GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method and system

Abstract

The present invention discloses a method and system for reconstructing GNSS‑R sea surface height measurement precision and along-track spatial resolution, the method comprising: processing raw data to obtain a complex waveform; processing the complex waveform to obtain a power waveform, and solving to obtain the average height measurement precision corresponding to N mirror points; correcting the mirror point delay, inverting to obtain the vertical distance from the sea surface of the receiver's sub-satellite point to the reference ellipsoid, and solving to obtain the average height measurement accuracy corresponding to N mirror points; obtaining the average height measurement precision and average height measurement accuracy obtained with different signal processing times; combining the relationship between along-track spatial resolution and signal processing time, to achieve reconstruction of GNSS‑R sea surface height measurement precision and along-track spatial resolution. The present invention comprehensively analyzes the relationship between airborne iGNSS‑R height measurement precision and along-track spatial resolution from the two perspectives of height measurement precision and accuracy, and provides a reference for the balance consideration of related parameters of future satellite-borne iGNSS‑R height measurement tasks.

Description

Technical Field

[0001] The present invention belongs to the cross-technical fields of satellite altimetry, oceanographic mapping, etc., and in particular relates to a method and system for reconstructing the sea surface altimetry accuracy and along-track spatial resolution of a GNSS-R. Background Art

[0002] Global Navigation Satellite System Reflectometer (GNSS-R technology) can use GNSS reflected signals to measure a series of physical parameters on the earth's surface, including sea surface wind speed, sea surface height, and soil moisture. Among them, GNSS-R sea surface altimetry, as a new type of dual-base radar remote sensing technology, can obtain the height of the earth's reflecting surface relative to the reference ellipsoid by measuring the path delay between the direct signal and the reflected signal. Since Martin-Neira et al. first proposed the concept of GNSS-R sea surface altimetry in 1993, this technology has been verified on various platforms such as ground-based, airborne, and satellite. Compared with traditional radar altimeters, GNSS-R sea surface altimetry has the advantages of low cost, multi-directional simultaneous observation, and high temporal and spatial earth coverage.

[0003] According to the signal processing method for obtaining path delay, GNSS-R sea surface altimetry currently mainly adopts cGNSS-R (conventional GNSS-R) altimetry and iGNSS-R (interferometric GNSS-R) altimetry. cGNSS-R technology uses the replica code of the locally generated transmission signal to perform cross-correlation with the received reflected signal for a certain time (usually 1ms) after delay and Doppler frequency compensation to obtain the path delay. Currently, only the C / A code is publicly available in the GNSS signal code structure. However, the low bandwidth of the C / A code (2.046MHz) limits the altimetry accuracy and along-track spatial resolution of cGNSS-R. In order to overcome the bandwidth limitation, ESA proposed the PARIS IoD mission in 2011, aiming to realize the proposed satellite-borne signal interferometric processing, namely iGNSS-R altimetry technology. iGNSS-R technology performs complex cross-correlation between the reflected signal and the direct signal, which will make full use of the full power spectrum of the GNSS transmission signal, improve the clarity of the autocorrelation function, and thus maximize the altimetry accuracy and along-track spatial resolution.

[0004] Altimeter accuracy and along-track spatial resolution are key indicators that characterize the high performance of iGNSS-R. Altimeter accuracy can be divided into altimeter precision and altimeter accuracy. Altimeter precision is mainly affected by zero-mean random error; altimeter accuracy is mainly determined by the total absolute measurement error affected by additional random and deterministic errors. Along-track spatial resolution characterizes the ability of iGNSS-R altimeter to sample along-track space, which can be expressed as: R a_t =v SP ·T coh ·Nincoh (v SP represents the speed of the mirror point along the track; T coh Represents the signal coherent integration time; T incoh Represents the number of incoherent accumulations of the signal). The spatial resolution along the track can be improved by reducing the signal integration time. However, the reduction in the number of independent waveform samples will increase the influence of speckle noise, resulting in a decrease in the signal-to-noise ratio, which in turn increases the uncertainty of the mirror point delay, and ultimately reduces the accuracy and precision of the height measurement. There are currently no iGNSS-R height measurement satellites in orbit, but ESA has released several iGNSS-R height measurement mission plans in recent years, such as PARIS IoD (PARIS In-orbit Demonstrator, proposed in 2011), GEROS-ISS (GNSS reflectometry, radio occultation, and scatterometry onboard the International Space Station, proposed in 2011); Cookie constellation (proposed in 2016); G-TERN (GNSS Transpolar Earth Reflectometry exploration system, proposed in 2018). Figure 1 As shown in the figure, airborne iGNSS-R altimetry is a pre-research technology for satellite-borne iGNSS-R altimetry. At present, some mid-altitude flight experiments have been carried out. Among them, the most typical one is that the Institute of Space Sciences of Catalonia (IEEC) has successively carried out two iGNSS-R flight missions with an altitude of about 3km in the Baltic Sea, which are called PIRA and SPIR respectively. Based on this data, relevant scholars have carried out a series of studies on the accuracy of iGNSS-R altimetry and along-track spatial resolution. In 2014, Cardellach et al. used PIRA mission data to focus on analyzing the performance of cGNSS-R and iGNSS-R altimetry accuracy under different signal processing times, and simulated and analyzed the performance of future low-orbit iGNSS-R altimetry. The results showed that increasing the signal processing time will significantly improve the altimetry accuracy. In 2017, Ribo et al. first published the signal-to-noise ratio results of SPIR waveforms in different frequency bands of multiple GNSS. In the same year, Li et al. studied the altimetry accuracy performance of SPIR data with signal processing time of 1s, 10s and 100s. In 2019, Fabra et al. analyzed the altimetry accuracy of SPIR data with a signal processing time of 10 seconds. The results showed that the altimetry accuracy ranged from 0.09 to 0.66 m depending on the signal source and satellite altitude information. However, there is currently no comprehensive analysis of the altimetry precision and accuracy of airborne iGNSS-R and the spatial resolution along the track. Summary of the invention

