determined by a lower spot scan rate possibly far below a double Nyquist frequency is recalculated at a frequency actually occurring prior to the spot scanning. This occurs through reflecting the test signal already in digital form on the Nyquist frequency.

Description

The invention relates to a method for determining the Wind speed from its own movement with the Wind field moving scattering particles, their speed by measuring the Doppler frequency shift one with egg ner conical scanning movement from one to a moving one System housed laser beam emitted and sent the particles towards one after the heterodyne or Homodyne receiver working backscattered laser Light radiation of constant frequency is determined so that due to the conical scanning movement of the radiation in a measuring arrangement a sinusoidal measuring signal Speed components is obtained, the amplitude a measure of the wind force and its phase shift The measure of the wind direction is before the detection of the Doppler shift and a subsequent digital loading Calculation of the wind field using an analog / digital converter is digitized.

Such methods are from US 5 724 125 A, DE 38 20 654 A1 or also known from US 5 367 371 A.

In a method for determination known from DE 40 13 702 C2 measurement of the wind speed, the analog measurement signal with sampled at a frequency that is a multiple of the highest occurring Doppler frequency, while in WO 97/06525 A1 the sampling frequency 5 to 10 times before is preferably 2.5 times the signal frequency.

With the laser Doppler method the wind speed is said to be from the inherent movement of the scattering particles, d. H. from the smallest, a so-called aerosol-forming particle of dust that forms with move the wind field, be determined.  

For this purpose, a laser light radiation of constant frequency is sent out, scattered back on the particles in the direction of a laser radiation receiver and registered there. If the scattering particles have a speed component in the direction of beam propagation (Line of Sight; LOS), the backscattered radiation has a characteristic frequency shift f _D due to the known Doppler effect:

where v _{LOS is} the velocity component in the beam direction and λ is the wavelength of the laser radiation.

A speed component of 1 m / s results in a Doppler shift of approx. 200 kHz for CO ₂ laser radiation (wavelength λ = 10.6 µm).

Since the frequency of the laser radiation (approx. Terahertz) is too high tuning frequency exceeds a few powers of ten a direct measurement is not possible.

The laser light received is therefore a laser radiation similarly high (heterodyne reception) or equal (homodyne frequency) (so-called local oscillator) like the Sen that overlays. This gives you a beat frequency, the in the order of magnitude of the frequency ver shift lies.

In order to determine the direction and speed of the wind field, the measuring volume is additionally illuminated in a known manner under different directions. In this context, reference is made to FIGS . 1 and 2.

Fig. 1 shows a conical scan of the wind field WIND at an elevation angle ϕ, where Θ is the instantaneous phase angle of the laser scanning beam. 1 on a Ke gelmantel in steps or continuously rotating outer radiation of the laser radiation according to FIG. 1 gives a sinusoidal profile of the speed components according to FIG. 2, wherein the amplitude of the wind force corrected via the elevation angle 9 and the phase shift of the wind direction corresponds.

The determination of the Doppler shift (frequency analysis) and the subsequent calculation of the wind field (sine adaptation) is carried out by electronic data processing (IT). To do this, it is necessary to use the analog voltage values digitize existing measurement data signals.

The most important parameter of the analog / digital converter to be used for digitizing the measurement data signals is, in addition to the voltage swing, the sampling rate, which is characteristic of the maximum or uniquely determined frequency. The so-called sampling theorem, which is also known as the Nyquist theorem, must be observed:

f _max ≦ 0.5 f _ADC . (2)

For the sinusoidal time signal (frequency beat), this means that there are at least two sampling points per oscillation period. If the signal frequency exceeds the limit value f _max , a full period is no longer recorded and the resulting frequency appears smaller (cyclic folding or aliasing effect).

Fig. 3 shows an example in the upper representation of a sinusoidal signal (signal frequency 86 Hz), in the middle representation the sampled time signal (sampling frequency 100 Hz, ie maximum detectable frequency 50 Hz) and in the lower representation the spectrum of the sampled signal . The resulting signal shows a frequency that is too low both in the time domain (middle representation in FIG. 3) and especially in the spectrum (lower representation in FIG. 3).

