Apparatus and method for compensating for doppler expansions

Abstract

An apparatus (100) for compensating for weather-independent Doppler expansions in radar signals of a weather radar system (200) is disclosed. The device comprises: a receiving device (110) for receiving a representation (50) of the radar signals, a calculation device (120) and a compensation device (130). The representation includes pixels of a range Doppler matrix. The calculation device (120) is designed to calculate azimuth angles (Azi) for the pixels (75) by means of fine bearing. The compensation device (130) is designed to correct weather-independent Doppler shifts for the pixels (75) based on the calculated azimuth angle (Azi; AziMopu) and thus to compensate for the weather-independent Doppler expansions and to provide them as a compensated representation (150).

Claims

1. An apparatus for compensating for weather-independent Doppler expansions in radar signals of a weather radar system, the apparatus comprising: a receiving device for receiving a representation of the radar signals, wherein the representation comprises pixels of a range Doppler matrix; a calculation device which is configured to calculate azimuth angles (Azi) for the pixels by fine bearing; and a compensation device which is designed to correct weather-independent Doppler shifts for the pixels based on the calculated azimuth angle (Azi; AziMopu) and thus to compensate for the weather-independent Doppler expansions and to provide them as a compensated representation.

2. The apparatus according to claim 1, wherein the calculation device is designed to use a phase monopulse method for fine bearing.

3. The apparatus according to claim 2, wherein the compensation device is designed to take into account one or more of the following causes for the weather-independent Doppler expansions when compensating: an intrinsic movement (v) of the weather radar system, a beam width, an offset angle (AziBeam).

4. The apparatus according to claim 3 wherein the weather radar system is designed to detect turbulent weather in a spatial area and to take a rough bearing (AziBeam), and wherein the calculation device is designed to calculate the azimuth angle (AziMopu) for each pixel of the spatial area relative to the rough bearing (AziBeam) via the fine bearing, and the compensation device is designed to compensate for the intrinsic movement (v) for each pixel of the spatial region.

5. A weather radar system having: an evaluation unit which is designed to receive radar signals and to generate the representation in the form of the range Doppler matrix based on the radar signals; and an apparatus according to claim 1.

6. The weather radar system according to claim 5, wherein the evaluation unit is designed to generate the representation using a pulse Doppler method.

7. The weather radar system according to claim 5, wherein the evaluation unit is designed to generate the representation in such a way that each pixel forms a Doppler value range and a distance value range.

8. The weather radar system according to claim 5, which further comprises an antenna system having an antenna half-width that is greater than 3° or greater than 5° or greater than 10°.

9. The weather radar system according to claim 8, wherein the antenna system is pivotable through a pivot angle of a maximum of 40°.

10. The weather radar system according to claim 5, wherein the evaluation unit is designed to initiate a rough bearing based on a current orientation of the antenna system and an antenna characteristic, and wherein the calculation device is designed to initiate the fine bearing within the rough bearing.

11. The weather radar system according to claim 5, wherein the evaluation unit is further designed to evaluate the compensated representation and, based thereon, to cause at least one of the following reactions: Representing turbulent weather in the spatial region on a display, Warning of turbulent weather when a threshold value is exceeded, Triggering an evasive maneuver for the aircraft to fly around the spatial area.

12. Aircraft with a weather radar system according to claim 5.

13. A method for compensating for weather-independent Doppler expansions in radar signals of a weather radar system, wherein the method comprises: Receiving a representation of the radar signals, wherein the representation comprises pixels of a range Doppler matrix; Calculating azimuth angles (Azi) for the pixels by fine bearing; Correcting weather-independent Doppler shifts for the pixels based on the calculated azimuth angle (Azi; AziMopu); Compensating for the weather-independent Doppler expansions; and Providing a compensated representation.

14. The apparatus according to claim 1, wherein the compensation device is designed to take into account one or more of the following causes for the weather-independent Doppler expansions when compensating: an intrinsic movement (v) of the weather radar system, a beam width, an offset angle (AziBeam).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The embodiments of the present invention will be better understood from the following detailed description and the accompanying drawings, which should not be construed as limiting the disclosure to the specific embodiments but are for explanation and understanding only.

(2) FIG. 1 shows an embodiment of an apparatus for compensating for weather-independent Doppler expansions.

