A method for measuring water level using low-orbit satellite navigation signal interference

Abstract

The present invention discloses a method for measuring water level by interference of low-orbit satellite navigation signals, and belongs to the field of low-orbit satellite remote sensing measurement and hydrological monitoring. The method first uses a high-gain antenna to receive direct signals and reflected signals emitted by low-orbit satellites, obtains SNR time series, and uses the isolation forest method to eliminate abnormal values ​​of the time series; then uses the L‑S spectrum analysis method to calculate the water level height of each satellite and each frequency point according to the interference fringe frequency of the direct signal and the reflected signal, and finally uses a multi-band, multi-satellite data fusion algorithm to improve the water level measurement accuracy. The present invention utilizes the advantages of low-orbit satellite navigation signals and the dense coverage of multiple satellites to solve the accuracy and spatiotemporal resolution problems of traditional GNSS‑IR in water surface height measurement, and extracts the water level height by performing time series analysis on the measurement data in a continuous time period, thereby providing real-time or quasi-real-time water level monitoring capabilities.

Description

Technical Field

[0001] The invention belongs to the field of low-orbit satellite remote sensing measurement and hydrological monitoring, and in particular relates to a method for measuring water level by interferometry of low-orbit satellite navigation signals. Background Art

[0002] At present, GNSS Interferometric Reflectometry (GNSS-IR) technology has made some progress in the measurement of soil moisture, snow thickness and water level. However, due to the low power and frequency of GNSS signals and the limitation of the number of satellites, it has problems of low signal strength, low accuracy and low measurement time resolution in water level measurement. With the large-scale deployment of low-orbit satellite constellations, its higher frequency, stronger ground signal power, high-density satellite network and fast operation speed have provided new possibilities for water level measurement.

[0003] Since the orbital altitude of low-orbit satellites is low, the spatial attenuation of signals is smaller, and the landing power is more than 10dB higher than the GNSS signal on average, which will greatly increase the power of reflected signals. The interference caused by direct signals and reflected signals will be more obvious, which is more conducive to the inversion measurement of water level, soil moisture, snow thickness, etc. Low-orbit satellite constellations are generally large in number and run at a fast speed. Observation data from multiple low-elevation satellites can be collected in the same time period, providing an important basis for short-term near-real-time high-precision measurement of water levels. In addition, low-orbit satellites have a higher signal frequency, which is more conducive to identifying small-scale water level changes. Despite the above advantages, there is no public solution in the prior art for measuring water levels using navigation signals from low-orbit satellites. Summary of the invention

[0004] In view of this, the present invention proposes a method for measuring water level by interferometry of low-orbit satellite navigation signals. The present invention is based on the interference phenomenon between the direct signal of low-orbit satellite navigation and the reflected signal on the water surface, and measures the water level by analyzing the signal-to-noise ratio (SNR) frequency of the low-orbit satellite navigation signal, thereby improving the accuracy and reliability of water level measurement.

[0005] The technical solution adopted by the present invention is:

[0006] A method for measuring water level by interferometry of low-orbit satellite navigation signals, comprising the following steps:

[0007] Step 1, receiving a direct signal from a low-orbit satellite and a water surface reflection signal by a receiver device;

[0008] Step 2: determine the Fresnel reflection area according to the receiver position, calculate the satellite elevation angle range, use polynomial fitting to eliminate the trend term of the direct signal, extract the signal-to-noise ratio change of the signal, and analyze the interference fringe frequency;

[0009] Step 3, use the isolation forest method to remove outliers with signal-to-noise ratio changes;

[0010] Step 4: Calculate the water surface height based on the interference fringe frequency and satellite altitude angle range using the LS spectrum analysis method:

[0011]

[0012] Where, H is the water surface height, H antenna is the height of the antenna phase center, f is the main frequency component of the interference fringes, that is, the frequency of the signal-to-noise ratio change, and λ is the wavelength of the signal.

