A method for measuring surface soil moisture content based on GNSS refraction mode

Abstract

The application provides a kind of surface soil moisture content measurement method based on GNSS refraction mode, which is measured by the power ratio of GNSS signal direct signal in air and GNSS refraction signal in soil to retrieve surface soil humidity, and the accurate acquisition of power ratio is realized by using the SNR observation value output by the GNSS receiving device connected with the antenna in soil (main antenna) and the antenna in air (auxiliary antenna);The measurement of surface soil humidity is realized by using the polynomial function, GNSS SNR observation value and soil temperature observation value.The method can use low-cost, navigation-type, consumer-grade GNSS receiver and antenna to collect data, realize continuous, near real-time, high-precision measurement of surface soil humidity, and has lower requirements for use environment, is easy to deploy and implement, and can effectively solve the technical problems of high-precision, real-time (near real-time) monitoring of surface soil humidity.

Description

Technical Field

[0001] This invention belongs to the field of terrestrial natural environment monitoring, and in particular relates to a method for measuring surface soil moisture content based on GNSS refraction mode. This method can be used for monitoring changes in surface soil moisture, regulating crop irrigation, monitoring and predicting meteorological drought, etc. Background Technology

[0002] Soil water is a crucial component of terrestrial water resources, the lifeline of agricultural ecosystems, and a link between the water and energy cycles in the terrestrial-atmosphere coupling system. Compared to deep soil moisture (below 10 cm), surface soil moisture (0-10 cm) is extremely sensitive to events such as solar radiation and atmospheric precipitation. Obtaining accurate, continuous, and near-real-time information on dynamic changes in soil moisture is vital for agricultural irrigation, hydrometeorology, and drought prevention. For example, in agriculture, appropriate regulation of soil moisture based on real-time monitoring data and crop water requirements can promote crop development and increase yield. In meteorology, surface soil moisture is a necessary input parameter for various drought prediction models and drought assessment indicators; accurate surface soil moisture information is the prerequisite and foundation for precise drought disaster prediction and assessment.

[0003] GNSS refers to all satellite navigation systems, including global, regional, and augmented systems, such as the US GPS, Russia's GLONASS, Europe's Galileo, China's BeiDou Navigation Satellite System, and related augmentation systems. GNSS refraction mode technology uses two sets of antennas and receivers to simultaneously acquire GNSS signals. One antenna is installed in the measured medium (e.g., soil) to acquire the refracted signal transmitted into the medium; the other antenna is installed in the air to acquire the direct signal. Based on the changes in the intensity and propagation speed of the refracted signal relative to the direct signal, parameters related to the electrical properties of the measured medium can be retrieved. Compared to GNSS reflected or interferometric signals, GNSS refraction signals are more sensitive to changes in parameters related to the electrical properties of the measured medium, making them ideal for measuring parameters such as soil moisture, snow density, and vegetation water content.

[0004] Existing soil moisture measurement methods or technologies can be divided into three categories: manual sampling and drying methods, FDR or TDR sensor measurement methods, and active or passive satellite microwave remote sensing methods. The main problems with existing technologies are as follows:

[0005] (1) Manual sampling and drying method. Manual sampling and drying is time-consuming and labor-intensive, and it is destructive to soil texture. It is also impossible to monitor soil moisture in situ and continuously.

[0006] (2) FDR or TDR sensor measurement method. This type of traditional sensor measurement method is more suitable for monitoring the moisture of deep soil (10cm and above). For surface soil (less than 10cm), the fluctuation of soil moisture will cause the soil structure to undergo a periodic "stretching-compression" cycle, which will destroy the continuity of the soil structure and cause gaps between the sensor probe and the soil, thus causing this type of sensor measurement method to fail.

[0007] (3) Satellite microwave remote sensing method. The error of the satellite microwave remote sensing method is relatively large (≥0.03cm). 3 cm -3 This method is particularly inaccessible in mountainous and other terrain-complex areas; its temporal resolution is also relatively low (≥3 days); data acquisition is cumbersome, and users have no control over the data source. Therefore, it cannot achieve high-precision, continuous, real-time (near real-time) monitoring of surface soil moisture.

