GNSS-R sea surface height measurement method based on novel beam stacking method

Abstract

The invention relates to the technical field of electronic signal processing and satellite height measurement, in particular to a GNSS-R (Global Navigation Satellite System-Receiver) sea surface height measurement method based on a novel stacked beam method, which can obviously improve the measurement precision, and is characterized in that a plurality of GNSS-R beam signals corresponding to a plurality of GNSS satellites are synthesized through the novel stacked beam method; the method is used for improving the signal-to-noise ratio of reflected signals received by GNSS-R satellites, and comprises the following steps: step 1, receiving and processing the reflected signals from a plurality of satellites; 2, synthesizing and superposing the received signals by using a beam forming technology; and step 3, analyzing the superposed signal to obtain sea surface height information.

Description

Technical field:

[0001] The invention relates to the technical field of electronic signal processing and satellite altimetry, and in particular to a GNSS-R sea surface altimetry method based on a novel stacked beam method which can significantly improve measurement accuracy. Background technology:

[0002] As a new remote sensing technology, GNSS-R technology has attracted more and more attention from researchers in recent years and has been widely used. Among the many application fields of GNSS-R technology, sea surface height measurement is an important research direction. The basic principle of GNSS-R sea surface height measurement is to establish a dual-base remote sensing system consisting of GNSS satellites, mirror reflection points and GNSS-R satellites. In the GNSS-R remote sensing system, GNSS satellites are used as radiation sources to transmit GNSS signals. These GNSS signals are reflected at the mirror reflection points, and the reflected GNSS signals are used as remote sensing signals to enter the GNSS-R satellite. In addition, the GNSS direct wave signal received by the GNSS-R satellite is used as a reference signal for the GNSS-R system. The GNSS-R satellite captures the GNSS reflected signal and the GNSS direct signal at the same time, and the GNSS-R satellite acts as the receiving end in the system. By measuring the transmission delay difference between the GNSS reflected signal and the GNSS direct signal, the sea surface height value can be inverted.

[0003] The current research focus in this field is to further improve its height measurement uncertainty. In terms of system applications, high-precision sea surface height data is the basic data for high-precision ocean physical models. Due to the limited transmission power of GNSS signals, the reflected signals are weak and the signal-to-noise ratio is low. To overcome this limitation, researchers have explored new methods and techniques to improve the measurement uncertainty and reliability of GNSS-R technology, aiming to enhance the signals received by the GNSS-R system. Some studies focus on improving hardware, such as developing more sensitive receivers and antennas to improve the signal-to-noise ratio of received signals, allowing the system to more effectively capture weak reflected signals. Other studies have explored advanced signal processing techniques, using complex algorithms to filter noise and enhance useful signals to improve measurement uncertainty. Although these efforts have made gradual progress, a breakthrough approach is still needed to significantly improve the uncertainty of GNSS-R sea surface height measurement. Summary of the invention:

[0004] In view of the shortcomings and deficiencies in the prior art, the present invention proposes a GNSS-R sea surface altimetry method based on a novel stacked beam method, which can significantly improve the altimetry uncertainty of the existing GNSS-R system.

[0005] The present invention is achieved by the following measures:

[0006] A GNSS-R sea surface height measurement method based on a novel stacked beam method, characterized in that a plurality of GNSS-R beam signals corresponding to a plurality of GNSS satellites are synthesized by the novel stacked beam method to improve the signal-to-noise ratio of the reflected signal received by the GNSS-R satellite, comprising the following steps:

[0007] Step 1: Receive and process reflected signals from multiple satellites;

[0008] Step 2: Use beamforming technology to synthesize and superimpose the received signals;

[0009] Step 3: Analyze the superimposed signal to obtain sea surface height information.

[0010] In the present invention, in the GNSS-R system, it is assumed that the direct signal and its reflected signal transmitted by the i-th GNSS satellite are S dir,i (t) and S ref,i (t), its expression is:

[0011] S dir,i (t) = A dir,i (t)cos(ω dir,i t+φ dir,i (t))(1),

[0012] S ref,i (t) = A ref,i (t)cos(ω ref,i t+φ ref,i (t))+n i (t)(2),

[0013] Among them, A dir,i (t), A ref,i (t) represents S dir,i (t) and S ref,i The amplitude of (t), ω dir,i ,ω ref,i represents its angular frequency, φ dir,i (t),φ ref,i (t) represents its phase, n i (t) represents the noise term of the reflected signal.

