A method for measuring soil moisture using an Android smartphone

Abstract

The present invention relates to a method for measuring soil humidity using an Android smart phone, belonging to the field of soil humidity measurement. The method includes: obtaining an original interference signal using the Android smart phone; decomposing the original interference signal using an empirical mode decomposition algorithm to obtain different frequency components; extracting a direct signal and a reflected signal according to the original interference signal; determining the vertical distance between the Android smart phone and the soil surface according to the different frequency components using an LSP spectrum estimation algorithm; screening the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smart phone to obtain a screened direct signal and a screened reflected signal; determining a peak power ratio according to the screened direct signal and the screened reflected signal using a Hilbert transform method; and determining the soil humidity according to the peak power ratio. The present invention can reduce the complexity of device installation.

Description

Technical Field

[0001] The present invention relates to the field of soil humidity measurement, and particularly to a method for measuring soil humidity using an Android smartphone. Background Art

[0002] China is located in the east of the Asian continent, with a span of more than 5,000 kilometers from east to west, north to south, crossing part of the temperate and tropical zones, and the soil types are complex. Accurately measuring the change of soil moisture content is one of the most important links in protecting the ecosystem and has always been the focus of attention of researchers. Soil moisture content, also known as soil humidity, is usually expressed by the ratio of the volume of free water to the volume of soil. Traditional measurement methods mainly include the drying and weighing method, the probe method, and optical remote sensing, which have problems such as complex operation, small detection range, and expensive equipment. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for measuring soil humidity using an Android smartphone to reduce the complexity of equipment installation.

[0004] To achieve the above object, the present invention provides the following solution:

[0005] A method for measuring soil humidity using an Android smartphone, comprising:

[0006] Obtaining an original interference signal using an Android smartphone;

[0007] Decomposing the original interference signal using an empirical mode decomposition algorithm to obtain different frequency components;

[0008] Extracting a direct signal and a reflected signal according to the original interference signal;

[0009] Determining the vertical distance between the Android smartphone and the soil surface according to the different frequency components using an LSP spectrum estimation algorithm;

[0010] Screening the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smartphone to obtain a screened direct signal and a screened reflected signal;

[0011] Determining a peak power ratio according to the screened direct signal and the screened reflected signal using a Hilbert transform method;

[0012] Determining the soil humidity according to the peak power ratio.

[0013] Optionally, before decomposing the original interference signal using an empirical mode decomposition algorithm to obtain different frequency components, it further includes:

[0014] Obtaining altitude angle and azimuth angle information of different satellites;

[0015] Determine the observation range according to the measurement requirements;

[0016] Screen the original interference signal according to the observation range, the elevation angle, and the azimuth angle information to obtain the original interference signals of the satellites whose elevation angles and azimuth angles are both within the observation range.

[0017] Optionally, the screening of the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smartphone to obtain a screened direct signal and a screened reflected signal specifically includes:

[0018] Judge whether the absolute value of the difference between the vertical distance and the actual installation height of the Android smartphone is less than a set threshold to obtain a judgment result;

[0019] If the judgment result is yes, use the direct signal as the screened direct signal and the reflected signal as the screened reflected signal;

[0020] If the judgment result is no, remove the direct signal and the reflected signal, and return to the step of "obtaining the original interference signal using the Android smartphone".

[0021] Optionally, the determination of the peak power ratio using the Hilbert transform method according to the screened direct signal and the screened reflected signal specifically includes:

[0022] Perform Hilbert transform on the screened reflected signal to obtain the peak power of the reflected signal;

[0023] Construct an analytic signal according to the screened reflected signal and the peak power of the reflected signal;

[0024] Determine the peak of the reflected signal according to the analytic signal;

[0025] Determine the peak power ratio according to the peak of the reflected signal and the screened direct signal.

[0026] Optionally, the determination of the peak of the reflected signal according to the analytic signal specifically includes:

[0027] Take the absolute value of the analytic signal to obtain the envelope of the screened reflected signal;

[0028] Determine the peak of the reflected signal according to the peaks of multiple oscillatory cycle signals in the envelope of the screened reflected signal.

[0029] Optionally, the determination of the peak power ratio according to the peak of the reflected signal and the screened direct signal specifically includes:

[0030] Determine the peak power of the reflected signal according to the peak of the reflected signal;

[0031] Determine the peak power of the direct signal according to the filtered direct signal;

[0032] Divide the peak power of the reflected signal by the peak power of the direct signal to obtain the peak power ratio.