[0008] In addition to the methods mentioned above, two other methods for measuring soil moisture using GNSS signals have emerged in recent years, both domestically and internationally: GNSS reflectance measurement (GNSS-R) and GNSS interferometry (GNSS-IR). These two methods mainly have the following problems:

[0009] (1) GNSS reflection measurement method. This method requires a specially designed left-hand circularly polarized GNSS antenna to receive reflected signals from the ground surface, and requires dedicated GNSS equipment to process the signals to generate soil moisture values. This measurement method is very costly and the equipment used consumes a great deal of energy, making it unsuitable for large-scale, networked ground soil moisture monitoring applications.

[0010] (2) GNSS interferometry. This method mainly uses a geodetic receiver to collect SNR sequences at low satellite elevation angles for phase inversion to retrieve soil moisture. Due to measurement noise and antenna phase center variations, the soil moisture measurement accuracy of this method is relatively low (≥0.03 cm). 3 cm -3 Furthermore, it has stringent environmental requirements, necessitating a large area (≥1000m) around the station. 2 The ideal terrain should be flat, unobstructed, and free from significant vegetation cover, but real-world environments often do not meet these requirements, making practical application difficult.

[0011] The GNSS refraction mode soil moisture measurement method disclosed in this invention can achieve continuous, near real-time, and high-precision measurement of surface soil moisture using data collected by a low-cost, navigation-type, consumer-grade GNSS receiver and antenna. It has low requirements for the operating environment, is easy to deploy and implement, and can effectively solve the technical problem of high-precision, real-time (near real-time) monitoring of surface soil moisture. Summary of the Invention

[0012] To address the aforementioned technical problems, the present invention aims to provide a method for measuring surface soil moisture content based on GNSS refraction modes. This method fundamentally solves the problem that existing soil moisture measurement methods or technologies cannot effectively monitor surface soil moisture (0-10cm) continuously, in real-time, and with high precision.

[0013] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0014] A method for measuring surface soil moisture content based on GNSS refraction mode, the method comprising the following steps:

[0015] Step (1): First, prepare the equipment used for measurement. The equipment used in this method includes 2 GNSS antennas, 2 GNSS receivers, 2 temperature sensors and 1 electromagnetic shielding plate.

[0016] Step (2) Deploy a GNSS antenna and a temperature sensor in the soil on the ground surface to obtain GNSS SNR and soil temperature observation data; use a column to raise the electromagnetic shielding plate a certain distance away from the ground, with a maximum height of 1.5m; deploy another GNSS antenna at the center of the electromagnetic shielding plate; the horizontal distance between the center points of the two GNSS antennas is greater than or equal to 2m, and the installation directions of the main antenna and the auxiliary antenna should be the same or similar in both the horizontal and vertical planes, with a maximum difference in installation direction of less than 45°; use the SNR observation value output by the GNSS receiving equipment connected to the GNSS antenna in the soil and the GNSS antenna in the air to accurately obtain the power ratio;

[0017] Step (3) converts the GNSS SNR observations obtained from the two GNSS antennas and the soil temperature observations obtained from the underground temperature sensor into soil moisture observations. Specifically, the relationship between the power ratio, soil temperature, and soil moisture is determined as a polynomial function of no less than second order and no more than third order. The function uses the power ratio and soil temperature as independent variables and soil moisture as dependent variable. The coefficients of the polynomial function are obtained by using the discrete data sequences of the power ratio, soil temperature, and soil moisture measured simultaneously, based on the least squares parameter estimation method. The surface soil moisture is measured using this polynomial function, the GNSS SNR observations, and the soil temperature observations.

[0018] Preferably, in step (1), the gain mode of the GNSS antenna is right-hand circular polarization gain; the GNSS antenna is of the type without a protective cover or a rectifier; if the antenna has a shell, the gap between the shell and the antenna electrical feed point is less than 1 cm; the sampling frequency of the GNSS receiver is higher than 0.01 Hz; the GNSS receiver can output SNR observation values, and the minimum resolution of the SNR observation values ​​is greater than 0.2 dB-Hz; the measurement accuracy of the temperature sensor is better than 1 ℃; the electromagnetic shielding plate is implemented in a circle or rectangle, and the minimum distance between any two ends and the center is greater than twice the wavelength of the GNSS electromagnetic wave.

