CN110109092B - Radar speed measurement method based on time reversal in multipath environment - Google Patents
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- G—PHYSICS
- G01—MEASURING; TESTING
- 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
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
- G01S13/50—Systems of measurement based on relative movement of target
- G01S13/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
- G01S13/581—Velocity or trajectory determination systems; Sense-of-movement determination systems using transmission of interrupted pulse modulated waves and based upon the Doppler effect resulting from movement of targets
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- G—PHYSICS
- G01—MEASURING; TESTING
- 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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- 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
- G01S7/418—Theoretical aspects
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Abstract
The invention discloses a radar speed measurement method based on time reversal in a multipath environment, which mainly solves the problem that the prior art has low signal-to-noise ratio of echo signals and poor speed measurement performance in the multipath environment. The implementation scheme is as follows: transmitting a pulse train signal to obtain radar echo data; normalizing the energy of echo data and performing time reversal to obtain an echo signal; obtaining a time echo signal by retransmitting the time echo signal; and performing pulse compression processing and Doppler filtering on the time reversal echo signals to obtain the Doppler frequency of the moving target, and solving the radial velocity of the target by using the relation between the Doppler frequency of the target and the radial velocity of the target. The invention carries out energy normalization time inversion on the echo signals and retransmits the echo signals, so that the time reversal signals form energy accumulation at the target, the signal-to-noise ratio of the time reversal echoes is improved, the radar speed measurement performance is effectively improved, and the method can be used for measuring the speed of the low-altitude multipath environment target.
Description
Technical Field
The invention belongs to the technical field of radars, and particularly relates to target speed measurement, in particular to a radar speed measurement method based on time reversal in a multipath environment, which can be used for measuring the speed of a low-altitude multipath environment target.
Background
When the radar measures the speed of a target in a low-altitude environment on the sea surface, the radar is influenced by multipath effects, namely echoes received by the radar comprise direct wave signals from the target and multipath wave signals reflected by a reflecting surface. When the direct wave signal and the multipath wave signal are superposed, the finally formed signal is different from the signal of the direct wave which enters the receiver through one-way irradiation in amplitude and phase, and the amplitude of the vector superposed signal is lower than that of the direct wave signal. Multipath effects cause the amplitude of the echo signal to decay, reducing the signal-to-noise ratio of the echo signal. In a multipath environment, if a traditional pulse accumulation method is used for measuring the speed, the method is influenced by multipath and noise, the signal-to-noise ratio of echo signals is low, the speed measurement precision is low, and the speed of a target cannot be accurately estimated.
Aiming at the influence of multipath effect on the measurement of the target speed of the radar in the sea surface low-altitude environment, the existing method mainly inhibits multipath signals from two aspects of an airspace and a time domain to obtain the real speed information of the target. In the aspect of airspace, multipath signals cannot enter a radar receiving antenna by designing a reasonable antenna position or controlling the direction of an antenna directional diagram, and the method can inhibit the influence of multipath effect to a certain extent, but cannot be applied to all situations, and has high implementation cost and poor flexibility. In the time domain, techniques such as narrow correlation may be utilized, but may introduce estimation errors.
In summary, the pulse accumulation velocity estimation method in the prior art is affected by the multipath effect, and the accuracy is low under the condition of low signal-to-noise ratio, and the problems of high cost and small application range exist when multipath is suppressed by the space domain and time domain methods.
Disclosure of Invention
The invention aims to provide a radar speed measurement method based on time reversal aiming at the defects of the method, so that the signal-to-noise ratio of echo signals is improved by effectively utilizing multipath information, and the radar speed measurement performance is improved.
In order to achieve the purpose, the technical scheme of the invention comprises the following steps:
(1) The radar generates a chirp burst signal s (t) and transmits the chirp burst signal s (t) to the environment to obtain a radar echo signal s r (t)；
(2) Radar echo signal s r (t) carrying out energy normalization and time reversal processing to obtain radar time reversal signal
(3) Will be time-reversed
Re-transmitted to the environment to obtain a time-echo signal of
(4) Time-setting echo signal
Sequentially carrying out frequency mixing, filtering, pulse compression and Doppler filtering to obtain the Doppler frequency f of the echo signal of the moving target along the direction of the direct wave d1 And echo signals along multipath wave directionsLe frequency f d2 ；
(5) According to the Doppler frequency f of the echo signal of the moving target along the direction of the direct wave d1 To find a target radial velocity v d ：
And lambda is the wavelength of the transmitted signal, and the target speed parameter is measured by the radar based on time reversal in the multipath environment.
