Phase correction system for multichannel foresight array imaging radar

Abstract

The invention discloses a phase correction system for a multi-channel foresight array imaging radar, and belongs to the field of radar imaging. Comprising the steps that a microwave switch is arranged at the rear end of an antenna array element of a receiving antenna array, and a one-to-N power divider is arranged in front of the microwave switch; the signal generator is connected with the microwave switch through the one-to-N power divider on one hand, and is connected with the transmitting antenna through the power amplifier unit on the other hand; according to the invention, based on the foresight array imaging radar under the FMCW system, a practical and feasible phase correction system is provided by considering the influence of inconsistent receiving channel phases on imaging quality in practical engineering, and the system is embedded into an imaging algorithm, so that the influence of inconsistent multi-channel receiving phases of the foresight array imaging radar under the FMCW system is solved.

Description

Technical Field

[0001] The present invention relates to the field of radar imaging, and in particular to a phase correction system for a multi-channel forward-looking array imaging radar. Background Art

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] Array antenna technology can realize radar forward imaging. It only needs to place a uniformly distributed array antenna in a plane parallel to the ground along the vertical heading direction to realize imaging of the area in front of and below the platform. By placing the array antenna as the receiving antenna, the two targets on both sides of the route that are symmetrical to a certain antenna array element must be asymmetrical relative to other antenna array elements. The echo signals of different antenna array elements can solve the previous ambiguity problem; and although the Doppler frequency gradient in this mode is very low compared to the side-looking imaging radar, a long synthetic aperture can still be obtained, so high cross-heading resolution can be obtained.

[0004] “FS Imaging Method of Airborne Forward-Looking Array SAR Based on FMCW”, [J] Journal of the Graduate School of the Chinese Academy of Sciences, 2013, 30(3): 372-379. “Study on the Influence and Compensation of Array Antenna Micromotion on Forward-Looking SAR Imaging”, [J] Journal of Electronics and Information Technology, 2011, 33(4): 831-837. “Study on Motion Compensation of Airborne Forward-Looking Array SAR” Journal of Radars, 2013, 2(2): 168-178. The above three papers studied the imaging algorithm of forward-looking array imaging radar, proposed an improved frequency scaling algorithm (FSA), and combined with the geometric model of forward-looking array imaging radar, considered the deformation and flutter of the antenna due to factors such as engine vibration and the influence of the actual airborne platform attitude change, analyzed the micromotion characteristics and motion error model of the forward-looking array imaging radar antenna, and proposed micromotion compensation method and motion error correction method. However, the forward-looking array radar is essentially a multi-channel receiving system, which is usually composed of a multi-channel receiving antenna, a radio frequency switch matrix and a down-conversion channel. The research in the above literature has not considered the impact of the phase inconsistency of this receiving channel on the imaging quality. Summary of the invention

[0005] The purpose of the present invention is to provide a phase correction system for a multi-channel forward-looking array imaging radar to solve the problem of phase inconsistency between receiving channels of a forward-looking array imaging radar.

[0006] The technical solution of the present invention is as follows:

[0007] A phase correction system for a multi-channel forward-looking array imaging radar comprises: a microwave switch is arranged at the rear end of an antenna array element of a receiving antenna array, and a 1-to-N power divider is arranged in front of the microwave switch; a signal generator is connected to the microwave switch through the 1-to-N power divider on the one hand, and is connected to a transmitting antenna through a power amplifier unit on the other hand.

[0008] Furthermore, the microwave switch switches between adjacent array elements at a fixed frequency, the microwave switch is synchronized with the signal generator, and its switching frequency is set to the transmission repetition frequency of the FMCW.

[0009] Furthermore, the microwave switch internally includes N receiving channels and an N-to-1 switch, and each receiving channel has two optional input signals, which can be an echo signal from an antenna array element or a correction signal from a power divider.

[0010] Furthermore, under the control of the synchronization signal, the signal generator generates a broadband radar signal, which is amplified by the power amplifier unit and radiated by the transmitting antenna; the radar echo signal is then reflected by the target area and received by the receiving antenna array, and the microwave switch, under the control of the synchronization signal, selects to output the radar echo signal according to a specific rule; the output signal is conjugate multiplied with the reference signal, and the result of the multiplication is sampled by AD and can be used for subsequent radar imaging processing.