The actual frequency of 86 Hz in the example given is folded back to a low frequency by sub-sampling, in the example to 14 Hz (see the illustration below in FIG. 3). As a vivid further example of the aliasing effect, backward rotating wheels, in particular spoke wheels, are on moving vehicles, e.g. B. on a Postkut cal, called in movies (scanning 16 to 64 frames per second).

As is well known, the aliasing effect can be achieved with digitali sizing of electrical signals by using an An ti aliasing filters, i. H. a low pass filter with a Pass frequency less than half the sampling frequency is to avoid. In addition, the low-pass filter Bandwidth of the electrical signal limited. The Si Signal / noise ratio is improved because the detector roughness without anti-aliasing filter also in the sample area is folded back richly.

The frequency range is thus limited to half the sampling rate according to equation (2). For a stationary laser Doppler system, e.g. B. in a ground measuring station, for example, the measuring range when using a 10 MHz analog / digital converter is limited to v _LOS ≦ ± 26.5 m / s at a laser wavelength of 10.6 µm.

In a moving system that z. B. on an airplane or Satellite is attached, superimposed on top of that Measuring range, the own movement which is in or at beam position  counter to the direction of flight leads to sampling frequencies that are not technically feasible.

An earth satellite typically has speeds of 7 up to 10 km / s above ground. This makes maximum Doppler servers shifts from 1 to 2 GHz, projected in the beam direction, generated because here the wind field is also scanned conically becomes. The sampling rate of the digitization of the measurement signal performing analog / digital converter must therefore at least 2 up to 4 GHz.

As a result of the high sampling rate, one also gets accordingly enough samples, i. H. the analog / digital converter must have a very high memory depth. In the previous example the required memory depth would be 450 to 900 kW words each Laser shot for a measurement volume of only 30 km. On the market located analog / digital converters meet these two For changes only according to the relation either very quickly (1 to 2 GHz), but then only with a small memory depth (typically 512 words), or vice versa.

The invention has for its object in one of moving system, especially from an earth satellite existing laser Doppler measurement method of the type mentioned to create a way of using yourself Common analog / digital converter digitizing the Measurement signals for the later calculation of the wind speed can be carried out.

According to the invention, which relates to a method of the beginning referred type, this object is achieved in that in the measuring arrangement during the digitization of the analog measurement signals in the analog / digital converter with a so-called Ali  as sampling is carried out, i.e. with a sampling rate that under the Ab fulfilling the sampling theorem (Nyquist theorem) sampling rate, and that due to the lower sampling rate, which can be far below the double Nyquist frequency, certain shift of the higher frequency, d. H. the cycli c folding that actually existed before scanning frequency is calculated back, which is reflected by mirroring the measurement signal already available in digital form at the Nyquist Frequency.

The method according to the invention is thus characterized that high frequencies using the alias effect can be detected despite a sampling rate that is too low per se nen. The lower sampling rate, which is far below the dop pelt Nyquist frequency (sampling theorem) may be determined the shift of the higher frequencies (convolution). The back calculation of the actual frequency before the Ab sensing is done easily by EDP, since the data is already digi valley are available.

An advantageous development of the method according to the Er invention consists of a bandpass filtering, which is based on a mitt lere, detected by the measuring arrangement and thus known Fre is centered and a band adapted to the residual signal has wide. It is thus guaranteed that no overload alias frequencies occur, i.e. a non-rever Sible mixing of the folded spectral ranges takes place. By the use of the bandpass filter is also advantageous the signal-to-noise ratio is considerably improved sert.

The method according to the invention should be designed that both the detection bandwidth and the bandwidth  of the analog / digital converter (aperture time as short as possible) corresponds to the highest frequency to be expected.

In addition, the slope of the bandpass filter should be one reasonable compromise corresponding to the measurement requirements between steep slope and change in group duration exhibit.

