Unmanned aerial vehicle-borne GNSS-R sea surface height measurement method based on subcarriers

Abstract

The application provides a subcarrier-based unmanned aerial vehicle-borne GNSS-R sea surface height measurement method, and belongs to the technical field of satellite navigation and remote sensing height measurement. The method simultaneously receives direct signals and sea surface reflected signals of a global navigation satellite system (GNSS) on an unmanned aerial vehicle platform, performs double-frequency joint tracking processing on the direct signals to eliminate phase inconsistency errors introduced by independent tracking; on this basis, interference processing is performed on the direct signals and the reflected signals, a virtual subcarrier interference signal is constructed, high-frequency error components are suppressed through long-time coherent integration processing, and a continuous and stable virtual subcarrier interference phase is obtained; further, the geometric relationship among GNSS satellites, the unmanned aerial vehicle platform and the sea surface is combined to realize the inversion calculation of the sea surface height change. The method can effectively improve the precision and stability of GNSS-R sea surface height measurement in the unmanned aerial vehicle-borne dynamic environment, has the advantages of clear processing flow, strong noise resistance and suitability for implementation on a low-cost unmanned aerial vehicle platform.

Description

A Subcarrier-Based UAV-borne GNSS-R Sea Surface Altimetry Method Technical Field

[0001] This invention belongs to the high-tech field of satellite navigation and remote sensing, specifically relating to a technical method for measuring sea surface altitude by using the subcarrier phase of the Global Navigation Satellite System Reflected Signal (GNSS-R). Background Technology

[0002] Since its inception in the 1990s, Global Navigation Satellite System (GNSS) Reflected Signal (GNSS-R) technology has been widely applied in Earth observation, such as sea state monitoring, soil moisture measurement, vegetation analysis, snowfall assessment, and environmental parameter acquisition. GNSS-R's advantages lie in its low cost, all-weather capability, and high spatiotemporal resolution, making it a rapidly developing new remote sensing method in recent years. In sea surface altimetry, GNSS-R technology can effectively complement traditional tide gauges and radar satellite altimetry methods.

[0003] Existing GNSS-R altimeter techniques mainly include the signal-to-noise ratio (SNR) method, the code delay method, and the carrier phase method. The SNR method can achieve centimeter-level accuracy, but it requires interferometric signal sequences at low elevation angles, resulting in a long formation time and low temporal resolution. The code delay method supports real-time monitoring, but its accuracy is limited by the chip width and is generally lower than other methods. The carrier phase method can achieve higher accuracy, but in most cases, the phase of the GNSS reflected signal is difficult to track, and phase unwrapping and ambiguity resolution are also challenging. For observation methods using UAV platforms, the following unresolved technical challenges remain: First, the platform's dynamic nature leads to poor signal coherence; traditional single-frequency signals have short wavelengths and are highly susceptible to high-frequency errors such as platform vibration, leading to measurement failure. Second, when using dual-frequency signals for optimization, existing technologies often employ independent dual-frequency tracking strategies, ignoring the inherent correlation between the two frequencies and introducing additional phase inconsistency errors. This makes achieving high-precision, continuous, and stable sea surface altitude inversion on low-cost, highly dynamic UAV platforms a significant challenge for the industry. The aforementioned technical bottlenecks have largely limited the engineering application capabilities of GNSS-R sea surface altimetry in complex sea conditions, high-dynamic platforms, and refined spatiotemporal monitoring tasks. New technologies are urgently needed to overcome the bottlenecks of signal tracking instability and high-frequency error suppression, so as to support the further development of GNSS-R technology in high-precision, fast-response, and multi-platform sea surface altimetry applications. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a subcarrier-based UAV-borne GNSS-R sea surface altimetry method. This method solves the problems of phase inconsistency, high-frequency errors, and poor signal coherence in existing GNSS-R phase altimetry on dynamic platforms. Furthermore, by jointly tracking dual-frequency direct signals, constructing virtual subcarrier interference signals, and performing long-time coherent integration processing, this method achieves high-precision and stable sea surface altitude inversion, making it suitable for low-cost UAV platforms.

[0005] The present invention adopts the following solution:

[0006] A subcarrier-based UAV-borne GNSS-R sea surface altimetry method includes the following steps:

[0007] S1. Receive direct signals and sea surface reflected signals from the Global Navigation Satellite System (GNSS) on the UAV platform using both direct and reflected antennas. Perform down-conversion, synchronization, and quantization sampling on the received dual-frequency signals to obtain intermediate frequency data for the direct and reflected signals. Perform joint tracking processing on the dual-frequency direct signals in adjacent frequency bands, jointly estimate the code phase and carrier phase of the direct signals, and construct a code copy of the direct signals.