[0019] Preferably, the GNSS antenna and temperature sensor in the soil in step (2) are deployed as follows:

[0020] (1) Select a rectangular area with an area greater than or equal to 50cm × 50cm on the ground surface. The ground surface of the selected area should be flat and without obvious systematic tilt. The standard deviation of the ground elevation of the selected area should be less than 2cm.

[0021] (2) Cut vertically along the center line of the selected area to expose the soil profile. On the soil profile, at the center of the area, dig a horizontal mounting hole according to the size of the GNSS antenna. The mounting hole is 0-10cm away from the ground surface. Fix the GNSS antenna in the horizontal mounting hole and install the temperature sensor close to the GNSS antenna.

[0022] (3) The excavated soil was backfilled to the original position and leveled to complete the deployment of the underground GNSS antenna and temperature sensor.

[0023] Preferably, in step (3), the GNSS SNR observation values ​​measured by the two GNSS antennas and the soil temperature observation values ​​measured by the underground temperature sensor are converted into soil moisture observation values; depending on the timeliness, the conversion can be performed by two methods: real-time conversion method and near-real-time conversion method, wherein the real-time conversion method has the following steps:

[0024] (1) Elevation angle determination: Based on the satellite elevation angle output by the GNSS receiving equipment connected to the GNSS antenna in the soil, determine whether the SNR observation value is usable. If the satellite elevation angle is greater than or equal to 20°, the GNSS SNR observation value measured by the GNSS antenna is usable; otherwise, the observation data of the satellite is unusable.

[0025] (2) Calculate the power ratio; Based on the GNSS SNR observation value of the GNSS antenna, calculate the power ratio of the direct signal to the refracted signal; The calculated power ratio is expressed in units of ratio or decibels;

[0026] (3) Correct the power ratio; based on the satellite elevation angle, correct the antenna gain difference in the power ratio;

[0027] (4) Calculate the surface soil moisture value; calculate the soil moisture value based on the corrected power ratio and soil moisture value, and the calculation method is a polynomial function of no less than second order and no more than third order.

[0028] Preferably, the steps of the near real-time conversion method are as follows:

[0029] (1) Elevation angle filtering: Based on the input satellite elevation angle, determine whether the SNR observation value is within the range of 50°-60°. If it is within the range, save the SNR observation value and satellite elevation angle data. Determine whether the difference between the maximum and minimum satellite elevation angles of all saved data is greater than or equal to 8°. If it is greater than 8°, use the saved data sequence; otherwise, discard the satellite observation data sequence.

[0030] (2) Fitting the SNR sequence; Using the satellite elevation angle that stores the data sequence as the independent variable and the SNR observation value of the GNSS antenna deployed in the soil as the dependent variable, a linear fitting method is used to obtain a linear function; Substitute the satellite elevation angle into the linear function, and the obtained function value is used as the fitted SNR observation value of the GNSS antenna in the soil; Perform the same operation on the SNR observation value of the GNSS antenna deployed on the ground surface to obtain the fitted SNR observation value.

[0031] (3) Calculate the average power ratio; calculate the power ratio at each measurement time from the fitted SNR observations of the two GNSS antennas; average the power ratios at all times to obtain the average power ratio for the observation period.

[0032] (4) Calculate the surface soil moisture value; calculate the surface soil moisture value based on the average power ratio and average soil temperature within the range of satellite elevation angle of 50°-60°; the calculation formula adopts a third-order polynomial or a second-order polynomial; the coefficients of the polynomial are determined by the least squares parameter estimation method based on the average power ratio, average soil temperature and average soil moisture of synchronous observation; the polynomial coefficients determined by the least squares method are suitable for near real-time measurement of soil moisture in the GNSS refraction mode in this area, and the lag of the measurement data is less than 3 hours.

[0033] The beneficial effects of this invention are reflected in:

[0034] (1) This invention utilizes the sensitivity of GNSS refracted signal intensity to soil moisture. Two right-hand circularly polarized GNSS antennas are fixed in the soil to receive refracted signals and fixed on the ground surface to receive direct signals, respectively. By measuring the power ratio of the direct signal to the refracted signal, accurate measurement of surface soil moisture is achieved. Under the premise of accurate determination of the polynomial model coefficients, the measurement error of surface soil moisture is less than 1% (or 0.01 cm). 3 cm -3 ).