[0011] Furthermore, the instantaneous output of the microwave switch can only be connected to the output of a certain antenna array element.

[0012] Furthermore, the correction signal is generated by a signal generator and connected to a microwave switch after power division; each correction signal in the microwave switch is time-sharingly passed through an N-to-1 switch and then conjugate-multiplied with the reference signal, and finally each conjugate-multiplied signal is AD-collected, cached and phase-extracted in turn.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] Based on the forward-looking array imaging radar under the FMCW system, the present invention considers the influence of the phase inconsistency of the receiving channels on the imaging quality in actual engineering, provides a practical and feasible phase correction system, and embeds this system into the imaging algorithm, thereby solving the influence of the phase inconsistency of the multi-channel receiving of the forward-looking array imaging radar under the FMCW system. The phase correction system can also be extended to the radar signal processing process based on phase information of other multi-channel receiving system radars. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the principle block diagram of the imaging radar correction mode;

[0016] Figure 2 It is a block diagram of a forward-looking array imaging radar project implementation;

[0017] Figure 3 This is the geometric relationship diagram of the forward-looking array imaging radar;

[0018] Figure 4 The influence of channel phase consistency on radar echo;

[0019] Figure 5 This is the flow chart of the FS algorithm of the forward-looking array imaging radar based on FMCW;

[0020] Figure 6 The imaging results of multiple point targets (a), the magnified results of the intermediate point targets (b) and their cross-sectional views (c)(d);

[0021] Figure 7 The imaging results of multiple point targets (a), the magnified results of the intermediate point targets (b) and their cross-sectional views (c)(d)

[0022] (Phase difference ±45°);

[0023] Figure 8 The imaging results of multi-point targets (a), the magnified results of the intermediate point targets (b) and their cross-sectional views (c) and (d) (under the condition of phase difference ±90°);

[0024] Fig. 9 The imaging results of multi-point targets (a), the magnified results of the intermediate point targets (b) and their cross-sectional views (c) and (d) (under the condition of phase difference of ±180°);

[0025] Fig.10 The imaging results of multi-point targets (a), the magnified results of the intermediate point targets (b) and their cross-sectional views (c) and (d) (when the power divider phase difference is ±10°). DETAILED DESCRIPTION

[0026] It should be noted that relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0027] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0028] Embodiment 1

[0029] In practical applications, the forward-looking array imaging radar under the FMCW system has the advantages of small size, light weight, and easy installation on light and small platforms such as drones. Because it uses array antennas to achieve radar imaging, the forward-looking array radar system is essentially a multi-channel system.

[0030] like Figure 3 As shown in , the transmitting antenna is located at h directly below the center of the array, and all array elements located on the array antenna receive echo signals, and all array elements are distributed with equal spacing.

[0031] For FMCW signals, the Dechirp echo reception method is generally used. Without considering the influence of signal amplitude, the signal form after radar echo reception is:

[0032]

[0033] Among them, t' represents the distance to time, y n represents the position of the λth array element in the cross-row upward direction, λ is the wavelength, c is the speed of light, γ is the modulation frequency of the transmitted signal, and R ref is the slant distance from the center of the scene, and R' represents the equivalent instantaneous slant distance.

[0034] The first phase term in formula (1) represents the Doppler process, the second phase term represents the distance information including the range migration, and the third term is the residual video phase (RVP term). The data after Dechirp is not aligned in the range domain because of the influence of this RVP term. The first and second phase terms are the phase information necessary for imaging; the third phase term affects the imaging quality and must be corrected in the imaging process.

[0035] A practical engineering implementation block diagram of a forward-looking array imaging radar is shown below: Figure 2 As shown in the figure, the signal generator generates a broadband radar signal (such as a linear frequency modulation signal) under the control of the synchronization signal, which is radiated by the transmitting antenna after being amplified by the power amplifier unit; the radar echo signal is then received by the receiving antenna array after being reflected by the target area, and the microwave switch selects the output of the radar echo signal according to a specific rule under the control of the synchronization signal. The instantaneous output of the microwave switch can only be connected to the output of a certain antenna array element, and then conjugate multiplied with the reference signal (Dechirp processing), and the result of this multiplication is sampled by AD (the sampling rate is much smaller than the radar signal bandwidth at this time) for subsequent radar imaging processing.