In addition, in the method according to the invention, con constant, gradual frequency shifts exi bull.

This means that signals are suitable which have, for example, a constant shift by the local oscillator on the receiving side or by a step-by-step scanning of the atmosphere. On the other hand, the use of continuous scanning ( Fig. 1) would be less suitable.

The method according to the invention is described below for a Measuring arrangement arranged on a satellite platform for Measurement of the wind speed using drawings tert. Show it:

Fig. 1 is a schematic diagram of the conical scanning of the wind field already explained;

FIG. 2 is a function illustration, also already explained, of the speed components resulting from the conical scanning according to FIG. 1;

Fig. 3 in three mutually recorded, already described representations, the aliasing effect due to an insufficient sampling rate;

Fig. 4 frequency ranges of the possible Doppler shifts in the satellite-based method according to the inven tion;

Fig. 5, the reflection of the higher frequencies at too low a sampling rate ,;

Figures 6 and 7, the geometric or the weft arrangement be apt scheme. A satellite laser Doppler measurement using the method according to the invention it, and

Fig. 8 is a according to the method according to the invention made alias sampling for four different shot positions.

The reduction in the sampling rate according to the invention by utilizing the alias effect is explained below on the basis of the spectrum representations (f = frequency) in FIGS. 4 and 5.

The signal at the output of the detector of the measuring arrangement contains the following frequencies:

• - the own movement of the satellite platform,
• - the earth's rotation
• - The group speed of the scattering particles.

These frequencies add up in the direction of the laser beam (Vek gate sum of the components in the direction of beam propagation). The  actual useful signal, namely the Doppler shift due to the speed of the scattering particles, the the smallest spectral latitude from a satellite.

In the previously mentioned example, this is a spectral width of 10 MHz for a measuring range of v _LOS ≦ ± 26.5 m / s. This useful signal is shifted in the frequency domain by the earth's rotation (approx. 100 MHz, depending on the geographical latitude of the satellite) and the satellite's own movement. FIG. 4 shows an outline of the spectral regions involved (only positive frequencies, not to scale).

The largest frequency shift (1 to 2 GHz) always occurs when the measuring axis, i.e. the laser beam direction LOS, points in or opposite to the axis of movement of the satellite. This signal would have to be according to the sampling theorem with a sampling rate f _ADC , which is twice as high as the maximum possible frequency, in the analog / digital converter according to the measuring arrangement

f _ADC ≧ 2. (f _{max, satellite} + f _{max, earth rotation} + f _{max, useful signal} ) (3)

can be digitized, in this example approx. 2 to 4 GHz.

If this signal is now digitized with a lower sampling rate, then those frequencies which have higher frequency components than equation (2) are in the frequency domain at the Nyquist frequency f _NYQ corresponding to half the sampling rate. mirrored.

Fig. 5 shows the same frequency components as Fig. 4, but digitized with a sampling rate that is half as large as that which would actually be required to fulfill the sampling theorem. This mirroring shown in FIG. 5, or also cyclic convolution, only occurs if, in addition to the violation of the sampling theorem, no anti-aliasing filter (with a suitable blocking frequency) is used.

This signal distortion that it would normally suppress applies with the correct sampling rate and with one correctly dimensioned anti-aliasing filter, can now be Trap of the moving system, as is the case with the Earth satellite Case is to be used with higher frequency components to measure lower sampling rate.

The actual useful signal plus the frequency shift due to the earth's rotation is much narrower than that Frequency shift due to satellite movement (about the factor 10). Is the approximate measuring axis (position of the sen de- and receiving mirror) depending on the axis of movement known of the satellite, so it is possible to fold the Si gnal to scale back again.

For the digitized values shown in FIG. 5, this means:

• - The direction of the measuring beam is known (here roughly in Direction or opposite to the path axis of the Satellites)
• - The useful signal plus the frequency shift due to the earth rotation is therefore before digitization in a frequency range that is well above the Nyquist frequency f _Nyq . lies
• - The digitized signal must therefore only at the Nyquist frequency f _NYq . be mirrored to get the actual frequency.

