CN109358325B - Terrain inversion method of radar altimeter under rugged terrain background - Google Patents
Terrain inversion method of radar altimeter under rugged terrain background Download PDFInfo
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- G01S13/882—Radar or analogous systems specially adapted for specific applications for altimeters
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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- G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
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Abstract
A terrain inversion method of a radar altimeter under an undulating terrain background comprises the following steps: acquiring a radar echo beat signal; acquiring a frequency spectrum of an echo beat signal; detecting the leading edge of the beat frequency spectrum; obtaining the Doppler frequency of a scattering point corresponding to the leading edge of the beat frequency spectrum; obtaining a Doppler frequency difference value; judging whether the Doppler frequency difference is smaller than the Fourier spectrum resolution, if so, calculating the flight height of the aircraft under the flat terrain background, and outputting the flight height and the terrain features of the aircraft; otherwise, extracting a plurality of groups of super-resolution spectrum peaks; matching super-resolution spectrum peaks; inverting the topographic relief characteristic; and outputting the flight altitude and the terrain features of the aircraft. According to the invention, by extracting a plurality of groups of super-resolution spectrum peaks and carrying out spectrum pairing by using a minimum variance spectrum pairing method, the distances and Doppler frequencies of a plurality of scattering points are obtained, and the topographic relief features are inverted. The method can be used for accurate height measurement of a low-altitude aircraft under the undulating terrain background and inversion of the topographic relief features.
Description
Technical Field
The invention belongs to the technical field of wireless communication, and further relates to a terrain inversion method of a radar altimeter under an undulating terrain background in the technical field of radar data processing. The method can be applied to inversion of the fluctuation characteristics of the earth surface topography by using the echo data of the radar altimeter.
Background
The radar altimeter is a device which is mounted on an aircraft and measures the height from a flying platform to the ground by using the time delay of electromagnetic wave propagation. The terrain inversion is to reversely deduce the fluctuation characteristics of the terrain by utilizing the altimetry data of the radar altimeter. At present, two main types of methods commonly used for inverting the terrain by utilizing radar height measurement data are analytic methods and statistical methods. The analytical method is mainly based on a response function, surface gravity anomaly calculated by utilizing radar height measurement data is utilized, and high-resolution terrain is inverted by referring to a terrain model; the statistical method is mainly based on the least square configuration theory of the random process. The altitude information of the aircraft is an important reference index for controlling the flight state, and the existing radar altimeter can measure the altitude of a nadir point, but the reliability of determining the safe flight altitude by only using the altitude information provided by the existing radar altimeter is low. In the flying process, the platform control system is unknown about the surrounding terrain environment, most scenes faced by the radar altimeter have certain height fluctuation, and height misjudgment may exist in nadir height information obtained by utilizing the traditional radar altimeter. In order to adapt to the undulating terrain environment, improve the height measurement accuracy and reduce the height measurement error, a new height extraction and terrain inversion method is needed, so that the frequency modulation continuous wave radar altimeter can accurately measure the height in the undulating terrain and invert the topographic relief characteristics around the flight platform, and a more effective reference is provided for flight decision.
The patent document "millimeter wave radar altimeter for plant protection rotor unmanned aerial vehicle" (patent application number 201610725723.7, application publication number CN 107783107 a) of the company, continental loran science and technology limited discloses a height inversion method using a millimeter wave radar altimeter. The method uses discrete Fourier transform to carry out spectrum analysis on echo beat signals, combines a constant false alarm detection method to detect a frequency spectrum, and then calculates the height of the millimeter wave radar by a frequency (namely echo beat frequency) corresponding to the maximum frequency spectrum peak value which passes a threshold. The method improves the height measurement precision of the radar altimeter. However, the method still has the disadvantage that the frequency corresponding to the maximum peak value which passes the threshold is the echo beat frequency of the scattering point under the radar, however, the frequency corresponding to the maximum peak value under the terrain relief background is not necessarily the echo beat frequency of the scattering point under the radar, and the frequency corresponding to the maximum peak value which passes the threshold is the echo beat frequency of the scattering point under the radar, so that the radar height inversion has ambiguity and cannot be used for the relief terrain feature inversion.
The university of electronic technology discloses a method for inverting the position of an aircraft by using a radar altimeter in the patent document 'radar altimeter and a method for measuring the position of the aircraft by using the radar altimeter' (patent application number 200610022520.8, application publication number CN 101017202A). The method comprises the steps of accurately calculating the distance between an aircraft and a target through the phase difference of signals received by two antennas, calculating the azimuth of the target by using Doppler information, and performing correlation on the measured topographic data and topographic data prestored in a digital reference map to invert the position information of the aircraft. The method can improve the ranging precision of the radar altimeter and reduce ranging errors. However, the method still has the disadvantages that the ground digital topographic map of the flight area where the aircraft is located needs to be prestored in advance, the accuracy of the aircraft position measurement is related to the accuracy of the digital topographic map, and when the coverage accuracy of the digital topographic map is low, the measurement accuracy is low, so that the method is not suitable for high-accuracy height measurement of low-altitude aircraft.
Disclosure of Invention
The invention aims to provide a terrain inversion method of a radar altimeter under an undulating terrain background, which can realize the inversion of the terrain features of the frequency modulation continuous wave radar altimeter under the undulating terrain background and improve the height measurement precision under the undulating terrain background.
The basic idea of the invention is as follows: estimating and distinguishing spectral peaks of target echo beat frequency spectrums with strong near scattering distance by utilizing a super-resolution spectrum, extracting a plurality of groups of super-resolution spectral peaks, pairing the spectral peaks in positive and negative frequency modulation cycles, calculating the distance and Doppler frequency of scattering points by utilizing the structural characteristics of the spectral peaks in the positive and negative frequency modulation cycle frequency spectrums according to the pairing result, determining the height of the aircraft according to the relation between the attitude angle of the aircraft and the Doppler frequency of the scattering points under the radar, and then reversely performing topographic relief characteristics by a geometric model.