[0013] A method for measuring water level by interferometry of low-orbit satellite navigation signals, comprising the following steps:

[0014] Step 1, receiving direct signals and water surface reflection signals of different frequency bands from different low-orbit satellites through a receiver device;

[0015] Step 2: determine the Fresnel reflection area according to the receiver position, calculate the satellite elevation angle range, use polynomial fitting to eliminate the trend term of the direct signal, extract the signal-to-noise ratio change of the signal, and analyze the interference fringe frequency;

[0016] Step 3, use the isolation forest method to remove outliers with signal-to-noise ratio changes;

[0017] Step 4: Use the LS spectrum analysis method to calculate the water surface height measured by each satellite signal in each frequency band according to the interference fringe frequency and the satellite altitude angle range:

[0018]

[0019] Where, H is the water surface height, H antenna is the height of the antenna phase center, f is the main frequency component of the interference fringes, that is, the frequency of the signal-to-noise ratio change, and λ is the wavelength of the signal;

[0020] Step 5, calculate the weights of different frequency bands of the same satellite:

[0021]

[0022] Among them, w i is the weight of the i-th frequency band, SNR i is the average signal-to-noise ratio of the ith frequency band, σ i is the signal measurement error of the i-th frequency band;

[0023] Step 6, calculate the weights of different satellites:

[0024]

[0025] Among them, w k is the weight of the kth satellite, θ k is the standard deviation of the elevation angle of the kth satellite, M is the number of received satellites, is the average elevation angle within the satellite arc, θ i is the instantaneous elevation angle within the satellite arc segment, and N is the number of elevation angle samples within the satellite arc segment;

[0026] Step 7: Take the weighted average of the water level heights obtained from different satellites and different frequency bands to obtain the final water level height:

[0027]

[0028] Among them, N is the satellite signal frequency format, M is the number of received satellites, The final water level measurement result.

[0029] Compared with the existing GNSS-IR technology, the present invention has the following advantages:

[0030] 1. Stronger signal power: Low-orbit satellite navigation signals are stronger than traditional GNSS signals. The signal-to-noise ratio change caused by the reflected signal is more obvious, which is more conducive to extracting dSNR frequency and improving the sensitivity of water level measurement.

[0031] 2. Higher frequency: Low-orbit satellite navigation signals have high-frequency data (Ka band), which can reflect small changes in water level more finely and are suitable for high-resolution water level monitoring.

[0032] 3. Advantages of low-elevation-angle reception: In the future, there will be a large number of low-orbit satellites with wide distribution, providing more opportunities for low-elevation-angle reception. It is more suitable for receiving reflected signals under low-elevation-angle conditions and optimizing water level measurement effects.

[0033] 4. High temporal and spatial resolution: The high-density distribution of low-orbit satellites can achieve high-frequency and continuous water level monitoring, meeting the needs of real-time dynamic hydrological monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the principle of water level measurement in an embodiment of the present invention.

[0035] Figure 2 1 is a flow chart of water level measurement in an embodiment of the present invention.

[0036] Figure 3 It is the dSNR time series diagram. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] A method for measuring water level through interference of low-orbit satellite navigation signals. The method receives navigation signals transmitted by low-orbit satellites and signals reflected from the water surface, and uses the advantages of high landing power and strong reflected signals of low-orbit satellite navigation signals and high operating speed of low-orbit satellites to calculate the water level through the interference phenomenon of direct and reflected signals.

[0039] like Figure 1 As shown in the figure, the path for the low-orbit satellite signal to reach the ground receiver includes: the signal path directly transmitted from the satellite to the receiver (direct signal path) and the path for the satellite signal to reach the receiver after being reflected by the water surface (reflected signal path). The change in water level will cause the length of the reflection path to change, thereby affecting the frequency of the interference fringes. To this end, the high power and high frequency of the low-orbit satellite landing signal, as well as the advantages of the fast speed and large number of satellites, can be used to improve the accuracy and reliability of water level measurement.

[0040] See also Figure 1 and Figure 2 , the method comprises the following steps:

[0041] Step 1: Receive direct signals and water surface reflection signals from low-orbit satellites, and store SNR observations with path difference information of direct and reflected signals.

[0042] In order to achieve high-precision reception and processing of low-orbit satellite navigation signals, ground receiving equipment must have functions such as high-gain reception capability, frequency filtering, signal demodulation, and interference signal extraction. Specifically, it includes:

[0043] High-gain antenna: In order to effectively receive low-orbit satellite signals, the receiver is equipped with a high-gain antenna to ensure sufficient signal strength. The antenna should be designed to be multi-band so that it can receive signals from different low-orbit satellite frequency bands (such as Ku / Ka bands).

