Method for monitoring water level changes based on GNSS multipath phase changes

Abstract

The application discloses a water level change monitoring method based on GNSS multipath phase change, and on the basis of a GNSS LSP method, a segmented, resampled and fitted SNR sequence is separated into a plurality of sinusoidal curves, multipath phase changes between sinusoidal curves of two different time periods are calculated, the reflection height changes of the two different time periods are calculated, and then water level change monitoring is carried out, so that the cost and the calculation time are saved, and the precision and the resolution of GNSS-R water level change monitoring are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of satellite navigation and positioning and water level monitoring, and particularly relates to a water level change monitoring method based on GNSS multipath phase shift. BACKGROUND

[0002] Reservoirs are used for various purposes, including regular distribution, flood control, hydroelectric power generation, and meeting the environmental needs of downstream habitats and ecosystems, so it is very important to monitor the water level changes of reservoirs.

[0003] Generally, the water level of a reservoir is mainly read directly by a water level gauge, however, the traditional water level gauge is easily damaged by storms and biological pollution, and is also affected by vertical crustal movement and human activities. With the development of satellite technology, Global Navigation Satellite System Reflectometry (GNSS-R) as a new remote sensing detection method has the characteristics of low cost, low power consumption, all-weather, large range, full automation, high spatial and temporal resolution, and research has confirmed that it can be used for water surface height monitoring. Therefore, by using the existing GNSS deformation monitoring system near the dam, the water level change of the reservoir can be monitored.

[0004] Currently, the existing technology usually processes a series of original SNRs, extracts the main frequency through the Lomb-Scargle periodogram (LSP) spectral analysis method, and then obtains the reflected height RH. However, this method has poor accuracy, requires a long arc segment, and has low resolution, and usually only one result is obtained every few tens of minutes, so it is necessary to study a GNSS-IR water level change monitoring method with higher accuracy and resolution. SUMMARY

[0005] The present application provides a water level change monitoring method based on GNSS multipath phase shift, which calculates the phase changes of multiple corresponding sinusoidal curves at different time periods to invert the reflected height changes at different time periods, thereby obtaining the water level change of the reservoir, as described in detail below:

[0006] A water level change monitoring method based on GNSS multipath phase shift, the method comprising:

[0007] By fitting a low-order polynomial to remove the direct signal component in the signal-to-noise ratio (SNR), a mathematical model of the SNR oscillation term δSNR sequence is obtained.

[0008] The signal-to-noise ratio oscillation term δSNR sequence is extracted into multiple sinusoidal curve sequences, and the multipath phase change of the sinusoidal curves corresponding to the δSNR sequence at two different time periods is calculated using fast Fourier transform.

[0009] Establish phase change The functional relationship between the reflection height and the change in reflection height is used to obtain the change in reflection height ΔH. k Then, the water level change value is calculated.

[0010] Specifically, the step of extracting the signal-to-noise ratio oscillation term δSNR sequence into multiple sinusoidal curve sequences and using fast Fourier transform to calculate the multipath phase change of the sinusoidal curves corresponding to the δSNR sequences at two different time periods involves:

[0011] The δSNR sequence is segmented by period. Within each period, the δSNR sequence is uniformly resampled relative to the sine value sinθ of the elevation angle. The δSNR sequence within the period is fitted with a sine function to extract the δSNR sequence into multiple sine curve sequences.

[0012] When the water level changes, the multipath relative phase change corresponding to the k-th sine curve is: Calculated using the FFT method.

[0013] Wherein, the establishment of phase change The functional relationship between the reflection height and the change in reflection height is used to obtain the change in reflection height ΔH. k Specifically:

[0014] Phase change With the change in reflection height △H k The functional relationship is expressed as:

[0015]

[0016] in, and These are the phases corresponding to the k-th sine curves of the δSNR sequences in the two time periods, θ. k H is the average elevation angle corresponding to the k-th sine curve of the δSNR sequence over two time periods. 0k and H 1k These are the average reflection heights corresponding to the k-th sine curves of the δSNR sequences for the two time periods, ΔH. k This represents the change in reflection height of the k-th sine curve caused by the change in water level between the initial and next time moments.

[0017] Further, the water level change value is calculated by establishing phase change and the function relationship with the reflection height change AH and the water level change AH', and the water level change value is obtained.

[0018] The beneficial effects of the technical solutions provided by the present application are:

[0019] 1. The GNSS station for deformation monitoring is used for water level change monitoring without additional cost, and the water level change based on the global unified reference ellipsoid can be obtained by combining the GNSS absolute positioning.

[0020] 2. The reflection height change value can be calculated for each sinusoidal curve fitted by the two different observation period δSNR sequences meeting the conditions, and multiple reflection height changes can be calculated for one trajectory, which greatly improves the calculation efficiency and accuracy compared with the previous LSP method.

