Satellite-borne GNSS-S multi-station radar right downward-looking large-breadth SAR (Synthetic Aperture Radar) imaging method

Abstract

The invention relates to a satellite-borne GNSS-S multi-station radar direct downward-looking large-breadth SAR imaging method. The method comprises the following steps that S1, direct downward-looking work is conducted on a multi-channel array antenna through the distance of a satellite-borne GNSS-S radar echo receiving system; s2, dividing the downward-looking detection area into a left side area, a middle area and a right side area, performing digital beam forming processing on the distance direction multi-channel echo signals by the receiving host, and recovering left side area echoes, middle area echoes and right side area echoes; and S3, based on a navigation satellite signal selection rule, performing direct downward view large-breadth imaging. According to the invention, the system has the advantage of direct downward view large-width SAR imaging, can achieve the synchronous imaging detection of a left side region, a right lower region and a right side region, and greatly improves the detection efficiency of the system.

Description

Technical Field

[0001] The present invention relates to the field of radar technology in the electronic information industry, and in particular to a satellite-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method. Background Art

[0002] For a long time, SAR refocusing imaging and high-precision positioning of moving ship targets on the sea surface have been a hot topic in scientific research. Spaceborne SAR systems can penetrate clouds and fog, and have all-day and all-weather sea observations, which are suitable for high-resolution imaging and detection applications of sea surface ship targets. However, existing spaceborne SAR systems are affected by the velocity vector of moving ships, resulting in low imaging quality, mainly manifested as energy defocusing and position offset. At present, there are many adaptive search and estimation methods to search for the speed of moving ship targets, but the existing single-station SAR or dual-station SAR lacks sufficient information dimensions or quantity, making it difficult to find the optimal evaluation function, which limits the search dimension or accuracy of the speed of moving ship targets, and can only achieve limited refocusing SAR imaging, but it is difficult to achieve high-precision uncontrolled positioning.

[0003] In 2020, my country's BeiDou-3 satellite navigation system was officially completed and put into operation. Together with the US GPS, it constitutes a global satellite navigation system (GNSS), providing a spatial diversity transmission source for new detection. Both at home and abroad, the use of reflected signals of navigation satellite signals (GNSS-R) has been realized to detect sea surface wind fields, and low-orbit satellite-mounted experiments have been realized. At the same time, many research institutions at home and abroad have used scattered signals of navigation satellite signals (GNSS-S) on the ground to realize satellite-to-ground dual-station SAR imaging, laying a technical foundation for satellite-borne GNSS-S multi-station radar detection.

[0004] Since the single-side imaging detection requires the receiving antenna to swing to a certain angle, the dual-side imaging detection efficiency is low, and the satellite platform needs to maneuver to achieve dual-side imaging. Existing satellite-borne radars are also unable to achieve imaging detection of the area directly below the satellite, which in turn affects the detection efficiency of the entire satellite-borne radar.

[0005] Therefore, designing a spaceborne GNSS-S multi-station radar downward-looking wide-band SAR imaging method is an urgent problem to be solved. Summary of the invention

[0006] In order to solve the technical problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a satellite-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method, which has the advantages of downward-looking wide-bandwidth SAR imaging and can realize synchronous imaging detection of the left area, the area directly below and the right area, thereby greatly improving the detection efficiency of the system.

[0007] To achieve the above-mentioned object of the invention, the present invention provides a space-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method, comprising the following steps:

[0008] Step S1, using the distance-direction multi-channel array antenna of the space-borne GNSS-S radar echo receiving system to work directly downward;

[0009] Step S2, dividing the downward detection area into three areas: a left area, a middle area, and a right area, and the receiving host performs digital beamforming processing on the range multi-channel echo signal to restore the left area echo, the middle area echo, and the right area echo;

[0010] Step S3: Based on the navigation satellite signal selection rule, perform downward-looking wide-width imaging.

[0011] According to a technical solution of the present invention, M navigation satellites of the global navigation satellite system are used as transmission sources to construct a satellite-borne GNSS-S radar echo receiving system, wherein the satellite-borne GNSS-S radar echo receiving system comprises a zenith direct antenna, the range multi-channel array antenna and a receiving host, wherein step S1 specifically comprises:

[0012] The array normal direction of the range multi-channel array antenna of the satellite-borne GNSS-S radar echo receiving system is parallel to the normal direction of the sea surface, and the beam pointing accuracy of the range multi-channel array antenna is better than 0.1°;

[0013] The signals of M navigation satellites illuminate the target area at the same time. The target area is a large sea surface area. The N channels in the range direction simultaneously receive the multi-source echo signals of the target area with a wide beam. The baseband complex echo signal of the N channels is denoted as s n (t),n=1,2,...,N,s n (t) is a one-dimensional continuous signal, and any of the echo signals includes scattered echo signals of M navigation satellite signals from the same sea surface, wherein t is a sampling time sequence in the fast direction;

[0014] N echo signals are transmitted with T s Segmented storage for time units.

