CN117826197A - Polarization calibration method of polarized GNSS-R receiver - Google Patents
Polarization calibration method of polarized GNSS-R receiver Download PDFInfo
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Abstract
The invention provides a polarization calibration method of a polarized GNSS-R receiver, which comprises the following steps: acquiring a delay-doppler plot DDM; acquiring Stokes parameters from a delay-Doppler plot DDM; defining 1 or more non-ideal effects to be compensated for; calibrating the non-ideal effect based on Stokes parameters; converting factors which need to be compensated for respectively different non-ideal effects into error matrixes with the same dimension; calibrating Stokes parameters based on the error matrix; and (4) recovering the calibrated Stokes parameter inversion into a delay-Doppler diagram DDM diagram. The invention corrects the existing polarization non-ideal effect by using Stokes parameters, and solves the problems of gain loss caused by the difference of the polarization direction of the receiver antenna and the polarization direction of the reflected signal, phase shift caused by the two-way Faraday rotation of the reflected signal through the ionized layer, and non-ideal states such as ovalization, phase shift and the like caused by the non-ideal axial ratio of the GNSS transmitting antenna.
Description
Technical Field
The invention relates to the technical field of global navigation satellite systems (Global Navigation Satellite System), in particular to a polarization calibration method of a polarized GNSS-R receiver.
Background
GNSS-R technology is a novel technology in the current field of remote sensing detection, which can be used to study climate change, ocean roughness and salinity, ice, wind speed, disaster monitoring, atmospheric and ionosphere measurements, etc., and in terms of soil detection polarized GNSS-R has been proven to be sensitive to different soil parameters such as freeze thawing, crop growth or soil humidity. Currently in-orbit GNSS-R satellites include, but are not limited to, UK-TDS-1, cyclone GNSS (CYGNSS), wind first satellite, wind cloud third satellite E, F, SMAP-R using a passive radiometer, and the like. All of these dedicated tasks carry a GNSS-R receiver as the main instrument and successfully collect the reflected signals of the global navigation satellite system in the microwave band.
Besides the physical information of the rough surface, the received satellite-borne GNSS reflected signals also have errors caused by factors such as ionosphere, troposphere, non-ideal receiver loading and the like, which can seriously affect the quality of subsequent products, in particular to parameter accuracy obtained based on two-dimensional Delay Doppler (DDM) inversion, such as sea wind, sea ice, soil humidity, vegetation water content and the like. The GNSS-R receiver load will therefore perform a field calibration on the correlated product data.
In global navigation satellite system reflectometers (GNSS-R), field calibration is mainly focused on observable data: bistatic Radar Cross Section (BRCS). It is generally assumed that the transmitted signal is ideally circularly polarized and that such an observable is calibrated using some known parameter, such as receiver antenna pattern gain or receiver noise floor. In addition, the power transmitted by each GNSS satellite is typically estimated by measuring the direct signal with the same receiver or using a look-up table and information from the system GNSS receiver carrier to noise ratio (C/N0) value.
The above-described load field calibration scheme is sufficient for data that is currently only used in marine applications because in marine applications most reflected signals are circular and the reflected signal is Left Hand Circular Polarized (LHCP). However, in future tasks, the load of polarized GNSS-R will receive elliptical reflected signals from complex surfaces such as soil, ice, atmosphere, etc., and thus proper calibration schemes are needed to cope with non-ideal conditions such as ovalization and phase shift of the transmitted signals. Meanwhile, when the linear polarized wave propagates in the magnetized plasma along the magnetic field direction, the polarized surface of the electromagnetic wave continuously rotates in the magnetized plasma by taking the advancing direction as an axis, and signals can be subjected to rotation influence caused by ionized layers both on the way of reaching the earth surface reflection point and the way of reaching the receiver, so that the polarized direction of the signals is not aligned with the polarized direction of the receiving antenna, and the receiving power is attenuated. In addition to this, there is an influence of misalignment of the polarization plane (X/Y plane) from the receiving antenna itself with the polarization plane (H/V plane) of the scattering surface, which seriously affects the GNSS-R product quality, which must be correspondingly compensated.
For example, one of the inventions of publication No. CN115508867a in the prior art discloses a system and a method for processing the co-correlation of signals of two antennas of a GNSS-R receiver, the system estimates specific parameters such as carrier-to-noise ratio, satellite doppler information, code phase and the like of signals by using direct signals received by an up-looking antenna, and uses these information to reconstruct correlation peaks of direct signal leakage and interference, generate a corresponding DDM image, and is used for assisting in the correlation process of reflected signals received by a down-looking antenna and local codes, and reject reconstructed DDM in the reflected correlation waveforms, thereby suppressing the direct leakage and interference and finally improving the quality of output DDM. The method adopts a method of eliminating and reconstructing DDM on the reflection related waveform for calibration.