[0005] The technology of the present invention solves the problem: overcomes the shortcomings of the prior art, provides a GNSS-R sea surface altimetry accuracy and along-track spatial resolution reconstruction method and system, obtains the corresponding altimetry accuracy by setting different signal processing times, and then comprehensively analyzes the relationship between the airborne iGNSS-R altimetry accuracy and along-track spatial resolution from the two perspectives of altimetry precision and accuracy through the relationship between the signal processing time and the along-track spatial resolution, providing a reference for the balance consideration of relevant parameters of future spaceborne iGNSS-R altimetry missions.

[0006] In order to solve the above technical problems, the present invention discloses a GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, comprising:

[0007] Step 1, obtain the raw data received by the SPIR upper and lower antennas;

[0008] Step 2, beamforming and cross-correlation processing of direct and reverse signals are performed on the original data to obtain a complex waveform;

[0009] Step 3, performing coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; and according to the power waveform, calculating the average height measurement accuracy corresponding to N mirror points;

[0010] Step 4: correct the mirror point delay according to the tropospheric delay, and invert the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid; and calculate the average height measurement accuracy corresponding to N mirror points according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid;

[0011] Step 5, by repeating steps 1 to 4, obtaining average height measurement precision and average height measurement accuracy obtained with different signal processing times;

[0012] Step 6: Based on the average height measurement accuracy and average height measurement precision obtained at different signal processing times, combined with the relationship between along-track spatial resolution and signal processing time, the along-track spatial resolution corresponding to the height measurement accuracy is obtained, thereby realizing the reconstruction of the GNSS-R sea surface height measurement accuracy and along-track spatial resolution.

[0013] In the above GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, the number of complex waveforms obtained by beamforming and direct and reverse signal cross-correlation processing is: 6×10 5 The coherent integration time of each complex waveform is 1ms.

[0014] In the above-mentioned GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, the complex waveform is processed by coherent integration and incoherent cumulative averaging to obtain a power waveform, including:

[0015] According to the following formulas (1) and (2), the complex waveform is processed by coherent integration and incoherent cumulative averaging to obtain the power waveform:

[0016]

[0017] W(T0+N incoh ·T coh / 2,τ)=|w(T0+N incoh ·T coh / 2,τ)| 2 ···(2)

[0018] Among them, w(T0+N incoh ·T coh / 2,τ) represents the waveform after coherent integration and incoherent cumulative averaging, T0 represents the start time of coherent integration and incoherent cumulative averaging, N incoh Indicates the number of incoherent accumulations, T coh represents the coherent integration time, c(nT coh ,τ) represents the complex waveform, t represents the time, τ represents the delay, W(T0+N incoh ·T coh / 2,τ) represents the power waveform.

[0019] In the above-mentioned GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, it also includes: before the complex waveform is subjected to coherent integration and incoherent cumulative averaging processing, the complex waveform is aligned; wherein, T coh =1s, 2s, 5s, 10s, N incoh =1000.