In order to achieve the purpose, the technical scheme of the invention comprises the following steps:
(1) Acquiring radar echo beat signals:
reading an echo beat signal sampled by a receiver in a positive and negative frequency modulation period of an altimeter radar transmitting signal from the receiver of the altimeter radar on the aircraft;
(2) Acquiring the frequency spectrum of the echo beat signal:
copying one group of echo beat signal data, processing the two groups of data in parallel, performing fast Fourier transform on original data to obtain a Fourier spectrum of the echo beat signal data, and performing a Capon super-resolution spectrum on the copied data to obtain a super-resolution spectrum of the echo beat signal data;
(3) Detecting the leading edge of the beat frequency spectrum:
respectively carrying out frequency spectrum threshold detection on Fourier frequency spectrums of a positive frequency modulation period and a negative frequency modulation period by using a self-adaptive threshold detection method to obtain a plurality of spectrum peaks of the Fourier frequency spectrums exceeding a threshold;
(4) Obtaining the Doppler frequency of scattering points corresponding to the leading edge of the beat frequency spectrum:
subtracting the frequency spectrum leading edge frequency value of the positive frequency modulation period from the frequency spectrum leading edge frequency value of the negative frequency modulation period, and dividing the difference value by 2 to obtain the Doppler frequency of the scattering point corresponding to the beat frequency spectrum leading edge;
(5) Obtaining a Doppler frequency difference value:
(5a) Calculating the Doppler frequency of a scattering point right below the altimeter radar antenna on the aircraft by using a Doppler frequency calculation formula;
(5b) Subtracting the Doppler frequency of a scattering point right below a radar antenna of the altimeter on the aircraft from the Doppler frequency of the scattering point corresponding to the leading edge of the beat frequency spectrum, and taking the absolute value of the difference value as a Doppler frequency difference value;
(6) Judging whether the Doppler frequency difference value is smaller than the Fourier spectrum resolution, if so, executing the step (7); otherwise, executing step (8);
(7) After calculating the flying height of the aircraft in the flat terrain background according to the following formula, executing the step (11):
wherein H represents the flight altitude of the aircraft under the background of flat terrain, c represents the speed of light, T represents a positive and negative frequency modulation period of an altimeter radar transmitting signal on the aircraft, B represents the working bandwidth of the altimeter radar on the aircraft, and f b- Representing the leading edge of the frequency spectrum of the negative modulation period, the value of which is the frequency value of the first peak of the Fourier spectrum in the negative modulation period, f b+ Representing the leading edge of a beat frequency spectrum of the positive frequency modulation period, the value of which is the frequency value of the first spectral peak of the Fourier frequency spectrum in the positive frequency modulation period;
(8) Extracting multiple groups of super-resolution spectrum peaks:
respectively carrying out frequency spectrum threshold detection on the super-resolution frequency spectrums of the positive and negative frequency modulation periods by using a self-adaptive threshold detection method to obtain a plurality of groups of spectrum peaks of the super-resolution frequency spectrums of the positive and negative frequency modulation periods from small to large;
(9) Matching super-resolution spectrum peaks:
matching spectral peaks of super-resolution frequency spectrums of positive and negative frequency modulation periods by adopting a minimum variance frequency spectrum matching method to obtain beat frequency pairs;
(10) Inverting topographic relief features:
(10a) Calculating the distance of a scattering point corresponding to each matched spectral peak by using a distance calculation formula of the frequency modulation continuous wave radar;
(10b) Subtracting the first frequency value from the second frequency value in each beat frequency pair, and dividing the difference by two to obtain the Doppler frequency of the scattering point;
(10c) Finding out the Doppler frequency f of scattering points which are closest to the scattering points right below the altimeter radar antenna on the aircraft u The distance value of the scattering point corresponding to the serial number is used as the flying height of the aircraft under the background of the undulating terrain;
(10d) The height of each scattering point relative to the ground is calculated as follows:
wherein h is k Representing the height of the kth scattering point relative to the ground,
representing the flight altitude of an aircraft in the context of undulating terrain, cos representing the cosine function, theta k Representing the pitch angle of the kth scattering point relative to an altimeter radar antenna on the aircraft;
(10e) Subtracting the height of each scattering point relative to the ground by the flight height of the aircraft in the context of the undulating terrain to obtain the vertical distance of the scattering point relative to an altimeter radar antenna on the aircraft:
(10f) The horizontal distance in the flight direction of each scattering point is calculated as follows:
l k ＝R k sinθ k
wherein l k Represents the horizontal distance of the kth scattering point along the flight direction, and sin represents a sine function;
(10g) Simulating the convex terrain of the scattering point relative to the ground by using a sine function peak with the height equal to the height of the scattering point relative to the ground to obtain the topographic relief characteristic of each scattering point relative to the horizontal plane of the scattering point under the altimeter radar antenna on the aircraft;
(11) Outputting the flight height and terrain features of the aircraft:
and outputting the flight heights of the aircrafts under different terrain backgrounds and the corresponding terrain features.
Compared with the prior art, the invention has the following advantages:
firstly, because the invention extracts a plurality of groups of super-resolution spectrum peaks in the super-resolution spectrum of the echo beat signal to obtain the distances and Doppler frequencies of a plurality of scattering points, the problems that in the prior art, the frequency corresponding to the maximum peak value of a threshold is taken as the echo beat frequency of the scattering points under the radar, so that the radar height inversion has ambiguity and cannot be used for the inversion of the undulating terrain features are solved, and the flight height of an aircraft and the corresponding terrain features can be accurately extracted under the flat terrain background and the undulating terrain background.