Disclosure of Invention
The following presents a simplified summary of embodiments of the invention in order to provide a basic understanding of some aspects of the invention. It should be understood that the following summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
In order to solve the non-ideal factors of the GNSS-R receiver from signals and loads, the invention has the following ideas: and performing polarization calibration on the delay-doppler plot DDM, and then replacing I/Q polarization data by using the processed delay-doppler plot DDM. By comparing the I/Q data streams and generating the DDM, the delay Doppler diagram DDM processed by the satellite is used as a calibration input, so that the method has lower data rate and higher signal to noise ratio, and is convenient for polarization calibration.
Specifically, the polarization calibration method of the polarized GNSS-R receiver comprises the following steps:
step 1: acquiring a Delay-Doppler Mapping (DDM); acquiring Stokes parameters from a delay-Doppler plot DDM;
step 2: defining 1 or more non-ideal effects to be compensated for;
step 3: calibrating the non-ideal effect defined in step 2 based on the stokes parameters;
step 4: converting factors which are required to be compensated by different non-ideal effects in the step 3 into error matrixes with the same dimension;
step 5: calibrating Stokes parameters;
step 6: and (3) recovering the calibrated Stokes parameter inversion into a DDM diagram, thereby realizing the calibration of the DDM diagram.
Further, in the step 1, the stokes parameter is obtained from the delay-doppler plot DDM, which specifically includes: constructing a polarization scene, and extracting polarization characteristics of GNSS reflected signals to obtain Stokes parameters; the Stokes parameter is a parameter describing the polarization state of electromagnetic waves, and the formula for extracting the Stokes parameter is as follows:
S 0 ＝＜|E RH | 2 ＞+＜|E RV | 2 ＞；
S 1 ＝＜|E RH | 2 ＞-＜|E RV | 2 ＞；
wherein: e (E) RH And E is RV Is the complex electric field of the right-hand circularly polarized transmitted signal after the cross-correlation of the two linear orthogonal polarization components (H and V) with the C/a code replica. Stokes parameters S 0 -S 3 Is commonly used for researching the polarization state of light or electromagnetic waves, and is described as follows:
S 0 (intensity parameter) represents the total electric field intensity, which is generally expressed in units 1, the other three values being normalized accordingly;
S 1 (polarization parameters) represent the linear polarization degree of electromagnetic waves, range: s is more than or equal to-1 1 ≤1；
S 2 (polarization parameter) represents the linear polarization degree of electromagnetic wave, and S 1 In contrast, its measurement is perpendicular to S 1 Degree of polarization in direction, range: s is more than or equal to-1 2 ≤1；
S 3 (circular polarization parameter) means degree of circular polarization, measurement of rotation property of electromagnetic wave, range: s is more than or equal to-1 3 ≤1。
As a possible solution, the non-ideal effect in step 2 includes polarization calibration of the receiver; the calibrating the non-ideal effect defined in the step 2 based on the stokes parameter in the step 3 specifically includes:
calculating the pointing direction of the axis of the receiver antenna:
δθ＝θ sp -θ T ；
wherein θ is T Andazimuth angle and pitch angle respectively of actual pointing direction of receiver antenna, theta sp And->Azimuth and pitch angles at the specular reflection point positions, respectivelyThe angle calculation uses a geocentric and geodetic coordinate system. Delta theta and->To target the receiver position as the origin, the angular position and pitch angle offsets in the clockwise direction of the specular reflection point, the angular calculation uses a geocentric fixed coordinate system.
As a possible solution, the non-ideal effect in step 2 includes two faraday rotations (FAR) calibration on the reflection path; the calibrating the non-ideal effect defined in the step 2 based on the stokes parameter in the step 3 specifically includes:
the angle omega of the Faraday rotation angle is estimated by the following formula:
wherein lambda is the wavelength of the reflected signal, TEC U The normalized total column electron content was obtained by inversion of the TEC plot provided by GPS.
As a possible solution, the non-ideal effect in step 2 includes calibration of the signal axis ratio of the transmitter; the calibrating the non-ideal effect defined in the step 2 based on the stokes parameter in the step 3 specifically includes:
inverting the full polarization model of the reflected signal using the GNSS-R L level data (GNSS-R L level data of GNSS satellite observations) and the ERA5 dataset;
performing preliminary calibration on the Stokes parameters obtained in the step 1, wherein the preliminary calibration comprises Faraday rotation calibration and receiver polarization calibration;
setting an optimizer and a loss function to obtain the calibrated Stokes parameters.
One possible scheme is as follows: the minimized loss function is as follows:
wherein m is the antenna axis ratio, S n (m) (n=0, 1,2, 3) is a full polarization model with unknown parameters m,is the corrected stokes parameter.
