Method and system for determining position of ground sampling point of satellite-borne linear frequency modulation system radar

Abstract

The invention discloses a method and a system for determining the position of a ground sampling point of a satellite-borne linear frequency modulation system radar. The method comprises the following steps: selecting a plurality of positions of known pointing vectors at equal angle intervals from each wave beam of a satellite-borne linear frequency modulation system radar as a group of coarse positioning points, and determining the positioning information of each coarse positioning point on the ground; according to the positioning information of each coarse positioning point, obtaining the accurate frequency of the echo of the coarse positioning point; performing fitting interpolation on the actual pointing vector of the sampling frequency point according to the obtained group of coarse positioning point pointing vectors and the accurate frequency of the corresponding echo to obtain the accurate pointing vector of the secondary positioning point; coordinate system transformation is carried out, and precise positioning of the satellite-borne linear frequency modulation system radar is achieved. According to the two-step positioning process combining coarse positioning based on the preset pointing vector and fine positioning based on the fitting interpolation actual pointing vector, the positioning processing time is greatly shortened, and the positioning precision can reach ten-meter magnitude or even higher.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite-borne radar positioning, and in particular relates to a method and system for determining the position of ground sampling points of satellite-borne linear frequency modulation radar. Background Art

[0002] Spaceborne linear frequency modulation radars such as microwave scatterometers have relatively low spatial resolution. The spatial resolution in the azimuth direction is directly determined by the beam width, which is generally 20 to 30 kilometers. In the distance direction, the beam footprint is divided into multiple equally spaced strips, which are generally spaced from a few kilometers to more than ten kilometers. As a remote sensing payload that operates continuously 24 hours a day, the preprocessing of satellite-borne microwave scatterometers, including positioning, needs to be completed in a shorter time than the observation time. For example, the length of a track of data is usually 100 minutes, so the positioning processing needs to be completed in about ten minutes. In order to reduce the computational complexity of positioning processing, microwave scatterometers such as SeaWinds and ASCAT use the strip center pointing vector of each strip determined in advance, and directly obtain the positioning result of the strip center sampling point through conventional coordinate system conversion methods, with a positioning accuracy of about 1 kilometer.

[0003] The wind field measurement radar carried by Fengyun-3 is different from traditional satellite-borne microwave scatterometers. Its distance sampling interval reaches 250 meters, and the surface interval near the center of the beam is about 300 to 400 meters. The number of measurement points per second is more than 100 times that of traditional microwave scatterometers such as SeaWinds and ASCAT. If the wind field measurement radar uses the direct positioning method of SeaWinds' coordinate system conversion, it is difficult to achieve the accuracy requirement of ten meters; if the positioning method of satellite-borne SAR is used, the processing time of one track of data will be much longer than the observation time length of the data itself, which cannot meet the requirements of real-time business operation.

[0004] The real-time positioning needs of space-borne linear frequency modulation radars such as the Fengyun-3 wind field measurement radar, which have a small ground sampling interval (on the order of hundreds of meters) and an extremely high data sampling rate (more than 10,000 measured data points per second). The existing direct positioning method can ensure a shorter processing time by converting the coordinate system by pointing vectors, but has the disadvantage of being unable to achieve a positioning accuracy of ten meters. At the same time, the existing SAR partial point iterative solution positioning method can achieve decimeter-level positioning accuracy but has the disadvantage of being extremely time-consuming to process. Summary of the invention

[0005] In view of the technical problem that these traditional positioning methods cannot take into account both positioning accuracy and processing time efficiency when used for space-borne linear frequency modulation radar, the purpose of the present invention is to overcome the above-mentioned defects of the prior art and propose a method and system for determining the ground sampling point position of space-borne linear frequency modulation radar.