Since the signal is already in digital form, the whole process easily with the help of electronic data accomplish processing (EDP).

An application of the method according to the invention for wind speed measurement in practice is described below with reference to FIGS. 6 and 7.

The application of alias scanning to a real measurement problem is illustrated using the example of a laser Doppler system stationed in space. Fig. 6 shows schematically and not to scale the measurement geometry with the satellite SAT, which moves vertically above an earth orbit TRACK on a path indicated by the arrow P and from the laser beam shots in the, viewed from the satellite SAT, lying on a conical surface Directions of propagation LOS to the earth ERD are given.

Of the scattering parties in such a direction the laser radiation is scattered back and in a measuring arrangement examined for Doppler shift to get out of it to determine the wind speed.

In Fig. 7, the shot arrangement belonging to the geometry according to Fig. 6 on the ground ERD, viewed from the receiver of the satellite SAT, is shown. The wind field should be measured at eight different angles, which are fixed (so-called step scan).

In the shot positions shown in Fig. 7 there are de frequency shifts (laser wavelength here 9.11 µm, frequencies in MHz) due to the own movement of the satellite (Table 1, second column).

The following Table 1 shows the eight shots 1 to 8 Frequency shift due to the Satelli's own movement ten (second column), plus frequency range due to earth rotation (third column), plus constant frequency difference (offset) between the transmitter laser and the local oscillator LO of the receiver (fourth column), here 40 MHz.

Table 1

The Earth's rotation is shown in Table 1 above for takes into account a polar orbit so that maximum frequency shifts occur when the satellite is above the Equator is located and the measuring beam LOS to the side of the flight direction device is transmitted (here ± 53 MHz).  

The useful signal also has a bandwidth of ± 11 MHz for a maximum detectable horizontal wind speed of ± 100 m / s.

In the simplest case where there is no frequency difference (offset) between the transmitter laser and the local oscillator LO, i.e. Ho there is only two different fre sequence shifts due to the movement of the satellite SAT: 750 MHz and 216 MHz. The sign of the shift (forward or backward to the path movement) exists re al. but is not perceived by the detector, since it only the range of services of the superimposed field of Emp capture radiation and local oscillator detected. In a different way presses, there will be no phase relationship between receive beam tion and local oscillator measured, since the individual frequencies in Terahertz (THz) range.

Case 1: 216 MHz

The frequency shift due to the satellite movement is minimal here (sideways to the orbit movement), but the possible shift due to earth rotation is maximal. The shifted spectrum therefore extends from 152 MHz to 280 MHz. If the detector signal is scanned with only 280 MHz when the measuring beam is in position 1, 2, 5 or 6, the spectrum is in the range 0 MHz (before scanning 280 MHz) to 128 MHz (previously 152 MHz) mapped, i.e. once mirrored at the Nyquist frequency (140 MHz). In this case, reference is made to FIG. 5.

Case 2: 750 MHz

The spectrum extends from 716 MHz to before sampling 784 MHz (Earth rotation less for polar satellite and beam direction in or against the satellite flight direction) for positions 3, 4, 7 and 8. A scan at 200 MHz tet the spectrum three times so that it is now in a range of 16 MHz (before sampling 784 MHz) to 84 MHz (before 716 MHz) appears.

There is also a constant frequency difference between transmitter laser and local oscillator, so there is hetero Dyn reception, which is applied to the sign of the wind to be able to determine the field better, it follows from the previously specified table 1 (fourth column) four different cases.

In the following, the advantageous development of the Ver driving according to the invention provided bandpass filtering he purifies, which among other things to suppress the high frequency th noise in the alias Ab provided according to the invention groping serves.