[0020] In the above-mentioned GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, the mirror point delay is corrected according to the tropospheric delay, and the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid is inverted, including:

[0021] Using the DER algorithm, the mirror point delay τ is estimated spec :

[0022]

[0023] in, represents the time delay corresponding to the maximum value of the derivative calculated from the measured waveform, represents the mirror point delay of the simulation waveform, represents the time delay corresponding to the maximum value point of the derivative calculated from the simulation waveform;

[0024] According to the satellite elevation angle θ at the mirror point, the tropospheric delay τ is estimated trop :

[0025]

[0026] Among them, H R Indicates the height of the iGNSS-R receiver, H trop represents the tropospheric height of the observation location;

[0027] Based on formulas (3) and (4), the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid is obtained: spir :

[0028]

[0029] Among them, τ antenna Indicates the antenna baseline delay error.

[0030] In the above-mentioned GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, the average height measurement accuracy corresponding to N mirror points is calculated based on the power waveform, including:

[0031] The waveform signal-to-noise ratio (SNR) of the power waveform is calculated. wf :

[0032]

[0033] Among them, A noise represents the average noise amplitude of the power waveform, σ noise Indicates A noise The standard deviation of

[0034] According to SNR wf , the height measurement accuracy σ is obtained by solving p :

[0035]

[0036] Where c is the speed of light in vacuum, θ S represents the satellite elevation angle at the mirror point, W(0) represents the signal power at the mirror point;

[0037] Based on formula (7), the height measurement accuracy of each mirror point is calculated in turn:

[0038] Then, the average height measurement accuracy corresponding to N mirror points is for:

[0039]

[0040] In the above GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, the average height measurement accuracy corresponding to N mirror points is calculated according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid, including:

[0041] According to the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid spir , the height measurement accuracy σ is obtained by solving formula (9) a :

[0042] σ a =SSH spir -SSH ref ···(9)

[0043] Among them, SSH ref represents the sea surface height of the reference model;

[0044] Based on formula (9), the height measurement accuracy of each mirror point is calculated in turn:

[0045] Then, the average height measurement accuracy corresponding to N mirror points is for:

[0046]

[0047] Correspondingly, the present invention also discloses a GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction system, comprising:

[0048] The acquisition module is used to obtain the raw data received by the SPIR upper and lower antennas;

[0049] A processing module is used to perform beamforming and cross-correlation processing of direct and reverse signals on the original data to obtain a complex waveform;

[0050] The first solution module is used to perform coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; and according to the power waveform, the average height measurement accuracy corresponding to N mirror points is obtained by solution;

[0051] The second solution module is used to correct the mirror point delay according to the tropospheric delay, and invert to obtain the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid; and according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid, solve to obtain the average height measurement accuracy corresponding to N mirror points;

[0052] A third solution module is used to repeat the execution process of the acquisition module to the second solution module to obtain the average height measurement precision and the average height measurement accuracy obtained at different signal processing times;

[0053] The reconstruction module is used to obtain the along-track spatial resolution corresponding to the height measurement accuracy based on the average height measurement accuracy and average height measurement precision obtained at different signal processing times, combined with the relationship between along-track spatial resolution and signal processing time, and thus realize the reconstruction of GNSS-R sea surface height measurement accuracy and along-track spatial resolution.

[0054] The present invention has the following advantages:

[0055] (1) The present invention discloses a GNSS-R sea surface altimetry precision and along-track spatial resolution reconstruction method and system. By setting different signal processing times, the corresponding altimetry precision is obtained. Then, through the relationship between the signal processing time and the along-track spatial resolution, the relationship between the airborne iGNSS-R altimetry precision and the along-track spatial resolution is comprehensively analyzed from the two perspectives of altimetry precision and accuracy, providing a reference for the balance consideration of related parameters of future spaceborne iGNSS-R altimetry missions.

[0056] (2) Taking the measured data of airborne iGNSS-R as an example, the present invention studies and analyzes the relationship between the height measurement accuracy and the spatial resolution along the track from the two perspectives of precision and accuracy. The results show that the reduction of the spatial resolution along the track will improve the height measurement accuracy, where the precision changes in the range of 0.16-1.13m and the accuracy changes in the range of 0.28-0.73m. However, this change is not linear, and the improvement of the height measurement accuracy will decrease with the increase of the spatial resolution along the track. The results obtained in the present invention can provide a technical reference for the reasonable configuration of parameters for future spaceborne iGNSS-R height measurement missions. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is a working principle diagram of an existing airborne iGNSS-R altimetry;

[0058] Figure 2 This is a flowchart of the steps of a method for reconstructing GNSS-R sea surface height measurement accuracy and along-track spatial resolution in an embodiment of the present invention;

[0059] Figure 3 Schematic diagram of the SPIR experimental data collection flight route used in the embodiments of the present invention;

[0060] Figure 4 is a schematic diagram of an iGNSS-R power waveform (different signal processing times) in an embodiment of the present invention;

[0061] Figure 5 is a schematic diagram of iGNSS-R height measurement accuracy (different signal processing times) in an embodiment of the present invention;

[0062] Figure 6 1 is a schematic diagram of the relationship between the SPIR height measurement accuracy and the along-track spatial resolution in an embodiment of the present invention;

[0063] Figure 7 is a schematic diagram of iGNSS-R height measurement accuracy (different signal processing times) in an embodiment of the present invention;

[0064] Figure 8 It is a schematic diagram of the relationship between the SPIR height measurement accuracy and the along-track spatial resolution in an embodiment of the present invention. DETAILED DESCRIPTION

[0065] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments disclosed in the present invention will be further described in detail below with reference to the accompanying drawings.