[0006] In view of this, the present invention proposes a method for determining the position of ground sampling points of a space-borne linear frequency modulation radar, the method comprising:

[0007] Step 1) selecting the positions of a number of known pointing vectors at equal angle intervals in each beam of the spaceborne linear frequency modulation radar as a group of coarse positioning points, and determining the positioning information of each coarse positioning point on the ground;

[0008] Step 2) obtaining the precise frequency of the echo of each coarse positioning point according to the positioning information of the coarse positioning point;

[0009] Step 3) for a set of coarse positioning point pointing vectors and the precise frequencies of the corresponding echoes, the actual pointing vectors of the sampling frequency points are fitted and interpolated to obtain the precise pointing vectors of the secondary positioning points;

[0010] Step 4) Perform coordinate system transformation to achieve precise positioning of the spaceborne linear frequency modulation radar.

[0011] Preferably, in step 1), a group of coarse positioning points includes two positions, the outermost and innermost positions of the radar main lobe beam in the pitch direction; the azimuth direction of each of the coarse positioning points is the same, and the pitch direction is at equal angle intervals, wherein the pitch angle of the i-th coarse positioning point is α i , azimuth is β i .

[0012] Preferably, the position of each rough positioning point on the ground in step 1) for:

[0013]

[0014] Among them, N, A and S are all 3×3 square matrices, the superscript ' represents the transpose of the matrix, the matrix N is the rotation matrix representing the transformation from the geodetic coordinate system to the Earth-centered Earth-fixed coordinate system, the matrix A is the rotation matrix representing the transformation from the geodetic coordinate system to the satellite flight coordinate system, and the matrix S is the rotation matrix representing the transformation from the satellite flight coordinate system to the instrument coordinate system; The center of the visual axis points to the unit vector in the instrument coordinate system, satisfying the following formula:

[0015]

[0016] Preferably, the step 2) comprises:

[0017] According to the positioning results of each rough positioning point, the slant distance R from the rough positioning point to the satellite and the relative speed V between the two are calculated. R , the precise frequency f of each coarse positioning point echo is obtained according to the following formula:

[0018]

[0019] Where f0 is the nominal operating center frequency of the spaceborne linear frequency modulation radar, λ0 is the corresponding wavelength, k is the frequency change rate of the linear frequency modulation signal, and f offset is the current beam azimuth frequency pre-bias, t dr is the center moment of the receiving window, c is the speed of light, and t represents time.

[0020] Preferably, the step 3) comprises: for a set of coarse positioning point pointing vectors The exact frequency {f} of the corresponding echo is interpolated by linear interpolation or cubic spline interpolation to obtain a set of sampling center frequencies {f C The pointing vector of the corresponding secondary positioning point

[0021] Preferably, the step 4) comprises:

[0022] Using the pointing vector of the secondary positioning point The precise observation pointing vectors of these secondary positioning points in the ECEF coordinate system are obtained by the positioning method of coordinate system transformation. Right now

[0023]

[0024] Among them, N, A and S are all 3×3 square matrices, the superscript ' represents the transpose of the matrix, the matrix N is the rotation matrix representing the transformation from the geodetic coordinate system to the ECEF coordinate system, the matrix A is the rotation matrix representing the transformation from the geodetic coordinate system to the satellite flight coordinate system, and the matrix S is the rotation matrix representing the transformation from the satellite flight coordinate system to the instrument coordinate system;

[0025] Thus, the precise surface position information of the sampling frequency point of the spaceborne linear frequency modulation radar can be obtained, including the longitude and latitude of the earth's surface and the local zenith angle.

[0026] On the other hand, the present invention provides a system for determining the position of ground sampling points of a spaceborne linear frequency modulation radar, characterized in that it includes:

[0027] A coarse positioning module is used to select the positions of a number of known pointing vectors with equal angle intervals in each beam of the spaceborne linear frequency modulation radar as a group of coarse positioning points, and determine the positioning information of each coarse positioning point on the ground;

[0028] The precise frequency acquisition module is used to obtain the precise frequency of the echo of each coarse positioning point according to the positioning information of the coarse positioning point;

[0029] A fitting interpolation module is used to fit and interpolate the actual pointing vector of the sampling frequency point based on a set of coarse positioning point pointing vectors and the precise frequency of the corresponding echo, so as to obtain the precise pointing vector of the secondary positioning point; and

[0030] The precise positioning module is used to perform coordinate system transformation to achieve precise positioning of the spaceborne linear frequency modulation radar.

