CN116699658A - GNSS-R carrier phase sea surface height measurement method based on double-frequency reflection signal combination - Google Patents
GNSS-R carrier phase sea surface height measurement method based on double-frequency reflection signal combination Download PDFInfo
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Abstract
The invention provides a GNSS-R carrier phase sea surface height measurement method based on double-frequency reflection signal combination, which belongs to the technical field of intersection such as satellite height measurement science, ocean surveying science and the like, and comprises the following steps: processing the direct signal, establishing a reference signal, generating a reflected signal waveform, extracting complex correlation results, extracting dual-frequency combined interference phases, and inverting the carrier phase sea surface height. Compared with the existing GNSS-R sea surface height measurement, the method provided by the invention adopts a direct signal assisted reflected signal open loop tracking method, and can acquire multiple kinds of observed values including pseudo codes and carrier phases; the method mainly comprises the step of acquiring a carrier phase observation value in a higher satellite altitude angle range when the sea surface is calm; when the sea condition is large, the carrier phase observation value can be effectively extracted and recovered under a low altitude angle, and a high-precision sea surface height measurement result can be obtained.
Description
Technical Field
The invention belongs to the technical field of intersection such as satellite altimetry and marine surveying and particularly relates to a GNSS-R carrier phase sea surface altimetry method based on double-frequency reflection signal combination.
Background
Mart I n-Neira et al of European space office originally proposed the concept of dual-antenna GNSS-R sea height measurement, and realized the sea height and longitude of the rice pole by using GPS L1 ranging code. The sea surface height measurement accuracy of decimeter level is realized by using B1C and B2a signals with larger bandwidth and code rate by the Shandong university Gao Fan and the like. In contrast, higher accuracy is obtained with coastal altimetry using carrier phase. In GNSS-R sea level altimetry, the "Rayleigh criterion" is widely used to distinguish between smooth and rough surfaces. According to this law, the actual roughness observed at low elevation angles is effectively reduced. Under the condition of low incident angle of GNSS reflected signals, the Cardelllach and the like realize satellite-borne carrier phase height measurement, and the precision is 4.1 cm when sampling is performed at 20 Hz; when 1Hz is sampled, the precision is in the centimeter level, which is equivalent to the special radar altimeter. However, for signal combining, this statement does not hold. Only when the individual wavelengths of L1 and L2 are available, a height measurement accuracy of 1cm can be achieved in calm pond experiments. The carrier phase measurements in the experiments were generated separately by tracking the L1 and L2 reflected signals, which is difficult to achieve in severe sea conditions. The Nguyen et al respectively adopts double-frequency phase observation values to measure the sea skimming reflection data observed by the Spire satellite, and the height measurement precision is 3 cm. At present, most carrier phase height measurement needs strict coherent conditions, the influence of sea surface roughness on continuous tracking of carrier phase of GNSS reflected signals is reduced by using low elevation angle GNSS signals, and wind and waves are lower than 6m/s and 1.5m effective wave height, so that the application field of the carrier phase height measurement is greatly limited.
In general, sea surface altimetry using carrier phase can achieve very high accuracy, but is currently mainly observed at calm sea surface low sweep angles. Although partial researches acquire double-frequency observation values, a double-frequency reflection signal combination method is not adopted to improve the practical application range, and how to realize higher satellite altitude angles and GNSS-R carrier phase height measurement under high sea conditions is an important difficulty for restricting the practical application.
Disclosure of Invention
The invention mainly solves the problem that when the altitude angle of the GNSS satellite is higher or the sea state of the sea is worse, the GNSS-R carrier phase can not realize sea height measurement. The method utilizes the correlation between the double-frequency GNSS reflected signals, and combines the double-frequency reflected signals, thereby realizing the effective recovery and extraction of the interference carrier phase observation value, and simultaneously allowing longer coherent integration time length so as to obtain the high-precision GNSS-R carrier phase sea surface height measurement result.
In order to achieve the above object, the present invention provides the following technical solutions:
a method for performing sea surface altimetry on a GNSS-R carrier phase based on double-frequency reflected signal combination, the method comprising the following steps:
step one: processing the direct signal;
step two: establishing a reference signal;
step three: generating a reflected signal waveform and extracting complex correlation results;
step four: extracting a double-frequency combined interference phase;
step five: carrier phase sea level altitude inversion.