[0036] Assume that the transmitted signal and the reference signal are:

[0037]

[0038] Among them, f c is the center frequency of the transmitted signal.

[0039] Assume that the distance from a point target to the radar is R i , then the target signal received by the radar is:

[0040]

[0041] After the target signal is conjugate-multiplied by the reference signal, the signal form after Dechirp reception can be obtained:

[0042] S2(y n ,t')=s r (y n ,t').[s ref (y n ,t')] * (5)

[0043] For radar imaging, the imaging quality is mainly contained in the phase information.

[0044] In formula (1), it is assumed that the phase of the receiving channel is completely consistent, which has nothing to do with the frequency and different receiving channels, but this is obviously impossible in practice. Considering the influence of processing level, process, equipment consistency, etc. on antenna array elements, RF cables, and RF channels, different antenna elements themselves, RF cables from each array element to the microwave switch, and the internal channels of the microwave switch will cause inconsistency in the phase of the receiving channel, which will lead to the deterioration of imaging quality.

[0045] Depend on Figure 2 It can be seen that the array antenna contains multiple receiving elements, each of which can independently output radar echo signals. If a set of Dechirp receiving and AD acquisition equipment is designed at the back end of each antenna element, the system complexity is obviously unimaginable.

[0046] An effective processing method is to arrange a microwave switch at the back end of the array element. The switch switches between adjacent array elements at a fixed frequency. The microwave switch needs to be synchronized with the signal generator, and its switching frequency is set to the FMCW transmission repetition rate. The advantage of this is that the radar echo signal can be received in time by switching the microwave switch. Only one set of Dechirp receiving and one set of AD acquisition equipment are needed at the back end to complete the echo data acquisition of the forward-looking array imaging radar system.

[0047] A dedicated phase correction mode can be added to the forward-looking array imaging radar system. A feasible implementation scheme is as follows Figure 1 shown.

[0048] A 1-to-N power divider is placed in front of the microwave switch. The correction signal is generated by the signal generator and connected to the microwave switch after power division. The microwave switch contains N receiving channels and an N-to-1 switch. Each receiving channel has two input signals to choose from. You can choose the echo signal from the antenna array element or the correction signal from the power divider. Each correction signal is time-sharingly passed through the N-to-1 switch and conjugated and multiplied with the reference signal (equivalent to Dechirp processing). Finally, each Dechirp signal is collected, cached, and phase extracted by AD.

[0049] The working sequence of the radar system in this mode is as follows:

[0050] 1. The switches of channels 1 to N inside the microwave switch are all switched to the ports connected to the 1-to-N power splitter;

[0051] 2. Set the distance value of the reference signal to 0;

[0052] 3. The control signal generator generates a correction signal output (its essence is still a broadband radar signal, which is the same as the conventional working mode);

[0053] 4. Control the N-to-1 switch so that the correction signal of channel 1 is output to the multiplier input, conjugate-multiplied with the reference signal, and AD extracts the phase under timing control;

[0054] 5. Control the N-to-1 switch so that the correction signal of channel 2 is output to the multiplier input, conjugate-multiplied with the reference signal, and AD extracts the phase under timing control;

[0055] 6. Traverse all N channels in turn and extract the corresponding phases.

[0056] The key to this mode is to control the signal generation, the timing of the N-to-1 switch and the AD acquisition timing, so that the output signal of the N-to-1 switch and the reference signal and the AD acquisition timing are perfectly matched.

[0057] The transmitted correction signal is consistent with formula (2), and the reference distance R is taken ref is zero, then the reference signal is:

[0058]

[0059] Depend on Figure 1 It can be seen that the correction signal is:

[0060]

[0061] The signal output by the N-to-1 switch inside the microwave switch is:

[0062] s c-out (y n ,t')=sc (y n ,t')·exp[jθ] (8)

[0063] Where θ is the phase introduced by different receiving channels.