[0015] According to a technical solution of the present invention, the step S2 specifically includes:

[0016] Step S21, dividing the downward detection area into three areas: a left area, a middle area, and a right area, defining the angle between the normal of the multi-channel array antenna surface and the normal of the sea level as the incident angle, taking the flight direction of the low-orbit satellite as the reference direction, defining the sea surface area with an incident angle greater than 7° on the left side of the flight direction of the low-orbit satellite as the left area, defining the sea surface area with an incident angle greater than 7° on the right side of the flight direction of the low-orbit satellite as the right area, and defining the sea surface area with an incident angle of 7° on the left side to 7° on the right side of the flight direction of the low-orbit satellite as the middle area;

[0017] Step S22: Receive N echo signals s from the host according to the incident angle range of each area. n (t) Perform DBF processing to restore the left echo, middle echo and right echo, and obtain 2P+Q high-gain narrow beam echo signals, still using T s is the time unit for N echo signals s n (t) Perform segmented DBF processing.

[0018] According to a technical solution of the present invention, the step S22 specifically includes:

[0019] Step S221: DBF processing is performed on the left echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the left echo area after DBF processing are recorded as Among them, w 1,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the left area;

[0020] Step S222: DBF processing is performed on the right echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the right echo area after DBF processing are recorded as Among them, w 2,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the right area;

[0021] Step S223: DBF processing is performed on the echoes in the middle area to form Q high-gain narrow beams covering Q small areas. The Q echo signals in the middle area after DBF processing are recorded as Among them, w 3,q,n ,q=1,2,...,Q;n=1,2,...,N are Q groups of complex weights corresponding to the Q DBF processes in the middle area.

[0022] According to a technical solution of the present invention, in step S3, the navigation satellite signal selection rule is:

[0023] The navigation satellite signal on the right is selected as a reference for SAR imaging of the echo in the left area. The angle between the line connecting the navigation satellite and the low-orbit satellite and the normal line of the sea surface is defined as the navigation satellite angle. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the right side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the left area. The number of navigation satellite signal sources that meet this condition is denoted as D, and the D navigation satellite signals are denoted as g. 1,d (t), d = 1, 2, ..., D, where D navigation satellite signals are output by the direct signal receiver and the reference signal generation unit of the receiving host;

[0024] The left navigation satellite signal is selected as a reference to perform SAR imaging on the echo in the right area. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the left side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the right area. The number of navigation satellite signal sources that meet this condition is recorded as E, and the E navigation satellite signals are recorded as g. 2,e (t), e = 1, 2, ..., E, the E navigation satellite signals are output by the direct signal receiver and the reference signal generation unit of the receiving host;

[0025] The navigation satellite signal with a large incident angle is selected as a reference for SAR imaging of the middle area echo. The low-orbit satellite flight direction is used as a reference, and the navigation satellite signals with a navigation satellite angle greater than 35° on the left and right sides of the low-orbit satellite flight direction are used as reference signals for SAR imaging processing in the middle area. The number of navigation satellite signal sources that meet this condition is F, and the F navigation satellite signals are denoted as g. 3,f (t), f = 1, 2, ..., F, the F navigation satellite signals are outputted by the direct signal receiver and the reference signal generating unit of the receiving host in coordination.

[0026] According to a technical solution of the present invention, step S3 specifically includes:

[0027] The wide sea surface echo signal received by looking directly downward is divided into three areas: the left area, the middle area and the right area for synchronous parallel SAR imaging processing. 1,d (t),d=1,2,...,D is used as the reference signal for SAR imaging of the left P narrow beam areas, and the signal g is used 2,e (t), e=1,2,...,E is used as the reference signal for SAR imaging of the P narrow beam areas on the right side, and the signal g is used 3,f (t), f = 1, 2, ..., F as the reference signal for SAR imaging of the middle Q narrow beam areas;

[0028] Perform SAR imaging on each narrow beam area;

[0029] The SAR images of multiple spatially continuous narrow beam areas are spatially spliced ​​to obtain a wide-width SAR image in the downward viewing situation.