Secondly, the invention adopts the minimum variance frequency spectrum pairing method to pair the super-resolution spectrum peaks, thereby overcoming the problems that the position measurement precision in the prior art is related to the precision of a digital topographic map, and when the coverage precision of the digital topographic map is lower, the measurement precision is low, so that the invention has higher measurement precision for the flight height measurement value of the aircraft under the rugged topography background.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a geometric model diagram of the present invention for inversion of topographic features;
FIG. 3 is a graph of simulation results for the present invention at an aircraft pitch angle of 0 degrees;
FIG. 4 is a graph of simulation results for the present invention at a 15 degree pitch angle of the aircraft;
FIG. 5 is a graph of simulation results for the present invention at an aircraft pitch angle of 30 degrees.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
The steps of the present invention will be further described with reference to fig. 1.
From a receiver of an altimeter radar on an aircraft, an echo beat signal sampled by the receiver in a positive and negative frequency modulation period of a signal transmitted by the altimeter radar is read.
And step 2, acquiring the frequency spectrum of the echo beat signal.
Copying one group of echo beat signal data, processing the two groups of data in parallel, performing fast Fourier transform on original data to obtain a Fourier spectrum of the echo beat signal data, and performing a Capon super-resolution spectrum on the copied data to obtain the super-resolution spectrum of the echo beat signal data.
When Capon super-resolution spectrum is carried out on the echo data sequence, only one snapshot is carried out, so that a covariance matrix is estimated by adopting a moving average method.
And 3, detecting the leading edge of the beat frequency spectrum.
And respectively carrying out frequency spectrum threshold detection on the Fourier frequency spectrums of a positive frequency modulation period and a negative frequency modulation period by using a self-adaptive threshold detection method to obtain a plurality of spectrum peaks of the Fourier frequency spectrums exceeding the threshold.
The steps of the self-adaptive threshold detection method are as follows:
and step 1, sequentially selecting a spectral line from the frequency spectrum as a current detection unit.
And step 2, taking the current detection unit as a reference, taking 10 spectral lines forward, forming a protection unit by two spectral lines close to the current detection unit, and forming a reference unit by other 8 spectral lines.
And step 3, calculating the average value of all spectral line amplitudes in the reference unit, and multiplying the average value by 3 to be used as a threshold value.
Step 4, judging whether the spectral line amplitude of the current detection unit is larger than a threshold value, if so, executing step 5; otherwise, executing step 1.
And 5, outputting the spectral line amplitude and the serial number of the current detection unit.
And 6, repeating the first step to the fifth step to obtain the spectral line amplitudes and the serial numbers of all the detection units exceeding the threshold, and sequentially combining the spectral lines with continuous serial numbers to obtain a plurality of spectral peaks of the frequency spectrum exceeding the threshold, wherein the frequency of the spectral peaks is from small to large.
And 4, obtaining the Doppler frequency of the scattering point corresponding to the leading edge of the beat frequency spectrum.
And subtracting the frequency spectrum leading edge frequency value of the positive frequency modulation period from the frequency spectrum leading edge frequency value of the negative frequency modulation period, and dividing the difference value by 2 to obtain the Doppler frequency of the scattering point corresponding to the beat frequency spectrum leading edge.
And 5, acquiring a Doppler frequency difference value.
And calculating the Doppler frequency of a scattering point right below the altimeter radar antenna on the aircraft by using a Doppler frequency calculation formula.
The Doppler frequency calculation formula is as follows:
wherein f is u Representing the doppler frequency of a scattering point directly below the altimeter radar antenna on the aircraft, v representing the speed of the aircraft, λ representing the operating wavelength of the altimeter radar on the aircraft, and α representing the pitch angle in the attitude angle of the aircraft read from the output data of the inertial navigation system of the aircraft.
And subtracting the Doppler frequency of a scattering point right below the altimeter radar antenna on the aircraft by using the Doppler frequency of the scattering point corresponding to the leading edge of the beat frequency spectrum, and taking the absolute value of the difference value as the Doppler frequency difference value.
Step 6, judging whether the Doppler frequency difference value is smaller than the Fourier spectrum resolution, if so, executing step 7; otherwise, step 8 is performed.
The Fourier spectrum resolution is a value obtained by dividing the circumferential rate pi by the data length of the echo beat signal of a positive frequency modulation period and a negative frequency modulation period.
And 7, calculating the flying height of the aircraft under the flat terrain background according to the following formula, and then executing the step (11):
where H represents the altitude of the aircraft in the context of flat terrain, c represents the speed of light, and T represents an altimeter radar on the aircraftA positive and negative frequency modulation period of the transmitted signal, B represents the operating bandwidth of the altimeter radar on the aircraft, f b- Representing the leading edge of the frequency spectrum of the negative modulation period, the value of which is the frequency value of the first peak of the Fourier spectrum in the negative modulation period, f b+ The leading edge of the beat spectrum, representing the positive cycle, has the value of the frequency of the first spectral peak of the fourier spectrum in the positive cycle.
The flat terrain refers to the fact that a scattering point corresponding to the front edge of a beat frequency spectrum in a positive frequency modulation period and a scattering point under an altimeter radar antenna on an aircraft are the same scattering point, and the terrain in the illumination range of the altimeter radar antenna on the aircraft is flat, wherein the Doppler frequency difference is smaller than the Fourier frequency spectrum resolution.