[0064] The signal obtained by conjugating the reference signal and multiplying the output signal of the N-to-1 switch is:

[0065] s c-out ·s ref =s·s * exp[jθ]=exp[jθ] (9)

[0066] From the above analysis, it can be seen that the result of the conjugate multiplication of each channel in the correction mode is only related to the phase of each receiving channel itself, and has nothing to do with other phases.

[0067] After AD acquisition, the phases of different receiving channels can be calculated in real time.

[0068] It should be noted that the phase consistency requirements of the 1-to-N power divider are relatively high, because the phase introduced by this part cannot be corrected through this correction method; in addition, the phase consistency of the antenna array element itself and the RF cable between the antenna array element and the microwave switch can only be controlled during the production process; if it is necessary to further correct the phase inconsistency of this part, it is necessary to adopt a system-level correction method, such as SAR external calibration.

[0069] Embodiment 2

[0070] This embodiment takes into account the inconsistency of the phase of the receiving channel and the influence of frequency on the phase. The phase introduced by different receiving channels at different frequencies, the target signal received by the radar becomes:

[0071]

[0072] Among them, θ can be expressed as follows

[0073]

[0074] The θ phase factor 1, 2…n represents N different receiving channels, and the phase factor 1, 2…m represents M different frequencies.

[0075] Conjugate multiplication can also be understood as the phase difference between two receiving channels. Therefore, in the above factors, each row represents the phase difference between a single receiving channel with different frequencies and the channel where the reference signal is located, and each column represents the phase difference between the receiving channels with different frequencies of the same signal and the channel where the reference signal is located. As can be seen from the above, this phase factor can be measured. Therefore, the signal form after Dechirp reception is:

[0076]

[0077] Figure 4 The simulation results of three point targets when the phase difference between the receiving channels changes in different ranges are simulated. These three point targets are the same in the range direction and 50m apart in the cross-course direction. The range directions of the three point targets in the results are different, which is range migration and needs to be corrected during the imaging process. We will not pay attention to it here. From the results, the greater the difference in the phase difference of the receiving channels, the greater the cross-course blur, so correction measures need to be considered.

[0078] When imaging with forward-looking array radar, the influence of channel phase inconsistency must be considered. According to the above analysis, its phase correction factor H pha for:

[0079] H pha =exp[-jθ n、m ] (13)

[0080] The rest of the radar imaging algorithm process is as follows Figure 5 The Doppler frequency shift correction factor H is dop , tilt factor H SK for:

[0081]

[0082] Frequency scaling function H FS , RVP correction function H RVPC , inverse scalar function H IFS They are:

[0083]

[0084] Block frequency shift factor H BV , secondary distance compression correction factor H SRC They are:

[0085] H BV (K Y ,ΔK R )=exp[-j(A Y R ref -R B )ΔK R ] (19)

[0086]

[0087] Reference signal H involved in azimuth compression aref for:

[0088] H aref (K Y ,r;R B ) = exp[jAY K RC R B ] (twenty one)

[0089] Finally, we get a focused image:

[0090]

[0091] To verify the accuracy of the above analysis and the effectiveness of the proposed phase correction method, multiple point target simulation analyses were carried out. The parameters are shown in Table 1, and the positions of the point targets are shown in Table 2.

[0092] Table 1 Simulation parameters

[0093] parameter Numeric parameter Numeric Carrier frequency 35GHZ Number of receiving elements 128 Sweep bandwidth 500MHZ Pulse repetition frequency 1KHZ Beam incident angle 60° Array source spacing 0.0086m Beam width 33° Aircraft height 800m Beam width 24° Carrier speed 50m / s

[0094] Table 2 Point target positions

[0095] Point Target P1 P2 P3 P4 P5 P6 P7 P8 P9 Slope distance / m 1550 1550 1550 1600 1600 1600 1650 1650 1650 Cross-course / m -50 0 50 -50 0 50 -50 0 50

[0096] Taking the point target at the coordinates (1600, 0, 1000) as the center, take points once at intervals of 50m in the positive and negative directions of the X axis and 50m in the positive and negative directions of the Y axis to obtain a rectangular simulation scene with 9 points. Figures 6 to 9 (a) in the figure are all imaging simulation results of 9 point targets, and the difference is the different influence of phase difference between receiving channels.