[0014] The present invention relates to a novel stacked beam method based on cGNSS-R. The replica signal in the GNSS-R satellite is a replica of the direct signal of the i-th GNSS satellite, which is used as a reference signal. Its expression is:

[0015] S rep,i (t) = A rep,i (t)cos(ω rep,i t+φ rep,i (t))(3),

[0016] Among them A rep,i (t) represents S rep,i The amplitude of (t), ω rep,i represents its angular frequency, φ rep,i (t) represents its phase; the reflected signal of the i-th GNSS satellite and its replica signal are correlated and integrated

[0017]

[0018] Substituting formula (2) and formula (3) into formula (4), we can get:

[0019]

[0020] According to the Wiener-Khinchin theorem, the GNSS-R signal power of the i-th GNSS satellite can be obtained by cross-correlating the reflected signal and the signal replica, which is expressed as

[0021] P s,i =Y i (0)(6).

[0022] The multi-satellite signal stacking synthesis in the present invention is specifically as follows: by accumulating the correlation integration results of the reflection signals of multiple GNSS satellites, the formula of the stacking synthesis signal of the novel stacking beam method can be obtained:

[0023]

[0024] It can be seen from formula (7) that the amplitude of the signal is superimposed through integration. In order to simplify the analysis, it is assumed that the amplitudes of multiple GNSS-R stacked signals are consistent, that is, P s,i =P s Therefore, after the correlation integration of the reflected signals from M GNSS satellites, the total power of the composite signal is M 2 ·P s .

[0025] The present invention can effectively improve the signal-to-noise ratio by stacking and synthesizing the GNSS-R signals from multiple GNSS satellites. Assuming that the signal power and noise power of each satellite are the same, that is, P s,i =P s And P N,i =P N , where P N,i represents the noise power of the receiving channel of the GNSS-R satellite receiving the i-th GNSS satellite. Through beam stacking, the signal term will be coherently accumulated, while the noise term will be incoherently accumulated. Therefore, the signal-to-noise ratio formula of the new stacked beam method is:

[0026]

[0027] Among them, SNR SS It represents the signal-to-noise ratio of the reflected signal from the i-th GNSS satellite received in the GNSS-R system. Assuming that the transmission signal power and noise power of the GNSS satellite are equal, the signal-to-noise ratio of its GNSS-R signal is consistent. That is, by superimposing the GNSS-R signals of M satellites, the signal-to-noise ratio is increased by M times, and the converted dB value is 10lg(M)dB.

[0028] For each GNSS-R signal, its signal-to-noise ratio can be obtained as follows:

[0029]

[0030] Among them, P t Indicates the transmission signal power of the GNSS satellite, G t and G r denote the transmit antenna gain of the GNSS satellite and the receive beam gain of the GNSS-R satellite, respectively; λ denotes the carrier wavelength of the GNSS satellite signal (for GPSL1 signal, λ = 19.042 cm); r t represents the distance of the link between the i-th mirror reflection point and the transmitting antenna of the GNSS satellite, r r represents the distance between the GNSS-R satellite receiving antenna and the i-th mirror reflection point, Γ lr represents the Fresnel reflection coefficient (typical seawater temperature is 25°, salinity is 35‰, Γ lr The value is 70.53+65.68i), P N =kT R B represents the noise power of the GNSS-R satellite, k represents the Boltzmann constant, T R represents the equivalent noise temperature of the reflected signal receiving channel, and B represents the bandwidth of the GNSS signal.

[0031] The signal-to-noise ratio of the GNSS-R system signal of the novel stacked beam method of the present invention is as follows:

[0032]

[0033] From the above content, it can be seen that the cGNSS-R altimeter system based on the new stacked beam method not only utilizes multi-channel GNSS reflection signals, but also improves the signal-to-noise ratio through the stacking synthesis process, thus creating favorable conditions for improving the comprehensive uncertainty of altimeter measurement.

[0034] The present invention further establishes height measurement uncertainty models for the traditional single beam method and the novel stacked beam method, respectively. The set of height measurement uncertainty mathematical models established in the present invention are derived based on the height measurement uncertainty models proposed in the prior art.

[0035]

[0036] Among them, N incoh represents the mean of the incoherent coefficient of the sample; ψ SSH Indicates the height measurement sensitivity. In the satellite-borne scenario, it can be considered that ψ SSH Depends on the autocorrelation characteristics of the signal, defined as and represents the amplitude of the average power at the mirror reflection point (i.e., τ = 0) and the leading-edge derivative (DER) of the power waveform, and c represents the speed of light.

[0037] Compared with the traditional single-beam method, the novel stacked-beam method of the present invention significantly improves the height measurement uncertainty of the GNSS-R system, thereby effectively improving the height measurement performance of the GNSS-R system. Description of the drawings:

[0038] Attached Figure 1 It is a schematic diagram of the height measurement scene of the present invention.