According to the height measurement principle of the frequency modulation continuous wave radar altimeter, only the scattering point corresponding to the front edge of the beat frequency spectrum in the positive and negative frequency modulation periods under the background of flat terrain and the scattering point under the altimeter radar antenna on the aircraft are the same scattering point, and the Doppler frequency of the scattering point under the radar antenna and the attitude angle of the aircraft have a fixed relation, so that whether the terrain is flat or not can be judged by judging whether the Doppler frequency difference is smaller than the Fourier spectrum resolution or not.
And 8, extracting a plurality of groups of super-resolution spectrum peaks.
And respectively carrying out frequency spectrum threshold detection on the super-resolution frequency spectrums of the positive and negative frequency modulation periods by using a self-adaptive threshold detection method to obtain a plurality of groups of frequency-from-small to-large spectrum peaks of the super-resolution frequency spectrums of the positive and negative frequency modulation periods.
The steps of the self-adaptive threshold detection method are as follows:
and step 1, sequentially selecting a spectral line from the frequency spectrum as a current detection unit.
And step 2, taking the current detection unit as a reference, taking 10 spectral lines forward, forming a protection unit by two spectral lines close to the current detection unit, and forming a reference unit by other 8 spectral lines.
And step 3, calculating the average value of all spectral line amplitudes in the reference unit, and multiplying the average value by 3 to be used as a threshold value.
Step 4, judging whether the spectral line amplitude of the current detection unit is larger than a threshold value, if so, executing step 5; otherwise, executing step 1.
And 5, outputting the spectral line amplitude and the serial number of the current detection unit.
And 6, repeating the first step to the fifth step to obtain the spectral line amplitudes and the serial numbers of all the detection units exceeding the threshold, and sequentially combining the spectral lines with continuous serial numbers to obtain a plurality of spectral peaks of the frequency spectrum exceeding the threshold, wherein the frequency of the spectral peaks is from small to large.
And 9, matching the super-resolution spectrum peaks.
And matching the spectral peaks of the super-resolution frequency spectrums with the positive and negative frequency modulation periods by adopting a minimum variance frequency spectrum matching method to obtain beat frequency pairs.
The minimum variance frequency spectrum pairing method comprises the following steps:
and step 1, sequentially selecting a spectral peak from the super-resolution frequency spectrum of the positive frequency modulation period from small to large according to frequency, and sequentially arranging the spectral line amplitudes of the spectral peaks to obtain an amplitude sequence of the super-resolution spectral peak of the positive frequency modulation period.
And 2, sequentially selecting a spectral peak from the super-resolution spectrum of the negative frequency modulation period from small to large according to the frequency, and sequentially arranging the spectral line amplitudes of the spectral peaks to obtain an amplitude sequence of the super-resolution spectral peak of the negative frequency modulation period.
And 3, calculating element values in a variance matrix of the super-resolution spectrum peak amplitude sequence difference according to the following formula:
δ i,j ＝var(A up (i,n)-A dn (j,n))
wherein, delta i,j Representing the ith row and the jth column element in the variance matrix of the spectrum peak amplitude difference sequence, the value of i is equal to the corresponding sequence number of the spectrum peak of the super-resolution spectrum of the positive frequency modulation period, i =1,2, ·, N, N represents the number of the spectrum peaks in the super-resolution spectrum of the positive frequency modulation period, the value of j is equal to the corresponding sequence number of the spectrum peak of the super-resolution spectrum of the negative frequency modulation period, j =1,2, ·, M, M represents the number of the spectrum peaks in the super-resolution spectrum of the negative frequency modulation period, var represents the variance solving operation, A represents the variance solving operation, and A represents the number of the spectrum peaks in the super-resolution spectrum of the negative frequency modulation period up Representing the amplitude of the positive frequency-modulated periodic spectrum, n representing n points in the sequence of peak amplitudes, A dn Representing the negative fm periodic spectral amplitude.
Step 4, selecting a row of elements each time from the first row of the variance matrix of the spectrum peak amplitude difference sequence, finding out the minimum value in the row of elements, and finding out the minimum value of the row where the minimum value of the row is located;
and 5, judging whether the minimum value of the row is equal to the minimum value of the row where the minimum value of the row is located, if so, executing the step 6, otherwise, executing the step 4.
And 6, respectively using the line number and the column number of the minimum value of the line as indexes, finding out the frequency of the spectral line with the maximum spectral peak amplitude corresponding to the serial number in the frequency spectrum from the super-resolution spectral peaks of the positive frequency modulation period and the negative frequency modulation period, and forming a beat frequency pair.
And step 10, inverting the topographic relief characteristic.
And calculating the distance of the scattering point corresponding to each matched spectral peak by using a distance calculation formula of the frequency modulation continuous wave radar.
The distance calculation formula of the frequency modulation continuous wave radar is as follows:
wherein R is k Denotes the distance of the kth scattering point corresponding to the paired spectral peak, f k+ Representing the first frequency value, f, of the kth beat frequency pair k- Representing the second frequency value in the kth beat frequency pair.
The second frequency value in each beat frequency pair is subtracted from the first frequency value, and the difference is divided by two to obtain the Doppler frequency of the scattering point.
Finding out the Doppler frequency f of scattering points which are closest to the scattering points right below the altimeter radar antenna on the aircraft u The distance value of the scattering point corresponding to the serial number is used as the flight altitude of the aircraft under the background of the undulating terrain.
The height of each scattering point relative to the ground is calculated as follows:
wherein h is k Representing the height of the kth scattering point relative to the ground,
representing the flight altitude of an aircraft in the context of undulating terrain, cos representing the cosine function, theta k The pitch angle of the kth scattering point is shown relative to the altimeter radar antenna on the aircraft.