[0097] like Figure 6 (a) The simulation is the imaging result when the receiving channels are completely consistent. The simulation parameters are shown in Table 3. Figure 6 (b), 6(c), and 6(d) are respectively the enlarged result diagram of the intermediate point target, the pulse pressure plane diagram along the range direction, and the pulse pressure plane diagram along the expanded heading direction. Figure 7 (a), 8(a), and 9(a) simulate the imaging results when the phase differences of the receiving channels are ±45°, ±90°, and ±180°, respectively. It can be found that the greater the phase difference between the receiving channels, the greater the impact on the imaging quality and the more serious the image distortion. In addition, the phase inconsistency between the receiving channels obviously has a greater impact on the cross-heading imaging quality. This is because the acquisition of cross-heading sampling data relies on different receiving channels as cross-heading sampling units. Figures 6 to 9 (b), (c), and (d) also verify this conclusion.

[0098] Table 3 Simulation parameters

[0099]

[0100] It can be seen that the phase difference of the receiving channel can be measured and embedded into Figure 5In addition, the effect of the correction signal power divider on the imaging quality needs to be considered. The consistency between channels must be controlled within a certain range, otherwise the above correction imaging algorithm will lose its meaning. A better approach is to control the phase consistency of the power divider within ±10°. The imaging result at this time is as follows: Fig.10 The impact on the image quality is basically negligible, and the PSLR in the range and cross-heading directions are -13.23dB and -13.21dB respectively. The phase consistency of the power divider is controlled within ±10°, which is feasible in the actual development process.

[0101] The above-mentioned embodiments only express the specific implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the protection scope of the present application. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the technical solution concept of the present application, and these all belong to the protection scope of the present application.

[0102] This background section is provided to generally present the context of the invention, and the work of the presently named inventors, the work to the extent described in this background section, and aspects of this section that did not constitute prior art at the time of application are neither explicitly nor implicitly admitted to be prior art to the present invention.

Claims

1. A phase correction system for a multi-channel forward-looking array imaging radar, characterized in that: include: A microwave switch is arranged at the rear end of the antenna array element of the receiving antenna array, and a 1-to-N power divider is arranged in front of the microwave switch; The signal generator is connected to the microwave switch through a 1-to-N power divider on the one hand, and is connected to the transmitting antenna through a power amplifier unit on the other hand.

2. A phase correction system for a multi-channel forward-looking array imaging radar according to claim 1, characterized in that: The microwave switch switches between adjacent array elements at a fixed frequency. The microwave switch is synchronized with the signal generator, and its switching frequency is set to the transmission repetition frequency of FMCW.

3. A phase correction system for a multi-channel forward-looking array imaging radar according to claim 2, characterized in that: The microwave switch contains N receiving channels and an N-to-1 switch. Each receiving channel has two input signals to choose from. It can choose the echo signal from the antenna array element or the correction signal from the power divider.

4. A phase correction system for a multi-channel forward-looking array imaging radar according to claim 3, characterized in that: Under the control of the synchronization signal, the signal generator generates a broadband radar signal, which is amplified by the power amplifier unit and radiated by the transmitting antenna. After being reflected by the target area, the radar echo signal is received by the receiving antenna array. Under the control of the synchronization signal, the microwave switch selects the output of the radar echo signal according to a specific rule. The output signal is conjugate multiplied with the reference signal, and the result of the multiplication is sampled by AD and can be used for subsequent radar imaging processing.

5. The phase correction system for a multi-channel forward-looking array imaging radar according to claim 4, characterized in that: The instantaneous output of the microwave switch can only be connected to the output of a certain antenna array element.

6. A phase correction system for a multi-channel forward-looking array imaging radar according to claim 5, characterized in that: The correction signal is generated by a signal generator and connected to a microwave switch after power division. Each correction signal in the microwave switch is time-sharingly passed through an N-to-1 switch and then conjugate-multiplied with the reference signal. Finally, each conjugate-multiplied signal is AD-collected, cached, and phase-extracted in turn.

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