[0033] Optionally, the expression of the soil humidity is:

[0034]

[0035] where Γ is the peak power ratio, and m v is the soil humidity.

[0036] According to the specific embodiments provided by the present invention, the following technical effects are disclosed:

[0037] The present invention uses an Android smartphone to obtain the original interference signal; decomposes the original interference signal using the empirical mode decomposition algorithm to obtain different frequency components; extracts the direct signal and the reflected signal according to the original interference signal; determines the vertical distance between the Android smartphone and the soil surface according to the different frequency components using the LSP spectrum estimation algorithm; screens the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smartphone to obtain the filtered direct signal and the filtered reflected signal; determines the peak power ratio according to the filtered direct signal and the filtered reflected signal using the Hilbert transform method; determines the soil humidity according to the peak power ratio. The measurement of the soil humidity can be directly realized by using an Android smartphone, thereby reducing the complexity of equipment installation. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings without creative efforts.

[0039] Figure 1 is a flowchart of the method for measuring soil humidity using an Android smartphone provided by the present invention;

[0040] Figure 2 is a schematic diagram of the method for measuring soil humidity using an Android smartphone provided by the present invention;

[0041] Figure 3 is a GNSS interference signal propagation geometric model diagram;

[0042] Figure 4 is a diagram of the collected original signal-to-noise ratio sequence. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0044] The object of the present invention is to provide a method for measuring soil moisture using an Android smart phone to reduce the complexity of equipment installation.

[0045] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] The Global Navigation Satellite System Interferometric Reflectometry (GNSS-IR) is a newly emerging passive remote sensing technology, which has the advantages of high precision, wide coverage, portable equipment, and no need for a specific signal source. The navigation antenna used by the surveying and mapping receiver is right-handed circular polarized (RHCP), which is mainly used to receive the GNSS signals transmitted in the line of sight. It can also receive the GNSS signals with a relatively low satellite elevation angle reflected by the soil at the same time, and superimpose with the direct signal to form an interference phenomenon. The GNSS antenna of the smart phone adopts a linear polarization design. Usually, when placed horizontally, it can receive the horizontal component of the right-handed circular polarized electromagnetic wave, and its power is 3 dB lower than that of the circular polarization. When the linear polarization is reflected by the reflecting surface, the polarity is not easily changed. Therefore, the smart phone can receive the linear polarization components of the reflected signals of almost all satellites with different elevation angles, which can make up for the defect that the circular polarization antenna cannot receive the reflected signals of high elevation angle satellites.

[0047] In traditional satellite signal reception, in addition to the direct signal propagating in the line of sight received by the receiver, there are also signals reflected through multiple paths, also known as multipath reflection signals. Although multipath signals can affect navigation and positioning, they have a certain correlation with the physical characteristics of the reflecting surface (such as soil moisture). For shore-based GNSS receivers, the frequencies of the direct and reflected signals received are almost the same, but due to their different propagation paths, there is a path delay for the reflected signal relative to the direct signal. When using the same antenna to receive the direct and reflected signals simultaneously, a relatively stable interference phenomenon will occur, forming an interference signal. Android smartphones record and store this interference signal in the device record in the form of the signal-to-noise ratio (SNR), and the storage form is Rinex. The change of the interference phenomenon depends on the reflection characteristics of the soil surface. Therefore, some important physical properties such as the soil moisture of the reflecting surface can be retrieved using this interference phenomenon. The mathematical model of the SNR of the interference signal between the direct signal and the reflected signal received by the receiver can be expressed by the following formula:

[0048]

[0049] Where, A d is the amplitude of the direct signal, A m is the amplitude of the reflected signal, is the phase difference between the direct signal and the reflected signal.

[0050] As Figure 1 and Figure 2 shown, a method for measuring soil moisture using an Android smartphone provided by the present invention first uses the linearly polarized navigation antenna of the Android smartphone to collect GNSS raw signal-to-noise ratio (SNR) data, then uses empirical mode decomposition (EMD) to extract different frequency components of the interference signal and separate the GNSS direct signal and the signal reflected by the ground, and then calculates the power reflectivity of the direct and reflected signals by calculating the peak power ratio, and finally uses an empirical model to calculate the soil moisture. Specifically, it includes:

[0051] Step 101: Obtain the original interference signal using an Android smartphone.