[0035] (2) The solution disclosed in this invention has the characteristics of all-day and all-weather measurement and is not affected by soil gaps; the technical solution disclosed in this invention can be implemented based on low-cost, navigation-type GNSS receiver chip and antenna to achieve continuous, real-time (near real-time) monitoring of surface soil moisture.

[0036] (3) Fixing the direct signal receiving antenna (auxiliary antenna) to the ground or an electromagnetic shielding plate can effectively avoid the influence of reflected signals on the direct signal and power ratio measurement results. Installing the refracted signal receiving antenna (main antenna) in the soil, ensuring a flat surface within a 50cm x 50cm area above the main antenna, will allow for accurate refracted signal acquisition. The deployment scheme disclosed in this invention is easy to implement, highly applicable, and can be used to measure surface soil moisture in most practical scenarios, such as mountainous and hilly areas. Attached Figure Description

[0037] Figure 1 This is a structural diagram of the electromagnetic shielding plate used in this invention.

[0038] Figure 2 This is a top view showing the deployment of GNSS antennas in the soil according to the present invention.

[0039] Figure 3 This is a deployment diagram (right view) of the GNSS antenna and temperature sensor in the soil in this invention;

[0040] Figure 4 This is a deployment diagram of the surface GNSS antenna in this invention (without shielding).

[0041] Figure 5 This is a deployment diagram of the surface GNSS antenna and electromagnetic shielding plate in this invention;

[0042] Figure 6 This is a flowchart of the real-time conversion method in this invention;

[0043] Figure 7 This is a flowchart of the near real-time conversion method in this invention;

[0044] Figure 8 This is a satellite arc distribution diagram in a practical application case of the present invention;

[0045] Figure 9 This is a practical application example of the invention, using a near-real-time soil moisture measurement method to obtain fitted SNR sequences under different soil temperature and humidity conditions;

[0046] Figure 10 This is a residual distribution diagram between the results of near-real-time soil moisture measurement and the measured values ​​of the soil moisture sensor in a practical application case of the present invention;

[0047] Figure 11 A comparison chart of daily average soil moisture measurements and sensor measurements in a practical application case of this invention. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings:

[0049] Example 1

[0050] A method for measuring surface soil moisture content based on GNSS refraction mode. In the implementation of this method, the configuration of the measuring equipment includes at least: 2 GNSS antennas, 2 GNSS receiving devices (such as GNSS receivers, GNSS modules, GNSS chips, and other hardware capable of outputting GNSS SNR observation values), and 1 temperature sensor; the optimal configuration is: 2 GNSS antennas, 2 GNSS receiving devices, 2 temperature sensors, and 1 electromagnetic shielding plate.

[0051] The specific requirements for the above-mentioned configuration device in the method of the present invention are as follows:

[0052] (1) The gain mode of the GNSS antenna is right-hand circular polarization gain;

[0053] (2) GNSS antennas should be of the type without a protective cover or a fairing; if the antenna has a housing, the gap between the housing and the antenna electrical feed point should be less than 1 cm.

[0054] (3) The sampling frequency of the GNSS receiving equipment is higher than 0.01Hz;

[0055] (4) The GNSS receiving equipment is capable of outputting SNR observation values, and the minimum resolution of the SNR observation values ​​is greater than 0.2 dB-Hz;

[0056] (5) The temperature sensor has a measurement accuracy better than 1℃.

[0057] Electromagnetic shielding plates can be circular or rectangular, with the minimum distance between any two ends and the center greater than twice the wavelength of GNSS electromagnetic waves. For example, for a square electromagnetic shielding plate, its side length should be greater than 40 cm; for a circular electromagnetic shielding plate, its diameter should be greater than 40 cm. The three-dimensional structure of the shielding plate is as follows: Figure 1 As shown, the shielding plate is composed of three tightly assembled layers, from top to bottom: a heat insulation layer, a support layer, and a shielding layer. The heat insulation layer is made of materials with low thermal conductivity, such as plastic or wood, and its thickness should be less than 5 cm. The support layer is made of materials with minimal structural deformation (such as aluminum, iron, and other metals), and its thickness should be less than 5 cm. The shielding layer is made of multiple layers of absorbing films capable of shielding L-band electromagnetic waves, tightly bonded together.