[0030] According to a technical solution of the present invention, performing SAR imaging on each narrow beam area specifically includes:

[0031] Divide the narrow beam coverage area into two-dimensional spatial grids along the flight direction and distance direction;

[0032] Arrange the one-dimensional echo signal into a two-dimensional echo signal, taking the time when the corresponding navigation satellite signal arrives at the low-orbit satellite as the starting point, with 2ms as the cycle and 1ms as the step, and transform the one-dimensional echo signal into a two-dimensional echo signal. 1,p (t), r 2,p (t), r 3,q (t) Arranged into a two-dimensional echo signal R 1,p (τ,t),R 2,p (τ,t),R 3,q (τ,t), where τ is the sampling time series in the slow direction;

[0033] The two-dimensional echo signal is pulse compressed in the range direction, and the corresponding navigation satellite signal is used as the reference signal to perform pulse compression processing on the two-dimensional echo signal to obtain a new two-dimensional echo signal with a high signal-to-noise ratio, that is, R 1,p (τ,t),R 2,p (τ,t),R 3,q (τ,t) is converted into

[0034] Remove the influence of navigation satellite message information on echo phase reversal, keep the phase of the multi-channel echo signal corresponding to "1" unchanged, and flip the phase of the multi-channel echo signal corresponding to "0" by 180°;

[0035] The two-dimensional echo signal is processed by back-projection imaging to obtain multiple SAR images of the same narrow beam area. The d-th SAR image of the p-th narrow beam area in the left area is denoted as x 1,p,d (x, y), the e-th SAR image of the p-th narrow beam area in the right area is χ 2,p,e (x, y), the fth SAR image of the qth narrow beam area in the middle area is x 3,q,f (x, y), where x and y are the spatial grid unit numbers in the distance direction and the azimuth direction, respectively.

[0036] According to another aspect of the present invention, a satellite-borne GNSS-S radar echo receiving system for implementing the satellite-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method as described in any one of the above technical solutions is proposed, wherein the satellite-borne GNSS-S radar echo receiving system comprises a zenith direct antenna, a range multi-channel array antenna and a receiving host;

[0037] The zenith direct antenna is used to receive direct signals from multiple navigation satellites;

[0038] The range multi-channel array antenna is used to receive multi-source echo signals from the sea surface;

[0039] The receiving host is used to process multi-source echo signals, and the processing process at least includes amplification, down-conversion, bandpass filtering, sampling quantization, digital IQ demodulation, DBF, and SAR imaging.

[0040] According to a technical solution of the present invention, the zenith direct antenna operates in the L band, with a frequency coverage of 1.2 GHz to 1.3 GHz, and a gain greater than 4 dB within the frequency band;

[0041] The range-direction multi-channel array antenna is composed of N linear array antennas arranged along the range direction, and the 3dB beam width of any of the linear array antennas in the range direction is greater than 100°. Any of the linear array antennas corresponds to an independent high-sensitivity digital receiver, and any of the linear array antennas includes P antenna radiating units. The frequency of the antenna radiating array elements covers 1.2GHz to 1.3GHz, and the gain within the frequency band is greater than 4dB.

[0042] According to a technical solution of the present invention, the receiving host includes a direct signal receiver, a reference signal generation unit, a frequency integrated management unit, N high-sensitivity digital receivers, a radar main control unit, a DBF processing unit, and a SAR imaging processing unit;

[0043] The direct signal receiver outputs the position and speed of the navigation satellite, the navigation message of the navigation satellite signal, and the navigation satellite pseudo code signal with Doppler modulation;

[0044] The reference signal generating unit is used to generate the reference signal required for GNSS-S radar imaging processing;

[0045] The frequency integrated management unit is used to generate the clock signal and local oscillator signal required by the receiving host;

[0046] The high-sensitivity digital receiver is used to amplify, down-convert, band-pass filter, sample, quantize and digitally demodulate the sea surface echo signal to obtain a baseband complex domain echo signal;

[0047] The radar main control unit is used to implement command control of the direct signal receiver, command control of the reference signal generation unit, segmented storage control of the echo data, and transmission control of the echo data;

[0048] The DBF processing unit is used to obtain high-gain narrow beams directed to different areas;

[0049] The SAR imaging processing unit is used to perform SAR imaging on the echo signals of multiple beam areas.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] The present invention proposes a method for downward-looking large-width SAR imaging of a satellite-borne GNSS-S multi-station radar. The method utilizes a range multi-channel array antenna to work downward, synchronously and in parallel receives a wide range of sea surface multi-source echo signals with multiple wide beam angles, divides the downward-looking detection area into three areas: a left area, a middle area and a right area. The receiving host performs digital beamforming processing on the range multi-channel echo signal to restore the left area echo, the middle area echo and the right area echo. The right navigation star signal is selected as a reference to perform SAR imaging on the left area echo, the left navigation star signal is selected as a reference to perform SAR imaging on the right area echo, and the large-incident-angle navigation star signal is selected as a reference to perform SAR imaging on the middle area echo, thereby realizing downward-looking large-width imaging and improving the detection efficiency of the system.