[0008] S2. Based on the code copy of the direct signal, perform time delay and Doppler compensation processing on the reflected signal, and perform interference processing on the compensated reflected signal and the corresponding direct signal to construct a dual-frequency interference signal. Combine the dual-frequency interference signal to construct a virtual subcarrier interference signal.

[0009] S3. Perform long-time coherent integration processing on the virtual subcarrier interference signal, with the integration time not less than a preset threshold. Perform phase unwrapping based on the obtained virtual subcarrier interference phase, and determine the phase ambiguity by combining the code measurement results of the corresponding frequency band. Combine the geometric relationship between the GNSS satellite, the UAV platform and the sea surface to invert the sea surface height change information.

[0010] Furthermore, in step S1, the specific method for joint tracking processing is as follows:

[0011] (1) Calculate the code tracking error for each frequency band using a normalized early and late amplitude discriminator;

[0012] (2) The overall code tracking error based on the power ratio of E5a and E5b signals is processed by weighted averaging, wherein the weighting parameters are determined according to the power ratio of the dual-frequency signals;

[0013] (3) Calculate the phase error using the phase discriminator of the phase-locked loop and combine it with the phase error of the dual-frequency signal;

[0014] (4) The Doppler frequency of E5b is calculated from the Doppler frequency of E5a according to the carrier frequency ratio. It is used to adjust the loop parameters for feedback, realize the effective joint tracking of the E5 direct signal, and construct a code copy of the direct signal by combining the tracking parameters of the direct signal.

[0015] Further, step S2, which involves constructing the dual-frequency interference signal and the virtual subcarrier interference signal, includes:

[0016] (1) Based on the code copy of the direct signal, the reflected signal is processed for time delay and Doppler compensation;

[0017] (2) Multiply the compensated reflected signal with the corresponding direct signal by complex conjugate multiplication to construct a dual-frequency interference signal:

[0018] ;

[0019] In the formula , and These are the components of the E5a and E5b interference signals, respectively; and This represents the power of the reflected signal. The time delay between the direct signal and the reflected signal; The frequency difference between the direct signal and the reflected signal can be approximately ignored in low dynamic scenes; The sinc function is related to the carrier wave. The cross-correlation function of the ranging code; This is the overall error; and This represents the interference phase between the direct signal and the reflected signal.

[0020] (3) By applying trigonometric function transformations to the in-phase and quadrature components of the dual-frequency interference signal, the corresponding components of the E5a and E5b interference signals are linearly weighted and combined to construct a virtual subcarrier interference signal:

[0021] ;

[0022] In the formula and These are the in-phase and quadrature components of the virtual subcarrier interference signal, respectively.

[0023] Furthermore, the long-time coherent integration processing and phase unwrapping processing involved in step S3 include:

[0024] (1) The virtual subcarrier interference signal is subjected to long-time coherent integration processing, and the integration is used as a low-pass filter. The integration time is proportional to the coherent gain.

[0025] (2) Use the arctangent function to extract the virtual subcarrier interference phase from the in-phase and quadrature components after integration;

[0026] (3) The extracted virtual subcarrier interference phase is unwrapped, and its long wavelength characteristics are used to detect and correct the phase period jump error in order to obtain continuous phase measurement;

[0027] (4) Using the code path delay measurement results of the corresponding frequency band, determine the ambiguity of the virtual subcarrier interference phase through calculation; calculate the path delay between the direct signal and the reflected signal based on the unwrapped and ambiguity determined virtual subcarrier interference phase measurement values.

[0028] The beneficial effects of this invention are as follows:

[0029] (1) By jointly tracking the dual-frequency direct signals of adjacent frequency bands, the phase inconsistency error introduced by independent tracking is effectively eliminated, and the stability and accuracy of signal tracking are improved. It is especially suitable for the application requirements of GNSS-R sea surface altimetry in dynamic environments of UAVs.

[0030] (2) Using virtual subcarrier interference signal construction and long-time coherent integration processing, taking advantage of its long wavelength of 19.5 meters, the path delay-related phase is converted into a low-frequency change, which facilitates phase unwrapping and rapid fixation of ambiguity. At the same time, it effectively suppresses high-frequency error components, improves the signal-to-noise ratio and phase continuity stability, and solves the limitations of traditional single-frequency carrier phase method which is susceptible to platform vibration and sea state interference.