In the minimized loss function, the label is obtained by a machine learning fitting modeThe training set is S n (m)，S n (m)＝S n +me jφ ，S n The Stokes parameter is a constant when antenna polarization calibration is not carried out, and fitting is carried out by setting the formula and the label; the m-antenna axis ratio is an unknown number and is needed to be used when calibrating the transmitter antenna for non-ideal effects.
Further, the step 4 converts the factors to be compensated for by the different non-ideal effects in the step 3 into error matrices with the same dimension, which specifically includes:
defining a matrix a describing error factors of the receive antennas and the transmit antennas:
wherein m is the antenna axial ratio,phase difference for H and V polarization; sigma (sigma) 1 Sum sigma 2 Normalized power of H and V polarization, e jφ ＝cos(φ)+jsin(φ)；
m is the antenna axis ratio, the receiver antenna (a x Representing the amplitude of the x-polarization direction, A y Representing the magnitude of the y-polarization direction):
e jφ in (a)For x and y biasPhase difference (deflection angle) of the vibrations. e, e jφ =cos (Φ) +jsin (Φ) (euler formula), when the phase offset is equal to 90 degrees (error free condition), e jφ In this case, only the influence of the antenna axis ratio remains, m=1 is circular polarization, and m+.1 is positive elliptical polarization. When the phase offset angle is not 90 degrees, that antenna is polarized in a partial ellipse.
Defining matrix C as an error factor describing faraday rotation:
wherein Ω is the angle of the faraday rotation angle obtained in step 3;
further, in the step 5, the calibration of the stokes parameter is shown as follows:
in the middle ofAnd S is n N=0, 1,2,3, respectively, for calibrated and uncalibrated stokes parameters; />And->Calibration matrix for transmitter and receiver, respectively, C -1 Is a calibration matrix for faraday rotation.
Wherein, step 1: the obtaining the delay-doppler plot DDM specifically includes:
step 11: generating a GNSS-R data set comprising a data set received by a direct antenna and a data set received by a reflected antenna, the data set received by the direct antenna comprising: the I/Q data set received by the direct antenna, GNSS satellite PVT (Position, velocity and Time) information and satellite clock bias; the data set received by the reflective antenna includes the I/Q data set of H, V channels received by the reflective antenna. The receiver employs two orthogonal linearly polarized receive antennas, and a pair of circularly polarized antennas of opposite significance may be used to receive vertically polarized and horizontally polarized and cross polarized directional reflection signals.
Step 12: calculating a specular reflection point position and a reflection time stamp according to the GNSS-R data set, specifically calculating a specular reflection point position (SP) by solving a following function which minimizes a reflection path length, and satisfying a condition that an incident angle at the specular reflection point position is equal to a reflection angle, and then restricting the specular reflection point position to the earth surface; wherein the following function is represented by the following formula:
S(r sp )＝|r Rx -r sp |+|r Tx -r sp |
wherein r is Rx 、r sp 、r Tx The position vectors of the GNSS-R receiver, the specular reflection point and the GNSS satellite in the geocentric and fixed rectangular coordinate system (ECEF) are respectively. It should be noted that r herein Tx The position of the time at which the signal propagates to the receiver can be extrapolated by GNSS satellite orbits. In terms of constraints, the earth's surface is separated into land and sea, so that the sea reflection is modeled using an average sea surface (MSS) model, such as DTU13. Land reflection uses terrain data such as space plane radar terrain mission (SRTM).
Step 13: constructing a Doppler model according to the specular reflection point positions and the reflection time stamps, and generating a local carrier I/Q signal (local replica signal) according to the Doppler model; the doppler model is a model of the positive performance of the reflection geometry constructed by specular reflection points, and includes both delay and doppler frequency parameters of the reflected signal.
Step 14: the I/Q data segments of the H and V channels are acquired from the GNSS-R data set, and correlated (convolved) with the local carrier I/Q signals to generate a delay-Doppler plot (DDM).