Compared with the prior art, the invention has the following advantages:
1. the signal-to-noise ratio of the echo signal is improved.
Under the multipath environment, the target detection performance is reduced due to the influence of noise if the signal-to-noise ratio is too low. According to the invention, the echo signals are subjected to time reversal and are retransmitted to the environment, and the time reversal signals generate a self-adaptive focusing effect on the channel through the time reversal, so that the signal-to-noise ratio of the time reversal echoes is enhanced.
2. The effective utilization of the multipath information is improved.
In order to weaken the influence of multipath effect on speed measurement, the traditional method adopts a method for inhibiting the multipath effect, the method carries out time reversal retransmission on the radar received echo signal, effectively utilizes the multipath information to carry out speed estimation, has lower cost and is suitable for various delayed multipath conditions.
3. The speed measurement performance is improved.
In the process of time reversal of a plurality of echo pulses, the invention effectively combines the advantages of a time reversal technology and a pulse accumulation technology and improves the velocity measurement performance.
Drawings
FIG. 1 is a flow chart of an implementation of the present invention;
FIG. 2 is a graph comparing the results of pulse compression using the pulse accumulation method with the method of the present invention at a signal-to-noise ratio of 10 dB;
FIG. 3 is a graph of the results of Doppler measurements using the pulse accumulation method at a signal-to-noise ratio of 10 dB;
FIG. 4 is a graph of the results of Doppler measurements made using the method of the present invention at a signal-to-noise ratio of 10 dB;
FIG. 5 is a graph of the results of Doppler measurements using the pulse accumulation method at a signal-to-noise ratio of-15 dB;
figure 6 is a graph of the results of doppler measurements made using the method of the present invention at a signal-to-noise ratio of-15 dB.
Detailed Description
The embodiments and effects of the present invention will be further described in detail below with reference to the accompanying drawings:
referring to fig. 1, the specific implementation steps of the present invention are as follows:
step 1: and acquiring a radar echo signal.
The chirp signal transmitted by the radar is:
wherein,
which represents the complex envelope of the signal,
T e for transmitting pulse width, T r Is the pulse transmission period, N is the number of transmission pulses, f 0 In order to be the center frequency of the signal,
frequency modulation slope, B frequency modulation bandwidth;
echo signals s received by radar r (t) is:
in the formula, s d (t) represents the portion of the signal reflected from the target through the direct wave channel, s m (t) represents the portion of the signal reflected from the object through the multipath wave channel, and n (t) is the variance σ 2 Eta is the multipath wave channel complex electrical energyThe magnetic scattering coefficient of the magnetic particles,
v d and v m Respectively representing the target radial velocity and the multi-path wave direction radial velocity in the direction of the direct wave, c representing the speed of light, t d Representing the transmission delay of the signal along the direction of the direct wave, delta tau representing the transmission delay difference between the signal along the direction of the multipath wave and the signal along the direction of the direct wave, N being the number of the transmitted pulses, T e For transmitting pulse width, T r For a pulse transmission period, f 0 Mu is the chirp rate for the signal center frequency.
And 2, step: for echo signal s r (t) carrying out energy normalization and time reversal processing to obtain time reversal emission signals
2a) From the transmitted signal s (t) and the echo signal s r (t) calculating an energy normalization factor K:
2b) Will echo s r (t) carrying out energy normalization and time reversal to obtain time reversal signal
Ignoring the constant term, one can get:
wherein * For conjugate operation, N is the number of transmitted pulses, T e For transmitting pulse width, T r For a pulse transmission period, f 0 Is the signal center frequency, mu is the chirp rate,
v d and v m Respectively representing the radial speed of the target in the direction of direct wave and the speed of the target in the direction of multipath wave, c representing the speed of light, t d The transmission delay of the signal along the direction of the direct wave is represented, eta is the complex electromagnetic scattering coefficient of the multipath wave channel, delta tau represents the transmission delay difference of the signal along the multipath wave direction and the signal along the direction of the direct wave,
representing the complex envelope of the signal, f d1 And f d2 Respectively representing Doppler frequencies of direct wave direction transmission signals and multipath wave direction transmission signals;
and step 3: according to the signal
Obtaining a time-of-arrival echo signal of
Will be time-reversed
Re-transmitted to the environment to obtain a time-echo signal of
Wherein,
v d and v m Respectively representing the radial speed of the target in the direction of direct wave and the speed of the target in the direction of multipath wave, c representing the speed of light, t d Representing the propagation delay of the signal in the direction of the direct wave,eta is complex electromagnetic scattering coefficient of multipath wave channel, delta tau is transmission delay difference of signal along multipath wave direction and signal along direct wave direction, w (t) is variance sigma 2 White gaussian noise signal.