[0031] Compared with the prior art, the advantages of the present invention are:

[0032] The technical solution of the present invention is based on the radar linear frequency modulation signal modulation principle, and derives an accurate expression of the radar receiving signal frequency that fully considers various influencing factors, thereby forward calculating the echo frequency corresponding to the preset pointing vector, and then using the fitting interpolation method for the preset pointing vector and the corresponding frequency data to obtain the actual pointing vector corresponding to the sampling frequency, thereby ensuring the continuity and consistency of the relationship between the sampling frequency and the pointing vector, and the positioning accuracy can reach the order of ten meters or even higher. At the same time, the technical solution of the present invention adopts a two-step positioning process that combines coarse positioning based on a preset pointing vector with fine positioning based on fitting and interpolation of the actual pointing vector, avoiding the multi-step repeated calculation process of the satellite-borne SAR using an iterative method to solve the sampling frequency-pointing vector relationship, which can greatly shorten the positioning processing time. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a frequency domain sampling spaceborne linear frequency modulation radar echo distance-frequency coupling characteristics, where Figure 1 (a) is the radar sampling points at different distances in the time domain, and Figure 1 (b) is the frequency point position corresponding to the radar sampling points at different distances in the frequency domain;

[0034] Figure 2 The present invention is a flow chart of a method for determining the position of ground sampling points of a spaceborne linear frequency modulation radar. DETAILED DESCRIPTION

[0035] The invention provides a method and system for determining the position of ground sampling points of a space-borne linear frequency modulation radar.

[0036] Spaceborne linear frequency modulation radar faces a dilemma between positioning accuracy and processing time when determining the spatial position of ground sampling points. The reason is that the time-frequency coupling makes the nonlinear relationship between its sampling frequency and pointing vector very complex and it is impossible to obtain an analytical solution. Microwave scatterometers such as SeaWinds use a direct positioning method called coordinate system conversion to replace the real pointing vector with an approximate pointing vector. In essence, it simplifies the complex relationship between sampling frequency and pointing vector and can only achieve kilometer-level positioning accuracy.

[0037] Compared with ground-based radar or airborne radar, the observation target distance of spaceborne earth observation radar is much farther. At the same time, the peak power of the radar transmitter is also constrained by the energy limit of the satellite platform. Therefore, spaceborne radar usually adopts pulse compression technology to increase the average transmission power and transmission energy of the radar by increasing the pulse width without changing the peak power, while obtaining a larger bandwidth to maintain the distance resolution of the narrow pulse. Among them, the linear frequency modulation waveform is the most commonly used pulse compression waveform, which can be defined as

[0038]

[0039] Where a(t) is the RF transmission signal The normalized amplitude (i.e., the maximum value is 1), t represents time, for rectangular pulses a(t)≡1, f0 is the nominal center frequency of the RF carrier, p t0 is the peak power of the transmission, T is the pulse width, and k is the frequency change rate of the linear frequency modulation signal (i.e., equal to B / T, where B is the signal bandwidth).

[0040] The echo received by the radar is compressed by convolution in the time domain or multiplication in the frequency domain with the reference signal. The compressed pulse obtained will show the time-frequency coupling characteristic, that is,

[0041] Δf=kt R

[0042] Where Δf is the offset frequency of the final output signal relative to the carrier frequency, and the delay time t of the radar receiving and transmitting signal is R It is determined by the distance from the ground sampling point to the radar, so the echoes of ground targets at different distances will appear at different frequencies.