[0052] Furthermore, compared with active satellite-borne radar imaging and detection systems, the present invention has the advantages of strong concealment and low system power consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0054] Figure 1 A schematic diagram of a downward-looking large-scale detection scenario of a space-borne GNSS-S multi-station radar according to an embodiment of the present invention is shown;

[0055] Figure 2 The components of a satellite-borne GNSS-S multi-station radar receiving system according to one embodiment of the present invention are schematically shown;

[0056] Figure 3 Schematically showing the footprint coverage of three partitioned beams recovered by range-oriented DBF processing according to one embodiment of the present invention;

[0057] Figure 4 Schematically showing beam coverage and navigation satellite selection in the left area according to an embodiment of the present invention;

[0058] Figure 5 Schematically showing beam coverage and navigation satellite selection in the right area according to an embodiment of the present invention;

[0059] Figure 6 Schematically showing the beam coverage and navigation satellite selection in the middle area according to an embodiment of the present invention;

[0060] Figure 7 The SAR imaging processing flow of a single beam area in the left region according to an embodiment of the present invention is schematically shown. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] like Figure 1 and Figure 2 As shown, the satellite-borne GNSS-S radar echo receiving system includes a zenith direct antenna 10, a range multi-channel array antenna 20 and a receiving host 30;

[0063] The zenith direct antenna 10 is used to receive direct signals from multiple navigation satellites;

[0064] The range-directional multi-channel array antenna 20 is used to receive multi-source echo signals from the sea surface;

[0065] The receiving host 30 is used to process multi-source echo signals, and the processing process at least includes amplification, down-conversion, bandpass filtering, sampling quantization, digital IQ demodulation, DBF, and SAR imaging.

[0066] The zenith direct antenna 10 operates in the L band, with a frequency coverage of 1.2 GHz to 1.3 GHz, and a gain greater than 4 dB within the frequency band;

[0067] The range multi-channel array antenna 20 is composed of N linear array antennas arranged along the range direction, where N is 8 to 16. The 3dB beam width of any linear array antenna in the range direction is greater than 100°. Any linear array antenna corresponds to an independent high-sensitivity digital receiver 304. Any linear array antenna includes P antenna radiating units, and the number of antenna units P of the linear array is 32 to 64. The frequency of the antenna radiating array element covers 1.2GHz to 1.3GHz, and the gain within the frequency band is greater than 4dB.

[0068] The receiving host 30 includes a direct signal receiver 301, a reference signal generating unit 302, a frequency integrated management unit 303, N high-sensitivity digital receivers 304, a radar main control unit 305, a DBF processing unit 306, and a SAR imaging processing unit 307;

[0069] The direct signal receiver 301 outputs the position and speed of the navigation satellite, the navigation message of the navigation satellite signal, and the navigation satellite pseudo code signal with Doppler modulation;

[0070] The reference signal generating unit 302 is used to generate the reference signal required for GNSS-S radar imaging processing;

[0071] The frequency integrated management unit 303 is used to generate the clock signal and the local oscillator signal required by the receiving host 30;

[0072] The high-sensitivity digital receiver 304 is used to amplify, down-convert, band-pass filter, sample, quantize and digitally demodulate the sea surface echo signal to obtain a baseband complex domain echo signal;

[0073] The radar main control unit 305 is used to implement the command control of the direct signal receiver 301, the command control of the reference signal generation unit 302, the segmented storage control of the echo data, and the transmission control of the echo data;

[0074] The DBF processing unit 306 is used to obtain high-gain narrow beams directed to different areas;

[0075] The SAR imaging processing unit 307 is used to perform SAR imaging on the echo signals of multiple beam regions.

[0076] like Figures 1 to 7 As shown, a space-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method of the present invention comprises the following steps:

[0077] Step S1, using the range multi-channel array antenna 20 of the satellite-borne GNSS-S radar echo receiving system to work directly downward, using M navigation satellites of the global navigation satellite system as the transmitting source, to construct a satellite-borne GNSS-S radar echo receiving system, the satellite-borne GNSS-S radar echo receiving system includes a zenith direct antenna 10, a range multi-channel array antenna 20 and a receiving host 30, specifically including:

[0078] The normal direction of the range multi-channel array antenna 20 of the satellite-borne GNSS-S radar echo receiving system is parallel to the normal direction of the sea surface, and the beam pointing accuracy of the range multi-channel array antenna 20 is better than 0.1°, that is, it works directly downward;

[0079] The signals of M navigation satellites illuminate the target area at the same time. The target area is a large sea surface area. The N channels in the range direction simultaneously receive the multi-source echo signals of the target area with a wide beam. The baseband complex echo signal of the N channels is denoted as s n (t),n=1,2,...,N,s n (t) is a one-dimensional continuous signal. Any echo signal includes the scattered echo signals of the same sea surface to the M navigation satellite signals, where t is the sampling time series in the fast direction;

[0080] N echo signals are transmitted with T s The time unit is segmented and stored to facilitate data caching, transmission and on-orbit processing. The time unit T s The value ranges from 3s to 30s.