[0052] Place the Android smartphone horizontally, with the earpiece side facing the area to be measured, set the smartphone screen to be always on, and open the open-source software GEO++ to collect GNSS raw data. During the collection process, the phone screen needs to be kept always on to complete the collection of the original signal-to-noise ratio signal.

[0053] Before decomposing the original interference signal using the empirical mode decomposition algorithm to obtain different frequency components, it further includes: acquiring the elevation angle and azimuth angle information of different satellites; determining the observation range according to the measurement requirements; screening the original interference signal according to the observation range, the elevation angle, and the azimuth angle information to obtain the original interference signal of the satellites whose elevation angle and azimuth angle are both within the measurement range.

[0054] Select the data of the test area according to the satellite azimuth. Assume the direction of the observation area with respect to the smartphone is: (AZM min , AZM max ), where AZM min is the starting value of the azimuth and AZM max is the ending value of the azimuth. If the calculated satellite azimuth is within the above direction range, then select the data at this time for the processing of step 102.

[0055] Step 102: Decompose the original interference signal using the empirical mode decomposition algorithm to obtain different frequency components.

[0056] Use the empirical mode decomposition algorithm to separate different frequency components in the interference signal, where the DC component is the direct signal and other frequency components are the reflected signals, so as to obtain the direct and reflected signal components.

[0057] Let the signal-to-noise ratio sequence of the original interference signal be x(t), as shown in Figure 4 . Figure 4 (a) in it is the SNR diagram of GPS satellite No. 24; Figure 4 (b) in it is the SNR diagram of GPS satellite No. 15; Figure 4 (c) in it is the SNR diagram of GPS satellite No. 18; Figure 4 (d) in it is the SNR diagram of GPS satellite No. 5; Proceed as follows:

[0058] 1) Initialize r0(t) = x(t), i = 1; r0(t) is the initial signal sequence and i is the i-th operation.

[0059] 2) Let h j (t) = r i (t); h j (t) is the j-th fitting time series and r i (t) is the i-th signal sequence.

[0060] 3) Use cubic spline to fit the envelope lines e max (t) and e min (t) of the upper and lower extreme points of the time series h(t), as well as the average value m(t) of the upper and lower extreme envelopes.

[0061] 4) Subtract the average value m(t) from the time series h(t) to obtain a new time series h j+1 (t), and determine whether h j+1 (t) satisfies the conditions of the intrinsic mode component. If it satisfies, let h i (t) = h j+1 (t); otherwise, let h j (t) = h j+1 (t) and repeat steps 3) and 4).

[0062] 5) Subtract the i-th decomposed mode component h i (t) from the time series r i (t) to obtain the residual component r i+1 (t). Determine whether there are more than two extreme values in r i+1 (t). If so, repeat 2) - 5); otherwise, end the decomposition.

[0063] Step 103: Extract the direct signal and the reflected signal according to the original interference signal.

[0064] Step 104: Determine the vertical distance between the Android smartphone and the soil surface according to the different frequency components by using the LSP spectrum estimation algorithm. Calculate the spectral characteristics of the reflected signal by using the LSP spectrum estimation algorithm, and calculate the vertical distance between the smartphone and the soil surface.

[0065] Step 105: Screen the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smartphone to obtain the screened direct signal and the screened reflected signal.

[0066] Step 105 specifically includes: judging whether the absolute value of the difference between the vertical distance and the actual installation height of the Android smartphone is less than a set threshold to obtain a judgment result. If the judgment result is yes, use the direct signal as the screened direct signal and the reflected signal as the screened reflected signal. If the judgment result is no, remove the direct signal and the reflected signal, and return to the step of "obtaining the original interference signal by using the Android smartphone".

[0067] Compare the vertical distance with the actual installation height of the smartphone for satellite data quality control.

[0068] If the two satisfy the following conditions:

[0069] |h' - H| < 0.2m

[0070] where h' is the calculated vertical distance and H is the actual installation height of the smartphone. If the difference between the two is less than 0.2m, it is considered that the signal carried by the current data is the signal reflected by the measured area, that is, the data is available.

[0071] If the above relationship is not satisfied, it is considered that the signal carried by the data at this time is the signal reflected from other regions or the interference signal, that is, the data is discarded.

[0072] Step 106: Determine the peak power ratio by using the Hilbert transform method according to the screened direct signal and the screened reflected signal.