[0043] The geo-positioning of satellite earth observation instruments refers to the use of satellite orbital position, attitude, speed and other data and the instrument observation geometry model data to calculate the longitude, latitude and other related auxiliary positioning information of each sampling point of the instrument on the ground. The basic calculation process of positioning processing is to point the beam center or the center of the visual axis of each observation point in the instrument coordinate system to the vector The observed pointing vector (including the pointing in both pitch and azimuth) is transformed into the Earth-centered Earth-fixed (ECEF) coordinate system That is, the series of transformations from the instrument coordinate system to the satellite flight coordinate system, then to the geodetic coordinate system, and finally to the ECEF coordinate system can be mathematically described by the following formula:

[0044]

[0045] Among them, N, A and S are all 3×3 square matrices, the superscript ' indicates the transpose of the matrix, the matrix S is the rotation matrix representing the transformation from the satellite flight coordinate system to the instrument coordinate system, the matrix A is the rotation matrix representing the transformation from the geodetic coordinate system to the satellite flight coordinate system, and the matrix N is the rotation matrix representing the transformation from the geodetic coordinate system to the ECEF coordinate system. The positioning accuracy of the instrument ground sampling points needs to be less than half of the spatial resolution of the sampling points (or the spatial interval of the sampling points), and the positioning accuracy can usually reach 1 / 5 to 1 / 10 of the spatial resolution.

[0046] For a spaceborne linear frequency modulation radar that performs frequency domain sampling, the radar first performs time domain sampling of the ground footprint covered by the same beam (as shown in Figure 1(a)), and then transforms the samples at different times (distances) into the frequency domain (as shown in Figure 1(b)). According to the time (distance)-frequency coupling characteristics of the linear frequency modulation signal, each echo frequency corresponds to a different ground sampling point distance, and different distances correspond to different beam directions.

[0047] For frequency domain sampling spaceborne linear frequency modulation radar, it is assumed that and are the position and velocity vector of the satellite and the ground sampling point in the inertial coordinate system (geodetic coordinate system) at a certain moment, respectively. Then the slant distance from the sampling point to the satellite is

[0048]

[0049] The frequency offset of the sampling point echo is

[0050]

[0051] Where c is the speed of light, λ is the wavelength of the radar signal, and f r is the range frequency shift, f d is the Doppler (velocity) frequency shift. Due to the frequency shift Δf and beam pointing There is a high-order functional relationship between them, and the beam pointing vector corresponding to a certain sampling frequency point cannot be directly solved. Therefore, the aforementioned conventional positioning method cannot be used to determine the position of the ground sampling point of the frequency domain sampling space-borne linear frequency modulation radar.

[0052] If the spaceborne linear frequency modulation radar only samples the signal in the time domain without performing frequency domain transformation, due to the time-frequency coupling characteristics of the linear frequency modulation signal, the delay time t of the radar receiving signal relative to the reflected signal is R for

[0053]

[0054] It can be seen that the delay time t is the same as that of frequency sampling. R With beam pointing There is a complex functional relationship between them, so the aforementioned conventional positioning method cannot be used to determine the position of the ground sampling point of the time-domain sampling space-borne linear frequency modulation radar.

[0055] Considering the Doppler frequency shift f alone d , and using the WGS84 earth ellipsoid model, we can get the following set of equations (i.e., the range-Doppler model):

[0056]

[0057] in, a and b are the lengths of the major and minor semi-axes of the Earth ellipsoid, respectively, and h is the surface elevation of the sampling point. The solution of this set of equations is the position of the sampling point in the geodetic coordinate system. The longitude, latitude and altitude of the sampling point can be obtained by transforming the coordinate system from the geodetic coordinate system to the ECEF coordinate system.