[0081] Step S2: The downward detection area is divided into three areas: the left area, the middle area and the right area. The receiving host 30 performs digital beamforming processing on the multi-channel echo signal in the range direction to restore the left area echo, the middle area echo and the right area echo. Figure 3 As shown, specifically including:

[0082] Step S21, dividing the downward detection area into three areas: a left area, a middle area, and a right area, defining the angle between the normal of the multi-channel array antenna surface and the normal of the sea level as the incident angle, taking the flight direction of the low-orbit satellite as the reference direction, defining the sea surface area with an incident angle greater than 7° on the left side of the flight direction of the low-orbit satellite as the left area, defining the sea surface area with an incident angle greater than 7° on the right side of the flight direction of the low-orbit satellite as the right area, and defining the sea surface area with an incident angle of 7° on the left side to 7° on the right side of the flight direction of the low-orbit satellite as the middle area;

[0083] Step S22: According to the incident angle range of each area, the host 30 receives N echo signals s in the range direction. n (t) Perform DBF processing to restore the left echo, middle echo and right echo, and obtain 2P+Q high-gain narrow beam echo signals, still using Ts is the time unit for N echo signals s n (t) Perform segmented DBF processing, including:

[0084] Step S221: DBF processing is performed on the left echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the left echo area after DBF processing are recorded as Among them, w 1,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the left area;

[0085] Step S222: DBF processing is performed on the right echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the right echo area after DBF processing are recorded as Among them, w 2,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the right area;

[0086] Step S223: DBF processing is performed on the echoes in the middle area to form Q high-gain narrow beams covering Q small areas. The Q echo signals in the middle area after DBF processing are recorded as Among them, w 3,q,n ,q=1,2,...,Q;n=1,2,...,N are Q groups of complex weights corresponding to the Q DBF processes in the middle area.

[0087] Step S3, based on the navigation satellite signal selection rule, performing downward-looking wide-band imaging, specifically comprising:

[0088] The wide sea surface echo signal received by looking directly downward is divided into three areas: the left area, the middle area and the right area for synchronous parallel SAR imaging processing. 1,d (t),d=1,2,...,D is used as the reference signal for SAR imaging of the left P narrow beam areas, and the signal g is used 2,e (t), e=1,2,...,E is used as the reference signal for SAR imaging of the P narrow beam areas on the right side, and the signal g is used 3,f (t), f = 1, 2, ..., F as the reference signal for SAR imaging of the middle Q narrow beam areas;

[0089] Perform SAR imaging on each narrow beam area;

[0090] The SAR images of multiple spatially continuous narrow beam areas are spatially spliced ​​to obtain a wide-width SAR image in the downward viewing situation.

[0091] Among them, Figures 4 to 6As shown, the navigation satellite signal selection rules are:

[0092] The navigation satellite signal on the right is selected as a reference for SAR imaging of the echo in the left area. The angle between the line connecting the navigation satellite and the low-orbit satellite and the normal line of the sea surface is defined as the navigation satellite angle. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the right side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the left area. The number of navigation satellite signal sources that meet this condition is denoted as D, and the D navigation satellite signals are denoted as g. 1,d (t), d = 1, 2, ..., D, where D navigation satellite signals are outputted by the direct signal receiver 301 and the reference signal generating unit 302 of the receiving host 30;

[0093] The left navigation satellite signal is selected as a reference to perform SAR imaging on the echo in the right area. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the left side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the right area. The number of navigation satellite signal sources that meet this condition is recorded as E, and the E navigation satellite signals are recorded as g. 2,e (t), e = 1, 2, ..., E, the E navigation satellite signals are outputted by the direct signal receiver 301 and the reference signal generating unit 302 of the receiving host 30 in coordination;

[0094] The navigation satellite signal with a large incident angle is selected as a reference for SAR imaging of the middle area echo. The low-orbit satellite flight direction is used as a reference, and the navigation satellite signals with a navigation satellite angle greater than 35° on the left and right sides of the low-orbit satellite flight direction are used as reference signals for SAR imaging processing in the middle area. The number of navigation satellite signal sources that meet this condition is F, and the F navigation satellite signals are denoted as g. 3,f (t), f = 1, 2, ..., F, the F navigation satellite signals are cooperatively output by the direct signal receiver 301 and the reference signal generating unit 302 of the receiving host 30.