[0039] Attached Figure 2 This is a schematic diagram of the GNSS-R signal-to-noise ratio, with the antenna gain value substituted into the simulation value.

[0040] Attached Figure 3 It is a schematic diagram showing the improvement effect of the present invention on height measurement uncertainty compared with the traditional single-beam method.

[0041] Attached Figure 4 Attached is a statistical diagram of the distribution of the number of mirror reflection height angles involved in the present invention. Figure 5 It is a working principle diagram of the antenna verification prototype in the present invention.

[0042] Attached Figure 6 It is a schematic diagram of the antenna verification prototype in the present invention.

[0043] Attached Figure 7 : This is a schematic diagram of the GNSS-R signal-to-noise ratio in the present invention, where the antenna gain value is substituted into the simulation value and the test value for comparison.

[0044] Attached Figure 8 A schematic diagram of uncertainty verification of height measurement in an embodiment of the present invention, wherein Figure 8 (a) shows the improvement of the height measurement uncertainty of the new stacked beam method compared with the traditional single beam method, and (b) shows the comparison between the simulation value and the test value after substituting the antenna gain value into the simulation value. Specific implementation method:

[0045] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0046] The present invention relates to a method for improving the comprehensive accuracy of GNSS-R sea surface altimetry by using a novel stacked beam, wherein sea surface altimetry is a very important application field in GNSS-R remote sensing. The altimetry system based on GNSS-R technology receives the reflected signals of GNSS satellites and obtains the sea surface height value by analyzing the characteristics of these signals. However, the current GNSS-R altimetry system still has certain limitations in terms of altimetry uncertainty. In the traditional single beam method (TSBM) commonly used in the current on-orbit GNSS-R project, due to the limited power of the GNSS satellite transmitted signal, the signal-to-noise ratio (SNR) of the GNSS signal reflected by the sea surface is very low, which limits the further improvement of remote sensing performance including altimetry uncertainty. In order to overcome this limitation, the present invention proposes a novel stacked beam method (NSBFM) for improving the comprehensive uncertainty of the GNSS-R sea surface altimetry system: the GNSS-R altimetry system using the novel stacked beam method superimposes multiple reflected signals of multiple GNSS satellites to form a stacked synthetic GNSS-R signal.

[0047] Compared with the system using the traditional single beam method, the stacked synthesized GNSS-R signal has a higher signal-to-noise ratio, which helps to ultimately improve the uncertainty of altitude measurement. The present invention conducts the following innovative research on the new stacked beam method: First, based on the theory of stacking synthesis processing of sea level scattering coherent signals, a theoretical model of the new stacked beam method is constructed, and based on the model, an analysis is performed to obtain performance parameters such as signal-to-noise ratio and altitude uncertainty. Secondly, a typical evaluation scenario is established to obtain performance parameters such as altitude uncertainty in a specific GNSS-R application environment. The evaluation results show that compared with the GNSS-R altitude measurement system based on the traditional single beam method, the signal-to-noise ratio of the GNSS-R altitude measurement system based on the new stacked beam method is improved by 4.7dB. Correspondingly, its altitude uncertainty is increased from 2.60m to 2.27m.

[0048] In order to verify the effectiveness of the constructed new stacked beam method theoretical model in practical applications, the present invention designed, manufactured and measured a new stacked beam method antenna verification prototype. The measured and simulated performance parameter values ​​of the antenna verification prototype were introduced into the evaluation scenario, and the evaluation results of the sea surface height measurement uncertainty in two cases were obtained. The evaluation results in the two cases are consistent, which proves the correctness of the new stacked beam method theoretical model and the improvement of the height measurement uncertainty of the GNSS-R system using this method.

[0049] Example:

[0050] Usually, in the GNSS-R sea surface height measurement system, the sea surface height is generally determined by analyzing the time delay difference between the GNSS direct signal and its sea surface reflected signal. Improving the uncertainty of height measurement is an important direction in ocean research, because it helps to deepen the understanding of sea level changes and other ocean phenomena. However, the current GNSS-R systems on orbit all use the traditional single beam method. Since the reflected GNSS signal is significantly weak, the signal-to-noise ratio (SNR) of its GNSS-R signal is low. This limits the possibility of further improving the uncertainty of sea surface height measurement. The new stacked beam method proposed in the present invention can improve the signal-to-noise ratio of the GNSS-R signal by integrating multiple signals using a new stacked beam antenna. The improvement in the signal-to-noise ratio further improves the uncertainty of sea surface height measurement, which is crucial for obtaining accurate sea surface height data.