Through the scheme, the invention provides a polarized GNSS-R receiver algorithm and a polarized calibration algorithm of a time-lapse Doppler map DDM; wherein the polarized GNSS-R receiver algorithm comprises: the method comprises the steps of position calculation of direct signals, prediction of specular reflection points, construction of a Doppler delay model and design of a reflection channel DDM processor, wherein a receiver antenna receives GNSS reflection signals in a linear polarization mode, data are divided into H and V channels to be processed, a complex delay Doppler map DDM of the corresponding channel is generated, and generation of subsequent full Stokes parameters is facilitated; in a polarization calibration algorithm of a delay-doppler plot DDM, a delay-doppler plot DDM generated based on a polarization GNSS-R receiver algorithm is used to characterize polarization characteristics of a reflected signal, on the basis of stokes parameters, three non-ideal effects (polarization calibration of a receiver, two faraday rotations (FAR) calibration on a reflection path, and calibration of a signal axis ratio of a transmitter) to be calibrated are provided by the invention, then calibration factors of different non-ideal effects are converted into a compensation matrix of the same dimension by a mode of constructing the matrix, and finally calibration of DDM data is completed by compensating the stokes parameters in a product mode. Compared with the prior art, the invention has the following beneficial effects:
1. on the basis of acquiring data by a polarized GNSS-R receiver, the invention calibrates the existing polarized nonideal effect by using Stokes parameters, and solves the problems of gain loss caused by the difference of the polarization direction of the receiver antenna and the polarization direction of a reflected signal, phase offset caused by two-way Faraday rotation generated by the reflected signal passing through an ionosphere for two times, and nonideal states such as ovalization, phase offset and the like caused by nonideal of the axis ratio of a GNSS transmitting antenna;
2. the algorithm provided by the invention can acquire the axial ratio information of the corresponding GNSS satellite, is convenient to apply to the subsequent product generation process, and improves the precision of the product;
3. meanwhile, the algorithm of the invention calibrates the polarization direction of the antenna to a certain extent, increases the power of the reflected signal and reduces the antenna load to the load.
Drawings
The invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which like or similar reference numerals are used to indicate like or similar elements throughout the several views. The accompanying drawings, which are included to provide a further illustration of the preferred embodiments of the invention and together with a further understanding of the principles and advantages of the invention, are incorporated in and constitute a part of this specification. In the drawings:
FIG. 1 is a flowchart of a polarized GNSS-R receiver algorithm according to the present invention;
FIG. 2 is a reflection channel flow of a circularly polarized antenna in a conventional GNSS-R receiver;
FIG. 3 is a reflection channel flow of a GNSS-R linearly polarized receiver according to the present invention;
FIG. 4 is a flow chart of signal polarization calibration according to the present invention;
FIG. 5 is a general processing scheme for complex DDM to obtain Stokes parameters;
FIG. 6 is a flowchart of two DDM diagrams obtained in the polarized GNSS-R receiver algorithm of the present invention;
fig. 7 is a schematic diagram of receiver polarization calibration.
Detailed Description
Embodiments of the present invention will be described below with reference to the accompanying drawings. Elements and features described in one drawing or embodiment of the invention may be combined with elements and features shown in one or more other drawings or embodiments. It should be noted that the illustration and description of components and processes known to those skilled in the art, which are not relevant to the present invention, have been omitted in the drawings and description for the sake of clarity.
The invention aims to design a calibration algorithm after data processing of a polarized GNSS-R receiver, which is used for calibrating non-ideal factors from signals and loads, wherein the non-ideal factors comprise receiver effect, scene antenna polarization misalignment, faraday rotation, transmitter non-ideal and the like, and products are compensated one by obtaining corresponding parameter values, so that the GNSS-R product with higher quality is obtained. In the prior art, the above mentioned non-idealities have affected the polarized radar receiver and the GNSS radio occultation receiver and have been solved, but the GNSS-R receiver has not yet been.
First, the present invention provides an algorithm flow for a polarized GNSS-R receiver. As shown in fig. 1, the polarized GNSS-R receiver algorithm steps are as follows:
in a first step, a GNSS-R data set is generated, including the I/Q data set received by the direct antenna, GNSS satellite PVT (position, velocity, time) information, and satellite clock, and the I/Q data set of H, V channels received by the reflection antenna. The receiver employs two orthogonal linearly polarized receive antennas, and a pair of circularly polarized antennas of opposite significance may be used to receive vertically polarized and horizontally polarized and cross polarized directional reflection signals.
And secondly, searching the positions of the specular reflection points and the reflection time stamps. The specular reflection point position (SP) is estimated by solving a follow-up function that minimizes the reflection path length while satisfying the condition that the incident angle at the specular reflection point is equal to the reflection angle, and then the specular reflection point position is constrained to the earth's surface. The following function is shown as follows:
S(r sp )＝|r Rx -r sp |+|r Tx -r sp |
wherein r is Rx 、r sp 、r Tx The position vectors of the GNSS-R receiver, the specular reflection point and the GNSS satellite in the geocentric and fixed rectangular coordinate system (ECEF) are respectively. It should be noted that r herein Tx The position of the time at which the signal propagates to the receiver can be extrapolated by GNSS satellite orbits. In terms of constraints, the earth's surface is separated into land and sea, so that the sea reflection is modeled using an average sea surface (MSS) model, such as DTU13. Land reflection uses terrain data such as space plane radar terrain mission (SRTM).