[0044] Multi-band filter: used to separate the required frequency range, filter out irrelevant frequencies, and reduce interference signals. Multi-band filter ensures that the device can receive low-orbit satellite signals in a specific frequency band and filter out possible interference signals.

[0045] Signal demodulation module: designed for different modulation methods of low-orbit satellite signals, demodulates the original signal containing water level interference characteristics. The demodulation module needs to support a higher sampling rate to ensure the time domain integrity of the signal.

[0046] Interference signal: The receiver captures the signal and continuously tracks it, outputs the SNR observation and stores it. The satellite elevation angle range in the Fresnel reflection area is calculated based on the receiver position and rough altitude, and the SNR data within the satellite elevation angle range is selected. The trend term of the direct signal is eliminated by quadratic polynomial fitting, and the signal-to-noise ratio change (dSNR) of the signal is extracted to obtain the signal dSNR time series.

[0047] Step 2: Calculate the satellite elevation angle range in the Fresnel reflection area according to the receiver position and rough altitude, use quadratic polynomial fitting to eliminate the trend term of the direct signal, extract the signal-to-noise ratio change (dSNR), and obtain the signal dSNR time series.

[0048] Step 3: Use the isolation forest method to remove outliers from the dSNR time series of each satellite and each frequency point, and regularly update the isolation forest model to ensure that the model can accurately capture the dSNR changes caused by different weather and environmental changes. Adjust the anomaly score threshold according to the measurement environment and system requirements to optimize the sensitivity of anomaly detection and balance detection accuracy.

[0049] Step 4: Calculate the water level of each satellite and each frequency point using the LS spectrum analysis method based on the interference fringe frequency and the satellite elevation angle. The formula is as follows:

[0050]

[0051] Where, H is the water surface height, H antenna is the antenna phase center height, f is the main frequency component of the interference fringes, that is, the frequency of dSNR, and λ is the wavelength of the navigation signal.

[0052] Step 5: Due to the large number of low-orbit satellites, dense orbit distribution and strong signals at low elevation angles, the observation data of multiple satellites can be fused to further improve the measurement accuracy and spatial coverage. The weights of different frequencies of the same satellite are calculated according to the following formula:

[0053]

[0054] Among them, w i is the weight of the i-th frequency band, SNR i is the average signal-to-noise ratio of the ith frequency band, σ i is the signal measurement error in the i-th frequency band.

[0055] Step 6: For the dSNR data of different satellites, the weights of different satellites are calculated based on the standard deviation of the elevation angle of the satellite ascending or descending orbit. The formula is:

[0056]

[0057] Among them, w k is the weight of the kth satellite, θ k is the standard deviation of the elevation angle of the kth satellite, M is the number of satellites received, is the average elevation angle within a certain satellite arc segment, θ i is the instantaneous elevation angle within a certain satellite arc segment, and N is the number of elevation angle samples within the satellite arc segment.

[0058] Step 7: Combine the weights of multiple frequency points and the weights of different satellites to calculate the water level height and further optimize the water surface height measurement results:

[0059]

[0060] Among them, N is the satellite signal frequency format, M is the number of received satellites, This is the final refined water level measurement result obtained.

[0061] Here is a more specific example:

[0062] A method for measuring water level by interferometry of low-orbit satellite navigation signals, which includes the main links of receiving signal processing, outlier removal, multi-satellite multi-frequency fusion analysis and water level height calculation. Figure 2 As shown, the following steps are included:

[0063] Step 1: Use a receiving device equipped with a high-gain antenna, a multi-band filter, a signal demodulation module, etc. to receive low-orbit satellite navigation signals at the lakeside or seaside to be tested. The high-gain antenna is aimed at the Fresnel reflection area of ​​the water surface to enhance the receiving power of the reflected signal, and the obtained SNR observation value is stored in a file.

[0064] Step 2: Calculate the satellite elevation angle range within the Fresnel reflection zone on the water surface according to the receiver position and rough altitude, then remove the SNR observation data that is no longer within the elevation angle range, use quadratic polynomial fitting to remove the trend term of the SNR observation data, extract the signal-to-noise ratio change (dSNR), and obtain the signal dSNR time series;

[0065] Step 3: Use the isolation forest method to remove outliers from the dSNR time series of each satellite and each frequency point; perform outlier detection on the obtained dSNR time series, generate an anomaly score ([0,1]) for each data point, and set a threshold (0.5). Points with scores higher than the threshold are considered to be outliers, and are removed to generate a "clean" time series. The isolation forest model is updated regularly to ensure that the model can accurately capture the changes in dSNR caused by different weather and environmental changes.