[0066] Precision and along-track spatial resolution are key parameters that characterize the high performance of iGNSS-R (Interferometric Global Navigation Satellite Systems Reflectometry) sea surface measurement, and the two are closely related through signal processing time. If a higher height measurement accuracy is obtained in the observation area, a longer signal processing time is required, which will result in a partial loss of along-track spatial resolution. On the contrary, obtaining a higher along-track spatial resolution requires more intensive time sampling, which ultimately leads to unsatisfactory height measurement accuracy. Taking the measured data of airborne iGNSS-R as an example, the present invention studies and analyzes the relationship between height measurement accuracy and along-track spatial resolution from the two perspectives of precision and accuracy. The results show that the reduction of along-track spatial resolution will improve the height measurement accuracy, where the precision range is 0.16-1.13m, and the accuracy range is 0.28-0.73m, but this change is not linear, and the magnitude of the height measurement accuracy improvement will decrease with the increase of the along-track spatial resolution value. The results of the present invention can provide a technical reference for the reasonable configuration of parameters for future spaceborne iGNSS-R height measurement tasks.

[0067] like Figure 2 In this embodiment, the GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method includes:

[0068] Step 101, obtaining raw data received by the SPIR upper and lower antennas.

[0069] Step 102, beamforming and direct and reverse signal cross-correlation processing are performed on the original data to obtain a complex waveform.

[0070] In this embodiment, after the raw data is received by the SPIR upper and lower antennas, it needs to be further processed to obtain the power waveform. The present invention adopts the raw data processing method to obtain a complex waveform c(t, τ) with a dry integration time of 1 ms through beamforming and cross-correlation of direct and reverse signals. The total waveform is 6×10 5Among them, t represents different time and τ represents the delay.

[0071] Step 103, performing coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; and according to the power waveform, calculating and obtaining the average height measurement accuracy corresponding to N mirror points.

[0072] In this embodiment, in order to reduce the influence of thermal noise and speckle noise and improve the signal-to-noise ratio of the power waveform, it is necessary to perform coherent integration and N incoh The power waveform is obtained by performing incoherent cumulative averaging.

[0073] Preferably, the specific calculation process of the power waveform can be as follows:

[0074] According to the following formulas (1) and (2), the complex waveform is processed by coherent integration and incoherent cumulative averaging to obtain the power waveform:

[0075]

[0076] W(T0+N incoh ·T coh / 2,τ)=|w(T0+N incoh ·T coh / 2,τ)| 2 ···(2)

[0077] Among them, w(T0+N incoh ·T coh / 2,τ) represents the waveform after coherent integration and incoherent cumulative averaging, T0 represents the start time of coherent integration and incoherent cumulative averaging, N incoh Indicates the number of incoherent accumulations, T coh represents the coherent integration time, c(nT coh ,τ) represents the complex waveform, t represents the time, τ represents the delay, W(T0+N incoh ·T coh / 2,τ) represents the power waveform.

[0078] Preferably, before performing coherent integration and incoherent cumulative averaging on the complex waveform, the complex waveform needs to be aligned. coh =1s, 2s, 5s, 10s, N incoh =1000.

[0079] Preferably, the specific calculation process of the average height measurement accuracy corresponding to the mirror point can be as follows:

[0080] The height measurement accuracy is closely related to the signal-to-noise ratio (SNR) of the iGNSS-R waveform.wf Defined as:

[0081]

[0082] Among them, A noise represents the average noise amplitude of the power waveform, σ noise Indicates A noise The standard deviation of .

[0083] According to SNR wf , the height measurement accuracy σ is obtained by solving p :

[0084]

[0085] Where c is the speed of light in vacuum, θ S represents the satellite elevation angle at the mirror point, and W(0) represents the signal power at the mirror point.

[0086] Based on formula (7), the height measurement accuracy of each mirror point is calculated in turn: Then, the average height measurement accuracy corresponding to N mirror points is for:

[0087]

[0088] Step 104, correct the mirror point delay according to the tropospheric delay, and invert to obtain the vertical distance from the sea surface of the receiver's sub-satellite point to the reference ellipsoid; and according to the vertical distance from the sea surface of the receiver's sub-satellite point to the reference ellipsoid, calculate the average height measurement accuracy corresponding to N mirror points.