(3) FIG. 2 shows a possible application scenario of the apparatus from FIG. 1.

(4) FIG. 3 illustrates further details for performing the fine bearing according to the embodiments.

(5) FIG. 4 shows an example of a simulation for radar signal processing.

(6) FIG. 5 shows an exemplary top view (azimuth plane) of the individual pixels in the recorded angular space area.

(7) FIG. 6 shows for example, a range Doppler matrix as used in the embodiments to represent the radar signals.

(8) FIG. 7 shows a flow diagram for a method for compensating for weather-independent Doppler expansions according to the embodiments.

DETAILED DESCRIPTION

(9) FIG. 1 illustrates an embodiment for an apparatus 100 for compensating for weather-independent Doppler expansions in radar signals of a weather radar system 200. The radar signals can be obtained from an antenna system 215 and, after preprocessing, can be forwarded into a desired representation. The weather radar system 200 can also comprise antennas 210, 220 which are suitable for fine bearing (for example by means of a phase monopulse method).

(10) The apparatus 100 comprises a receiving device 110 for receiving the representation 50 of the radar signals, wherein the representation comprises pixels of a range Doppler matrix. The apparatus 100 further comprises a calculation device 120 for performing a fine bearing of the azimuth angle for the pixels, which for this purpose can control the weather radar system 200, for example, so that it carries out the fine bearings for the pixels and sends corresponding data back to the apparatus 100. It is also possible that another system or other antennas that are not part of the weather radar system are used for fine bearing. Finally, the apparatus 100 comprises a compensation device 130 which is designed to correct weather-independent Doppler shifts for the pixels based on the calculated azimuth angle and thus to compensate for the weather-independent Doppler expansions. After the compensation, a compensated representation 150 can be provided.

(11) Embodiments can, in particular, calculate the exact azimuth angle for each pixel shown (e.g. for a distance) using the fine bearing and use formulas to compensate for the airspeed of the flying object and the offset from the straight line direction, so that the Doppler widenings provide more accurate results.

(12) FIG. 2 shows an example of an application of the apparatus 100 according to an embodiment for an aircraft 300 such as a drone (or in an airplane) that can have a considerable airspeed v in front of a potentially dangerous weather scenario 70, which should be avoided under radar control if necessary (for example along an evasive course 350).

(13) For this purpose, a radar system 200 having, for example, two or more antennas 210, 220 is installed on an aircraft 300, which antennas are able to calculate a correction of the azimuth angle AziBeam via a fine bearing. For this purpose, the radar system 200 is in particular designed to emit the radar beam AziBeam with an antenna half-width 240 into the spatial region 70 and to receive signals from there. The spatial region 70 includes, for example, a turbulent weather situation which, as a result of the relative speed of the aircraft 300, not only leads to a Doppler shift of the radar signals, but can also lead to a widening (the Doppler expansion).

(14) The aircraft 300 moves, for example, at its airspeed v and if the recorded spatial area 70 should have a dangerous weather situation, the flight object 300 can fly the evasive maneuver 350 based thereon to avoid flying through it. The evasive maneuver may be carried out purely automatically, controlled by radar.

(15) One of the main features of critical rain areas is the strongly changing wind speed in the rain area, which is referred to below as “turbulence.” This turbulence can be displayed with some larger on-board weather radar systems due to the different Doppler speeds in the reflections, and highlighted according to the strength (e.g. through color). However, this only works satisfactorily with small antenna half-widths (e.g. in the range of 3°). In radar devices having a larger antenna half-width, the half-width already inherently creates a Doppler widening of the reflected signals. This makes it difficult to detect the turbulence. Likewise, a Doppler expansion occurs in lateral offsets—due to the intrinsic movement and the width of the beam (beam width), and only because of the dynamics and not because of the scenario. This undesirable Doppler expansion masks the actual effect that is to be detected.

(16) In order to avoid this negative effect, embodiments use the monopulse capability of modern devices in order to compensate for the above-mentioned inherent beam width-related Doppler expansion to a certain extent within the scope of the signal processing. This reduces the undesired effect, so that the Doppler expansion due to the weather can also be reliably measured at larger offset angles and evasive maneuvers can be carried out in good time.