[0066] Step 4: Use LS spectrum analysis to calculate the water level of each satellite and each frequency point according to the interference fringe frequency and satellite elevation angle according to the following formula:

[0067]

[0068] The interference fringe frequency is Figure 3 shown.

[0069] Step 5: At this time, each satellite, each frequency point, and each arc segment will obtain the water level height calculation result, and the calculation results of different frequency points are weighted according to the following formula:

[0070]

[0071] Step 6: weight the water level calculation results obtained by different satellites according to the following formula:

[0072]

[0073] Step 7: Combine the two types of weights to obtain the refined high-precision water level measurement results after integrating satellites and frequencies:

[0074]

[0075] In summary, the present invention utilizes the advantages of low-orbit satellite navigation signals and dense coverage of multiple satellites to calculate the water level height through the SNR frequency of the received signal. The present invention adopts multi-band fusion and multi-satellite fusion methods, and improves the measurement accuracy through a weighted average algorithm, solving the accuracy and spatiotemporal resolution problems of traditional GNSS-IR in water surface height measurement. By utilizing the higher spatiotemporal resolution of low-orbit satellite signals, dynamic changes in water level height can be monitored. By performing time series analysis on the measurement data within a continuous time period, the changing trend of the water level height is extracted, thereby providing real-time or quasi-real-time water level monitoring capabilities. This method can be applied to height monitoring of oceans, lakes, rivers and other water areas, providing new technical support for hydrological monitoring and environmental research.

Claims

1. A method for measuring water level using low-orbit satellite navigation signal interference, characterized in that: The following steps are involved: Step 1, receiving a direct signal from a low-orbit satellite and a water surface reflection signal by a receiver device; Step 2: determine the Fresnel reflection area according to the receiver position, calculate the satellite elevation angle range, use polynomial fitting to eliminate the trend term of the direct signal, extract the signal-to-noise ratio change of the signal, and analyze the interference fringe frequency; Step 3, use the isolation forest method to remove outliers with signal-to-noise ratio changes; Step 4: Calculate the water surface height based on the interference fringe frequency and satellite altitude angle range using the LS spectrum analysis method: Where, H is the water surface height, H antenna is the height of the antenna phase center, f is the main frequency component of the interference fringes, that is, the frequency of the signal-to-noise ratio change, and λ is the wavelength of the signal.

2. A method for measuring water level using low-orbit satellite navigation signal interference, characterized in that: The following steps are involved: Step 1, receiving direct signals and water surface reflection signals of different frequency bands from different low-orbit satellites through a receiver device; Step 2: determine the Fresnel reflection area according to the receiver position, calculate the satellite elevation angle range, use polynomial fitting to eliminate the trend term of the direct signal, extract the signal-to-noise ratio change of the signal, and analyze the interference fringe frequency; Step 3, use the isolation forest method to remove outliers with signal-to-noise ratio changes; Step 4: Use the LS spectrum analysis method to calculate the water surface height measured by each satellite signal in each frequency band according to the interference fringe frequency and the satellite altitude angle range: Where, H is the water surface height, H antenna is the height of the antenna phase center, f is the main frequency component of the interference fringes, that is, the frequency of the signal-to-noise ratio change, and λ is the wavelength of the signal; Step 5, calculate the weights of different frequency bands of the same satellite: Among them, w i is the weight of the i-th frequency band, SNR i is the average signal-to-noise ratio of the ith frequency band, σ i is the signal measurement error of the i-th frequency band; Step 6, calculate the weights of different satellites: Among them, w k is the weight of the kth satellite, θ k is the standard deviation of the elevation angle of the kth satellite, M is the number of received satellites, is the average elevation angle within the satellite arc, θ i is the instantaneous elevation angle within the satellite arc segment, and N is the number of elevation angle samples within the satellite arc segment; Step 7: Take the weighted average of the water level heights obtained from different satellites and different frequency bands to obtain the final water level height: Among them, N is the satellite signal frequency format, M is the number of received satellites, The final water level measurement result.

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