The pitch angle of the scattering point relative to the altimeter radar antenna on the aircraft is the pitch angle of the scattering point in an altimeter radar antenna coordinate system on the aircraft, and the value of the pitch angle is calculated by the following formula:
wherein arccos represents an inverse cosine function, f dk Representing the doppler frequency of the k-th scattering point.
And subtracting the height of each scattering point relative to the ground by the flight height of the aircraft in the context of the undulating terrain to obtain the vertical distance of the scattering point relative to the altimeter radar antenna on the aircraft.
The horizontal distance in the flight direction of each scattering point is calculated as follows:
l k ＝R k sinθ k
wherein l k Denotes the horizontal distance of the kth scattering point in the flight direction, sin denotes the sine function.
And simulating the convex terrain of the scattering point relative to the ground by using a sine function peak with the height equal to the height of the scattering point relative to the ground, so as to obtain the topographic relief characteristic of each scattering point relative to the horizontal plane of the scattering point under the altimeter radar antenna on the aircraft.
Referring now to FIG. 2, the inversion of topographic features of the present invention is performedAnd (5) describing one step. The origin of coordinates O in the three-dimensional rectangular coordinate system in fig. 2 corresponds to a ground scattering point directly below the altimeter radar antenna on the current aircraft, the Y axis represents the direction of the projection of the flight direction of the aircraft in the horizontal plane, the unit is meter, the Z axis represents the vertical geocentric direction, the unit is meter with the vertical upward direction as the positive direction, the X axis represents the direction perpendicular to the flight path plane of the aircraft, the direction meeting the right-hand screw rule is the positive direction, and the unit is meter. In FIG. 2
The method comprises the steps of representing the flying height of an aircraft in the context of undulating terrain of the aircraft, representing the pitch angle of the aircraft by alpha, representing the pitch angle of a scattering point relative to an altimeter radar antenna on the aircraft by theta, representing the distance from the scattering point to the radar antenna by R, representing the horizontal distance of the scattering point along the flying direction by l, and representing the height of the scattering point relative to the ground by h. Two straight lines on both sides of the Z axis in fig. 2 represent radar main lobe beam edges, a broken line represents a portion of a radar beam blocked by the undulating topography, and a curve on the right side of the Z axis represents a section of the undulating topography. When the topographic features are inverted, the geometric relation between each scattering point and the aircraft can be established by using the geometric model, the distance R from each scattering point to the radar antenna is calculated according to the geometric relation, the horizontal distance l of each scattering point along the flight direction and the height h of each scattering point relative to the ground are calculated, then the sinc function peak with the height equal to the height of each scattering point relative to the ground is used for simulating the convex topography of each scattering point relative to the ground, the topographic features of each scattering point corresponding to the matched spectral peaks are obtained, and the topographic features in the irradiation range of the radar antenna of the aircraft are further obtained.
And step 11, outputting the flight altitude and the terrain features of the aircraft.
And outputting the flight heights of the aircrafts under different terrain backgrounds and the corresponding terrain features.
The effect of the present invention can be illustrated by the following simulation experiment:
1. simulation conditions are as follows:
the radar altimeter adopted in the experimental simulation has the working mode of symmetrical triangular modulation linear frequency modulation continuous wave, the center frequency is 4.1 GHz, the bandwidth is 200 MHz, the sampling rate of a receiver is 1 MHz, the wave beam width is 60 degrees, the wave beam shape is conical, the modulation period is 880 microseconds, and the half modulation period is 880 microseconds. The flying height of the aircraft is 50 meters, the flying speed is 200 meters per second, the yaw angle and the roll angle in the attitude angle of the aircraft are zero, and the pitch angle is 0 degree, 15 degrees and 30 degrees respectively. The flying terrain background of the aircraft is a valley, the profile of the valley is approximated by a parabola, and the three-dimensional terrain of the valley is a space curved surface with a generatrix as the parabola and the trend of the valley as a directrix by mathematical description.
2. Simulation content and result analysis:
in the simulation experiment, the terrain inversion simulation is carried out on the conditions that the pitch angle of the aircraft is 0 degree, 15 degrees and 30 degrees respectively by using the terrain inversion method of the frequency-modulated continuous wave radar altimeter under the undulating terrain background, and the obtained simulation result is shown in figures 3, 4 and 5.
Fig. 3 is a graph of the results of a simulation in which the pitch angle of the aircraft takes 0 degrees. Wherein, fig. 3 (a) is Capon super-resolution spectrogram of the echo beat signal of positive and negative frequency modulation cycles, and fig. 3 (b) is super-resolution spectrogram of the positive and negative frequency modulation cycles passing the threshold. The abscissa in fig. 3 (a) represents frequency in kilohertz and the ordinate represents amplitude. In fig. 3 (a), a curve marked with small triangles represents a spectrum curve of a positive frequency modulation period, and a curve marked with small circles represents a spectrum curve of a negative frequency modulation period.
As can be seen from fig. 3 (a), the positive and negative frequency modulation periodic spectrums overlap each other in the range of 0-200KHz, and useful information cannot be obtained from the spectrums. Therefore, the invention needs to further process the positive and negative frequency modulation periodic frequency spectrum to obtain more useful information from the frequency spectrum. Firstly, performing threshold detection on a frequency spectrum to obtain a frequency spectrum front edge, then judging the frequency spectrum front edge by utilizing the attitude information of the platform, and extracting a plurality of groups of spectrum peaks from the super-resolution spectrum when the terrain fluctuates, wherein the extraction result is shown in figure 3 (b). The abscissa in fig. 3 (b) represents frequency in kilohertz and the ordinate represents amplitude. In fig. 3 (b), the curve marked with small triangles represents the spectrum curve of the positive frequency modulation period, and the curve marked with small circles represents the spectrum curve of the negative frequency modulation period.