[0021] 3. In addition, the reservoir water surface is approximately planar, and even if the reflection points are not at the same position, the finally obtained reflection height change can be considered correct, and for the reservoir, the satellite trajectories can not be overlapped, and different satellites can be used in addition to the same satellite. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The flowchart of the water level change monitoring method based on GNSS multipath phase change is shown.

[0023] Figure 2 The schematic diagram of the water level change monitoring result based on GNSS multipath phase change is shown. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solutions and advantages of the present application clearer, the embodiments of the present application are further described in detail below.

[0025] Embodiment 1

[0026] A water level change monitoring method based on GNSS multipath phase change (GNSS-IR-PS) comprises the following steps:

[0027] Step 101: establishing a mathematical model of the signal-to-noise ratio oscillation term δSNR sequence;

[0028] The mathematical model removes the direct signal component in the signal-to-noise ratio SNR by fitting a low-order polynomial, and then obtains the mathematical model of the signal-to-noise ratio oscillation term δSNR sequence.

[0029] The key of establishing the mathematical model of the signal-to-noise ratio oscillation term δSNR in the step 101 is to establish the mathematical model of the signal-to-noise ratio SNR, which specifically includes: establishing the function of the path difference D between the direct signal and the reflected signal and the elevation angle θ and the reflected height H, the function of the phase difference (regarded as the phase of δSNR) between the direct signal and the reflected signal and the path difference D, and then establishing the mathematical model of the signal-to-noise ratio SNR according to the expression of the power of the near-ground direct-reflected combined signal:

[0030]

[0031] wherein, is the non-coherent term of the direct signal and the reflected signal, mainly including: the direct signal component, the signal-to-noise ratio oscillation term caused by the interference of the direct signal and the reflected signal, P d represents the power of the direct signal, P r represents the power of the reflected signal, and N0 represents the noise power spectral density.

[0032] The signal emitted from the GNSS satellite reaches the GNSS receiver, and the signal received by the receiver is actually a coherent signal formed by the interference of the direct signal and the reflected signal. Different reflection medium will cause different interference oscillation of the coherent signal. For the signal-to-noise ratio, when the SNR is taken as the function of the sine value of the elevation angle, the interference oscillation becomes clear. The direct signal component in the SNR can be removed by fitting and removing the low-order polynomial, and finally the detrended signal-to-noise ratio, i.e. the signal-to-noise ratio oscillation term δSNR, is obtained. The mathematical model of the δSNR sequence can be expressed as:

[0033]

[0034] wherein, A is the amplitude of δSNR, is the phase of δSNR, H is the reflected height, i.e. the vertical distance from the reflection point to the phase center of the receiver antenna, θ is the elevation angle from the satellite antenna to the phase center of the receiver antenna, the GNSS precise ephemeris is calculated, and λ is the wavelength of the GNSS signal.

[0035] Step 102: calculating the multipath phase change;

[0036] wherein, the step specifically includes: separating the δSNR sequence into a plurality of sinusoidal curve sequences, and calculating the multipath phase change of the sinusoidal curves corresponding to the δSNR sequences in two different time periods by using the Fast Fourier transform (FFT) method.

[0037] ​When the mathematical model δSNR is taken as a function of sinθ, the δSNR sequence is a curve similar to a sine function. The δSNR sequence is divided into periods, and the δSNR sequence is uniformly resampled with respect to the sine value sinθ of the elevation angle in each period. The δSNR sequence in the period is fitted with a sine function, and the δSNR sequence is extracted into a plurality of sine curve sequences. When the water level changes, the δSNR sequence is offset, and the sine curves corresponding to the two time periods are also offset. The multipath relative phase change of the kth sine curve is The calculation can be performed by an FFT method.

[0038] Step 103: Water level change value calculation.

[0039] The phase change is established in relation to the function of the reflection height change △H k , and the reflection height change value △H k is obtained, and then the water level change value is calculated.

[0040] Further, the phase change is established in relation to the function of the reflection height change △H k , and the reflection height change value △H k is obtained, and then the water level change value is calculated.

[0041]

[0042] wherein, and are the phases corresponding to the kth sine curve of the δSNR sequences of the two time periods, θ k is the average value of the elevation angles corresponding to the kth sine curve of the δSNR sequences of the two time periods, H 0k and H 1k are the average values of the reflection heights corresponding to the kth sine curve of the δSNR sequences of the two time periods, △H k represents the reflection height change value of the kth sine curve caused by the water level change from the initial time to the next time. After the reflection height change value is obtained, the water level change value can be calculated.

[0043] Further, the water level change value calculation is crucial in establishing the function relationship between the phase change and the reflection height change △H k , and then the water level change △H'.

[0044]

[0045] The embodiment of the present application introduces a water level change monitoring method based on GNSS multipath phase change (GNSS-IR-PS). The method is based on the delta SNR sequence of GNSS multipath phase change at different time periods, and the relative phase change of the multiple corresponding sinusoidal curves obtained by fitting. Through the FFT method, multiple phase changes of the delta SNR sequence at different time periods can be obtained, and from the mathematical model of the delta SNR sequence, the mathematical relationship between the phase change and the reflection height change can be obtained, and then the change values of multiple reflection heights can be obtained. For the SNR of two different time periods with overlapping trajectories, the phase change values at multiple different elevation angles can be obtained by this method, and then the corresponding multiple reflection height change values can be calculated, greatly improving the accuracy and resolution of the GNSS-IR method.