[0051] In high-precision GNSS-R sea surface height measurement applications, the system based on the new stacked beam method can simultaneously capture multiple GNSS reflection signals and superimpose them into a synthetic signal, thereby enhancing the GNSS-R signal strength. Therefore, the new stacked beam method helps to improve the signal-to-noise ratio of the GNSS-R signal, thereby improving the comprehensive uncertainty of height measurement.

[0052] There are two main types of GNSS-R systems, based on the method of obtaining time delay: cGNSS-R (traditional GNSS-R) system and iGNSS-R (interferometric GNSS-R) system. The following describes the working principles of these two GNSS-R systems. For the cGNSS-R system, the reflected signal from a specific GNSS satellite is correlated and matched with a signal copy in the GNSS-R satellite, and the code structure of the signal copy is the same as the code structure of the GNSS satellite signal. In the iGNSS-R system, the time delay difference is obtained by directly correlating the GNSS reflected signal with the GNSS direct signal. However, the iGNSS-R system cannot distinguish different GNSS satellites through the unique code structure, so it is impossible to integrate reflected signals from multiple satellites at the same time. Therefore, the iGNSS-R system cannot be used for the new stacked beam method. In order to meet the application requirements of the new stacked beam method for synthesizing reflected signals from multiple different GNSS satellites, the cGNSS-R system must be used. The geometric relationship between the new stacked beam method and GNSS-R sea surface height measurement is as follows: Figure 1 As shown, the mirror reflection points 1, 2, and 3 are located in the same grid on the earth's surface.

[0053] In the scenario of sea surface height measurement using the new stacked beam method, the signal-to-noise ratio can be improved by stacking and synthesizing the reflected signals of multiple GNSS satellites. The stacking synthesis process includes signal modeling, correlation integration of a single GNSS-R signal, stacking synthesis of multiple signals, and finally obtaining the signal-to-noise ratio of the stacked synthesis signal. The following will explain these steps and related formulas in detail:

[0054] for Figure 1 In the GNSS-R system shown in the figure, it is assumed that the direct signal and its reflected signal transmitted by the i-th GNSS satellite are S dir,i (t) and S ref,i (t), its expression is:

[0055] S dir,i (t) = A dir,i (t)cos(ω dir,i t+φ dir,i (t)) (1)

[0056] S ref,i (t) = A ref,i (t)cos(ω ref,i t+φ ref,i (t))+n i (t) (2)

[0057] Among them, A dir,i (t), A ref,i (t) represents S dir,i (t) and S ref,i The amplitude of (t), ω dir,i ,ω ref,i represents its angular frequency, φ dir,i (t),φ ref,i (t) represents its phase, n i (t) represents the noise term of the reflected signal.

[0058] For the new stacked beam method based on cGNSS-R used in this example, the replica signal in the GNSS-R satellite is a replica of the direct signal of the i-th GNSS satellite, which is used as the reference signal. Its expression is:

[0059] S rep,i (t) = A rep,i (t)cos(ω rep,i t+φ rep,i (t)) (3)

[0060] Among them A rep,i (t) represents S rep,i The amplitude of (t), ω rep,i represents its angular frequency, φ rep,i (t) represents its phase.

[0061] Correlation integration is performed on the reflected signal of the i-th GNSS satellite and its replica signal

[0062] Substituting formula (2) and formula (3) into formula (4), we can get

[0063]

[0064] According to the Wiener-Khinchin theorem, the GNSS-R signal power of the i-th GNSS satellite can be obtained by cross-correlating the reflected signal and the signal replica, which is expressed as

[0065] P s,i =Y i (0) (6)

[0066] By accumulating the correlation integral results of the reflected signals from multiple GNSS satellites, the formula for the stacked composite signal of the new stacked beam method can be obtained:

[0067]

[0068] From formula (7), we can see that the amplitude of the signal is superimposed through correlation integration. To simplify the analysis, it is assumed that the amplitudes of multiple GNSS-R stacked signals are consistent, that is, P s,i =P s Therefore, after the correlation integration of the reflected signals from M GNSS satellites, the total power of the composite signal is M 2 ·P s .

[0069] By stacking and synthesizing the GNSS-R signals from multiple GNSS satellites, the signal-to-noise ratio can be effectively improved. In addition, during the stacking and synthesis process, the noise term n of the multi-channel GNSS-R signals is i (t) are independent of each other, and the expected value of each noise term is close to zero. Therefore, the relevant integrated power value of the synthetic signal component of the new stacked beam method will increase as the number of GNSS satellites increases according to the square sum, while the noise power component of the synthetic signal increases according to the linear sum. Therefore, through the stacking process, the signal-to-noise ratio is improved.