[0095] like Figure 7 As shown, SAR imaging is performed on each narrow beam area, specifically including the following steps:

[0096] The narrow beam coverage area is divided into two-dimensional space grids along the flight direction and distance direction, and the grid size is 20m to 200m;

[0097] Arrange the one-dimensional echo signal into a two-dimensional echo signal, taking the time when the corresponding navigation satellite signal arrives at the low-orbit satellite as the starting point, with 2ms as the cycle and 1ms as the step, and transform the one-dimensional echo signal into a two-dimensional echo signal. 1,p (t), r 2,p (t), r 3,q (t) Arranged into a two-dimensional echo signal R 1,p(τ,t),R 2,p (τ,t),R 3,q (τ,t), where τ is the sampling time series in the slow direction;

[0098] The two-dimensional echo signal is pulse compressed in the range direction, and the corresponding navigation satellite signal is used as the reference signal to perform pulse compression processing on the two-dimensional echo signal to obtain a new two-dimensional echo signal with a high signal-to-noise ratio, that is, R 1,p (τ,t),R 2,p (τ,t),R 3,q (τ,t) is converted into

[0099] Remove the influence of navigation satellite message information on echo phase reversal, keep the phase of the multi-channel echo signal corresponding to "1" unchanged, and flip the phase of the multi-channel echo signal corresponding to "0" by 180°;

[0100] The two-dimensional echo signal is processed by back projection (BP) imaging to obtain multiple SAR images of the same narrow beam area. The d-th SAR image of the p-th narrow beam area in the left area is denoted as x 1,p,d (x, y), the e-th SAR image of the p-th narrow beam area in the right area is χ 2,p,e (x, y), the fth SAR image of the qth narrow beam area in the middle area is X 3,q,f (x, y), where x and y are the spatial grid unit numbers in the distance direction and the azimuth direction, respectively.

[0101] A satellite-borne GNSS-S multi-station radar downward-looking large-width SAR imaging method of the present invention utilizes a range-oriented multi-channel array antenna to work downward-looking, synchronously and parallelly receives a large range of sea surface multi-source echo signals with multiple wide beam angles, divides the downward-looking detection area into three areas: a left area, a middle area, and a right area, and performs digital beamforming processing on the range-oriented multi-channel echo signal by the receiving host, recovers the left area echo, the middle area echo, and the right area echo, selects the right navigation star signal as a reference to perform SAR imaging on the left area echo, selects the left navigation star signal as a reference to perform SAR imaging on the right area echo, and selects the large-incident-angle navigation star signal as a reference to perform SAR imaging on the middle area echo. The present invention has the advantages of downward-looking large-width SAR imaging, can realize synchronous imaging detection of the left area, the area directly below, and the right area, and greatly improves the detection efficiency of the system.

[0102] Parts of the present invention that are not described in detail belong to the well-known technology in the art.

[0103] It should be noted that although the embodiments of the present invention described above are illustrative, they are not intended to limit the present invention, and therefore the present invention is not limited to the above specific embodiments. Without departing from the principles of the present invention, any other embodiments obtained by those skilled in the art under the guidance of the present invention are deemed to be within the protection of the present invention.

Claims

1. A space-borne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method, characterized in that: The following steps are involved: Step S1, using the distance-direction multi-channel array antenna (20) of the satellite-borne GNSS-S radar echo receiving system to work directly downward; Step S2, dividing the downward detection area into three areas: a left area, a middle area and a right area, and the receiving host (30) performs digital beamforming processing on the range multi-channel echo signal to restore the left area echo, the middle area echo and the right area echo; Step S3: Based on the navigation satellite signal selection rule, perform downward-looking wide-band imaging.

2. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 1, characterized in that: Using M navigation satellites of a global navigation satellite system as transmission sources, a satellite-borne GNSS-S radar echo receiving system is constructed, wherein the satellite-borne GNSS-S radar echo receiving system comprises a zenith direct antenna (10), the range multi-channel array antenna (20) and a receiving host (30), wherein step S1 specifically comprises: The array surface normal direction of the range multi-channel array antenna (20) of the satellite-borne GNSS-S radar echo receiving system is parallel to the normal direction of the sea surface, and the beam pointing accuracy of the range multi-channel array antenna (20) is better than 0.1°; The signals of M navigation satellites illuminate the target area at the same time. The target area is a large sea surface area. The N channels in the range direction simultaneously receive the multi-source echo signals of the target area with a wide beam. The baseband complex echo signal of the N channels is denoted as s n (t),n=1,2,...,N,s n (t) is a one-dimensional continuous signal, and any of the echo signals includes scattered echo signals of M navigation satellite signals from the same sea surface, wherein t is a sampling time sequence in the fast direction; N echo signals are transmitted with T s Segmented storage for time units.

3. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 2 is characterized in that: The step S2 specifically includes: Step S21, dividing the downward detection area into three areas: a left area, a middle area, and a right area, defining the angle between the normal of the multi-channel array antenna surface and the normal of the sea level as the incident angle, taking the flight direction of the low-orbit satellite as the reference direction, defining the sea surface area with an incident angle greater than 7° on the left side of the flight direction of the low-orbit satellite as the left area, defining the sea surface area with an incident angle greater than 7° on the right side of the flight direction of the low-orbit satellite as the right area, and defining the sea surface area with an incident angle of 7° on the left side to 7° on the right side of the flight direction of the low-orbit satellite as the middle area; Step S22: According to the incident angle range of each area, the host (30) receives N echo signals s in the range direction. n (t) Perform DBF processing to restore the left echo, middle echo and right echo, and obtain 2P+Q high-gain narrow beam echo signals, still using T s is the time unit for N echo signals s n (t) Perform segmented DBF processing.

4. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 3, characterized in that: The step S22 specifically includes: Step S221: DBF processing is performed on the left echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the left echo area after DBF processing are recorded as Among them, w 1,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the left area; Step S222: DBF processing is performed on the right echo area to form P high-gain narrow beams covering P small areas. The P echo signals in the right echo area after DBF processing are recorded as Among them, w 2,p,n ,p=1,2,...,P;n=1,2,...,N are the P groups of complex weights corresponding to the P DBF processes in the right area; Step S223: DBF processing is performed on the echoes in the middle area to form Q high-gain narrow beams covering Q small areas. The Q echo signals in the middle area after DBF processing are recorded as Among them, w 3,q,n ,q=1,2,...,Q;n=1,2,...,N are Q groups of complex weights corresponding to the Q DBF processes in the middle area.

5. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 4, characterized in that: In step S3, the navigation satellite signal selection rule is: The navigation satellite signal on the right is selected as a reference for SAR imaging of the echo in the left area. The angle between the line connecting the navigation satellite and the low-orbit satellite and the normal line of the sea surface is defined as the navigation satellite angle. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the right side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the left area. The number of navigation satellite signal sources that meet this condition is denoted as D, and the D navigation satellite signals are denoted as g. 1,d (t), d = 1, 2, ..., D, D navigation satellite signals are outputted in coordination by a direct signal receiver (301) and a reference signal generating unit (302) of a receiving host (30); The left navigation satellite signal is selected as a reference to perform SAR imaging on the echo in the right area. The flight direction of the low-orbit satellite is used as a reference, and the navigation satellite signal with a navigation satellite angle greater than 10° on the left side of the low-orbit satellite flight direction is used as the reference signal for SAR imaging processing in the right area. The number of navigation satellite signal sources that meet this condition is recorded as E, and the E navigation satellite signals are recorded as g. 2,e (t), e = 1, 2, ..., E, the E navigation satellite signals are outputted in coordination by a direct signal receiver (301) and a reference signal generating unit (302) of a receiving host (30); The navigation satellite signal with a large incident angle is selected as a reference for SAR imaging of the middle area echo. The low-orbit satellite flight direction is used as a reference, and the navigation satellite signals with a navigation satellite angle greater than 35° on the left and right sides of the low-orbit satellite flight direction are used as reference signals for SAR imaging processing in the middle area. The number of navigation satellite signal sources that meet this condition is F, and the F navigation satellite signals are denoted as g. 3,f (t), f = 1, 2, ..., F, the F navigation satellite signals are outputted in coordination by a direct signal receiver (301) and a reference signal generating unit (302) of a receiving host (30).

6. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 5, characterized in that: The step S3 specifically includes: The wide sea surface echo signal received by looking directly downward is divided into three areas: the left area, the middle area and the right area for synchronous parallel SAR imaging processing. 1,d (t),d=1,2,...,D is used as the reference signal for SAR imaging of the left P narrow beam areas, and the signal g is used 2,e (t), e=1,2,...,E is used as the reference signal for SAR imaging of the P narrow beam areas on the right side, and the signal g is used 3,f (t), f = 1, 2, ..., F as the reference signal for SAR imaging of the middle Q narrow beam areas; Perform SAR imaging on each narrow beam area; The SAR images of multiple spatially continuous narrow beam areas are spatially spliced ​​to obtain a wide-width SAR image in the downward viewing situation.