[0058] Spaceborne synthetic aperture radar (SAR) uses the distance (that is, spatial position vector or pointing vector)-Doppler (frequency) model for positioning. However, the three equations of this system of equations are nonlinear equations and cannot be solved analytically. Since the spatial resolution (meter level) of spaceborne SAR is very high and the positioning accuracy needs to be in the decimeter level, iterative methods (such as Newton iteration method) are often used to obtain high-precision numerical solutions during positioning. Considering that the computational complexity of cyclic iterative numerical approximation is large, and there are many sampling points in the observation of spaceborne SAR, in actual positioning, the iterative method is often used to solve only some sampling points, and then these points are used as reference points to deduce the recursive relationship between other sampling points and the reference points to reduce the computational complexity. Even so, the positioning of spaceborne SAR is very computationally resource-intensive.

[0059] 1. Spaceborne pulse compression radar coarse positioning

[0060] In the ground footprint covered by the same beam of the spaceborne linear frequency modulation radar, several equally spaced positions are selected as coarse positioning points. The pointing vectors of these points (i.e., the elevation angle and azimuth angle in the instrument coordinate system) are known. These points include the outermost and innermost positions of a main lobe beam of the radar in the pitch direction to achieve complete coverage of the spatial position of the radar main beam (including the echo frequency sampling range). The number of coarse positioning points selected needs to be a compromise between the positioning calculation time and the interpolation accuracy of the actual pointing vector of the sampling frequency point. The azimuth directions of these coarse positioning points of the same beam are all the same, and the pointing directions in the pitch direction can be selected at equal angle intervals. Assume that the pitch angle of one of the coarse positioning points is α i , azimuth is β i , then in the instrument coordinate system the center of the visual axis points to the unit vector It can be expressed as

[0061]

[0062] In this way, the positions of these rough positioning points on the ground can be determined by the conventional coordinate system transformation positioning method, that is,

[0063]

[0064] 2. High-precision calculation of coarse positioning point frequency

[0065] According to the positioning result of the rough positioning point, the slant distance R from the current rough positioning point to the satellite and the relative speed V between the two can be calculated. R , thereby calculating the frequency offset Δf of the coarse positioning point echo, that is,

[0066]

[0067] For the nominal working center frequency f0, wavelength λ0, and current beam azimuth frequency pre-bias f offset The center time of the receiving window is t dr For a spaceborne radar, the echo frequency of the current coarse positioning point is

[0068]

[0069] The above formula is only an approximate expression of the frequency of the signal detected by the spaceborne linear frequency modulation radar. In order to obtain an accurate expression of the frequency of the signal detected by the coarse positioning point echo, it is necessary to further consider the coupling between the speed and time of the linear frequency modulation radar. For this reason, the RF transmission signal of the linear frequency modulation radar is considered to be

[0070]

[0071] Where a(t) is the normalized amplitude of the transmitted signal. For rectangular pulses, a(t)≡1, p t0 is the peak power of the transmission, T is the pulse width, and k is the frequency change rate of the linear frequency modulation signal. Then the RF receiving signal can be expressed as

[0072]

[0073] The function K determines the signal amplitude from the distributed target scattering unit, t d is the radar delay

[0074]

[0075] Where Δt is the internal delay of the radar. d =t′∈[-T / 2,T / 2](transmitting time), we can get the receiving time

[0076]

[0077] The received signal after down-conversion and de-skewing is:

[0078]

[0079] Considering the receiving link response, the received baseband signal is

[0080]

[0081] Ignore the receive link response h rx The impact on the phase (or frequency) part of the signal, then the exponential part of the received baseband signal is

[0082]

[0083] Substitute the radar delay t d And expand it into the power of t, we get the following formula

[0084]

[0085] Taking the derivative of the above formula with respect to t, we get the exact expression of the received signal frequency.

[0086]

[0087] Therefore, R and V obtained by coarse positioning R , t and radar parameters f0, λ0, k, f offset ,t dr The precise frequency of the coarse positioning point echo can be calculated. Since there is a complex nonlinear relationship between the coarse positioning point pointing vector and the ground positioning results such as R, the pointing vectors with equal angle intervals will obtain the coarse positioning point echo frequencies with non-uniform intervals.