Further, the "processing direct signal" in the first step is specifically implemented as follows:
(1) Receiving GNSS direct signals by using an upward-looking right-handed antenna, receiving GNSS reflected signals by using a downward-looking left-handed antenna, and performing down-conversion and sampling quantization;
(2) Meanwhile, processing the dual-frequency direct signal, and acquiring code phase, carrier frequency, carrier phase and navigation message information of the dual-frequency direct signal by adopting a PLL and a DLL third-order tracking loop;
(3) Positioning by utilizing a plurality of GNSS satellites to obtain the accurate position and the geodetic altitude of a receiver, and calculating the altitude angle and the azimuth angle of the GNSS satellites;
(4) And setting an azimuth angle range according to the installation position of the down-looking antenna, selecting the satellite signals which can be effectively received and reflected by the sea surface, marking and carrying out the next processing.
Further, the "establishing a reference signal" in the second step is specifically implemented as follows:
(1) Estimating the position of the specular reflection point by using GNSS satellite ephemeris and a receiver positioning result;
(2) Calculating a pseudo code path difference between the direct signal and the reflected signal according to the specular reflection point position:
(3) According to the geometric difference of the direct signal and the reflected signal, synchronizing the direct signal and the reflected signal, and establishing a local reference signal;
(4) And correlating the local reference signal with the reflected signal to realize accurate tracking of the carrier wave of the reflected signal.
Further, the "reflected signal waveform generation and complex correlation result extraction" described in the third step is specifically implemented as follows:
(1) Taking the frequency of a reference signal as the carrier frequency, the carrier phase of the reference signal as the carrier phase, and the code phase as the center, respectively taking two chip range windows in front and back, taking 0.1 chip as the step length in the windows, generating a signal set, and correlating with the reflected signal which is tracked in the step two to generate a reflected signal waveform:
(2) Obtaining the position of a specular reflection point on the waveform of the reflected signal by adopting a first derivative method;
(3) And selecting a sampling point of the waveform front edge closest to the specular reflection point as a reference point, and extracting in-phase and quadrature components of the interference complex field of the reflected signals at the waveform reference points of different frequency points.
Further, the "dual-frequency combined interference phase extraction" described in the fourth step is specifically implemented as follows:
(1) Combining the reflected signals;
(2) The signal to noise ratio is improved by long-time coherent integration;
(3) Extracting a carrier phase observation value of the combined signal by adopting a four-quadrant arctangent function;
(4) And calculating root mean square no difference of the carrier phase observation values, detecting and eliminating the coarse difference, unwrapping the carrier phase observation values, and obtaining continuous carrier phase observation values.
Further, the carrier phase sea surface altitude inversion in the fifth step is implemented as follows:
(1) Calculating integer ambiguity by using the pseudo code path delay obtained in the second step and the carrier delay obtained in the fourth step;
(2) And calculating the sea surface height through the satellite altitude angle in the first step.
The beneficial effects of the invention are as follows:
(1) The method comprises the steps of firstly adopting an open loop tracking method to a reflected signal according to the relation between a direct signal and the reflected signal, ensuring carrier phase tracking precision, selecting a proper reference point on a reflected signal waveform, and controlling the range of a sea surface reflected signal area; the method comprises the steps of performing broadband combination on the double-frequency reflected signals by adopting a signal combination method, and weakening the error generated by sea surface roughness by utilizing the correlation of the reflected signals among different frequency points in the same area, so that the performance of carrier phase height measurement under high sea surface roughness is improved, and the usable satellite altitude angle range under the condition of calm sea surface is improved;
(2) Compared with the existing GNSS-R sea surface height measurement, the method provided by the invention adopts a direct signal assisted reflected signal open loop tracking method, and can acquire multiple kinds of observed values including pseudo codes and carrier phases: the method mainly comprises the step of acquiring a carrier phase observation value in a higher satellite altitude angle range when the sea surface is calm; when the sea condition is large, the carrier phase observation value can be effectively extracted and recovered under a low altitude angle, and a high-precision sea surface height measurement result can be obtained.