From the step 3 (b), it can be seen that the positive and negative frequency modulation periodic frequency spectrums have three spectral peaks, the third spectral peak from the left is very strong in amplitude, the spectral peak positions are basically overlapped, but the first two spectral peaks are close in amplitude and not very different in shape, the third spectral peak of the positive and negative frequency modulation periodic frequency spectrums can be correctly paired by using the existing spectral amplitude or spectral area pairing method, but the first two spectral peaks of the positive and negative frequency modulation periodic frequency spectrums are difficult to correctly pair. Therefore, the invention provides a minimum variance spectral peak pairing method, and the spectral peaks of the positive and negative frequency modulation periodic frequency spectrums can be correctly paired by utilizing the similarity of the spectral peaks. After the super-resolution spectrum peaks in the graph (3 b) are paired, the height of the aircraft under the undulating terrain background is calculated by using the following distance calculation formula of the frequency modulation continuous wave radar, the height of the aircraft under the undulating terrain background is 50.09 m, the difference with the real height is 0.09 m, and the percentage error is 0.18%.
Wherein R is k Denotes the distance, f, of the k-th scattering point corresponding to the paired spectral peak k+ Representing the first frequency value, f, of the kth beat frequency pair k- Representing the second frequency value in the kth beat frequency pair.
Fig. 4 is a graph of the results of a simulation in which the pitch angle of the aircraft is taken to be 15 degrees. Fig. 4 (a) is a Capon super-resolution spectrogram of a positive and negative frequency modulation periodic echo beat signal, wherein an abscissa in fig. 4 (a) represents frequency in kilohertz, an ordinate represents amplitude, a curve marked with a small triangle represents a spectrum curve of a positive frequency modulation period, and a curve marked with a small circle represents a spectrum curve of a negative frequency modulation period. Fig. 4 (b) is a graph of super-resolution peaks of positive and negative frequency modulation cycles crossing a threshold, the abscissa in fig. 4 (b) representing frequency in kilohertz and the ordinate representing amplitude, the curve marked with small triangles representing the spectral curve of the positive frequency modulation cycle and the curve marked with small circles representing the spectral curve of the negative frequency modulation cycle.
As can be seen from fig. 4 (a), the pitch angle of the aircraft becomes larger, and the frequency difference between the maximum spectral peaks of the super-resolution spectrum of the positive and negative chirp periods increases. After the super-resolution spectrum peaks in the graph (4 b) are paired, the height of the aircraft under the undulating terrain background is calculated by using a distance calculation formula of a frequency modulation continuous wave radar, the height of the aircraft under the undulating terrain background is 50.10 meters, the difference between the height and the actual height is 0.1 meter, and the percentage error is 0.2%.
Fig. 5 is a graph of the results of a simulation in which the pitch angle of the aircraft takes 30 degrees. Where fig. 5 (a) is a Capon super-resolution spectrogram of a positive and negative frequency modulated periodic echo beat signal, the abscissa in fig. 5 (a) represents frequency in kilohertz and the ordinate represents amplitude. In fig. 5 (a), a curve marked with small triangles represents a spectrum curve of a positive frequency modulation period, and a curve marked with small circles represents a spectrum curve of a negative frequency modulation period. Fig. 5 (b) is a plot of the super-resolution peak of positive and negative chirp cycles across the threshold, with the abscissa of fig. 5 (b) representing frequency in kilohertz and the ordinate representing amplitude. In fig. 5 (b), the curve marked with small triangles represents the spectrum curve of the positive frequency modulation period, and the curve marked with small circles represents the spectrum curve of the negative frequency modulation period.
Fig. 4 and 5 are graphs of simulation results when only the attitude angle (15 degrees, 30 degrees) of the aircraft is changed under the same conditions. Comparing Capon super-resolution spectra of the positive and negative frequency modulation cycle echo beat signals in fig. 3 (a), fig. 4 (a) and fig. 5 (a), it can be seen that the frequency difference between the maximum spectral peaks of the positive and negative frequency modulation cycles increases with the increase of the pitch angle of the aircraft, because the larger the pitch angle is, the larger the doppler frequency of the scattering point right below the radar antenna is, so that the frequency difference between the maximum spectral peaks of the positive and negative frequency modulation cycles is larger. The frequency difference between the maximum spectral peaks of the positive and negative frequency modulation cycles is twice the doppler frequency of the scattering point directly below the radar antenna.
The minimum variance spectral peak pairing method adopted in the super-resolution spectral peak pairing process is used for respectively pairing the super-resolution spectral peaks with the threshold crossing of the positive and negative frequency modulation periods in the images 3 (b), 4 (b) and 5 (b), and then the pairing result is used for carrying out height extraction and topographic relief feature inversion to obtain the height extraction result shown in the table 1 and the topographic relief feature inversion result shown in the table 2.
TABLE 1 nadir height extraction results
TABLE 2 results of topographical inversion
The nadir point in table 1 is a scattering point right below a radar antenna of an altimeter on an aircraft, the measured value is a value obtained after the frequency spectrum of the positive and negative frequency modulation cycles is processed by the method, and the error is an absolute value obtained by subtracting a true value from the measured value. The closest point in table 2 refers to the scattering point with the smallest distance among the scattering points corresponding to the super-resolution peak matching result, the vertical distance refers to the distance of the scattering point relative to the aircraft along the vertical direction, and the horizontal distance refers to the horizontal distance of the scattering point along the flight direction of the aircraft.