[0089] In this embodiment, accurate estimation of the mirror point delay is the key to ensuring the height measurement accuracy of iGNSS-R. The delay of the reflected signal relative to the direct signal can be obtained by re-tracking based on the iGNSS-R waveform. In addition, it is also necessary to correct the delay deviation of the troposphere and antenna baseline. Among them, waveform re-tracking is to compare the measured waveform with the simulated waveform. The simulated waveform corresponding to each measured waveform of the present invention is obtained through the ZV model provided by the open source software "wavpy". Currently, the commonly used waveform tracking algorithms are mainly DER, MAX and HALF. The present invention adopts the DER algorithm to estimate the mirror point delay.

[0090] The specific inversion process of the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid is as follows:

[0091] Using the DER algorithm, the mirror point delay τ is estimated spec :

[0092]

[0093] in, Indicates the measured waveform ( Figure 3 ) is the time delay corresponding to the maximum value of the derivative calculated in, represents the mirror point delay of the simulation waveform, Indicates the time delay corresponding to the maximum value point of the derivative calculated from the simulation waveform.

[0094] The troposphere is mainly distributed in the space from the ground to 15 km. Due to the high humidity on the sea surface and the different transmission paths of the direct and reflected signals in the air, the deviation caused by the troposphere on the airborne iGNSS-R delay estimation cannot be ignored. The present invention estimates the tropospheric delay τ based on the satellite elevation angle θ of the mirror point. trop :

[0095]

[0096] Among them, H R Indicates the height of the iGNSS-R receiver, H trop Indicates the tropospheric height of the observation location; H trop 8621m is available.

[0097] After correcting the mirror point delay error, combined with the airborne iGNSS-R geometric relationship, the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid can be inverted spir :

[0098]

[0099] Among them, τ antenna It represents the antenna baseline delay error, which is obtained by the path difference between the upward-looking antenna and the downward-looking antenna relative to the mirror point when the airborne position and attitude information are known.

[0100] Preferably, the specific calculation process of the average height measurement accuracy can be as follows:

[0101] The present invention defines the difference between the measured SSH and the SSH given by the model as the accuracy, that is, the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid spir , the height measurement accuracy σ is obtained by solving formula (9) a :

[0102] σ a =SSH spir -SSH ref ···(9)

[0103] Among them, SSH ref Represents the sea surface height of the reference model.

[0104] Based on formula (9), the height measurement accuracy of each mirror point is calculated in turn: Then, the average height measurement accuracy corresponding to N mirror points is for:

[0105]

[0106] Step 105 , by repeating steps 101 to 104 , the average height measurement precision and the average height measurement accuracy obtained with different signal processing times are obtained.

[0107] Step 106, based on the average height measurement accuracy and average height measurement precision obtained at different signal processing times, combined with the relationship between the along-track spatial resolution and the signal processing time, the along-track spatial resolution corresponding to the height measurement accuracy is obtained, thereby realizing the reconstruction of the GNSS-R sea surface height measurement accuracy and the along-track spatial resolution.

[0108] Based on the above embodiments, the data used in the scheme of the present invention, as well as the final results and discussions are described in detail below.

[0109] data

[0110] The present invention mainly uses SPIR (software PARIS interferometric receiver) airborne flight data, DTU15 global average sea surface model and DTU global ocean tide model. The SPIR airborne flight data is used to invert the sea surface height, and the DTU15 global average sea surface model and DTU10 global ocean tide model are used to construct the SSH verification model.

[0111] SPIR Data

[0112] The SPIR airborne mission was an airborne GNSS-R altimetry program implemented by IEEC on December 3, 2015. The mission location was in the Baltic Sea near Helsinki, Finland (such as Figure 4 As shown). The key to this flight mission is that the SPIR receiver developed by IEEC was used for the first time. Unlike previous GNSS-R receivers (such as GOLD-RTR, PIRA), SPIR can collect original complex GNSS reflection signals from 16 front ends at a sampling rate of 80MHz, and the operating frequency band includes all commonly used GNSS L1, L2 and L5 bands. Eight-element antennas are installed on the top and belly of Aalto's Skyvan aircraft to collect GNSS direct signals from above and GNSS reflection signals from the sea surface. The present invention uses 40001-40600 (GPS SOD) observation data of GPS PRN01 satellite in the L1 band for a total of 10 minutes.

[0113] SSH authentication data

[0114] When performing airborne iGNSS-R sea surface height inversion, it is necessary to compare and verify with the measured sea surface data to determine the accuracy of the spaceborne sea surface height inversion. Due to the lack of measured data, the verification model is used to verify the accuracy of the sea surface height inversion.