[0088] 3. Sampling frequency actual pointing vector fitting interpolation

[0089] Through the previous calculation, we get a set of rough positioning point pointing vectors And the corresponding echo frequency {f}. Since the radar detects the received signal in the frequency domain, the center frequency of the demodulation signal corresponding to the i-th frequency unit is

[0090]

[0091] Where N FFT Indicates the number of frequency units, B ADC is the receiver bandwidth. The center frequencies of the frequency cells are evenly spaced, which is obviously inconsistent with the echo frequency of the coarse sampling point.

[0092] As mentioned above, the coarse positioning point points to the vector The relationship between the corresponding echo frequency {f} is complex and cannot be directly expressed by an expression. To this end, we can use interpolation methods such as linear interpolation and cubic spline interpolation and the two sets of values ​​calculated above. {f}, perform piecewise interpolation calculation to obtain a set of sampling center frequencies {f C The pointing vector of the corresponding secondary positioning point

[0093] 4. Spaceborne pulse compression radar precise positioning

[0094] Using the obtained accurate pointing vector of the secondary positioning point The precise observation pointing vectors of these secondary positioning points in the ECEF coordinate system are determined by conventional coordinate transformation positioning methods. Right now

[0095]

[0096] In this way, the precise surface position information of the sampling frequency point of the space-borne linear frequency modulation radar can be obtained, including the longitude and latitude of the earth's surface, the local zenith angle, etc.

[0097] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0098] Example 1

[0099] Embodiment 1 of the present invention proposes a method for determining the location of ground sampling points for a spaceborne linear frequency modulation radar. Figure 2 As shown, the specific steps include:

[0100] 1. Spaceborne pulse compression radar coarse positioning

[0101] First, select several known pointing vectors with equal angle intervals in the ground footprint covered by the same beam of the spaceborne linear frequency modulation radar as coarse positioning points. These points should include the outermost and innermost positions of the radar main lobe beam in the elevation direction to achieve complete coverage of the radar main beam spatial position. The azimuth direction pointing of these coarse positioning points of the same beam are the same, and the pointing direction of the elevation direction is selected at equal angle intervals. Assume that the elevation angle of one of the coarse positioning points is α i , azimuth is β i , then in the instrument coordinate system the center of the visual axis points to the unit vector It can be expressed as

[0102]

[0103] In this way, the positions of these rough positioning points on the ground can be determined by the conventional coordinate system transformation positioning method, that is,

[0104]

[0105] The more positioning points are selected during coarse positioning, the closer the pointing vector obtained by fitting interpolation will be to the actual pointing vector of the radar sampling frequency. Of course, the coarse positioning process will also take longer. Since the positioning calculation of coordinate system transformation can be implemented by matrix operation rather than point-by-point operation in practical operation, the proportion of time consumed by radar coarse positioning due to the increase in positioning points will be much smaller than the proportion of the increase in the number of positioning points. Taking the Fengyun-3 wind field measurement radar as an example, the number of coarse positioning points is selected to be twice the total number of sampling points in the frequency domain, which can meet the final positioning accuracy requirement of ten meters.

[0106] 2. High-precision calculation of coarse positioning point frequency

[0107] According to the positioning result of the rough positioning point, the slant distance R from the current rough positioning point to the satellite and the relative speed V between the two are calculated. R , and then use the parameters f0, λ0, k, f of the spaceborne linear frequency modulation radar offset ,t dr , thus obtaining the precise frequency of the coarse positioning point echo

[0108]

[0109] Since there is a complex nonlinear relationship between the coarse positioning point pointing vector and the ground positioning result, the pointing vectors with equal (elevation) angle intervals will obtain the coarse positioning point position echo frequencies with non-uniform intervals.