[0073] Step 106 specifically includes:

[0074] Perform Hilbert transform on the screened reflected signal to obtain the peak power of the reflected signal.

[0075] Construct an analytic signal according to the screened reflected signal and the peak power of the reflected signal.

[0076] Determine the peak of the reflected signal according to the analytic signal. The determining the peak of the reflected signal according to the analytic signal specifically includes: taking the absolute value of the analytic signal to obtain the envelope of the screened reflected signal; determining the peak of the reflected signal according to the peaks of multiple oscillation period signals in the envelope of the screened reflected signal.

[0077] Determine the peak power ratio according to the peak of the reflected signal and the screened direct signal. The determining the peak power ratio according to the peak of the reflected signal and the screened direct signal specifically includes: determining the peak power of the reflected signal according to the peak of the reflected signal; determining the peak power of the direct signal according to the screened direct signal; dividing the peak power of the reflected signal by the peak power of the direct signal to obtain the peak power ratio.

[0078] Calculate the peak power ratio of the direct and reflected signals in the screened data, and calculate the average value of four oscillation periods of the same satellite.

[0079] First, use the Hilbert transform to extract the peak power of the reflected signal. The specific steps are as follows:

[0080] Assume that the original reflected interference signal is:

[0081]

[0082] A(t) is the amplitude of the original interference signal, is the initial phase of the original interference signal, f is the carrier frequency of the GNSS signal electromagnetic wave, t is the time, w = 2πf, and w is the angular frequency.

[0083] Perform Hilbert transform on it. The signal after Hilbert transform is:

[0084]

[0085] H[·] is the symbol of Hilbert transform, and τ is the integration variable.

[0086] Its frequency characteristic is as follows:

[0087]

[0088] is the frequency characteristic of the original signal, X(jw) is the Fourier transform result of the original signal, j is the imaginary symbol, and H(jw) is the Hilbert transform operator.

[0089] Where:

[0090]

[0091] Construct an analytic signal Its real part is the original reflected signal, and its imaginary part is the signal after Hilbert transform:

[0092]

[0093] Finally, we can get:

[0094]

[0095] Where: e jwt is the complex carrier signal, is the complex envelope. Taking the absolute value of the analytic signal can obtain the envelope of the original signal.

[0096] Finally, take the average value of the peak values of 3 oscillation periods of the envelope signal of the original signal as the peak value of the reflected signal, and calculate the peak power ratio Γ:

[0097]

[0098] Where, A r is the peak power of the reflected signal, and A r is the peak power of the direct signal.

[0099] Step 107: Determine the soil humidity according to the peak power ratio. Use the peak power ratio as the input and calculate the soil humidity through a semi-empirical model.

[0100] The expression of the soil humidity is:

[0101]

[0102] Where, Γ is the peak power ratio, and m v is the soil humidity. The soil humidity formula of the present invention is an improved Topp empirical formula.

[0103] The present invention provides a method for measuring soil humidity using an Android smartphone, which can improve the spatial resolution, coverage rate, and at the same time reduce the storage space occupancy rate, and the device is easy to install.

[0104] The present invention also provides a system for applying a method of measuring soil humidity using an Android smartphone. The system includes an original signal-to-noise ratio acquisition module, a satellite information extraction module, a direct and reflected signal separation module, a spectrum estimation module, and a soil humidity inversion module. As Figure 3 shown, first, the original signal-to-noise ratio acquisition module uses the Android smartphone GNSS antenna to collect the original signal-to-noise ratio data and records it in the form of Rinex2.0; then, the satellite information extraction module jointly extracts the elevation angle and azimuth angle information of the satellite using the GNSS ephemeris information and the smartphone positioning information; then, the direct and reflected signal separation module uses empirical mode decomposition to separate different frequency components in the interference signal and extracts the DC component of the direct signal and the low-frequency component reflected by the soil; secondly, the spectrum estimation module uses the LSP (Lomb-Scargle Periodogram) spectrum estimation method to calculate the spectral characteristics of the reflected signal; finally, the soil humidity inversion module calculates the peak power ratio of the direct and reflected signals and uses an empirical model to calculate the true value of the soil humidity.

[0105] The original signal-to-noise ratio acquisition module first uses GEO++ software to collect the original signal-to-noise ratio data, stores it in the Rinex2.0 format, and transmits the data to the host computer through the USB-A interface.