(17) In order to determine the critical weather situation, a fine bearing is carried out according to the embodiments, which fine bearing calculates the exact position of an exemplary pixel 75 within the antenna half-width 240. For this purpose, a phase monopulse method can be used, for example, in which one or more transmission pulse(s) are emitted and at least two receiving antennas 210, 220 receive reflected received signals in order to calculate a precise angle AziMopu of the pixel 75 as a fine bearing from a phase difference of the reflected received signals. The fine bearing can, for example, be calculated relative to a rough bearing AziBeam (=radiation direction of the antenna signal). The direction of view (rough bearing AziBeam) itself is also known.

(18) When the precise azimuth angle AziMopu of the pixel 75 has been determined, the radar signals can be compensated using the known relative speed v and provided as a compensated representation 150 of the radar signals.

(19) According to the embodiments, the radar beam is emitted with a beam width 240 in an azimuth angle AziBeam and is pivotable with a pivot angle of, for example, a maximum of 40 degrees (or in a range of 30 to 50 degrees measured from the airspeed vector v). The received radar signals are analyzed for each beam angle AziBeam and can be represented in a range Doppler matrix, so that the recorded spatial area includes a large number of pixels 75, each of which corresponds to a distance range (range gate) and a Doppler range (Doppler gate) and in their entirety form the range Doppler matrix. The Doppler range corresponds to the Doppler shift (e.g. frequency shift as a result of the relative movement) for the respective pixel. Since the transmitted signal has a certain frequency, the received signals should, if there is no turbulence, form more or less a frequency line in the range Doppler matrix in the forward direction. As already mentioned, a widening of this line can have various causes and the embodiments compensate for weather-independent effects.

(20) FIG. 3 shows further details for performing the fine bearing in order to determine the azimuth angle AziMopu. For this purpose, FIG. 3 shows a front part of the aircraft 300 having the radar system 200. The radar system 200 comprises a first antenna 210 (for example, a left antenna AL) and a second antenna 220 (for example, right antenna AR), which are spaced apart from one another by a base width 225. The radar system 200 also includes an evaluation unit 230. The evaluation unit 230 comprises a device 231 for calculating differences and a device 232 for forming sums, which are designed to form sums/differences between the received signals of the first antenna 210 and the second antenna 220. In a detection device 233 of the evaluation unit 230, for example, a ratio can be formed from the difference signal and the sum signal, from which the fine bearing angle AziMopu for the pixel 75 is calculated or can be calculated by simple mathematical transformations.

(21) With the help of the exact offset of the vehicle's own airspeed vector v, the Doppler effect can be calculated on the basis of its intrinsic movement and thus the position of each pixel in the range Doppler matrix can be changed (since the Doppler value changes) and thus compensated (the weather-independent component is excluded in the respective Doppler value). As a result, in the ideal case only a Doppler widening of the line will remain, which was caused by the weather.

(22) The exact azimuth angle therefore results from:
Azi=AziBeam+AziMopu,  (1)
wherein AziBeam represents the angle according to the rough bearing and AziMopu represents the correction according to the fine bearing. With this azimuth angle Azi, the Doppler shift fd.sub.DGAirspeed is calculated, which is caused by the known airspeed v:
fd.sub.DGAirspeed=cos(Azi)*2v/lambda,  (2)
wherein lambda is the radar wavelength (e.g. 0.03 m). This means that the purely weather-induced Doppler shift fd.sub.DGcompensated for the relevant Doppler gate (i.e. of the pixel 75) can be compensated for by forming the difference with the pixel value fd.sub.DG in the range Doppler matrix:
fd.sub.DGcompensated=fd.sub.DG−fd.sub.DGAirspeed.  (3)

(23) The Doppler expansion in Doppler gates (AziBeam, beam width, V) results for the situation from FIG. 2 from the difference between the Doppler values at the left and right azimuth edge of the radar beam (see edge lines of range 240):

(24) Difference Doppler=
(Cos(AziBeam−beam width/2)−Cos(AziBeam+beam width/2))*2V/lamda=sin(AziBeam)*beam width*2*V/lamda=AziBeam*beam width*2V/lamda,  (4)
wherein a common Taylor approximation was used for small angles (sin(X)=X applies to small angles). In Doppler gates, this results in the straight line
Doppler difference [DG]=AziBeam(beam width 2V/lamda)/DG width.  (5)