[0046] Embodiment 2

[0047] The present application will be further described in detail below in combination with the drawings and embodiments. It should be understood that the embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0048] XK01 station is selected for processing and illustration. XK01 records data at a sampling interval of 30 seconds. The antenna is installed on the reservoir bank, and the reservoir water surface is generally calm and will be regularly impounded and discharged. By comparing the GNSS signal-to-noise ratio sequences of the same satellite and the same frequency on DOY 1 and 2 in 2021, the reflection height change value is obtained, and the reservoir water level change value is further obtained.

[0049] The water level change monitoring method based on GNSS multipath phase change (GNSS-IR-PS) of the embodiment of the present application comprises the following steps: Through the multipath phase change Calculate the reflection height change △H k Three main parts.

[0050] Figure 1 The water level change monitoring method based on GNSS multipath phase change (GNSS-IR-PS) provided by the embodiment of the present application is shown in the flowchart. The embodiment of the present application comprises the following steps:

[0051] Step 201: data acquisition;

[0052] Download the GNSS observation file and precise ephemeris file of XK01 station to obtain the SNR at each time, solve the corresponding satellite position, and then obtain the azimuth angle, elevation angle and other information of the satellite relative to the receiver.

[0053] Step 202: extract the signal-to-noise ratio oscillation item delta SNR sequence;

[0054] First, a function of signal-to-noise ratio (SNR) and the sine of elevation angle sinθ is established. The second-order polynomial is fitted from the SNR data and removed to eliminate the influence of the direct signal, thus obtaining the SNR oscillation term δSNR sequence.

[0055] Step 203: Divide the signal-to-noise ratio (SNR) oscillation term δSNR sequence into periodic segments. Within each period, resample uniformly based on the sine value of the elevation angle, sinθ. Fit the sequence using a sine function within each period to obtain multiple sine curve sequences. When the water level changes, the SNR oscillation term δSNR sequence at different time periods will have a certain shift, and the obtained sine curves will also shift. Therefore, the multipath phase change of multiple sine curve sequences at two different time periods can be calculated using the FFT method.

[0056] Step 204: Establish multipath phase transformation The functional relationship between the reflection height change ΔH and the water level change ΔH' is used to calculate the reflection height change value, and then the water level change value.

[0057] The calculation results of this embodiment are illustrated as follows: Figure 2 As shown in the figure, the water level heights in DOY 1 and 2 of 2021 were different, resulting in different signal-to-noise ratio (SNR) oscillation terms (δSNR) sequences. The SNR sequences were segmented by period, resampled, and fitted with a sine function. The multipath phase change of the two sine curves in the fifth period was obtained using the FFT method. The reflection height change ΔH5 is -53.4219, and the change in reflection height is -6.9354 cm. Therefore, the water level change value obtained in this embodiment of the invention is 6.9354 cm, while the water level change value obtained by the water level gauge at the corresponding time is 6.7028 cm, a difference of 0.2326 cm. The LSP method, however, can only obtain one result for the entire δSNR sequence, resulting in a water level change value difference of 4.81 cm compared to the water level gauge. This demonstrates the effectiveness of this method and its significantly improved accuracy and resolution.

[0058] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not limited, and any device that can perform the above functions is acceptable.

[0059] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for monitoring water level changes based on GNSS multipath phase changes, characterized in that, The method comprises: Removing a direct signal component in a signal-to-noise ratio (SNR) by fitting a low-order polynomial to obtain a mathematical model of an SNR oscillation term δSNR sequence; Segmenting the δSNR sequence according to a period, uniformly resampling a sine value sinθ of the δSNR sequence relative to an elevation angle in each period, fitting a sine function to the δSNR sequence in the period, and extracting the δSNR sequence into a plurality of sinusoidal curve sequences; The relative phase change of multipath when water level changes is Calculated by FFT method; establishing phase change and the function relationship between the reflection height change and the water level change, the reflection height change value ΔH is obtained k , and then the water level change value is calculated Phase change with the function of the reflection height change ΔH k is expressed as: wherein, and are the phase corresponding to the kth sinusoid of the δSNR sequence of the two time periods, θ k is the average of the elevation angle corresponding to the kth sinusoid of the δSNR sequence of the two time periods, H 0k and H 1k are the average of the reflection height corresponding to the kth sinusoid of the δSNR sequence of the two time periods, ΔH k denotes the change in the reflection height of the kth sinusoid caused by the water level change from the initial time to the next time.

2. The water level change monitoring method based on GNSS multipath phase change according to claim 1, characterized in that, The water level change value is calculated by establishing a phase change and the function relationship between the reflection height change AH and the water level change AH', and the water level change value is calculated.

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