And 4, step 4: time-setting echo signal
And preprocessing to obtain target Doppler information.
4a) The time anti-echo signals are sequentially subjected to frequency mixing and filtering to obtain baseband complex signals of the received time anti-echo signals
Comprises the following steps:
wherein, * in order to perform the conjugate operation,
representing the complex envelope of the signal, f d1 For transmitting the Doppler frequency, t, of the signal in the direction of the direct wave d Representing the propagation delay of the signal in the direction of the direct wave, eta is the complex electromagnetic scattering coefficient of the multipath wave channel, f d2 Transmitting Doppler frequency of signals in a multipath wave direction, wherein delta tau represents the transmission delay difference of the signals in the multipath wave direction and the signals in a direct wave direction;
4b) Let the impulse response of the matched filter needed for pulse compression be:
using the impulse response to the baseband complex signal
Performing pulse compression to obtain output signal s after pulse compression o (t) is:
in the formula,
is a convolution symbol;
4c) For the output signal s after pulse compression o (t) performing fast Fourier transform to obtain the Doppler frequency f of the transmission signal in the direction of the direct wave d1 And Doppler frequency f of multipath wave direction transmission signal d2 。
Baseband complex signals based on time anti-echo signals
The expression (c) shows that the output signal s after pulse compression o (t) the filtering result obtained after the fast Fourier transform processing is 2f d1 ，2f d2 And f d1 +f d2 The Doppler frequency f of the transmission signal in the direction of the direct wave can be obtained according to the filtering result obtained after the fast Fourier transform processing d1 And Doppler frequency f of multipath wave direction transmission signal d2 。
And 5: doppler frequency f of signal transmission by direct wave direction d1 Obtaining the target speed:
and lambda is the wavelength of the transmitted signal, and the radar target speed measurement based on time reversal in the multipath environment is completed.
The effects of the present invention can be further verified by the following simulation experiments.
1. Experimental scenario
The radar transmits 64 linear frequency modulation pulse signals, the center frequency of the signals is 8GHz, the bandwidth is 50MHz, the pulse repetition period is 50us, the height of an antenna is 50m, the height of a target is 60m, the target distance is 1km, the modulus of the multipath reflection coefficient is 0.9, the radial speed of the target is 60m/s, and the Doppler frequencies of the signals of the target along the direct wave direction and the signals along the multipath wave direction are 3200Hz and 3184Hz respectively.
2. Analysis of Experimental Contents and Experimental results
Experiment 3, in the case of SNR of 10dB, doppler measurement was performed by the method of the present invention, and the result is shown in FIG. 4 (a), in which FIG. 4 (b) is a partially enlarged view of FIG. 4 (a), the ordinate of FIG. 4 (b) is the number of pulses, the number of pulses in the result of Doppler measurement by the method of the present invention was 53, and the Doppler frequency obtained by the method of the present invention was 2f d1 ，f d1 The final doppler frequency of the target direct wave direction signal is 3125Hz, and the corresponding target radial velocity is 58.59m/s, and it can be seen from fig. 3 and 4 that the results of doppler measurement using the conventional method and the method of the present invention are the same, which is due to the limitation of the number of transmitted pulses, thereby limiting the final velocity measurement accuracy.
Experiment 4, in the case that the SNR is-10 dB, the doppler measurement is performed by using the conventional method, and the result is shown in fig. 5, and it can be seen from fig. 5 that the doppler information of the target cannot be measured by the conventional method, which indicates that the conventional velocity measurement method fails in the case of low SNR.
Experiment 5, under the condition that the SNR is-10 dB, the Doppler measurement is carried out by using the method of the invention, and the result is shown in figure 6, and as can be seen from figure 6, under the condition of low SNR, the method of the invention can measure the Doppler information of the target, the measured Doppler frequency of the signal in the direction of the direct wave of the target is 3125Hz, and the corresponding radial velocity of the target is 58.59m/s.
As can be seen from fig. 3, 4, 5 and 6, in the case of high SNR, the doppler measurement results using the conventional method and the method of the present invention are the same due to the limitation of the number of transmitted pulses, but in the case of low SNR, the conventional method cannot measure the doppler information of the target, whereas the method of the present invention can measure the doppler information of the target.
In conclusion, the method effectively combines the time reversal technology and the coherent accumulation technology, utilizes the multipath information, improves the SNR of the target echo signal and improves the speed measurement performance.