(25) For the exemplary values from Table 1 (see below) the following results:
Doppler difference [DG]=0.8DG*AziBeam [degrees].  (6)

(26) TABLE-US-00001 TABLE 1 gives definitions of variables and representative sample values, as they were also used in the simulation below. Variable units Value [m, s, range or rad, Hz] Meaning value AziBeam Deviation of the beam center [0.35] line of sight from the speed Here 10.7 vector in the azimuth plane Beam width 3 dB one-way beam width in azimuth. 10    Base width Distance partial antenna center  0.115 points in azimuth AziMopu Azimuth in the beam, for which a [−5.5]   fine bearing is taken here by monopulse Azi Azimuth of the RD cell relative to <40 degrees the velocity vector The following applies here: Azi = AlphaBeam + AziMopu Phi Half of the phase between right and [0.90] left channel or phases between total antenna center (between the channels) and individual antenna center D Doppler [0, prf] [0.3000] DG Doppler gate DG width Width of Doppler gate 46.875 each (width) DGMax Maximum DG DG dimension RD matrix R Range (distance) [0, 12000] RG Range gate RG width 150 each (width) RGMax Maximum evaluated RG RG dimension RD matrix v Amount of speed, e.g. of the drone 195 m/s (example) S/N Signal power to noise power e.g. 100 linear or 20 dB AL, AR Signal amplitudes left or right antenna N Noise power S Signal power (here from a Doppler gate) ANxx Noise amplitudes, channels or ANRr, ANRi “direction” real or imaginary ANLr, ANLi e.g. NRr noise amplitude right channel (real part) ErrAzi Calculated measurement error 0.34 monopulse bearing lambda Radar wavelength 0.03

(27) Errors that occur are compensated for in accordance with further embodiments. For example, a fine bearing error of the pixel 75 may arise, which according to the phase monopulse method in azimuth can be expressed as follows:
AziError(Azi)=(lamda/(pi*base width))/(sqrt(2*S/N),  (7)
wherein the base width is the antenna spacing for antennas 210, 220 for the phase monopulse direction bearing and S/N is the signal-to-noise ratio. This error estimate provides a tolerance range of, for example, +/−0.34 degrees for the values as they are mentioned in Table 1.

(28) Another potential error lies in the calculation of the Doppler range (DGate). The width of a Doppler gate corresponds to an angle difference in azimuth in the example from Table 1 of approximately
Beam width/number of DGs in the radar beam=10.7/9=1.2 degrees.  (5)

(29) In addition, a discretization error in degrees must be taken into account, which can be calculated in the example in Table 1 as follows:
deltaDiscreteDG=(beam width/DG)/sqrt(12)=(10.7/9)/3.46=0.34 degrees.  (6)

(30) The bearing error and the discretization error happen to be the same for S/N=20 dB. Both errors together correspond to about ½ the width of the Doppler gate. It is therefore to be expected that the ideal Doppler gate will often be flipped over to the left and right, which can be seen in the simulation of the radar signal processing in FIG. 4 below.

(31) FIG. 4 shows an example of a simulation for the radar signal processing. The dependence of the amplitude sum on the Doppler shifts is shown in detail. The signal-to-noise ratio S/N is approx. 20 dB and a widening of, for example, 9 Doppler gates without compensation was considerably reduced according to the embodiment by means of the compensation by fine bearing. The exemplary signals accumulate in a range between the Doppler gates 17 and 19. In the simulation, each Doppler gate has, for example, a frequency range of 46.875 Hz, with the maximum representing the Doppler shift as a result of the relative movement between the flying object and the rainy area and the widening being a measure of how strongly turbulence is present in the respective spatial area, some of which strengthens or weakens the Doppler shift. The weather-independent contributions to the Doppler widening were calculated out according to the embodiments.

(32) The widening of a total of 9 Doppler gates in this example is reduced to approximately +/−1 Doppler gate by the compensation method according to embodiments, wherein a sigma of +/−4.5 Doppler gates was assumed. Therefore, the embodiments provide an improvement by a factor of approximately 4.

(33) FIG. 5 shows an exemplary top view (azimuth plane) of the individual pixels in the recorded angular space area 240 which is illuminated by the antenna (for example, +/−3 dB sensitivity range of the antenna). The approach geometry can be taken from FIG. 2 and the underlying exemplary values can be taken from Table 1. In this representation 50, each box is a pixel which correlates to a specific distance and an azimuth angle and for which the Doppler shift is calculated.