[0106] The satellite information extraction module calculates the elevation angle and azimuth angle of each visible satellite through the collected GNSS data positioning information and GNSS ephemeris information.

[0107] The direct and reflected signal separation module uses the empirical mode decomposition algorithm to separate different frequency components in the interference signal and extracts the DC component of the direct signal and the low-frequency component reflected by the ground.

[0108] The spectrum estimation module uses the LSP spectrum estimation algorithm to estimate the spectrum of the reflected signal, and performs data quality control according to the deviation degree between the vertical distance from the smartphone to the soil surface calculated by the spectrum estimation and the actual measured distance.

[0109] The soil humidity inversion module first extracts the peak power of the direct and reflected signals respectively, calculates the power reflectivity of the soil to the GNSS signal through the peak power ratio of 3 oscillation periods. The soil humidity is calculated through the soil humidity inversion empirical model.

[0110] The present invention can improve the spatial resolution of soil humidity measurement, the coverage area of soil humidity measurement, reduce the storage space occupancy rate of data in the measurement device and the complexity of device installation.

[0111] In the present specification, the various embodiments are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the various embodiments, reference can be made to each other.

[0112] Specific examples are used herein to illustrate the principles and implementation manners of the present invention. The descriptions of the above embodiments are only for helping to understand the method and its core idea of the present invention. At the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation on the present invention.

Claims

1. A method for measuring soil humidity using an Android smartphone, characterized in that, Comprising: Obtaining an original interference signal by using an Android smart phone; Decomposing the original interference signal by using an empirical mode decomposition algorithm to obtain different frequency components; Extracting a direct signal and a reflected signal according to the original interference signal; Determining the vertical distance between the Android smart phone and the soil surface according to the different frequency components by using a Lomb-Scargle Periodogram spectral estimation algorithm; Screening the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smart phone to obtain a screened direct signal and a screened reflected signal; Determining a peak power ratio according to the screened direct signal and the screened reflected signal by using a Hilbert transform method; Determining soil humidity according to the peak power ratio; The screening the direct signal and the reflected signal according to the vertical distance and the actual installation height of the Android smart phone to obtain a screened direct signal and a screened reflected signal specifically includes: Judging whether the absolute value of the difference between the vertical distance and the actual installation height of the Android smart phone is less than a set threshold value to obtain a judgment result; If the judgment result is yes, taking the direct signal as the screened direct signal and taking the reflected signal as the screened reflected signal; If the judgment result is no, removing the direct signal and the reflected signal, and returning to the step of "obtaining an original interference signal by using an Android smart phone".

2. The method for measuring soil humidity using an Android smartphone according to claim 1, wherein Before the decomposing the original interference signal by using an empirical mode decomposition algorithm to obtain different frequency components, it further includes: Obtaining altitude angle and azimuth information of different satellites; Determining an observation range according to measurement requirements; Screening the original interference signal according to the observation range, the altitude angle and the azimuth information to obtain an original interference signal of a satellite whose altitude angle and azimuth are both within the measurement range.

3. The method for measuring soil humidity using an Android smartphone according to claim 1, wherein The determining a peak power ratio according to the screened direct signal and the screened reflected signal by using a Hilbert transform method specifically includes: Performing a Hilbert transform on the screened reflected signal to obtain a peak power of the reflected signal; Constructing an analytic signal according to the screened reflected signal and the peak power of the reflected signal; Determining a peak of the reflected signal according to the analytic signal; Determining a peak power ratio according to the peak of the reflected signal and the screened direct signal.

4. The method for measuring soil humidity using an Android smartphone according to claim 3, characterized in that, The determining a peak of the reflected signal according to the analytic signal specifically includes: Taking the absolute value of the analytic signal to obtain an envelope of the screened reflected signal; Determining a peak of the reflected signal according to peaks of multiple oscillatory cycle signals in the envelope of the screened reflected signal.

5. The method for measuring soil humidity using an Android smartphone according to claim 3, characterized in that, The determining a peak power ratio according to the peak of the reflected signal and the screened direct signal specifically includes: Determining a peak power of the reflected signal according to the peak of the reflected signal; Determining a peak power of the direct signal according to the screened direct signal; Dividing the peak power of the reflected signal by the peak power of the direct signal to obtain a peak power ratio.

6. The method for measuring soil humidity using an Android smartphone according to claim 1, characterized in that, The expression of the soil humidity is: Among them, Γ is the peak power ratio, and m v is the soil moisture content.

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