And thirdly, respectively acquiring I/Q data segments of the H channel and the V channel and a Doppler model, wherein the data segments are used for open-loop tracking of the reflected signals and generation of a DDM diagram, and the Doppler model is a model of positive performance of reflection geometry constructed through specular reflection points and comprises two parameters of time delay and Doppler frequency of the reflected signals. The model may assist the receiver in constructing a local replica signal.
Fourth, a delay-doppler plot (DDM) is acquired. And (3) carrying out correlation processing on the local replica signal and the H channel data fragment (horizontal polarization data) and the V channel data fragment (vertical polarization data) obtained in the third step, and respectively storing the results, wherein the method is the same as that of a conventional reflection receiver.
The third step of Doppler delay model construction comprises the following steps:
the code delay and Doppler frequency of the reflected signal are respectively marked as tau by adopting a master-slave scheme R (t) and f R (t) delaying τ according to the direct signal code T (t), doppler frequency f T (t), specular reflection points SP modeling it:
wherein R is T (t) andrepresenting the distance and the distance change rate of the direct signal path, R R (t) and->Representing the distance and the rate of change of the distance of the reflected signal path, δt being the time delay between the direct and reflected signals, cps being the chip rate, f c C is the propagation speed of electromagnetic waves, which is the carrier frequency of the GNSS signal;
direct signal code delay τ T (t), doppler frequency f T (t) rate of distance changeCan be obtained by direct signal capturing and tracking; reflection path R R (t) and->Can be calculated from the position of the specular reflection point SP. The determination of specular reflection points requires the incorporation of geographical information. Most of the current specular reflection point determinations are based on the general empirical model WGS84 coordinate system. To obtain finer specular reflection points, the following can also be operated: by differentiating the environment of the reflection point, e.g. the reflection point being a marine surface environmentAn average sea surface Model (MSS), such as DTU13, is used. If the reflection point is a land environment, then terrain data such as space plane radar terrain mission (SRTM) is employed. Both of these actual terrain models are prior art and will not be described in detail herein.
The GNSS-R receiver of the present invention differs from existing receivers in terms of antennas and data reflection channels, among other things, for further improvements in the acquired delay-doppler plot DDM. Conventional GNSS-R receivers are circularly polarized antennas that receive I and Q composite signals (i+qj) from one signal. In this regard, a conventional GNSS-R reflection channel flow (one path) is typically 1ms, resulting in 1 DDM, the schematic diagram of which is shown in FIG. 2.
Referring to FIG. 3, the reflection channel flow (two paths) of the GNSS-R linearly polarized receiver of the invention generates 2 DDMs (H-channel data segment DDMs) at the same time H And V-channel data segment DDM V ). Therefore, the invention extracts two characteristics of horizontal polarization and vertical polarization, researches the polarization characteristic of signals, and is convenient for the subsequent algorithm to correct the signals, such as the correction of ovalization and the correction of polarization direction. In order to adapt to two paths of reflection channels, the GNSS-R receiver adopts a linear polarization receiver, and an antenna receives signals from horizontal and vertical directions respectively, namely two paths of signals according to horizontal polarization and vertical polarization; the number of the processing modules is two, and the two paths of signals are processed respectively. The reflection channel of the conventional receiver is a data carrier demodulation time division I/Q channel, and the polarized receiver generates two data when receiving, which needs to demodulate the independent carrier and then finally generates two DDMs, wherein the two DDMs represent signal information in different bias directions and possibly also contain information from the ground. Because the conventional antennas are all circularly polarized, no one in the prior art designs two processing modules on the reflection channel block; in order to process double-line polarization, two processing modules are creatively adopted, and polarization characteristics are extracted in a distributed mode to be processed respectively.
In addition, in the above-mentioned GNSS-R receiver algorithm, the direct signal position calculation and specular reflection point prediction are performed by using a general-purpose GNSS navigation receiver internal position calculation algorithm and specular reflection point prediction algorithm, and a DDM processor, which are not the invention points, and therefore will not be described in detail herein.