The previously mentioned anti-aliasing low-pass filter prevents not just the effect of refolding high-frequency Signal components, but for this reason also improves that Signal to noise ratio. Through the folding operation not only the useful signal, but also the entire detector noise reflected in a lower frequency range. Consequently add up all noise components, i.e. noise components, which already exist in this spectral range even without reflection with the tie caused by the folding frequency components.  

To improve the signal-to-noise ratio the implementation of the method according to the invention in counter Set to the low-pass anti-aliasing filter bandpass filter puts. It also ensures that finally the folded frequency range is detected.

The bandpass filters are exactly on the respective Fre frequency shift due to satellite movement (see table le 1, second column) plus a possible local oscillator Frequency shift centered. The upper and lower limits frequency must be of the spectral latitude (earth rotation plus useful gnal) of the residual signal.

For the use of real bandpass filters (3 dB attenuation at the cut-off frequencies, 7th order filter), the sampling frequencies must be adjusted accordingly. The table 2 below shows the resulting sampling frequencies when using bandpass filters corresponding to the shot position for all four cases, that is to say with heterodyne reception, and the basic data of these bandpass filters.

Table 2

Fig. 8 shows for the alias scanning used the Fre frequency space with the respective spectra and scanning frequencies from Table 2 for the four different shot positions provided there. Fig. 8 contains in the upper left corner a legend regarding the frequency bands that the bandpass filter comprises.

In general, the bandpass filter must always be known Frequency be centered, the knowledge of this Fre results from the measuring arrangement and thus this frequency is also detected, e.g. B. the beam direction in relation to Direction of propagation in the moving system or the difference frequency between transmitter laser and local oscillator.

In the example given, the maximum sampling rate is loud Table 2 at 520 MHz. The memory depth of the for the Digita lization to be used analog / digital converter must then et wa be 120 kwords per shot.

The comparison with the system theoretical values, the demge compared to a very high sampling rate of about 1.8 GHz and a shows a very large memory depth of about 400 kW words the alias scan gain according to the procedure ren according to the invention, which advantageously the Mög correspond to the state of the art the and commercially available analog / digital converters brings.

Claims (6)

1. A method for determining the wind speed from the egg movement of scattering particles moving with the wind field, the speed of which is measured by measuring the Doppler frequency shift of a cone-shaped scanning movement of a laser beam source housed on a moving system and sent to the particles in the direction to a receiver working according to the heterodyne or homodyne method, backscattered laser light radiation of constant frequency is determined, so that due to the conical scanning movement of the radiation in a measuring arrangement, a sinusoidal measuring signal of the speed components is obtained, the amplitude of which is a measure of the wind strength and its phase shift A measure of the wind direction and that is digitized before the detection of the Doppler shift and a subsequent digital calculation of the wind field by means of an analog / digital converter, characterized in that ß in the measuring arrangement in the digitization of the analog measurement signal in the analog / digital converter with a so-called alias scanning is used, that is, with a sampling rate that is below the sampling theorem (Nyquist theorem) and the sampling rate the lower sampling rate, which may be far below twice the Nyquist frequency, be certain shift of the higher frequency, ie the cyclic convolution to which the frequency actually present before the sampling is calculated back, which is reflected by mirroring the measurement signal already in digital form the Nyquist frequency he follows. 2. The method according to claim 1, characterized by a band pass filtering, based on a medium, through the measuring arrangement detected and thus known frequency is centered and a has bandwidth adapted to the residual signal. 3. The method according to claim 1 or 2, characterized in that the detection bandwidth of the highest expected Fre quenz corresponds. 4. The method according to any one of the preceding claims, characterized characterized that the bandwidth of the analog / digital wall corresponds to the highest frequency to be expected. 5. The method according to claim 2, characterized in that for Bandpass filtering a filter with an edge steepness ver is used, the one corresponding to the measurement requirements Compromise between steep slopes and changing groups has term. 6. The method according to any one of the preceding claims, characterized characterized by constant, step-by-step frequency shifts, especially through a gradual, on egg a conical path (Step Scan) of the At atmosphere or by a constant shift through the Lo caloscillator in the receiver.