Drawings
FIG. 1 is a schematic overall flow diagram of the method of the present invention;
FIG. 2 is a specific flow chart of steps one to three in the method of the present invention;
FIG. 3 is a flowchart showing steps four to five in the method of the present invention;
FIG. 4 is a schematic diagram of direct signal tracking in an embodiment of the present invention, where (a) is the L1 and L5 frequency point code phase tracking error, (b) is the L1 and L5 frequency point carrier phase tracking error, (c) is the L5 frequency point pilot channel and data channel tracking result, and (d) is the L1 and L5 frequency point signal-to-noise ratio;
FIG. 5 is a schematic illustration of the satellite altitude change of GNSS satellites broadcasting L1 and L5 signals at high wind speeds;
FIG. 6 is a waveform diagram of a reflected signal generated by an embodiment of the present invention;
FIG. 7 is a complex interference field of a single frequency reflected signal and a combined signal of a GEO satellite according to an embodiment of the invention;
FIG. 8 is a carrier phase observation of a single frequency reflected signal and combined signal from a GEO satellite in accordance with an embodiment of the invention;
FIG. 9 is a graph of carrier phase altimetry results for high sea conditions in accordance with an embodiment of the present invention;
fig. 10 is the sea level altitude and sea level wind speed obtained in a shore-based experiment.
Detailed Description
The invention is further described below with reference to the accompanying drawings:
embodiment 1 referring to fig. 1-3, a method for measuring sea surface height of GNSS-R carrier phase based on dual-frequency reflection signal combination is disclosed, based on dual-frequency reflection signal combination, shore-based experiments are performed under high sea conditions, the experimental data are about 7 hours long, the average wind speed is 5.55m/s, the maximum wind speed is 10m/s, and two frequency points broadcast by GEO satellites in QZSS (japan quasi zenith satellite navigation system) are used: l1 and L5, the specific steps are as follows:
step one: direct signal processing
In order to realize accurate tracking of the carrier wave of the GNSS sea surface reflected signals, direct signals are processed firstly:
(1) The up-looking right-handed antenna and the down-looking left-handed antenna respectively acquire direct signals and reflected signals, the sampling quantization is carried out at the front end of the radio frequency, the sampling rate is 62MHz,2 bits of quantization is carried out, and the data is recorded for 7 hours;
(2) The method comprises the steps of capturing four QZSS satellites, namely J01, J02, J03 and J07, of the collected direct signals by adopting a code parallel capturing mode;
(3) The code phase and frequency of the captured signal are utilized, a PLL (phase locked loop) and a DLL (delay locked loop) third-order loop are adopted for closed-loop tracking, and after tracking, the information such as the code phase, the carrier frequency, the carrier phase, the navigation message and the like of the dual-frequency direct signal can be output; the output frequency is 1KHz, taking J07 (GEO satellite) as an example, and the output result is shown in FIG. 4;
(4) Positioning by using more than four GNSS satellites to obtain the accurate position and the earth height of the receiver;
(5) Calculating the altitude and azimuth of the GNSS satellite by using the receiver position and the GNSS satellite ephemeris obtained by positioning, wherein FIG. 5 is a satellite altitude change chart of the GNSS satellite broadcasting L1 and L5 signals at high wind speed;
(6) And setting an azimuth angle range according to the installation position of the down-looking left-handed antenna, selecting the satellite signals capable of effectively receiving the satellite signals reflected by the sea surface, marking and carrying out the next processing.
Step two: reference signal establishment
(1) Estimating the position of a specular reflection point by utilizing GNSS satellite ephemeris and a receiver positioning result and adopting a Newton iteration method;
(2) According to the specular reflection point, the GNSS satellite position and the receiver position calculate the pseudo code path difference between the direct signal and the reflected signal, and the calculation formula is as follows:
；
wherein ,for GNSS satellite position>For LEO satellite position, < >>Is the position of the point of specular reflection,for the path delay between direct signal and reflected signal, < >>Is the speed of light.