[0115] The DTU15 global mean sea surface model (MSS) is a global high-resolution mean sea surface model released by the Technical University of Denmark (DTU) in December 2015. The biggest improvement of DTU15 compared to previous versions is the use of improved Cryosat-2LRM, SAR and SAR-In data, and the weight of ICESat data used in the Arctic Ocean region is reduced compared to DTU10MSS and DTU13MSS. Its grid resolution is 2′×2′.

[0116] DTU10 global ocean tide model is also released by DTU. DTU10 takes into account the wavelength and water depth of the diurnal and semi-diurnal tides, and uses the depth-based dynamic difference method to interpolate the correction value into the FES2004 grid. The load tide uses the calculation results of FES2004. Its grid resolution is 7.5′×7.5′.

[0117] The present invention combines the DTU15 global average sea surface model and the DTU10 global tide model to form an SSH verification model: SSH ref =DTU15 MSS +DTU10 Tide .

[0118] Results and Discussion

[0119] Power waveform

[0120] The iGNSS-R power waveform is the data basis for obtaining high-precision measurement. Among them, the high-precision measurement is directly related to the signal-to-noise ratio of the waveform. To obtain high-precision measurement, it is necessary to first re-track and calculate the delay of the mirror point based on the power waveform. In this study, the coherent integration time is set to 1ms, 2ms, 5ms and 10ms respectively, and the number of incoherent cumulative averages is 1000 times, then the signal processing time is 1s, 2s, 5s and 10s respectively. The present invention uses a total of 10 minutes of SPIR data, and the corresponding number of power waveforms is 600, 300, 120 and 60 respectively.

[0121] Figure 4 The iGNSS-R power waveform results under different signal processing times are given. Figure 4It can be seen that although the increase in signal processing time will increase the amplitude of waveform noise, the improvement of waveform power amplitude is more obvious. According to formula (6), the signal-to-noise ratios corresponding to 1s, 2s, 5s and 10s power waveforms are 8.69dB, 11.09dB, 14.82dB and 17.04dB respectively.

[0122] Relationship between height measurement accuracy and along-track spatial resolution

[0123] like Figure 5 As shown in Figure 1, based on the SPIR waveform data obtained by signal processing, the height measurement accuracy corresponding to each mirror point is calculated according to formula (7). Figure 5 It can be seen that the iGNSS-R height measurement accuracy shows an obvious "fault-like" distribution with the difference of signal processing time, which shows that the signal-to-noise ratio has a great influence on the height measurement accuracy. According to formula (8), the present invention calculates the average height measurement accuracy under different along-track spatial resolutions and Figure 6 The relationship between the average height measurement accuracy and the along-track spatial resolution is given.

[0124] Depend on Figure 6 It can be seen that the iGNSS-R height measurement accuracy gradually increases with the decrease of the along-track spatial resolution. However, this dependence is not linear. When the along-track spatial resolution changes in the range of 50-150m, the height measurement accuracy changes relatively significantly. Then, as the along-track spatial resolution value continues to increase, the fluctuation of the height measurement accuracy becomes relatively weaker. This change is similar to the relationship between height measurement accuracy and signal-to-noise ratio. According to formula (7), Will increase with SNR wf The increase in signal processing time will increase W(0), but this trend will gradually weaken. In addition, although the increase in signal processing time will increase W(0), the steeper waveform front will also cause W(0)' to increase. In this case, the change in W(0) / W(0)' is not obvious. SNR wf It is positively correlated with the along-track spatial resolution through signal processing time. Therefore, with the increase of along-track spatial resolution, the improvement of height measurement accuracy gradually weakens.

[0125] Relationship between height measurement accuracy and along-track spatial resolution

[0126] According to the SPIR waveform data and DER re-tracking algorithm, the SSH corresponding to the mirror point and the height measurement accuracy are calculated by combining formulas (5) and (9), as follows: Figure 7 Shown

[0127] according to Figure 7 It can be seen that the SSH obtained from the SPIR data inversion is in good agreement with the model value. spir With SSH refThe deviation between them gradually decreases, and the discreteness becomes smaller. The main reasons for the deviation are as follows:

[0128] (1) In this airborne altimetry inversion, only the effects of tropospheric error and antenna baseline delay error were considered. Other error terms such as aircraft positioning error and sea surface tide were not included in the model calculation.

[0129] (2) The signal processing time range is 1-10s. Although the signal-to-noise ratio of the obtained reflection signal is improved to a certain extent, there is still a certain amount of flicker noise and thermal noise in the signal.