[0110] 3. Sampling frequency actual pointing vector fitting interpolation

[0111] The spaceborne linear frequency modulation radar detects the received signal in the frequency domain. The center frequency of the demodulation signal corresponding to the i-th frequency unit is

[0112]

[0113] Where N FFT Indicates the number of frequency units, B ADC is the receiver bandwidth. The center frequencies of the frequency units are evenly spaced. The coarse positioning point pointing vectors obtained in the previous two implementation steps are The corresponding echo frequency {f} uses a piecewise interpolation method to obtain a set of sampling center frequencies {f C} for each frequency value to calculate the corresponding secondary positioning point pointing vector

[0114] Commonly used interpolation methods include linear interpolation, quadratic interpolation, Newton interpolation, spline interpolation, etc. As these interpolation methods increase from linear interpolation to spline interpolation, the computational complexity gradually increases, and the smoothness of the obtained segmented curve is also better. In addition, the selection of the interpolation method is also related to the number of coarse positioning points. When the number of positioning points is large enough, linear interpolation can also obtain a better curve fitting effect. Considering that most interpolation methods have fast calculation algorithms and the time consumed in calculation is not sensitive to the increase in the number of positioning points, the Fengyun-3 wind field measurement radar uses the cubic spline interpolation method when fitting and interpolating the actual pointing vector of the sampling frequency point.

[0115] 4. Spaceborne pulse compression radar precise positioning

[0116] Using the obtained accurate pointing vector of the secondary positioning point The precise observation pointing vectors of these secondary positioning points in the ECEF coordinate system are determined by conventional coordinate transformation positioning methods. Right now

[0117]

[0118] In this way, the precise location information of the sampling frequency point of the space-borne linear frequency modulation radar on the surface can be obtained, including the longitude and latitude of the earth's surface, the local zenith angle, etc.

[0119] Example 2

[0120] Embodiment 2 of the present invention proposes a system for determining the ground sampling point position of a space-borne linear frequency modulation radar, which is implemented based on the method of embodiment 1. The system includes:

[0121] A coarse positioning module is used to select the positions of a number of known pointing vectors with equal angle intervals in each beam of the spaceborne linear frequency modulation radar as a group of coarse positioning points, and determine the positioning information of each coarse positioning point on the ground;

[0122] The precise frequency acquisition module is used to obtain the precise frequency of the echo of each coarse positioning point according to the positioning information of the coarse positioning point;

[0123] The fitting interpolation module is used to fit and interpolate the actual pointing vector of the sampling frequency point based on the obtained set of coarse positioning point pointing vectors and the precise frequency of the corresponding echo, so as to obtain the precise pointing vector of the secondary positioning point;

[0124] The precise positioning module is used to perform coordinate system transformation to achieve precise positioning of the spaceborne linear frequency modulation radar.

[0125] Innovation:

[0126] 1. A step-by-step positioning method for spaceborne linear frequency modulation radar that combines coarse positioning based on a preset pointing vector with fine positioning based on fitting and interpolation of the actual pointing vector.

[0127] 2. Use various influencing parameters to consider the complete accurate expression of the frequency of the received signal of the space-borne linear frequency modulation radar, and realize the accurate calculation of the echo frequency at the corresponding point of the pointing vector.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A method for determining the location of ground sampling points of a spaceborne linear frequency modulation radar, the method comprising: Step 1) selecting the positions of a number of known pointing vectors at equal angle intervals in each beam of the spaceborne linear frequency modulation radar as a group of coarse positioning points, and determining the positioning information of each coarse positioning point on the ground; Step 2) obtaining the precise frequency of the echo of each coarse positioning point according to the positioning information of the coarse positioning point; Step 3) for a set of coarse positioning point pointing vectors and the precise frequencies of the corresponding echoes, the actual pointing vectors of the sampling frequency points are fitted and interpolated to obtain the precise pointing vectors of the secondary positioning points; Step 4) Perform coordinate system transformation to achieve precise positioning of the spaceborne linear frequency modulation radar.