[0031] (3) The present invention effectively overcomes the bottleneck of existing technologies in complex sea conditions and refined spatiotemporal monitoring tasks. The overall processing flow of the method is simple and efficient, and the anti-noise interference capability is strong. It is suitable for rapid deployment and implementation of low-cost, high-dynamic UAV platforms, and has higher engineering practicality and expansion potential. It can better support the application needs in fields such as marine environmental monitoring, tidal change detection and disaster early warning. Attached Figure Description

[0032] Figure 1 is a flowchart of the UAV-based sea surface altitude measurement algorithm based on GNSS-R subcarrier phase according to the present invention;

[0033] Figure 2 shows the GNSS-R altimeter geometry based on the UAV platform of this invention;

[0034] Figure 3 is a schematic diagram of the power spectral density and carrier frequency of E5a and E5b of the present invention, showing the signal structure of virtual subcarrier modulation;

[0035] Figure 4 is a comparison diagram of the path delay of the E5a code, E5b code and virtual subcarrier interferometric phase measurement of the present invention;

[0036] Figure 5 is a comparison diagram of the original subcarrier interferometric phase measurement, the 1-min moving average subcarrier interferometric phase measurement and the on-site tide level in this invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] The present invention will now be described in further detail with reference to the accompanying drawings:

[0039] This embodiment describes a subcarrier-based UAV-borne GNSS-R sea surface altitude measurement method. The embodiment was conducted on April 29, 2024, in Weihai City, Shandong Province (37°32′2.60″N, 122°2′44.06″E). The signal source was a Galileo E5 signal. The specific processing flow is shown in Figure 1. According to the specific circumstances of this embodiment, the method for UAV-borne subcarrier phase-based sea surface altitude measurement includes:

[0040] S1, GNSS signal reception and dual-frequency direct signal joint tracking processing:

[0041] (1) Use a quadcopter UAV platform with a flight altitude of about 50 meters. Equipped with a right-handed positioning antenna pointing upward (to receive direct signals) and a left-handed reflective antenna pointing downward (to receive reflected signals from the sea surface). The specific height measurement geometry is shown in Figure 2.

[0042] (2) Data post-processing was implemented using a software-defined receiver (20MHz intermediate frequency, 80MHz sampling frequency, 2-bit quantization). The received E5a and E5b signals were down-converted, synchronized, and quantized to obtain intermediate frequency data. The on-site tide level was provided by a 26GHz radar altimeter and GNSS, which were used as a comparison value to verify the accuracy of the altimeter results.

[0043] (3) To eliminate the phase inconsistency error introduced by independent dual-frequency tracking, it is necessary to perform joint tracking processing on the dual-frequency direct signals (E5a and E5b) in adjacent frequency bands. First, the code tracking error of each frequency band is calculated using a normalized early and late amplitude discriminator:

[0044] ;

[0045] In the formula, The code loop tracking error is represented by IE and QE, which are the in-phase and quadrature components of the leading branch, respectively, and IL and QL, which are the in-phase and quadrature components of the lagging branch, respectively.

[0046] (4) The overall code tracking error based on the power ratio of E5a and E5b signals is processed by weighted averaging, wherein the weighting parameters are determined according to the power ratio of the dual-frequency signals:

[0047] ;

[0048] In the formula, The total code tracking error is the result of joint processing of E5a and E5b. This is a weighting factor (valued at 0.5, based on the power ratio of E5a and E5b). and The code tracking errors for E5a and E5b are calculated separately, and the phase error is calculated using the phase discriminator of the phase-locked loop, combined with the phase error of the dual-frequency signals:

[0049] ;

[0050] In the formula This is the phase error;

[0051] (5) Using the Doppler frequency relationship between E5a and E5b signals:

[0052] ;

[0053] In the formula and The Doppler frequency shifts of the E5a and E5b signals are respectively. and The carrier frequencies of the E5a and E5b signals are used to adjust the loop parameters and achieve effective joint tracking of the E5 direct signal. The tracking parameters of the combined direct signal are used to construct a code copy of the direct signal.

[0054] S2. Reflection signal compensation, construction of dual-frequency interference signal and virtual subcarrier interference signal:

[0055] Based on code copies, time delay and Doppler compensation are applied to the reflected signal.