7. The spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method according to claim 6, characterized in that: The performing SAR imaging on each narrow beam area specifically includes: Divide the narrow beam coverage area into two-dimensional spatial grids along the flight direction and distance direction; Arrange the one-dimensional echo signal into a two-dimensional echo signal, taking the time when the corresponding navigation satellite signal arrives at the low-orbit satellite as the starting point, with 2ms as the cycle and 1ms as the step, and transform the one-dimensional echo signal into a two-dimensional echo signal. 1,p (t), r 2,p (t), r 3,q (t) Arranged into a two-dimensional echo signal R 1,p (τ,t),R 2,p (τ,t),R 3,q (τ,t), where τ is the sampling time series in the slow direction; The two-dimensional echo signal is pulse compressed in the range direction, and the corresponding navigation satellite signal is used as the reference signal to perform pulse compression processing on the two-dimensional echo signal to obtain a new two-dimensional echo signal with a high signal-to-noise ratio, that is, R 1,p (τ,t),R 2,p (τ,t),R 3,q (τ,t) is converted into Remove the influence of navigation satellite message information on echo phase reversal, keep the phase of the multi-channel echo signal corresponding to "1" unchanged, and flip the phase of the multi-channel echo signal corresponding to "0" by 180°; The two-dimensional echo signal is processed by back-projection imaging to obtain multiple SAR images of the same narrow beam area. The d-th SAR image of the p-th narrow beam area in the left area is denoted as x 1,p,d (x, y), the e-th SAR image of the p-th narrow beam area in the right area is χ 2,p,e (x, y), the fth SAR image of the qth narrow beam area in the middle area is X 3,q,f (x, y), where x and y are the spatial grid unit numbers in the distance direction and the azimuth direction, respectively.

8. A spaceborne GNSS-S radar echo receiving system for implementing the spaceborne GNSS-S multi-station radar downward-looking wide-bandwidth SAR imaging method as described in any one of claims 1 to 7, characterized in that: The satellite-borne GNSS-S radar echo receiving system comprises a zenith direct antenna (10), a range multi-channel array antenna (20) and a receiving host (30); The zenith direct antenna (10) is used to receive direct signals from multiple navigation satellites; The range multi-channel array antenna (20) is used to receive multi-source echo signals from the sea surface; The receiving host (30) is used for processing multi-source echo signals, and the processing process at least includes amplification, down-conversion, band-pass filtering, sampling quantization, digital IQ demodulation, DBF, and SAR imaging.

9. The spaceborne GNSS-S radar echo receiving system according to claim 8, characterized in that: The zenith direct antenna (10) operates in the L band, with a frequency coverage of 1.2 GHz to 1.3 GHz, and a gain greater than 4 dB within the frequency band; The range multi-channel array antenna (20) is composed of N linear array antennas arranged in the range direction, the 3dB beam width of any of the linear array antennas in the range direction is greater than 100°, any of the linear array antennas corresponds to an independent high-sensitivity digital receiver (304), and any of the linear array antennas includes P antenna radiation units, the frequency of the antenna radiation array elements covers 1.2GHz to 1.3GHz, and the gain within the frequency band is greater than 4dB.

10. The spaceborne GNSS-S radar echo receiving system according to claim 8, characterized in that: The receiving host (30) includes a direct signal receiver (301), a reference signal generating unit (302), a frequency integrated management unit (303), N high-sensitivity digital receivers (304), a radar main control unit (305), a DBF processing unit (306), and a SAR imaging processing unit (307); The direct signal receiver (301) outputs the position and speed of the navigation satellite, the navigation message of the navigation satellite signal, and the navigation satellite pseudo code signal with Doppler modulation; The reference signal generating unit (302) is used to generate a reference signal required for GNSS-S radar imaging processing; The frequency integrated management unit (303) is used to generate a clock signal and a local oscillation signal required by the receiving host (30); The high-sensitivity digital receiver (304) is used to amplify, down-convert, band-pass filter, sample, quantize and digitally demodulate the sea surface echo signal to obtain a baseband complex domain echo signal; The radar main control unit (305) is used to implement command control of the direct signal receiver (301), command control of the reference signal generation unit (302), segmented storage control of echo data, and transmission control of echo data; The DBF processing unit (306) is used to obtain high-gain narrow beams directed to different areas; The SAR imaging processing unit (307) is used to perform SAR imaging on the echo signals of multiple beam regions.

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