2. The method for determining the ground sampling point position of a spaceborne linear frequency modulation radar according to claim 1, characterized in that: In step 1), a group of coarse positioning points includes the outermost and innermost positions of the radar main lobe beam in the pitch direction; the azimuth direction of each coarse positioning point is the same, and the pitch direction is at equal angle intervals, wherein the pitch angle of the i-th coarse positioning point is α i , azimuth is β i .

3. The method for determining the ground sampling point position of a spaceborne linear frequency modulation radar according to claim 2, characterized in that: The position of each rough positioning point on the ground in step 1) for: Among them, N, A and S are all 3×3 square matrices, the superscript ' represents the transpose of the matrix, the matrix N is the rotation matrix representing the transformation from the geodetic coordinate system to the Earth-centered Earth-fixed coordinate system, the matrix A is the rotation matrix representing the transformation from the geodetic coordinate system to the satellite flight coordinate system, and the matrix S is the rotation matrix representing the transformation from the satellite flight coordinate system to the instrument coordinate system; The center of the visual axis points to the unit vector in the instrument coordinate system, satisfying the following formula:

4. The method for determining the ground sampling point position of a spaceborne linear frequency modulation radar according to claim 1, characterized in that: The step 2) comprises: According to the positioning results of each rough positioning point, the slant distance R from the rough positioning point to the satellite and the relative speed V between the two are calculated. R , the precise frequency f of each coarse positioning point echo is obtained according to the following formula: Where f0 is the nominal operating center frequency of the spaceborne linear frequency modulation radar, λ0 is the corresponding wavelength, k is the frequency change rate of the linear frequency modulation signal, and f offset is the current beam azimuth frequency pre-bias, t dr is the center moment of the receiving window, c is the speed of light, and t represents time.

5. The method for determining the ground sampling point position of a spaceborne linear frequency modulation radar according to claim 1, characterized in that: The step 3) comprises: for a set of coarse positioning point pointing vectors The exact frequency {f} of the corresponding echo is interpolated by linear interpolation or cubic spline interpolation to obtain a set of sampling center frequencies {f C The pointing vector of the corresponding secondary positioning point 6. The method for determining the ground sampling point position of a spaceborne linear frequency modulation radar according to claim 5, characterized in that: The step 4) comprises: Using the pointing vector of the secondary positioning point The precise observation pointing vectors of these secondary positioning points in the ECEF coordinate system are obtained by the positioning method of coordinate system transformation. Right now Among them, N, A and S are all 3×3 square matrices, the superscript ' represents the transpose of the matrix, the matrix N is the rotation matrix representing the transformation from the geodetic coordinate system to the ECEF coordinate system, the matrix A is the rotation matrix representing the transformation from the geodetic coordinate system to the satellite flight coordinate system, and the matrix S is the rotation matrix representing the transformation from the satellite flight coordinate system to the instrument coordinate system; Thus, the precise surface position information of the sampling frequency point of the spaceborne linear frequency modulation radar can be obtained, including the longitude and latitude of the earth's surface and the local zenith angle.

7. A system for determining the location of ground sampling points of a spaceborne linear frequency modulation radar, characterized in that: include: A coarse positioning module is used to select the positions of a number of known pointing vectors with equal angle intervals in each beam of the spaceborne linear frequency modulation radar as a group of coarse positioning points, and determine the positioning information of each coarse positioning point on the ground; The precise frequency acquisition module is used to obtain the precise frequency of the echo of each coarse positioning point according to the positioning information of the coarse positioning point; The fitting interpolation module is used to fit and interpolate the actual pointing vector of the sampling frequency point based on the obtained set of coarse positioning point pointing vectors and the precise frequency of the corresponding echo, so as to obtain the precise pointing vector of the secondary positioning point; and The precise positioning module is used to perform coordinate system transformation to achieve precise positioning of the spaceborne linear frequency modulation radar.

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