(3) According to the geometric difference of the direct signal and the reflected signal, the direct signal and the reflected signal are synchronized, a local reference signal is established, and the calculation formula is as follows:
；
；
；
wherein ,for the code phase of the reflected signal, < >>For the code phase of the direct signal, +.>For the code frequency to be the same,for reflecting signal carrier phase +.>For direct signal carrier phase +.>For carrier frequency +.>For the reflected signal frequency, +.>For direct signal carrier frequency, < >>The path delay is expressed as: />；
(4) And correlating the local reference signal with the reflected signal to realize accurate tracking of the carrier wave of the reflected signal.
Step three: reflected signal waveform generation and complex correlation result extraction
After the open loop tracking of the reflected signal is completed, the carrier wave is accurately tracked, so that the reflected signal waveform can be generated, and further processing is performed:
(1) Taking the frequency of a reference signal as the carrier frequency, the carrier phase of the reference signal as the carrier phase, and the code phase as the center, respectively taking two chip range windows in front and back, taking 0.1 chip as the step length in the windows, generating a signal set, and correlating with the reflected signals which are tracked in the step two (the following formula), so as to generate reflected signal waveforms of different frequency points: as shown in fig. 6;
；
；
wherein ,is the integration time; />Time delay for different local signals; />Is the center frequency of the reflected signal; />For GNSS signal code>Is a carrier wave.
(2) Obtaining the position of a specular reflection point on the waveform of the reflected signal by adopting a first derivative method;
(3) Selecting a sampling point, which is nearest to the specular reflection point from the front edge of the waveform, as a reference point (only related to the sampling rate), and extracting in-phase and quadrature components of interference complex fields of reflected signals at the waveform reference points of different frequency points; wherein the in-phase and quadrature components of the interference complex field of the reflected signals at different frequency points (L1 and L5) can be expressed as:
the frequency point L1 can be expressed as:
；
the frequency point L5 can be expressed as:
；
wherein I and Q are quadrature and in-phase components of the reflected signal at the reference point, respectively;reflecting signal amplitude values for different frequency points; />Is a path delay; />For the autocorrelation function of direct signal ranging code and reflected signal ranging code, +.> and />The phase difference of the direct signal and the reflected signal of the frequency point L1 and the frequency point L5 is obtained; />Is a noise term;
step four: dual-frequency combined interferometric phase extraction
After the reflected signal is extracted to interfere with the complex field, the single-frequency signal carrier phase observation value can be extracted, however, the sea surface height measurement cannot be carried out under the high sea condition because the single-frequency signal carrier phase observation value is greatly influenced by the sea surface roughness. Therefore, double frequency combining is required on this basis.
(1) Combining the reflected signals to obtain an interference complex field after combining the dual-frequency reflected signals, as shown in fig. 7;
the dual-frequency reflected signal interferes with the complex field:
；
；
the double-frequency reflection signal combination mode comprises the following steps:
；
interference complex field of the combined reflected signal:
；
wherein , and />Is the in-phase and quadrature components of the combined signal; />Interference complex fields for the combined signal at the reference point; /> and />Interference complex fields of the two frequency point reflection signals respectively; />Is a complex number unit.
(2) Under the shore-based condition, the height of the receiver is low, and the combined interference complex field in (1) adopts the coherent integration time length of 60 seconds, so that the signal-to-noise ratio of the signal is effectively improved;
(3) Extracting a combined signal carrier phase observation using a four-quadrant arctangent function using:
；
in the formula ,the carrier phase observation value is obtained after the combination of the double-frequency reflected signals;
(4) The root mean square of the carrier phase observations is calculated without a difference, the coarse difference is detected and eliminated, and the carrier phase observations are unwrapped to obtain continuous carrier phase observations, as shown in fig. 8.
Step five: carrier phase sea level altitude inversion
(1) And calculating the integer ambiguity by using the pseudo code path delay obtained in the second step and the carrier delay obtained in the fourth step, wherein the calculation formula is as follows:
；
wherein ,is a pseudo-range observation; />Is the carrier phase observation; />The geometrical distance between the GNSS satellite specular reflection point and the receiver; />Is the speed of light; />Is path delay; />Is an ionospheric error; />Is a tropospheric error; m is a multipath effect; />Ambiguity in the carrier phase observations; />、/>Is observation noise;
(2) The sea surface altitude is calculated through the satellite altitude angle in the first step, and the result is shown in fig. 9, and the calculation formula is as follows:
；
wherein H is the earth height determined by the GNSS receiver; SSH is the sea level; ρ is the path delay between the direct signal and the reflected signal;is the vector between the direct antenna and the reflective antenna phase center; />A unit vector that is a link between a GNSS satellite of the satellite and the receiver; θ is the satellite altitude.