[0070] Assume that the signal power and noise power of each satellite are the same, that is, P s,i =P s And P N,i =P N Among them, P N,i represents the noise power of the receiving channel of the GNSS-R satellite receiving the i-th GNSS satellite. Through beam stacking, the signal terms are coherently accumulated, while the noise terms are incoherently accumulated. Therefore, the signal-to-noise ratio formula of the new stacked beam method is:

[0071]

[0072] Among them, SNR SS It represents the signal-to-noise ratio of the reflected signal from the i-th GNSS satellite received in the GNSS-R system. Assuming that the transmission signal power and noise power of the GNSS satellite are equal, the signal-to-noise ratio of its GNSS-R signal is consistent. That is, by superimposing the GNSS-R signals of M satellites, the signal-to-noise ratio is increased by M times, and the converted dB value is 10lg(M)dB.

[0073] In addition, for each GNSS-R signal, its signal-to-noise ratio can be obtained as follows:

[0074]

[0075] Among them, P t Indicates the transmission signal power of the GNSS satellite, G t and G r They represent the transmit antenna gain of the GNSS satellite and the receive beam gain of the GNSS-R satellite, respectively. λ represents the carrier wavelength of the GNSS satellite signal (for GPSL1 signal, λ

[0076] =19.042cm), r t represents the distance of the link between the i-th mirror reflection point and the transmitting antenna of the GNSS satellite, r r represents the distance between the GNSS-R satellite receiving antenna and the i-th mirror reflection point, Γ lr represents the Fresnel reflection coefficient (typical seawater temperature is 25°, salinity is 35‰, Γ lr The value is 70.53+65.68i), P N =kT R B represents the noise power of the GNSS-R satellite, k represents the Boltzmann constant, T R represents the equivalent noise temperature of the reflected signal receiving channel, and B represents the bandwidth of the GNSS signal.

[0077] The GNSS-R system based on the traditional single-beam method can only process the reflected signal of one GNSS satellite. Therefore, the signal-to-noise ratio of the GNSS-R system signal based on the traditional single-beam method is

[0078] Substituting formula (9) into formula (8), we can obtain the signal-to-noise ratio of the GNSS-R system signal of the new stacked beam method:

[0079]

[0080] From the above content, it can be seen that the cGNSS-R altimeter system based on the new stacked beam method not only utilizes multi-channel GNSS reflection signals, but also improves the signal-to-noise ratio through the stacking synthesis process, thus creating favorable conditions for improving the comprehensive uncertainty of altimeter measurement.

[0081] In order to evaluate the improvement effect of the GNSS-R system in terms of height measurement uncertainty, this example establishes height measurement uncertainty models for the traditional single beam method and the new stacked beam method. The set of height measurement uncertainty mathematical models established in the present invention are derived based on the height measurement uncertainty models proposed in the literature.

[0082]

[0083] Among them, N incoh represents the mean of the incoherent coefficient of the sample; ψ SSH Indicates the height measurement sensitivity. In the satellite-borne scenario, it can be considered that ψ SSH Depends on the autocorrelation characteristics of the signal, defined as and represents the amplitude of the average power at the mirror reflection point (i.e., τ = 0) and the leading-edge derivative (DER) of the power waveform, and c represents the speed of light.

[0084] This example will evaluate the theoretical improvement effect of GNSS-R height measurement uncertainty by adopting a new stacked beam method. To this end, this example constructs a GNSS-R height measurement scenario for height measurement uncertainty evaluation. In this evaluation scenario, three on-orbit GNSS constellation systems are set up, including the Global Positioning System (GPS), the Galileo system (Galileo) and the Beidou system (BDS) to maximize the number of radiation sources (GNSS satellites). The evaluation scenario established in the present invention includes 31 medium earth orbit (MEO) satellites in the GPS system, 24 MEO satellites in the Galileo system, and 27 MEO satellites in the Beidou system. For GNSS-R satellites, the evaluation scenario established in the present invention refers to the typical on-orbit GNSS-R satellite TDS-1. The purpose of this setting is to ensure the reliability of the evaluation results. In addition, the antenna configuration of the new stacked beam method used in this evaluation scenario includes three stacked beams, each with a gain of 10dB. In contrast, the traditional single-beam GNSS-R antenna is only configured with one beam, and its antenna gain is 10dB. After establishing the evaluation scenario, the present invention substitutes the simulation parameters into formula (10) and formula (11), and then substitutes them into formula (12) to calculate the signal-to-noise ratio and height measurement uncertainty of the traditional single-beam method and the new stacked-beam method, respectively.