[0058] The specific implementation of the method of the present invention is as follows:

[0059] This method requires the acquisition of GNSS SNR and soil temperature observation data, which can be obtained through GNSS equipment and temperature sensors, respectively. Implementing this method requires deploying a set of GNSS antennas and temperature sensors in the soil at the ground surface; the deployment method of the GNSS antenna (as the main antenna) and temperature sensor in the soil is as follows... Figure 2 and Figure 3 As shown, the deployment process is as follows:

[0060] (1) Select a rectangular area with an area greater than or equal to 50cm × 50cm on the ground surface. The ground surface of the selected area should be flat and without obvious systematic tilt. The standard deviation of the ground elevation of the selected area should be less than 2cm.

[0061] (2) Cut vertically along the centerline of the selected area to expose the soil profile. On the soil profile, at the center of the area, dig a horizontal mounting hole according to the size of the GNSS antenna. The mounting hole should be 0-10cm from the ground surface. Fix the GNSS antenna in the horizontal mounting hole and install the temperature sensor in close contact with the GNSS antenna.

[0062] (3) Backfill the excavated soil to the original position and level it to complete the deployment of the underground GNSS antenna (main antenna) and temperature sensor;

[0063] In addition to the GNSS antenna and temperature sensor deployed in the soil, this solution also requires the deployment of one GNSS antenna on the ground surface (or above the ground). The deployment method of this surface GNSS antenna (as a secondary antenna) is as follows: Figure 3 and Figure 4 As shown. If the minimum configuration is selected, the auxiliary antenna is directly fixed to the ground surface. If the optimal configuration is selected, the electromagnetic shielding plate is raised off the ground at a certain distance using a column, with a maximum height of 1.5m. The auxiliary antenna is fixed at the center of the electromagnetic shielding plate, and the temperature sensor is fixed tightly against the auxiliary antenna. Regardless of whether a configuration with a shielding plate is used, the horizontal distance between the center of the auxiliary antenna and the center of the main antenna should be greater than or equal to 2m, and the installation directions of the main antenna and the auxiliary antenna should be the same or similar in both the horizontal and vertical planes, with a maximum difference of less than 45° in installation direction.

[0064] Specifically, converting the GNSS SNR observations obtained from the main and auxiliary antennas, and the soil temperature observations obtained from the underground temperature sensor, into soil moisture observations is one of the key aspects of this invention. Depending on the timeliness, this conversion can be performed using two methods: real-time conversion and near-real-time conversion. In practical applications, these two methods can be implemented individually or in combination. The specific implementation processes of the two methods are described below.

[0065] (1) Real-time conversion method

[0066] The input parameters for the real-time soil moisture conversion method are the real-time GNSS SNR observations output from the receivers connected to the main and auxiliary antennas, the satellite elevation angle, and the real-time soil temperature observations; the technical process for implementing the real-time conversion method is as follows: Figure 6 As shown, the process is described below:

[0067] ① Elevation Angle Determination. Based on the satellite elevation angle output by the main antenna receiver, determine whether the SNR observation value is usable. If the satellite elevation angle is greater than or equal to 20°, the SNR observation value of the main and auxiliary antennas is usable; otherwise, the observation data of the satellite is unusable.

[0068] ② Calculate the power ratio. Based on the GNSS SNR observations of the main and auxiliary antennas, calculate the power ratio of the direct signal to the refracted signal; the calculated power ratio is expressed in units of ratio or decibels, and the calculation methods for the two unit forms are (Formula 1) and (Formula 2), respectively.

[0069] Power ratio = 10·log 10 (SNR observation value of auxiliary antenna - SNR observation value of main antenna) (Formula 1)

[0070] Power ratio = SNR observation value of auxiliary antenna - SNR observation value of main antenna (Formula 2)

[0071] ③ Power ratio correction. Based on the satellite elevation angle, the power ratio is corrected for antenna gain differences. The power ratio correction in terms of ratio and decibels is implemented using (Formula 3) and (Formula 4), respectively.

[0072]

[0073] Corrected power ratio = Power ratio - G(θ) (Formula 4)

[0074] In the above formula, G(θ) is the antenna gain value when the satellite elevation angle is θ, and the unit of gain is decibels; the gain value is obtained by third-order B-spline interpolation based on the measured antenna gain data.