The calibration algorithm provided by the invention can be applied to the DDM data processed by the polarized GNSS-R receiver, and in the algorithm scheme, a polarized scene of GNSS reflection is defined first for expressing the polarization characteristics of the reflected signals. And then defining different non-ideal effects to be compensated, obtaining a corresponding calibration matrix by analyzing various non-ideal effects, compensating the polarized scene, and finally realizing the non-ideal calibration of the DDM. Specifically, the method for calibrating polarization of the polarized GNSS-R receiver according to the embodiment of the present invention has an algorithm flow chart shown in fig. 4, and the detailed steps are as follows:
step one, constructing a polarization scene. Before the non-ideality is calibrated, the polarization characteristics of the GNSS reflected signals need to be extracted, namely stokes parameters, which are parameters describing the polarization state of electromagnetic waves, and can replace the total intensity (I), polarization (p), shape parameters of polarized ellipses and the like of polarized radiation, and specific parameters of decomposition, such as the degree of polarization (DoP), relative phase (d), ellipticity parameters (x) and the like, are further inverted by using the stokes parameters. Stokes parameters can be obtained from complex DDMs of H and V polarized channels, known as full stokes DDMs, i.e. products generated by the polarized GNSS-R receiver described above. Fig. 5 shows a general processing scheme for obtaining stokes parameters from a complex DDM. The formula for extracting Stokes parameters is:
S 0 ＝＜|E RH | 2 ＞+＜|E RV | 2 ＞
S 1 ＝＜|E RH | 2 ＞-＜|E RV | 2 ＞
wherein: e (E) RH And E is RV Two of the transmitted signals being right-hand circularly polarisedThe complex electric field after the cross-correlation of the linear orthogonal polarization components (H and V) with the C/A code replica.
Referring to fig. 6, two DDM maps (DDM) are obtained in a polarized GNSS-R receiver algorithm H And DDM V ) The two DDMs are obtained by different polarization channels, and the DDM diagram is a two-dimensional matrix. In fig. 6, it is assumed that incoherent integration is not performed, only 1ms signals are extracted for coherent integration, that is, the flow of the polarized GNSS-R receiver algorithm is performed only once, and two DDM diagrams are generated at a time. Assuming that each DDM map has N values in total, four Stokes parameters can be extracted from the two DDM maps, and the process is recorded as a step-by-step polarization scene construction; each of the parameters obtained is a sub-parameter of the stokes parameter. The present invention applies stokes parameters to CYGNSS data or polarized receiver data for the first time.
And step two, defining different non-ideal effects to be compensated. The invention gives three non-ideal effects to be calibrated, namely polarization calibration of the receiver, two Faraday rotations (FAR) calibration on the reflection path, and calibration of the signal axis ratio of the transmitter.
And thirdly, calibrating the non-ideal effect proposed in the second step. All calibration effects can be applied to the Stokes parameters, the extracted Stokes parameters are calibrated, and after calibration, the calibrated Stokes parameters are used for calculating set indexes such as reflectivity and the like, namely the DDM diagram is calibrated.
The calibration method provided by the invention is based on the Stokes parameters defined in the first step, and factors which are required to be compensated for respectively different non-ideal effects are converted into error matrixes with the same dimension in the fourth step, so that only the respective calibration method is described in the fourth step.
(1) Receiver polarization calibration: it is necessary to calculate the pointing direction of the axis of the receiver antenna.
δθ＝θ sp -θ T
Wherein θ is T Andazimuth angle and pitch angle respectively of actual pointing direction of receiver antenna, theta sp And->The geocentric geodetic coordinate system is used for the angular calculation for azimuth and pitch angles, respectively, at the specular reflection point location.
Delta theta sumTo target the receiver position as the origin, the angular position and pitch angle offsets in the clockwise direction of the specular reflection point, the angular calculation uses a geocentric fixed coordinate system.
In the prior art, the non-ideal effect of the receiver is that the direction of the specular reflection point is not aligned when the receiving antenna receives the signal, one disadvantage of the adoption of the linear polarization antenna is that the receiving point is aligned when the signal is received, so that the signal can be completely received, if the criterion is not aligned, the received signal power is low, a certain deviation is generated in the phase, and the like, therefore, the polarization calibration of the receiver is realized by calculating how much the antenna is deviated. Referring to fig. 7, the offset calculation involves two angles, one azimuth and one pitch. The two angles are found and then calibrated in combination with a calibration method for receiver non-ideal effects.
(2) Faraday rotation calibration: the faraday rotation causes the polarization direction of the reflected signal to be different from the polarization direction of the receiver antenna, so that the angle of the faraday rotation angle needs to be estimated during calibration, and the estimation method is as follows:
wherein lambda is the wavelength of the reflected signal, TEC U The normalized total column electron content was obtained by inversion of the TEC plot provided by GPS.
(3) Calibration of transmitter signal axis ratio: calculating the full stokes parameters of the linear polarized antenna requires considering the axial ratio of the GNSS satellite emissions, and the calibration of the axial ratio needs to be obtained by fitting through machine learning, specifically 1, inverting the full polarization model of the reflected signal using the GNSS-R L and ERA5 datasets. 2. And (3) carrying out preliminary calibration on the Stokes parameters obtained in the step one, wherein the preliminary calibration comprises Faraday rotation calibration and receiver polarization calibration. 3. An optimizer and a loss function are set. A minimized loss function is provided herein as follows:
s in n (m) (n=0, 1,2, 3) is a full polarization model with unknown parameters m,is the corrected stokes parameter.