The effect of the method is verified through a shore-based experiment, and fig. 10 shows sea surface height and sea surface wind speed obtained in the experiment, wherein Case1 and Case2 are respectively the time periods of data acquisition twice. Case1 is referred to as a low sea state and Case2 is referred to as a high sea state for distinction. The average wind speed of Case2 is 5.55m/s, at this time, the wave fluctuation on the sea, the carrier phase cannot be extracted and recovered under the condition of the traditional GNSS-R single-frequency carrier phase height measurement, as shown by L1 and L5 frequency points in fig. 7 and 8, and the carrier phase can be better extracted and recovered by adopting the method of the invention to combine the signals of the frequency point L1 and the frequency point L5. At this time, under high wind speed, the satellite altitude changes of the GNSS satellites broadcasting the L1 and L5 signals are shown in fig. 5, and the carrier phases can be better recovered within the range of 5-75 degrees, so that the carrier phase height measurement under the high sea condition can be realized by adopting the method.
TABLE 1 altimetric results under high sea conditions
After the GNSS-R dual-frequency reflection signal combination method is used, compared with the traditional method, the carrier phase observation value is recovered under the high sea condition of Case2, and sea surface height measurement is realized. As shown in Table 1, the root mean square error RMSE is 4-5 cm, and the optimal value can be 3.49 cm, compared with the sea surface altitude change true value, as shown in the height measurement result under Case 2. The method realizes the carrier phase height measurement within the range of 5-75 degrees from the height angle of the used satellite, overcomes the defect that the original method only realizes high-precision height measurement at a glancing angle, namely a low satellite height angle, and improves the practicability of the carrier phase height measurement method.
By using the technical scheme of the invention or under the inspired by the technical scheme of the invention, a similar technical scheme is designed by a person skilled in the art, so that the technical effects are achieved, and the technical effects fall into the protection scope of the invention.
Claims (6)
1. The method for detecting the sea surface height of the GNSS-R carrier phase based on the double-frequency reflection signal combination is characterized by comprising the following steps:
step one: processing the direct signal;
step two: establishing a reference signal;
step three: generating a reflected signal waveform and extracting complex correlation results;
step four: extracting a double-frequency combined interference phase;
step five: carrier phase sea level altitude inversion.
2. The method for detecting sea surface height of GNSS-R carrier phase based on dual-frequency reflection signal combination according to claim 1, wherein the "processing direct signal" in the step one is specifically implemented as follows:
(1) Receiving GNSS direct signals by using an upward-looking right-handed antenna, receiving GNSS reflected signals by using a downward-looking left-handed antenna, and performing down-conversion and sampling quantization;
(2) Meanwhile, processing the dual-frequency direct signal, and acquiring code phase, carrier frequency, carrier phase and navigation message information of the dual-frequency direct signal by adopting a PLL and a DLL third-order tracking loop;
(3) Positioning by utilizing a plurality of GNSS satellites to obtain the accurate position and the geodetic altitude of a receiver, and calculating the altitude angle and the azimuth angle of the GNSS satellites;
(4) And setting an azimuth angle range according to the installation position of the down-looking antenna, selecting the satellite signals which can be effectively received and reflected by the sea surface, marking and carrying out the next processing.
3. The method for detecting sea surface height of GNSS-R carrier phase based on dual-frequency reflection signal combination according to claim 1, wherein the step two is characterized in that "establishing reference signals", which comprises the following specific implementation processes:
(1) Estimating the position of the specular reflection point by using GNSS satellite ephemeris and a receiver positioning result;
(2) Calculating a pseudo code path difference between the direct signal and the reflected signal according to the specular reflection point position:
；
wherein ,for GNSS satellite position>For LEO satellite position, < >>Is the position of the point of specular reflection,for the path delay between direct signal and reflected signal, < >>Is the speed of light;
(3) According to the geometric difference of the direct signal and the reflected signal, synchronizing the direct signal and the reflected signal, and establishing a local reference signal:
；
；
；
wherein ,for the code phase of the reflected signal, < >>For the code phase of the direct signal, +.>For code frequency +.>For reflecting signal carrier phase +.>For direct signal carrier phase +.>For carrier frequency +.>For the frequency of the reflected signal,for direct signal carrier frequency, < >>The path delay is expressed as: />；
(4) And the local reference signal is correlated with the reflected signal, so that the accurate tracking of the carrier wave of the double-frequency reflected signal is realized.