[0075] ④ Calculate the surface soil moisture value. Calculate the soil moisture value based on the corrected power ratio and soil moisture value using the third-order polynomial shown in (Formula 5). Alternatively, the second-order polynomial shown in (Formula 6) can also be used to convert the power ratio and soil temperature value into a soil moisture value, but the conversion accuracy is slightly lower than that of (Formula 5).

[0076] Soil moisture =

[0077] x0+x1γ+x2T+x3γT+x4γ 2 +x5T 2+x6γ 2 T+x7γT 2 +x8γ 3 +x9T 3 (Formula 5)

[0078] Soil moisture = x0 + x1γ + x3γT + x4γ 2 (Formula 6)

[0079] In the polynomial above, γ represents the power ratio, T represents the soil temperature (unit: °C), and {xi} represents the polynomial coefficients. The power ratio is expressed in ratios or decibels, but the polynomial coefficients corresponding to these two units need to be determined separately. The polynomial coefficients vary depending on the soil type, the burial depth of the main antenna, and the unit of the power ratio. In the implementation of this scheme, the polynomial coefficients {xi} are determined using the least squares parameter estimation method based on the power ratio, surface soil moisture value, and soil temperature value obtained from synchronous observations. The obtained polynomial coefficients are suitable for real-time monitoring of soil moisture in the GNSS refraction mode in this region.

[0080] By implementing the above four processes, the GNSS SNR and soil temperature observations collected in real time by the main and auxiliary antennas can be converted into real-time surface soil moisture values.

[0081] (2) Near real-time conversion method

[0082] The input parameters for the near-real-time soil moisture conversion method are the same as those for the real-time method, namely: GNSS SNR observations measured by the primary and secondary antennas, satellite elevation angle, and soil temperature observations; the technical process for implementing the near-real-time conversion method is as follows: Figure 7 As shown, the process is described below:

[0083] ① Elevation Angle Filtering. Based on the input satellite elevation angle, determine whether the SNR observation value is within the range of 50°-60°. If it is within this range, save the SNR observation value and satellite elevation angle data. Determine whether the difference between the maximum and minimum satellite elevation angles of all saved data is greater than or equal to 8°. If it is greater than 8°, use the saved data sequence; otherwise, discard the satellite observation data sequence.

[0084] ② Fitting the SNR sequence. Using the satellite elevation angle of the stored data sequence as the independent variable and the SNR observation value of the main antenna as the dependent variable, a linear fitting method is used to obtain a linear function; the satellite elevation angle is substituted into this linear function, and the resulting function value is used as the fitted SNR observation value of the main antenna; the same operation is performed on the SNR observation value of the auxiliary antenna to obtain the fitted SNR observation value of the auxiliary antenna.

[0085] ③ Calculate the average power ratio. Substitute the fitted SNR observation values ​​of the main antenna and auxiliary antenna into (Formula 1) or (Formula 2) to calculate the power ratio at each measurement time. After averaging the power ratios at all times, the average power ratio for the observation period is obtained.

[0086] ④ Calculate the surface soil moisture value. Calculate the surface soil moisture value based on the average power ratio and average soil temperature within the satellite elevation angle range of 50°-60°. The calculation formula uses either the third-order polynomial shown in Formula 5 or the second-order polynomial shown in Formula 6. The polynomial coefficients are determined using the least squares parameter estimation method based on the average power ratio, average soil temperature, and average soil moisture from synchronous observations. The polynomial coefficients determined by the least squares method are suitable for near-real-time measurement of soil moisture in the GNSS refraction mode in this region, with a measurement data lag of less than 3 hours.

[0087] By implementing the above four processes, the GNSS SNR and soil temperature observations collected by the main and auxiliary antennas can be converted into near-real-time surface soil moisture values.

[0088] Practical application examples of the method of this invention

[0089] The following is a specific implementation example of the near-real-time soil moisture measurement method disclosed in this invention. This embodiment was completed at a meteorological observation field in Jiangjin District, Chongqing City, and the equipment configuration scheme adopted is the lowest configuration scheme disclosed in this invention. Two ANN-MB navigation antennas manufactured by u-blox were used as the GNSS antennas, and two sets of M8N chips generated by u-blox were used as the GNSS receiving equipment. Soil temperature was measured using an MT-ABCAC010 multi-functional temperature and humidity sensor manufactured by Dalian Zhongshan District Huayu Electronic Products Co., Ltd. The near-real-time soil moisture measurement results disclosed in this invention were compared with the soil moisture values ​​measured by this sensor.