And step four, converting factors which are required to be compensated for by different non-ideal effects in the third step into error matrixes with the same dimension. A matrix a is defined here to describe the error factors of the receive and transmit antennas.
Wherein m is the antenna axial ratio,is the phase difference of the H and V polarizations. Sigma (sigma) 1 Sum sigma 2 Normalized power for H and V polarization, respectively.
Matrix C is defined as the error factor describing faraday rotation.
Wherein Ω is the angle of the Faraday rotation angle obtained in step three
And fifthly, calibrating Stokes parameters, wherein the calibration is shown in the following formula.
In the middle ofAnd S is n N=0, 1,2,3 for the calibrated and uncalibrated stokes parameters, respectively. />And->Calibration matrix for transmitter and receiver, respectively, C -1 Is a calibration matrix for faraday rotation.
Specifically, the calibration procedure is to construct a matrix formula on stokes parameters first, and then multiply the stokes parameters by an inverse matrix of the error factor.
More specifically, a matrix formula is first constructed:
S n ＝A T ·C·A R ·S FSA
wherein A is T 、C、A R Error factors of the transmitting antenna, faraday rotation and receiving antenna respectively, S FSA For forward scattering on its substrate (prior art), S n (S 0 、S 1 、S 2 、S 3 ) The parameters consist of error factors and a base.
The stokes parameter is then multiplied by the inverse of the error factor (S n ＝A T ·C·A R ·S FSA ) The calibrated Stokes parameters can be obtainedThe inverse matrices of the error factors are +.>C -1 、/>
The corrected Stokes parameters are obtained to calculate the corresponding index, e.g. to calculate the surface reflectivity<E RV | 2 >The earth surface reflectivity is used for a main observed quantity formula of soil inversion;
and step six, the calibrated Stokes parameter inversion is restored into a DDM diagram. Calibration of the DDM graph is achieved.
By adopting the scheme, the invention replaces the I/Q polarized data with the processed DDM, compares the I/Q data flow and generates the DDM, and adopts the DDM processed by the satellite as the calibration input to have lower data rate and higher signal to noise ratio, thereby facilitating the polarization calibration. Although the calibration scheme is designed based on a linearly polarized GNSS receiver, it is equally applicable to a generally circularly polarized GNSS-R receiver. However, in practice, the method of polarization calibration is applicable to both linear polarization and circular polarization, and is designed based on linear polarization only in the quadrature book. The other undescribed portions of the GNSS-R receiver algorithm of the present invention are substantially identical to those in conventional receivers, and reference is made to conventional receiver algorithms.
The methods of the present invention are not limited to being performed in the time sequence described in the specification, but may be performed in other time sequences, in parallel or independently. Therefore, the order of execution of the methods described in the present specification does not limit the technical scope of the present invention.
While the invention has been disclosed in the context of specific embodiments, it should be understood that all embodiments and examples described above are illustrative rather than limiting. Various modifications, improvements, or equivalents of the invention may occur to persons skilled in the art and are within the spirit and scope of the following claims. Such modifications, improvements, or equivalents are intended to be included within the scope of this invention.
Claims (10)
1. A method for calibrating polarization of a polarized GNSS-R receiver, characterized by: comprising the following steps:
step 1: acquiring a delay-doppler plot DDM; acquiring Stokes parameters from a delay-Doppler plot DDM;
step 2: defining 1 or more non-ideal effects to be compensated for;
step 3: calibrating the non-ideal effect defined in step 2 based on the stokes parameters;
step 4: converting factors which are required to be compensated by different non-ideal effects in the step 3 into error matrixes with the same dimension;
step 5: calibrating Stokes parameters based on the error matrix in the step 4;
step 6: and (4) recovering the calibrated Stokes parameter inversion into a delay-Doppler diagram DDM diagram.
2. The polarization calibration method of a polarized GNSS-R receiver according to claim 1, wherein: in the step 1, the stokes parameter is obtained from the delay-doppler plot DDM, which specifically includes: constructing a polarization scene, and extracting polarization characteristics of GNSS reflected signals to obtain Stokes parameters; the Stokes parameter is a parameter describing the polarization state of electromagnetic waves, and the formula for extracting the Stokes parameter is as follows:
S 0 ＝＜|E RH | 2 ＞+＜|E RV | 2 ＞；
S 1 ＝＜|E RH | 2 ＞-＜|E RV | 2 ＞；
wherein: s is S 0 -S 3 Is Stokes parameter, E RH And E is RV Is rightThe complex electric field after cross-correlation of the two linear orthogonal polarization components (H and V) of the circularly polarized transmitted signal with the C/a code replica.