As can be seen from simulation experiment results, the method for extracting the height can accurately extract the height of the nadir in an undulating terrain scene, can effectively avoid the problem of misjudgment of the height of the nadir caused by topographic relief, and can obtain higher height measurement precision and Doppler measurement precision, wherein the height measurement precision error is within 0.2 percent, and the Doppler measurement precision error is within 0.2 KHz. By adopting the method to invert the topographic relief characteristics, the topographic relief characteristics of a plurality of scattering points in the irradiation range of the radar antenna of the aircraft can be obtained, more reference information is provided for flight decisions, and the safety distance between the platform and the barrier is ensured.
Claims (8)
1. A terrain inversion method of a radar altimeter under a relief terrain background is characterized in that a plurality of groups of super-resolution spectrum peaks are extracted, a minimum variance spectrum pairing method is adopted to pair the spectrum peaks of the super-resolution spectrum with a positive and negative frequency modulation period, and the relief features of the terrain are inverted, wherein the method comprises the following steps:
(1) Acquiring radar echo beat signals:
reading an echo beat signal sampled by a receiver in a positive and negative frequency modulation period of an altimeter radar transmitting signal from the receiver of the altimeter radar on the aircraft;
(2) Acquiring the frequency spectrum of the echo beat signal:
copying one group of echo beat signal data, processing the two groups of data in parallel, performing fast Fourier transform on original data to obtain a Fourier spectrum of the echo beat signal data, and performing a Capon super-resolution spectrum on the copied data to obtain a super-resolution spectrum of the echo beat signal data;
(3) Detecting the leading edge of the beat frequency spectrum:
respectively carrying out frequency spectrum threshold detection on Fourier frequency spectrums of a positive frequency modulation period and a negative frequency modulation period by using a self-adaptive threshold detection method to obtain a plurality of spectrum peaks of the Fourier frequency spectrums exceeding the threshold;
(4) Obtaining the Doppler frequency of scattering points corresponding to the leading edge of the beat frequency spectrum:
subtracting the frequency spectrum leading edge frequency value of the positive frequency modulation period from the frequency spectrum leading edge frequency value of the negative frequency modulation period, and dividing the difference value by 2 to obtain the Doppler frequency of the scattering point corresponding to the beat frequency spectrum leading edge;
(5) Obtaining a Doppler frequency difference value:
(5a) Calculating the Doppler frequency of a scattering point right below the altimeter radar antenna on the aircraft by using a Doppler frequency calculation formula;
(5b) Subtracting the Doppler frequency of a scattering point right below a radar antenna of the altimeter on the aircraft from the Doppler frequency of the scattering point corresponding to the leading edge of the beat frequency spectrum, and taking the absolute value of the difference value as a Doppler frequency difference value;
(6) Judging whether the Doppler frequency difference value is smaller than the Fourier spectrum resolution, if so, executing the step (7); otherwise, executing step (8);
(7) And (3) according to the following formula, after the flying height of the aircraft in the flat terrain background is calculated, executing the step (11):
wherein H represents the flying height of the aircraft under the flat terrain background, c represents the light speed, T represents a positive and negative frequency modulation period of an altimeter radar transmitting signal on the aircraft, B represents the working bandwidth of the altimeter radar on the aircraft, f b- The leading edge of the frequency spectrum representing the negative modulation period, whose value is the frequency value of the first spectral peak of the Fourier frequency spectrum in the negative modulation period, f b+ Representing the leading edge of a beat frequency spectrum of the positive frequency modulation period, the value of which is the frequency value of the first spectral peak of the Fourier frequency spectrum in the positive frequency modulation period;
(8) Extracting a plurality of groups of super-resolution spectrum peaks:
respectively carrying out frequency spectrum threshold detection on the super-resolution frequency spectrums of the positive and negative frequency modulation periods by using a self-adaptive threshold detection method to obtain a plurality of groups of spectrum peaks of the super-resolution frequency spectrums of the positive and negative frequency modulation periods from small to large;
(9) Matching super-resolution spectrum peaks:
matching spectral peaks of super-resolution frequency spectrums of positive and negative frequency modulation periods by adopting a minimum variance frequency spectrum matching method to obtain beat frequency pairs;
(10) Inverting the topographic relief characteristic:
(10a) Calculating the distance of a scattering point corresponding to each matched spectral peak by using a distance calculation formula of the frequency modulation continuous wave radar;
(10b) Subtracting the first frequency value from the second frequency value in each beat frequency pair, and dividing the difference by two to obtain the Doppler frequency of the scattering point;
(10c) Finding out the Doppler frequency f of scattering points which are closest to the scattering points right below the altimeter radar antenna on the aircraft u The distance value of the scattering point corresponding to the serial number is used as the flying height of the aircraft under the background of the undulating terrain;
(10d) The height of each scattering point relative to the ground is calculated as follows:
wherein h is k Representing the height of the kth scattering point relative to the ground,
representing the flight altitude of an aircraft in the context of undulating terrain, cos representing the cosine function, theta k Representing the pitch angle of the kth scattering point relative to an altimeter radar antenna on the aircraft;
(10e) Subtracting the height of each scattering point relative to the ground by the flight height of the aircraft in the context of the undulating terrain to obtain the vertical distance of the scattering point relative to an altimeter radar antenna on the aircraft:
(10f) The horizontal distance in the flight direction of each scattering point is calculated as follows:
l k ＝R k sinθ k
wherein l k Represents the horizontal distance of the kth scattering point along the flight direction, and sin represents a sine function;
(10g) Simulating a convex terrain of the scattering point relative to the ground by using a sinc function peak with the height equal to the height of the scattering point relative to the ground to obtain the topographic relief characteristic of each scattering point relative to a horizontal plane on which the scattering point is positioned under the altimeter radar antenna on the aircraft;
(11) Outputting the flight height and terrain features of the aircraft:
and outputting the flight heights of the aircraft under different terrain backgrounds and corresponding terrain features.