[0090] In this example, the main antenna is installed 3.5 cm below the ground surface using the deployment scheme disclosed in this invention to measure the soil moisture value at that location; the auxiliary antenna is installed on the ground. The example implementation period is from November 17, 2020 to November 16, 2022, a total of two years. During the implementation period, soil moisture values ​​are calculated using GPS SNR observation sequences from 20 satellites across 40 50°-60° elevation angle intervals. The satellite segment distribution used in this example is as follows: Figure 8 As shown.

[0091] Figure 9The main antenna fitted SNR sequence (fitted SNRM) and the auxiliary antenna fitted SNR sequence (fitted SNRA) under different soil temperature and humidity conditions are obtained using the near real-time soil moisture measurement method disclosed in this invention. Here, (SM,ST)1, (SM,ST)2, and (SM,ST)3 represent soil moisture and temperature conditions of 0.249 cm⁻¹, respectively. 3 cm -3 and 21.6℃, 0.365cm 3 cm -3 and 15.6℃, 0.430cm 3 cm -3 and 31.2℃. A comparison of the results of the near-real-time soil moisture measurement method disclosed in this invention with the measurement results of a soil moisture sensor, such as... Figure 10 As shown; the average error of the measurement results is -0.001cm. 3 cm -3 The standard deviation is 0.009 cm. 3 cm -3 The root mean square error is 0.009 cm. 3 cm -3 .

[0092] The daily average soil moisture measurement value is obtained by averaging the near-real-time soil moisture measurement results over a day; during the example implementation, the comparison between the daily average soil moisture measurement value of the method disclosed in this invention and the sensor measurement value is shown below. Figure 11 As shown, the average error of the daily average measurement results is -0.001 cm. 3 cm -3 The standard deviation is 0.009 cm. 3 cm -3 The root mean square error is 0.009 cm. 3 cm -3 .

[0093] Any technical solution that achieves the above-mentioned technical effects by utilizing the technical solutions described in this invention, or by designing similar technical solutions by those skilled in the art under the inspiration of the technical solutions described in this invention, falls within the protection scope of this invention.

Claims

1. A method for measuring the water content of the surface soil based on the GNSS refraction pattern, characterized by, The method comprises the following steps: Step (1): First, prepare the equipment used for measurement, the equipment used in the method comprises two GNSS antennas, two GNSS receiving devices, two temperature sensors and one electromagnetic shielding plate; wherein the gain mode of the GNSS antenna is right-hand circular polarization gain; Step (2): One GNSS antenna and temperature sensor are arranged in the soil on the ground surface to obtain GNSS SNR and soil temperature observation data; the electromagnetic shielding plate is elevated by a stand to a certain distance from the ground surface, and the maximum height of the electromagnetic shielding plate is 1.5 m; the other GNSS antenna is arranged at the center position of the electromagnetic shielding plate; the horizontal distance between the center points of the two GNSS antennas is greater than or equal to 2 m, and the installation directions of the main antenna and the auxiliary antenna should be the same or similar in the horizontal plane and the vertical plane, and the maximum difference of the installation directions is less than 45°; the accurate acquisition of the power ratio is realized by using the SNR observation values output by the GNSS receiving devices connected with the GNSS antennas in the soil and the GNSS antennas in the air; Step (3): The GNSS SNR observation values measured by the two GNSS antennas and the soil temperature observation values measured by the underground temperature sensor are converted into soil humidity observation values; specifically, the relationship among the power ratio, the soil temperature and the soil humidity is determined as a polynomial function not lower than the second order and not higher than the third order; the function takes the power ratio and the soil temperature as independent variables and takes the soil humidity as the dependent variable; the coefficients of the polynomial function are obtained by the method of least squares parameter estimation based on the synchronous measured discrete data sequences of the power ratio, the soil temperature and the soil humidity; the measurement of the surface soil humidity is realized by using the polynomial function, the GNSS SNR observation values and the soil temperature observation values.