3. The polarization calibration method of a polarized GNSS-R receiver according to claim 1, wherein: the non-ideal effects in step 2 include polarization calibration of the receiver, two faraday rotation calibration on the reflection path, and calibration of the transmitter signal axis ratio.
4. A method of polarization calibration of a polarized GNSS-R receiver according to claim 3, wherein: the calibrating polarization of the receiver based on the stokes parameter in the step 3 specifically includes: calculating the pointing direction of the axis of the receiver antenna:
wherein θ is T Andazimuth angle and pitch angle respectively of actual pointing direction of receiver antenna, theta sp And->Respectively using a geocentric earth fixed coordinate system for calculating the azimuth angle and the pitch angle of the specular reflection point; delta theta and->To target the receiver position as the origin, the angular position and pitch angle offsets in the clockwise direction of the specular reflection point, the angular calculation uses a geocentric fixed coordinate system.
5. The method for polarization calibration of a polarized GNSS-R receiver of claim 4, wherein: the calibrating the two faraday rotations on the reflection path based on the stokes parameter in the step 3 specifically includes:
the angle omega of the Faraday rotation angle is estimated by the following formula:
wherein lambda is the wavelength of the reflected signal, TEC U The normalized total column electron content was obtained by inversion of the TEC plot provided by GPS.
6. The polarization calibration method of a polarized GNSS-R receiver of claim 5, wherein: the calibrating the transmitter signal axial ratio based on the stokes parameter in the step 3 specifically includes:
inverting the full polarization model of the reflected signal using the GNSS-R L level 1 data and ERA5 dataset;
performing preliminary calibration on the Stokes parameters obtained in the step 1;
setting an optimizer and a loss function to obtain the calibrated Stokes parameters.
7. The method for polarization calibration of a polarized GNSS-R receiver of claim 6, wherein: the minimized loss function is of the formula:
wherein m is the antenna axis ratio, S n (m) (n=0, 1,2, 3) is a full polarization model with unknown parameters m,is the corrected stokes parameter.
8. The polarization calibration method of a polarized GNSS-R receiver of claim 5, wherein: converting factors to be compensated for by different non-ideal effects in the step 3 into error matrixes with the same dimension, wherein the method specifically comprises the following steps:
defining a matrix a describing error factors of the receive antennas and the transmit antennas:
wherein m is the antenna axial ratio,phase difference for H and V polarization; sigma (sigma) 1 Sum sigma 2 Normalized power for H and V polarization, respectively; e, e jφ ＝cos(φ)+jsin(φ)；
Defining matrix C as an error factor describing faraday rotation:
where Ω is the angle of the faraday rotation angle obtained in step 3.
9. The method for polarization calibration of a polarized GNSS-R receiver of claim 8, wherein: in the step 5, the calibration of stokes parameters is shown as follows:
in the middle ofAnd S is n N=0, 1,2,3, respectively, for calibrated and uncalibrated stokes parameters; />And->Calibration matrix for transmitter and receiver, respectively, C -1 Is a calibration matrix for faraday rotation.
10. The polarization calibration method of a polarized GNSS-R receiver according to claim 1, wherein: the step 1: the obtaining the delay-doppler plot DDM specifically includes:
step 11: generating a GNSS-R data set comprising a data set received by a direct antenna and a data set received by a reflected antenna, the data set received by the direct antenna comprising: the direct antenna receives the I/Q data set, the GNSS satellite PVT information and the satellite clock error; the data set received by the reflection antenna comprises an I/Q data set of H, V channels received by the reflection antenna;
step 12: calculating a specular reflection point position and a reflection time stamp according to the GNSS-R data set, specifically calculating the specular reflection point position by solving a following function minimizing the reflection path length, and satisfying the condition that the incident angle at the specular reflection point position is equal to the reflection angle, and then restricting the specular reflection point position to the earth surface; wherein the following function is represented by the following formula:
S(r sp )＝|r Rx -r sp |+|r Tx -r sp |；
wherein r is Rx 、r sp 、r Tx The position vectors of the GNSS-R receiver, the specular reflection point and the GNSS satellite under a geocentric and geodetic rectangular coordinate system are respectively;
step 13: constructing a Doppler model according to the specular reflection point position and the reflection time stamp, and generating a local carrier I/Q signal according to the Doppler model; the Doppler model is a model of positive performance of reflection geometry constructed by specular reflection points and comprises two parameters of time delay and Doppler frequency of a reflected signal;
step 14: and acquiring I/Q data segments of the H and V channels according to the GNSS-R data set, and performing correlation processing on the I/Q data segments and the local carrier I/Q signals so as to generate a delay-Doppler map DDM.
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