[0130] (3) Model reference value SSH ref It is obtained by inverting observation data from multiple remote sensing and gravity satellites over many years and has good stability. However, airborne measurements only conduct a single measurement of the observation area, and the results have a certain degree of randomness, which leads to a certain deviation between the measurement results and the model data.

[0131] According to formula (10), the present invention calculates the average height measurement accuracy at different along-track spatial resolutions and Figure 8 The relationship between the average height measurement accuracy and the along-track spatial resolution is given.

[0132] according to Figure 8 It can be seen that the relationship between the altitude measurement accuracy of the airborne iGNSS-R and the spatial resolution along the track is similar to the relationship between the altitude measurement accuracy and the spatial resolution along the track. The altitude measurement accuracy increases with the decrease of the spatial resolution along the track. This is because the waveform front of the iGNSS-R becomes steeper after the spatial resolution along the track is reduced, thereby improving the accuracy of the time delay of the mirror reflection point.

[0133] Reference to altimetry satellites

[0134] This invention can provide some references for the design of future iGNSS-R sea surface altimetry satellites:

[0135] (1) Future iGNSS-R sea altimetry satellite hardware design requires higher gain upward and downward receiving antennas. The iGNSS-R altimetry accuracy decreases with the spatial resolution along the track mainly because the increase in signal processing time increases the signal-to-noise ratio of the waveform. The signal-to-noise ratio of the waveform can also be improved by increasing the antenna gain, which can reduce the signal processing time while ensuring the SNR, thereby improving the spatial resolution along the track. Of course, the increase in gain will also increase the mass of the antenna payload, which needs to be balanced.

[0136] (2) Future iGNSS-R sea surface altimetry satellite receivers should be designed to accommodate all available GNSS satellites. Studies have shown that the signal-to-noise ratio will be significantly reduced when the mirror elevation angle is lower than 45°, which will result in poor altimetry accuracy and along-track spatial resolution. Compared with GPS as the only transmitting source, iGNSS-R under GNSS has a better geometric structure and can obtain reflection events with higher mirror elevation angles, thereby improving the signal-to-noise ratio.

[0137] (3) Future iGNSS-R sea surface altimetry satellites will need to use signals with larger bandwidths. Signal bandwidth is one of the main factors limiting the altimetry performance of GNSS-R. iGNSS-R breaks through the limitation that cGNSS-R can only use GPS L1 C / A code. If future iGNSS-R satellites can use signals with higher bandwidth than GPS L1, such as GPS L5, Galileo E1 and Beidou-3 B2b, then the altimetry accuracy and along-track spatial resolution will be better.

[0138] On the basis of the above embodiments, the present invention further discloses a GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction system, including: an acquisition module, used to acquire the original data received by the SPIR upper and lower antennas; a processing module, used to perform beamforming and direct and reverse signal cross-correlation processing on the original data to obtain a complex waveform; a first solution module, used to perform coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; and according to the power waveform, solve and obtain the average height measurement accuracy corresponding to N mirror points; a second solution module, used to correct the mirror point delay according to the tropospheric delay, and invert to obtain the receiver satellite downlink delay. The vertical distance from the sea surface of the point to the reference ellipsoid surface; and according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid surface, the average height measurement accuracy corresponding to the N mirror points is solved; the third solution module is used to repeat the execution process of the above acquisition module to the second solution module to obtain the average height measurement accuracy and the average height measurement accuracy obtained at different signal processing times; the reconstruction module is used to obtain the along-track spatial resolution corresponding to the height measurement accuracy according to the average height measurement accuracy and the average height measurement accuracy obtained at different signal processing times, combined with the relationship between the along-track spatial resolution and the signal processing time, so as to realize the reconstruction of the GNSS-R sea surface height measurement accuracy and the along-track spatial resolution.

[0139] As for the system embodiment, since it corresponds to the method embodiment, the description is relatively simple, and the relevant parts can be referred to the description of the method embodiment part.

[0140] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0141] The contents not described in detail in the specification of the present invention belong to the common knowledge of the professionals in this field.