2. The GNSS-based refraction pattern based measurement of surface soil moisture content method of claim 1, wherein, In step (1), the GNSS antenna is of the type without protective cover or without fairing; if the antenna has a shell, the gap between the shell and the electrical feed point of the antenna is less than 1 cm; the sampling frequency of the GNSS receiving device is higher than 0.01 Hz; the GNSS receiving device can output SNR observation values, and the minimum resolution of the SNR observation values is greater than 0.2 dB-Hz; the measurement accuracy of the temperature sensor is better than 1℃; the electromagnetic shielding plate is realized in a circular or rectangular shape, and the minimum distance between any two ends and the center line is greater than 2 times the wavelength of the GNSS electromagnetic wave.

3. The GNSS-based refraction pattern based measurement of surface soil moisture content method of claim 1, wherein, In step (2), the GNSS antenna and the temperature sensor in the soil are arranged as follows: (1) Select a rectangular area with an area greater than or equal to 50 cm x 50 cm on the ground surface, and the ground surface of the selected area should be flat and have no obvious systematic tilt, and the standard deviation of the ground elevation of the selected area should be less than 2 cm; (2) Cut the soil profile vertically along the center line of the selected area, and at the center of the soil profile in the selected area, horizontally excavate an installation hole according to the size of the GNSS antenna, and fix the GNSS antenna in the installation hole, and the temperature sensor is close to the GNSS antenna; (3) Fill the excavated soil back to the original position and smooth it to complete the deployment of the underground GNSS antenna and the temperature sensor.

4. The GNSS-based refraction pattern based measurement of surface soil moisture content method of claim 1, wherein, In step (3), the GNSS SNR observation values measured by the two GNSS antennas and the soil temperature observation values measured by the underground temperature sensor are converted into soil humidity observation values; according to different time effectiveness, the conversion is carried out through two methods: a real-time conversion method and a near-real-time conversion method, wherein the real-time conversion method has the following steps: (1) Height angle discrimination; according to the satellite elevation angle output by the GNSS receiving device connected with the GNSS antenna in the soil, it is determined whether the SNR observation value is available, if the satellite elevation angle is greater than or equal to 20°, the GNSS SNR observation value measured by the GNSS antenna is available; otherwise, the observation data of the satellite is unavailable; (2) Calculation of power ratio; according to the GNSS SNR observation value of the GNSS antenna, the power ratio of the direct signal and the refracted signal is calculated; The calculated power ratio is in the unit of ratio or decibel; (3) Correction of power ratio; according to the satellite elevation angle, the power ratio is corrected for the difference in antenna gain; (4) Calculation of surface soil humidity value; according to the corrected power ratio and the soil humidity value, the soil humidity value is calculated, and the calculation method is a polynomial function of not less than second order and not higher than third order.

5. The GNSS-based refraction pattern based measurement of surface soil moisture content method of claim 4, wherein, The steps of the near-real-time conversion method are as follows: (1) Height angle screening; according to the input satellite elevation angle, it is determined whether the SNR observation value is within the range of 50°-60°, if it is within the range, the SNR observation value and the satellite elevation angle data are saved; it is determined whether the difference between the maximum and minimum satellite elevation angles of all saved data is greater than or equal to 8°, if it is greater than 8°, the saved data sequence is used; otherwise, the satellite observation data sequence is discarded; (2) Fitting of SNR sequence; using the satellite elevation angle of the saved data sequence as the independent variable and the GNSS antenna SNR observation value deployed in the soil as the dependent variable, a linear function is obtained by using a linear fitting method; the function value obtained by substituting the satellite elevation angle into the linear function is taken as the fitting SNR observation value of the GNSS antenna in the soil; The same operation is performed on the GNSS antenna SNR observation value deployed on the ground surface to obtain the fitting SNR observation value; (3) Calculation of power ratio average value; the fitting SNR observation values of the two GNSS antennas are used to calculate the power ratio at each measurement time; after averaging all the power ratios, the power ratio average value of the observation period is obtained; (4) Calculation of surface soil humidity value; according to the power ratio average value and the soil temperature average value within the range of 50°-60° of the satellite elevation angle, the surface soil humidity value is calculated; The calculation formula adopts a third-order polynomial or a second-order polynomial; the coefficients of the polynomial are determined by using the least square parameter estimation method according to the simultaneously observed power ratio average value, soil temperature average value and soil humidity average value; the polynomial coefficients determined by the least square method are suitable for the near-real-time measurement of GNSS refractive mode soil humidity, and the lag of the measurement data is less than 3 hours.

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