[0056] (1) Based on the code copy of the direct signal, the reflected signal is processed for time delay and Doppler compensation;

[0057] (2) Multiply the compensated reflected signal with the corresponding direct signal by complex conjugate multiplication to construct a dual-frequency interference signal:

[0058] ;

[0059] In the formula and These are the components of the E5a and E5b interference signals, respectively; and This represents the power of the reflected signal. The time delay between the direct signal and the reflected signal; The frequency difference between the direct signal and the reflected signal can be approximately ignored in low dynamic scenes; The sinc function is related to the carrier wave. The cross-correlation function of the ranging code; This is the overall error; and This represents the interference phase between the direct signal and the reflected signal.

[0060] (3) By applying trigonometric function transformations to the in-phase (I) and quadrature (Q) components of the dual-frequency interference signal, the corresponding components of the E5a and E5b interference signals are linearly weighted and combined to construct a virtual subcarrier interference signal:

[0061] ;

[0062] In the formula and These represent the in-phase and quadrature components of the virtual subcarrier interference signal, respectively. Figure 3 shows the spectral density of E5a and E5b and the signal structure of the virtual subcarrier modulation.

[0063] S3. Long-time coherent integral processing, virtual subcarrier interferometric phase extraction and sea surface height inversion calculation:

[0064] (1) Perform long-time coherent integration processing on the virtual subcarrier interference signal:

[0065] ;

[0066] In the formula, The integral time is the coherent integration time. This integral acts as a low-pass filter. The integration time is proportional to the coherent gain. The integration time is greater than one carrier period of a single-frequency signal (set to 1000ms in the experiment to match the sampling rate of the field data). At the same time, the low-frequency path delay-related phase components are preserved, thereby improving the signal-to-noise ratio.

[0067] (2) Extract the virtual subcarrier interference phase from the integrated in-phase and quadrature components using the arctangent function:

[0068] ;

[0069] (3) Perform phase unwrapping on the extracted virtual subcarrier interference phase:

[0070] ;

[0071] In the formula, This represents the virtual subcarrier interference phase value after unwrapping. The phase unwrapping function utilizes the long wavelength characteristics of the subcarrier to detect and correct phase period jump errors in order to obtain continuous phase measurements.

[0072] (4) Using the code path delay measurement results of the corresponding frequency band, the ambiguity of the virtual subcarrier interference phase is determined by calculation:

[0073] ;

[0074] In the formula, For integer ambiguity, Path delay measured for E5a code;

[0075] (5) Calculate the path delay between the direct signal and the reflected signal based on the virtual subcarrier interferometric phase measurements with unwrapping and ambiguity determined:

[0076] ;

[0077] In the formula, Figure 4 shows a comparison of the path delay between the direct signal and the reflected signal, using E5a code, E5b code, and virtual subcarrier interferometric phase measurement. The subcarrier path delay has higher accuracy.

[0078] (6) Combining path delay, subcarrier wavelength, and the elevation of the UAV positioning antenna ( Provided by GNSS RTK, with an accuracy of 5cm), GNSS satellite elevation angle. Unit vector between receiver and satellite and the baseline vector between the direct antenna and the reflective antenna. Calculate sea level height (SSH):

[0079] ;

[0080] In low-dynamic scenarios, the Doppler frequency shift difference can be approximately ignored, and the ionospheric and tropospheric errors are greatly eliminated through differential operations. The error below the positioning antenna caused by water surface evaporation is not discussed in this work. The original virtual subcarrier interferometric phase altimetry results have residual high-frequency errors, which can be further suppressed by 1-min moving average filtering. The results show good consistency with the on-site tide level (see Figure 5).

[0081] S4. Results of the Example:

[0082] This embodiment processes GNSS data from Galileo (E27, E30, E34) satellites. The sea surface height determined through the above steps is compared with the actual tide level measured using a 26GHz radar altimeter to verify the reliability of the altimeter results. The virtual subcarrier interferometric phase path delay is compared with the path delays of E5a and E5b codes. The root mean square error (RMSE) of E5a code is 0.94m, and that of E5b code is 0.65m, with a subcarrier phase of 0.21m, demonstrating that the subcarrier method is more accurate than traditional code-delay altimeter measurements. The original 1-second subcarrier phase altimeter result has residual high-frequency errors; after 1-min moving average filtering, it is consistent with the actual tide level. After processing multiple satellites, the RMSE between the subcarrier phase altimeter and the actual tide level is better than 10cm.