Claims (5)
1. A radar speed measurement method based on time reversal in a multipath environment is characterized by comprising the following steps:
(1) The radar generates a chirp burst signal s (t) and transmits the chirp burst signal s (t) to the environment to obtain a radar echo signal s r (t)；
(2) Radar echo signal s r (t) carrying out energy normalization and time reversal processing to obtain radar time reversal signal
(3) Will be time-reversed
Re-transmitted to the environment to obtain a time-echo signal of
(4) Time-setting echo signal
Sequentially carrying out frequency mixing, filtering, pulse compression and Doppler filtering to obtain the moving target edge straightnessDoppler frequency f of echo signal in wave arrival direction d1 And the Doppler frequency f of the echo signal in the direction of the multipath wave d2 ；
(5) According to the Doppler frequency f of the echo signal of the moving target along the direction of the direct wave d1 Finding the target radial velocity v d ：
And lambda is the wavelength of the transmitted signal, and the target speed parameter measurement of the radar based on time reversal in the multipath environment is completed.
2. The method of claim 1, wherein chirp signal s (t) and radar echo signal s (1) are combined r (t) in the following specific form:
s r (t)＝s d (t)+s m (t)+n(t)＝s(γ d (t-t d ))+ηs(γ m (t-t d -Δτ))+n(t)
in the formula,
which represents the complex envelope of the signal,
T e for transmitting pulse width, T r Is a pulse transmitting period, and N is the number of transmitting pulses; f. of 0 In order to be the center frequency of the signal,
is the chirp rate, B is the bandwidth of the tone, s d (t) represents the portion of the signal reflected from the target through the direct wave channel, s m (t) represents the portion of the signal reflected from the object through the multipath wave channel, η is the multipath wave channel complex electromagnetic scattering coefficient,
v d and v m Respectively representing the target radial velocity and the multi-path wave direction radial velocity in the direction of the direct wave, c representing the speed of light, t d Representing the transmission delay of the signal along the direction of the direct wave, delta tau representing the transmission delay difference of the signal along the direction of the multipath wave and the signal along the direction of the direct wave, n (t) being regarded as a Gaussian white noise signal, and the variance being sigma 2 。
3. The method of claim 1, wherein in (2) the radar echo signal s is used r (t) carrying out energy normalization and time reversal processing, and realizing the following steps:
2a) From the transmitted signal s (t) and the echo signal s r (t), calculating an energy normalization factor K:
wherein K is a positive number;
2b) For echo s r (t) carrying out energy normalization and time reversal to obtain time reversal signal
Wherein * Is a conjugate operation.
4. The method of claim 1, wherein the time-echo signal obtained in (3) is
The form is as follows:
wherein eta is the complex electromagnetic scattering coefficient of the multipath wave channel,
v d and v m Respectively representing the target radial velocity and the multi-path wave direction radial velocity in the direction of the direct wave, c representing the speed of light, t d Representing the propagation delay of the signal along the direction of the direct wave, delta tau representing the propagation delay difference of the signal along the direction of the multipath wave and the signal along the direction of the direct wave, and w (t) is the variance sigma 2 White gaussian noise signal.
5. The method according to claim 1, wherein the mixing process, the pulse compression process and the doppler filtering are sequentially performed on the time-delayed echo signals in (4), and are specifically implemented as follows:
4a) Time-setting echo signal
Mixing to obtain baseband complex signal of time anti-echo signal
In the formula, K represents an energy normalization factor;
representing the complex envelope of the signal, t d Representing the propagation delay of the signal in the direction of the direct wave, eta being the multipath wave signalTrace complex electromagnetic scattering coefficient, delta tau represents the transmission delay difference of signal along multipath wave direction and signal along direct wave direction, f d1 Doppler frequency, f, of signals in the direction of the direct wave for a moving object d2 The Doppler frequency of the echo signal along the direction of the multipath wave;
4b) For is to
The signal is processed by pulse compression, the impulse response function of the matched filter required by the pulse compression is
Obtaining a pulse-compressed signal s o (t) is:
4c) For the pulse-compressed signal s o (t) Doppler filtering is carried out, fast Fourier transform processing is carried out on the Doppler filtering, and the Doppler frequency f of the echo signal of the moving target along the direction of the direct wave is obtained d1 And the Doppler frequency f of the echo signal in the direction of the multipath wave d2 ：
Baseband complex signal based on time echo signal
The expression (c) shows that the output signal s after pulse compression o (t) the filtering result obtained after the fast Fourier transform processing is 2f d1 ，2f d2 And f d1 +f d2 The Doppler frequency f of the transmission signal in the direction of the direct wave can be obtained according to the filtering result obtained after the fast Fourier transform processing d1 And Doppler frequency f of multipath wave direction transmission signal d2 。
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