4. The method for detecting sea surface height of GNSS-R carrier phase based on dual-frequency reflected signal combination according to claim 1, wherein the reflected signal waveform generation and complex correlation result extraction in the third step are specifically implemented as follows:
(1) Taking the frequency of a reference signal as the carrier frequency, the carrier phase of the reference signal as the carrier phase, and the code phase as the center, respectively taking two chip range windows in front and back, taking 0.1 chip as the step length in the windows, generating a signal set, correlating with the reflected signal which is tracked in the step two, and generating reflected signal waveforms of different frequency points:
；
；
wherein ,is the integration time; />Time delay for different local signals; />Is the center frequency of the reflected signal; />For GNSS signal code>Is a carrier wave;
(2) Obtaining the position of a specular reflection point on the waveform of the reflected signal by adopting a first derivative method;
(3) Selecting a sampling point of the waveform front edge nearest to the specular reflection point as a reference point, and extracting in-phase and quadrature components of the interference complex field of the reflected signal at the waveform reference points of different frequency points;
frequency bin 1 is represented as:
；
frequency bin 2 is denoted as:
；
wherein I and Q are quadrature and in-phase components of the reflected signal at the reference point, respectively;reflecting signal amplitude values for different frequency points;is a path delay; />For the autocorrelation function of the direct signal ranging code and the reflected signal ranging code, and />The phase difference of the direct signal and the reflected signal of the frequency point 1 and the frequency point 2 is obtained; />Is a noise term.
5. The method for detecting sea surface height of GNSS-R carrier phase based on dual-frequency reflection signal combination according to claim 1, wherein the dual-frequency combined interference phase extraction in the fourth step is implemented as follows:
(1) Combining the interference complex fields of the dual-frequency reflected signal using:
；
；
；
；
wherein , and />Is the in-phase and quadrature components of the combined signal; />Interference complex fields for the combined signal at the reference point; /> and />Interference complex fields of the reflected signals of the frequency point 1 and the frequency point 2 respectively; />Is a complex number unit;
(2) The signal to noise ratio is improved by long-time coherent integration;
(3) Extracting a carrier phase observation value of the combined signal by adopting a four-quadrant arctangent function:
；
in the formula ,the carrier phase observation value is obtained after the combination of the double-frequency reflected signals;
(4) And calculating root mean square no difference of the carrier phase observation values, detecting and eliminating the coarse difference, unwrapping the carrier phase observation values, and obtaining continuous carrier phase observation values.
6. The method for measuring the sea level of the GNSS-R carrier phase based on the combination of the dual-frequency reflection signals according to claim 1, wherein the carrier phase sea level height inversion in the fifth step is realized by the following steps:
(1) And calculating the integer ambiguity by using the pseudo code path delay obtained in the second step and the carrier delay obtained in the fourth step, wherein the calculation formula is as follows:
；
wherein ,is a pseudo-range observation; />Is the carrier phase observation; />The geometrical distance between the GNSS satellite specular reflection point and the receiver; />Is the speed of light; />Is path delay; />Is an ionospheric error; />Is a tropospheric error; m is a multipath effect; />Ambiguity in the carrier phase observations; />、/>Is observation noise;
(2) And (3) calculating sea surface height through the satellite height angle in the first step, wherein a calculation formula is as follows:
；
wherein H is the earth height determined by the GNSS receiver; SSH is the sea level; ρ is the path delay between the direct signal and the reflected signal;is the vector between the direct antenna and the reflective antenna phase center; />GNSS satellites for satellites are connected to receiversA unit vector; θ is the satellite altitude.
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| CN117607908A (en) * | 2024-01-23 | 2024-02-27 | 山东大学 | GNSS-R code phase sea surface height measurement method and device of unmanned aerial vehicle platform |
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