Claims

1. A GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method, characterized in that: include: Step 1, obtain the raw data received by the SPIR upper and lower antennas; Step 2, beamforming and cross-correlation processing of direct and reverse signals are performed on the original data to obtain a complex waveform; Step 3, performing coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; And according to the power waveform, the average height measurement accuracy corresponding to N mirror points is calculated; Step 4: correct the mirror point delay according to the tropospheric delay, and invert the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid; and calculate the average height measurement accuracy corresponding to N mirror points according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid; Step 5, by repeating steps 1 to 4, obtaining average height measurement precision and average height measurement accuracy obtained with different signal processing times; Step 6: Based on the average height measurement accuracy and average height measurement precision obtained at different signal processing times, combined with the relationship between the along-track spatial resolution and the signal processing time, the along-track spatial resolution corresponding to the height measurement accuracy is obtained, thereby realizing the reconstruction of the GNSS-R sea surface height measurement accuracy and the along-track spatial resolution; The complex waveform is processed by coherent integration and incoherent accumulation and averaging to obtain the power waveform, including: According to the following formulas (1) and (2), the complex waveform is processed by coherent integration and incoherent cumulative averaging to obtain the power waveform: W(T0+N incoh ·T coh / 2,τ)=|w(T0+N incoh ·T coh / 2,τ)| 2 ···(2) Among them, w(T0+N incoh ·T coh / 2,τ) represents the waveform after coherent integration and incoherent cumulative averaging, T0 represents the start time of coherent integration and incoherent cumulative averaging, N incoh Indicates the number of incoherent accumulations, T coh represents the coherent integration time, c(nT coh ,τ) represents the complex waveform, t represents the time, τ represents the delay, W(T0+N incoh ·T coh / 2,τ) represents the power waveform; According to the tropospheric delay, the mirror point delay is corrected, and the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid is inverted, including: Using the DER algorithm, the mirror point delay τ is estimated sp : in, represents the time delay corresponding to the maximum value of the derivative calculated from the measured waveform, represents the mirror point delay of the simulation waveform, represents the time delay corresponding to the maximum value point of the derivative calculated from the simulation waveform; According to the satellite elevation angle θ at the mirror point, the tropospheric delay τ is estimated trop : Among them, H R Indicates the height of the iGNSS-R receiver, H trop represents the tropospheric height of the observation location; Based on formulas (3) and (4), the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid is obtained: spir : Among them, τ antenna Indicates the antenna baseline delay error; According to the power waveform, the average height measurement accuracy corresponding to N mirror points is calculated, including: The waveform signal-to-noise ratio (SNR) of the power waveform is calculated. wf : Among them, A noise represents the average noise amplitude of the power waveform, σ noise Indicates A noise The standard deviation of According to SNR wf , the height measurement accuracy σ is obtained by solving p : Where c is the speed of light in vacuum, θ S represents the satellite elevation angle at the mirror point, W(0) represents the signal power at the mirror point; Based on formula (7), the height measurement accuracy of each mirror point is calculated in turn: Then, the average height measurement accuracy corresponding to N mirror points is for: According to the vertical distance from the sea surface of the receiver's sub-satellite point to the reference ellipsoid, the average height measurement accuracy corresponding to N mirror points is calculated, including: According to the vertical distance SSH from the sea surface of the receiver sub-satellite point to the reference ellipsoid spir , the height measurement accuracy σ is obtained by solving formula (9) a : σ a =SSH spir -SSH ref ···(9) Among them, SSH ref represents the sea surface height of the reference model; Based on formula (9), the height measurement accuracy of each mirror point is calculated in turn: Then, the average height measurement accuracy corresponding to N mirror points is for:

2. The GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method according to claim 1 is characterized in that: The number of complex waveforms obtained by beamforming and cross-correlation of direct and reverse signals is: 6×10 5 The coherent integration time of each complex waveform is 1ms.

3. The GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction method according to claim 1 is characterized in that: Also includes: Before the complex waveform is subjected to coherent integration and incoherent cumulative averaging, the complex waveform is aligned; wherein, T coh =1s, 2s, 5s, 10s, N incoh =1000.

4. A GNSS-R sea surface height measurement accuracy and along-track spatial resolution reconstruction system for implementing the method as claimed in claim 1, characterized in that: include: The acquisition module is used to obtain the raw data received by the SPIR upper and lower antennas; A processing module is used to perform beamforming and cross-correlation processing of direct and reverse signals on the original data to obtain a complex waveform; The first solution module is used to perform coherent integration and incoherent cumulative averaging processing on the complex waveform to obtain a power waveform; And according to the power waveform, the average height measurement accuracy corresponding to N mirror points is calculated; The second solution module is used to correct the mirror point delay according to the tropospheric delay, and invert to obtain the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid; and according to the vertical distance from the sea surface of the receiver sub-satellite point to the reference ellipsoid, solve to obtain the average height measurement accuracy corresponding to N mirror points; A third solution module is used to repeat the execution process of the acquisition module to the second solution module to obtain the average height measurement precision and the average height measurement accuracy obtained at different signal processing times; The reconstruction module is used to obtain the along-track spatial resolution corresponding to the height measurement accuracy based on the average height measurement accuracy and average height measurement precision obtained at different signal processing times, combined with the relationship between along-track spatial resolution and signal processing time, and thus realize the reconstruction of GNSS-R sea surface height measurement accuracy and along-track spatial resolution.

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