[0083] In summary, the UAV-borne GNSS-R sea surface altimetry method designed in this invention, which combines tracking dual-frequency signals and virtual subcarrier interferometric signals, utilizes GNSS satellites as the signal source to achieve real-time, high-precision sea surface height monitoring in dynamic environments. Furthermore, the results show good correlation with on-site tide data. This invention overcomes the shortcomings of traditional GNSS-R altimetry methods on UAV platforms, such as poor signal coherence, severe high-frequency interference, and low temporal resolution. It can promote the application of GNSS-R technology and plays an important role in fields such as tide level change detection, marine environmental monitoring, and disaster early warning. The invention also features a clear processing flow, strong noise resistance, low power consumption, and the flexibility derived from the software-defined receiver allows for further modifications based on different application scenarios.

[0084] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A subcarrier-based UAV-borne GNSS-R sea surface altimeter measurement method, characterized in that, Includes the following steps: S1. Receive the direct signal and sea surface reflected signal of the Global Navigation Satellite System (GNSS) on the UAV platform through the direct antenna and the reflected antenna respectively, and perform down-conversion, synchronization and quantization sampling processing on the received dual-frequency signals to obtain the intermediate frequency data of the direct signal and the reflected signal; perform joint tracking processing on the dual-frequency direct signals of adjacent frequency bands, jointly estimate the code phase and carrier phase of the direct signal, and construct a code copy of the direct signal; the specific method of joint tracking processing is as follows: (1) Use a normalized early and late amplitude discriminator to calculate the code tracking error of each frequency band; (2) Process the overall code tracking error based on the power ratio of E5a and E5b signals by weighted averaging, where the weight parameters are determined according to the power ratio of the dual-frequency signals; (3) Use the phase discriminator of the phase-locked loop to calculate the phase error, and combine it with the phase error of the dual-frequency signals; (4) The Doppler frequency of E5b is calculated from the Doppler frequency of E5a according to the carrier frequency ratio, and is used to adjust the loop parameters to achieve effective joint tracking of the E5 direct signal. The tracking parameters of the direct signal are combined to construct a code copy of the direct signal; S2. Based on the code copy of the direct signal obtained in step S1, the reflected signal is subjected to time delay and Doppler compensation processing, and the compensated reflected signal is subjected to interference processing with the corresponding direct signal to construct a dual-frequency interference signal. The dual-frequency interference signal is combined to construct a virtual subcarrier interference signal; wherein, constructing the dual-frequency interference signal and the virtual subcarrier interference signal includes: (1) Based on the code copy of the direct signal, the reflected signal is subjected to time delay and Doppler compensation processing; (2) The compensated reflected signal is subjected to complex conjugate multiplication with the corresponding direct signal to construct a dual-frequency interference signal: In the formula 、 and 、 These are the components of the E5a and E5b interference signals, respectively; and This represents the power of the reflected signal. The time delay between the direct signal and the reflected signal; The frequency difference between the direct signal and the reflected signal can be approximately ignored in low dynamic scenes; The sinc function is related to the carrier wave. The cross-correlation function of the ranging code; This is the overall error; and (3) By applying trigonometric function transformation to the in-phase and quadrature components of the dual-frequency interference signal, the corresponding components of the E5a and E5b interference signals are linearly weighted and combined to construct a virtual subcarrier interference signal: In the formula and These are the in-phase and quadrature components of the virtual subcarrier interference signal, respectively; S3, perform long-time coherent integration processing on the virtual subcarrier interference signal obtained in step S2, with the integration time not less than a preset threshold, perform phase unwrapping based on the obtained virtual subcarrier interference phase, and determine the phase ambiguity by combining the code measurement results of the corresponding frequency band. Combined with the geometric relationship between the GNSS satellite, the UAV platform and the sea surface, the sea surface height change information is inverted.

2. The method for UAV-borne GNSS-R sea surface altimetry based on subcarrier as described in claim 1, characterized in that, The long-time coherent integration and phase unwrapping process described in step S3 includes: (1) performing long-time coherent integration on the virtual subcarrier interference signal, where the integration is used as a low-pass filter and the integration time is proportional to the coherent gain; (2) extracting the virtual subcarrier interference phase from the in-phase and quadrature components after integration using the arctangent function; (3) unwrapping the extracted virtual subcarrier interference phase and using its long-wavelength characteristics to detect and correct the phase period jump error to obtain continuous phase measurement; (4) using the code path delay measurement results of the corresponding frequency band to determine the ambiguity of the virtual subcarrier interference phase through calculation; and calculating the path delay between the direct signal and the reflected signal based on the unwrapped and ambiguity-determined virtual subcarrier interference phase measurement values.

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