2. The method of terrain inversion of radar altimeters in undulating terrain background of claim 1, wherein: the adaptive threshold detection method in the step (3) and the step (8) comprises the following steps:
the method comprises the steps that firstly, a spectral line is sequentially selected from a frequency spectrum to serve as a current detection unit;
secondly, taking 10 spectral lines forward on the basis of the current detection unit, forming a protection unit by two spectral lines close to the current detection unit, and forming a reference unit by other 8 spectral lines;
thirdly, calculating the average value of all spectral line amplitudes in the reference unit, and multiplying the average value by 3 to be used as a threshold value;
fourthly, judging whether the spectral line amplitude of the current detection unit is larger than a threshold value, if so, executing a fifth step; otherwise, executing the first step;
fifthly, outputting the spectral line amplitude and the serial number of the current detection unit;
and sixthly, repeating the first step to the fifth step to obtain the spectral line amplitudes and serial numbers of all the detection units exceeding the threshold, and sequentially combining the continuous serial numbers of the spectral lines to obtain a plurality of spectral peaks of the frequency spectrum exceeding the threshold, wherein the frequency of the spectral peaks is from small to large.
3. The method of terrain inversion of radar altimeters in undulating terrain background of claim 1, wherein: the doppler frequency calculation formula in step (5 a) is as follows:
wherein f is u Representing the doppler frequency of a scattering point directly below the altimeter radar antenna on the aircraft, v representing the speed of the aircraft, λ representing the operating wavelength of the altimeter radar on the aircraft, and α representing the pitch angle in the attitude angle of the aircraft read from the output data of the inertial navigation system of the aircraft.
4. The method of terrain inversion of radar altimeters in undulating terrain background of claim 1, wherein: the Fourier spectrum resolution in the step (6) is a value obtained by dividing the circumferential rate pi by the data length of the echo beat signal in a positive frequency modulation period and a negative frequency modulation period.
5. The method of terrain inversion for radar altimeters in undulating terrain background of claim 1, wherein: the flat terrain in the step (7) is deduced that the Doppler frequency difference is smaller than the Fourier spectrum resolution, the scattering point corresponding to the leading edge of the beat frequency spectrum and the scattering point right below the altimeter radar antenna on the aircraft are the same scattering point, and the terrain in the illumination range of the altimeter radar antenna on the aircraft is flat.
6. The method of terrain inversion for radar altimeters in undulating terrain background of claim 1, wherein: the minimum variance spectrum pairing method in the step (9) comprises the following steps:
the method comprises the steps that firstly, a spectrum peak is sequentially selected from a super-resolution spectrum of a positive frequency modulation period from small to large in frequency, and the spectral line amplitudes of the spectrum peak are sequentially arranged to obtain an amplitude sequence of the super-resolution spectrum peak of the positive frequency modulation period;
secondly, sequentially selecting a spectral peak from the super-resolution spectrum of the negative frequency modulation period from small to large according to the frequency, and sequentially arranging the spectral line amplitudes of the spectral peaks to obtain an amplitude sequence of the super-resolution spectral peak of the negative frequency modulation period;
thirdly, calculating element values in a variance matrix of the super-resolution spectrum peak amplitude sequence difference according to the following formula:
δ i,j ＝var(A up (i,n)-A dn (j,n))
wherein, delta i,j Representing the ith row and the jth column element in the variance matrix of the spectrum peak amplitude difference sequence, the value of i is equal to the corresponding sequence number of the spectrum peak of the super-resolution spectrum of the positive frequency modulation period, i =1,2, ·, N, N represents the number of the spectrum peaks in the super-resolution spectrum of the positive frequency modulation period, the value of j is equal to the corresponding sequence number of the spectrum peak of the super-resolution spectrum of the negative frequency modulation period, j =1,2, ·, M, M represents the number of the spectrum peaks in the super-resolution spectrum of the negative frequency modulation period, var represents the variance solving operation, A represents the variance solving operation, and A represents the number of the spectrum peaks in the super-resolution spectrum of the negative frequency modulation period up Representing the amplitude of the positive frequency-modulated periodic spectrum, n representing n points in the sequence of peak amplitudes, A dn Representing the amplitude of the negative frequency modulation periodic frequency spectrum;
fourthly, selecting a row of elements from the first row of the variance matrix of the spectrum peak amplitude difference sequence each time, finding out the minimum value in the row of elements, and finding out the minimum value of the row where the minimum value of the row is located;
step five, judging whether the minimum value of the row is equal to the minimum value of the row where the minimum value of the row is located, if so, executing the step six, otherwise, executing the step four;
and sixthly, respectively using the line number and the column number of the minimum value of the line as indexes, finding out the frequency of the spectral line with the maximum spectral peak amplitude corresponding to the serial number in the frequency spectrum from the super-resolution spectral peaks of the positive frequency modulation period and the negative frequency modulation period, and forming a beat frequency pair.
7. The method of terrain inversion of radar altimeters in undulating terrain background of claim 1, wherein: the distance calculation formula of the frequency modulation continuous wave radar in the step (10 a) is as follows:
wherein R is k Denotes the distance of the kth scattering point corresponding to the paired spectral peak, f k+ Representing the first frequency value, f, of the kth beat frequency pair k- Representing the second frequency value in the kth beat frequency pair.
8. The method of terrain inversion of radar altimeters in undulating terrain background of claim 3, wherein: the pitch angle of the scattering point relative to the altimeter radar antenna on the aircraft in the step (10 d) is a pitch angle of the scattering point in a coordinate system of the altimeter radar antenna on the aircraft, and the value of the pitch angle is calculated by the following formula:
wherein arccos represents an inverse cosine function, f dk Representing the doppler frequency of the k-th scattering point.
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