[0085] Figure 2The comparison of the signal-to-noise ratios of the GNSS-R system using the traditional single beam method and the new stacked beam method is shown. As shown in the figure, the signal-to-noise ratio of the GNSS-R system using the new stacked beam method is improved by 4.7dB compared with the traditional single beam method. By substituting the signal-to-noise ratio simulation values ​​of the traditional single beam method and the new stacked beam method into the evaluation scenario, the height measurement uncertainty is analyzed and obtained. The height measurement uncertainty data curve after analysis is shown in Figure 1. Figure 3 It can be seen that compared with the traditional single-beam method, the new stacked beam method improves the height measurement uncertainty of the GNSS-R system by 0.22m to 0.44m, thereby effectively improving the height measurement performance of the GNSS-R system.

[0086] From the above analysis results, it can be seen that the uncertainty of altitude measurement is closely related to the satellite altitude angle. Therefore, it is crucial to consider the probability distribution of the altitude angle when evaluating the comprehensive uncertainty of GNSS-R altitude measurement. Figure 4 The data curve of the altitude angle distribution obtained by statistics within 24 consecutive hours in the evaluation scenario established in this example is shown.

[0087] In order to obtain the value of the comprehensive uncertainty of height measurement, this example performs weighted averaging on the height measurement uncertainty data at different altitudes. In this weighted averaging process, the weight value is the distribution probability of the altitude angle, and the weighted average value of the altitude uncertainty is the comprehensive uncertainty of altitude measurement. In this way, the influence of the altitude angle distribution on the altitude uncertainty can be considered more accurately, and the comprehensive uncertainty data of altitude measurement obtained through the weighted averaging process will have more practical significance. The weighted average results of the evaluation show that the comprehensive uncertainty of altitude measurement of the traditional single beam method is 2.60m, while the comprehensive uncertainty of altitude measurement of the new stacked beam method is 2.27m. In comparison, the comprehensive uncertainty of altitude measurement of the new stacked beam method has improved by 0.33m, which shows that this method has a positive effect on improving the uncertainty of altitude measurement.

[0088] In order to verify the theoretical model of the new stacked beam method and evaluate the comprehensive uncertainty of the height measurement in the specific GNSS-R sea surface height measurement application scenario established in the previous article, this example also designed and manufactured a new stacked beam method antenna verification prototype. The antenna verification prototype uses artificial impedance surface technology to form multiple beams by reasonably distributing surface impedance, thereby meeting the requirements of the new stacked beam method.

[0089] In order to verify the design reliability of the new stacked beam antenna verification prototype, this example measured its performance in an electromagnetic shielding room to isolate external electromagnetic interference. The measurement results show that the new stacked beam antenna verification prototype has the ability to generate three stacked beams simultaneously, with gains of 10.3dB, 10.5dB, and 10.4dB respectively. Therefore, in the application scenario of the new stacked beam method, all three beams can be used for GNSS-R signal synthesis; in the scenario of the traditional single beam method, only one of the three beams is available.

[0090] Based on the above research, this example further verifies the possibility of improving the comprehensive uncertainty of GNSS-R sea surface height measurement. In this example, the antenna gain test values ​​of the traditional single beam method and the new stacked beam method are respectively substituted into the evaluation scenario to calculate and compare the comprehensive uncertainty of these two methods for GNSS-R sea surface height measurement. The results show that compared with the traditional single beam method, the new stacked beam method can improve the comprehensive uncertainty of sea surface height measurement.

[0091] Formula (12) shows that for the new stacked beam method, the main factor in improving the comprehensive uncertainty of height measurement is the number of GNSS reflected signals. However, the traditional single-beam antenna only generates a single beam, which cannot meet the number of beams required by the new stacked beam method. Therefore, it is necessary to develop a GNSS-R antenna specially customized for the new stacked beam method. Based on this demand, the present invention designs and manufactures a new stacked beam method antenna verification prototype. The new stacked beam method antenna verification prototype not only provides an effective experimental platform for theoretical research, but also enables the present invention to substitute more realistic antenna performance parameters into the evaluation scenario to obtain accurate height measurement uncertainty.