(34) The number of Doppler cells in the antenna pattern corresponds to the Doppler widening due to the radar's intrinsic movement in the beam. It is similar to a radar with a synthetic aperture (SAR system), but it also includes scenarios in which there are weather areas with added speed vectors of great variability.

(35) FIG. 6 shows, by way of example, a range Doppler matrix 50 as it is used in the embodiments as a representation 50 of the radar signals. The grid pattern of pixels is now not shown in the plan view (as in FIG. 5), but directly in relation to the distance and Doppler areas. The distance ranges (range gates) are shown on the vertical axis and the Doppler ranges (Doppler gates) are shown on the horizontal axis, wherein 9 Doppler gates are used for example, each with a frequency bandwidth of approx. 46 Hz. The exemplary pixel 75 is indicated again, with a distance value 32 and a Doppler value 19. For the example used, this means that the spatial area to the pixel is 32 distance gates of each 150 m wide (i.e. approx. 4.8 km distance) and has a Doppler shift of 19 Doppler gates of each approx. 46 Hz (i.e. a Doppler shift of, for example, 874 Hz), wherein the exemplary radar wavelength is 3 cm.

(36) FIG. 7 shows a flow chart for a method for compensating for weather-independent Doppler expansions in radar signals of a weather radar system 200. The method comprises: Receiving S110 a representation 50 of the radar signals, wherein the representation 50 comprises pixels of a range Doppler matrix; Calculating S120 azimuth angle Azi for the pixels 75 by fine bearing; Correcting S131 weather-independent Doppler shifts for the pixels 75 based on the calculated azimuth angle Azi; AziMopu; Compensating S132 for the weather-independent Doppler expansions; and Providing S133 a compensated representation 150.

(37) As further optional steps, the method can include all functionalities as described for the apparatus or the weather radar system.

(38) Advantages of the embodiments can be summarized as follows:

(39) Due to the compensation according to the embodiments, critical weather conditions can be reliably recognized at larger offset angles AziBeam from the speed vector v. In the quantitative estimation, it should be noted on the one hand that the weather-independent widening effect should be smaller than the critical sigma caused by the weather (standard deviation, i.e. weather-related widening) of at least 2-3 Doppler gates.

(40) In addition, there is a linear relationship between an angular expansion deltaDG in Doppler gates and the offset angle AziBeam from the velocity vector of:
deltaDG=0.8*offset angle.  (8)

(41) Without compensation, the critical 2-3 Doppler gates mean that offset angles of 3 to 4.5 degrees would be permitted. Weather-related widenings could still be reliably detected in this range. The compensation according to the embodiments leads to an improvement by a factor of 4 (see above), so that with compensation, offset angles of 12 to 18 degrees would be permitted (because of the linearity). This is a significant increase in the offset angle from the speed vector v, at which a critical weather situation should be able to be reliably detected. This also improves the overall success of evasive maneuvers.

(42) Overall, the Doppler expansions due to the turbulent weather can thus be measured even at higher offset angles, and evasive maneuvers can be flown in good time before the “weather” so as not to damage the autonomously flying drone 300, as an example. The method enables smaller on-board radar systems (e.g. 25 cm antenna size) to be able to detect turbulence, which for conventional on-board weather radar systems without monopulse capability would require an antenna size in the range of more than 50 cm.

(43) The features of the invention disclosed in the description, the claims, and the drawings may be essential for the realization of the invention either individually or in any combination.

LIST OF REFERENCE SIGNS

(44) 50 Representation of radar signals (e.g. as a range Doppler matrix) 70 Recorded spatial area 75 Pixels of the representation 100 Compensation apparatus 110 Receiving device 120 Calculation device 130 Compensation device 150 Compensated representation based on compensated radar signals 200 Weather radar system 215 Antenna system 210, 220 Antennas for a monopulse method 225 Base width of the monopulse antennas 230 Evaluation device 231, 232, . . . Components of the evaluation device 240 Beam width of the antenna system 300 Aircraft (drone, airplane, helicopter, etc.) 350 Evasive maneuvers Azi Azimuth angle AziBeam Rough bearing for azimuth angle AziMopu Fine bearing for azimuth angle v Relative speed (speed vector)