[0092] The working principle of the new stacked beam antenna verification prototype is as follows Figure 5 As shown. The new stacked beam antenna verification prototype uses a new artificial impedance surface to form multiple beams. At the same time, the impedance surface and the feed are located on the same plane, which helps to reduce the cross-sectional height of the antenna. The antenna generates a total of three stacked beams by dividing the impedance modulation surface into different areas. In addition, by combining the holographic tensor impedance surface with the monopole antenna, the present invention can design a corresponding impedance surface to control the beam. The new stacked beam antenna verification prototype is shown in FIG. Figure 6 As shown in the figure, the new stacked beam antenna verification prototype provides the hardware foundation for improving the signal-to-noise ratio of the GNSS-R system. The antenna prototype generates three stacked beams at the same time, and each beam receives the reflected signal from a specific GNSS satellite. The reflected signals of these three different GNSS satellites are transmitted to the GNSS-R receiver through three independent signal channels. The receiver stacks and synthesizes these three signals to form a synthetic signal to improve the signal-to-noise ratio and ultimately improve the height measurement uncertainty.

[0093] In this example, the new stacked beam method antenna verification prototype was tested in an electromagnetic shielding room. Under the test state, the beacon and the prototype were set relative to each other. The results show that the new stacked beam method antenna verification prototype has the ability to generate three stacked beams simultaneously, with gains of 10.3dB, 10.5dB and 10.4dB respectively. The gains of these three stacked beams are basically the same, which meets the application requirements of the new stacked beam method. Under this condition, the GNSS-R satellite receives reflected signals from three different GNSS satellites, thereby achieving the goal of multi-signal reception and processing. In addition, when the three beams work independently, the performance of each single beam meets the performance requirements of the traditional single beam method, and the gain value of each single beam can be directly used to evaluate the signal-to-noise ratio and height measurement uncertainty performance of the traditional single beam method.

[0094] Performance evaluation of GNSS-R altimetry scenarios

[0095] In the same evaluation scenario set previously, the signal-to-noise ratio of the new stacked-beam antenna verification prototype was evaluated. Figure 7 The comparison of the signal-to-noise ratios obtained by bringing the gain simulation values ​​and test values ​​of the new stacked beam antenna verification prototype into the evaluation scenario is shown. Obviously, the signal-to-noise ratio of the new stacked beam method is improved by 4.7dB compared to the traditional single beam method. In addition, the signal-to-noise ratio calculated by substituting the gain simulation value and the test value has good consistency, and the difference between the two is less than 0.1dB.

[0096] Figure 8 (a) shows the comparison of height measurement uncertainty after the gain test values ​​of the three stacked beams (used for the new stacked beam method) and the single beam (used for the traditional single beam method) of the new stacked beam method antenna verification prototype are respectively brought into the evaluation scenario. Figure 8 As shown in (a), compared with the traditional single-beam method, the GNSS-R system using the new stacked beam method improves the altitude uncertainty by 0.21m to 0.43m, thereby effectively improving the performance of the GNSS-R altitude measurement system. Figure 8 (b) shows the comparison of the height measurement uncertainty after the gain simulation value and gain test value of the new stacked beam antenna verification prototype are respectively introduced into the evaluation scenario. Figure 8As shown in (b), the altitude uncertainty obtained based on the gain simulation value and the gain test value shows good consistency, with a difference of less than 0.01m. By applying the same evaluation method as in the present invention, the comprehensive altitude uncertainty is obtained based on the weighted average of the altitude angle distribution probability. From the analysis results of the comprehensive altitude uncertainty, it can be seen that the comprehensive altitude uncertainty of the traditional single-beam method is 2.60m, while the comprehensive altitude uncertainty of the new stacked beam method is 2.27m. In contrast, the new stacked beam method improves the comprehensive altitude uncertainty by 0.33m. This evaluation result further verifies the effectiveness of the theoretical model of the new stacked beam method, and shows its improvement effect in measuring comprehensive altitude uncertainty.

Claims

1. A GNSS-R sea surface height measurement method based on a novel stacked beam method, characterized in that: A novel stacked beam method is used to synthesize multiple GNSS-R beam signals corresponding to multiple GNSS satellites to improve the signal-to-noise ratio of the reflected signal received by the GNSS-R satellite, including the following steps: Step 1: Receive and process reflected signals from multiple satellites; Step 2: Use beamforming technology to synthesize and superimpose the received signals; Step 3: Analyze the superimposed signal to obtain sea surface height information.

2. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 1 is characterized in that: In the GNSS-R system, it is assumed that the direct signal and its reflected signal transmitted by the i-th GNSS satellite are S dir,i (t) and S ref,i (t), its expression is: S dir,i (t)=A dir,i (t)cos(ω dir,i t+φ dir,i (t)) (1), S ref,i (t)=A ref,i (t)cos(ω ref,i t+φ ref,i (t))+n i (t) (2), Among them, A dir,i (t), A ref,i (t) represents S dir,i (t) and S ref,i The amplitude of (t), ω dir,i ,ω ref,i represents its angular frequency, φ dir,i (t),φ ref,i (t) represents its phase, n i (t) represents the noise term of the reflected signal.

3. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 1 is characterized in that: The new stacked beam method based on cGNSS-R is adopted. The replica signal in the GNSS-R satellite is a replica of the direct signal of the i-th GNSS satellite. As the reference signal, its expression is: S rep,i (t)=A rep,i (t)cos(ω rep,i t+φ rep,i (t)) (3), Among them A rep,i (t) represents S rep,i The amplitude of (t), ω rep,i represents its angular frequency, φ rep,i (t) represents its phase; Correlation integration is performed on the reflected signal of the i-th GNSS satellite and its replica signal Substituting formula (2) and formula (3) into formula (4), we can get: (5), according to the Wiener-Khinchin theorem, the GNSS-R signal power of the i-th GNSS satellite can be obtained by cross-correlating the reflected signal and the signal replica, which is expressed as P s,i =And i (0)(6)。 4. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 3 is characterized in that: The specific method of multi-satellite signal stacking synthesis is as follows: by accumulating the correlation integration results of the reflected signals of multiple GNSS satellites, the formula of the stacking synthesis signal of the new stacking beam method can be obtained: (7), it can be seen from formula (7) that the amplitude of the signal is superimposed through integration. In order to simplify the analysis, it is assumed that the amplitudes of multiple GNSS-R stacked signals are consistent, that is, P s,i =P s Therefore, after the correlation integration of the reflected signals from M GNSS satellites, the total power of the composite signal is M 2 ·P s .

5. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 4 is characterized in that: By stacking and synthesizing the GNSS-R signals from multiple GNSS satellites, the signal-to-noise ratio can be effectively improved. Assuming that the signal power and noise power of each satellite are the same, that is, P s,i =P s And P N,i =P N , where P N,i represents the noise power of the receiving channel of the GNSS-R satellite receiving the i-th GNSS satellite. Through beam stacking, the signal term will be coherently accumulated, while the noise term will be incoherently accumulated. Therefore, the signal-to-noise ratio formula of the new stacked beam method is: Among them, SNR SS It represents the signal-to-noise ratio of the reflected signal from the i-th GNSS satellite received in the GNSS-R system. Assuming that the transmission signal power and noise power of the GNSS satellite are equal, the signal-to-noise ratio of its GNSS-R signal is consistent. That is, by superimposing the GNSS-R signals of M satellites, the signal-to-noise ratio is increased by M times, and the converted dB value is 10lg(M)dB. For each GNSS-R signal, its signal-to-noise ratio can be obtained as follows: Among them, P t Indicates the transmission signal power of the GNSS satellite, G t and G r denote the transmit antenna gain of the GNSS satellite and the receive beam gain of the GNSS-R satellite, respectively; λ denotes the carrier wavelength of the GNSS satellite signal (for GPS L1 signal, λ = 19.042 cm); r t represents the distance of the link between the i-th mirror reflection point and the transmitting antenna of the GNSS satellite, r r represents the distance between the GNSS-R satellite receiving antenna and the i-th mirror reflection point, Γ lr represents the Fresnel reflection coefficient (typical seawater temperature is 25°, salinity is 35‰, Γ lr The value is 70.53+65.68i), P N =kT R B represents the noise power of the GNSS-R satellite, k represents the Boltzmann constant, T R represents the equivalent noise temperature of the reflected signal receiving channel, and B represents the bandwidth of the GNSS signal.

6. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 5 is characterized in that: The signal-to-noise ratio of the GNSS-R system signal of the new stacked beam method is as follows: From the above content, it can be seen that the cGNSS-R altimeter system based on the new stacked beam method not only utilizes multi-channel GNSS reflection signals, but also improves the signal-to-noise ratio through the stacking synthesis process, thus creating favorable conditions for improving the comprehensive uncertainty of altimeter measurement.

7. The GNSS-R sea surface height measurement method based on the novel stacked beam method according to claim 6 is characterized in that: Altitude measurement uncertainty models are established for the traditional single beam method and the novel stacked beam method, respectively. The set of altitude measurement uncertainty mathematical models established in the present invention are derived based on the altitude measurement uncertainty models proposed in the prior art. Among them, N incoh represents the mean value of the incoherence coefficient of the sample; ψ SSH Indicates the height measurement sensitivity. In the satellite-borne scenario, it can be considered that ψ SSH Depends on the autocorrelation characteristics of the signal, defined as and represents the amplitude of the average power at the mirror reflection point, i.e., τ = 0, and the leading edge derivative of the power waveform, and c represents the speed of light.

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