A GNSS occultation open-loop prediction method based on cubic spline interpolation algorithm

Abstract

The present invention discloses a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm, and relates to the field of occultation open-loop prediction. The method comprises: determining whether the current navigation satellite is atmospheric occulted using an occultation event prediction algorithm; obtaining position and velocity information of the atmospheric occultation and the low Earth orbit; performing occultation prediction calculations on the atmospheric occultation using an atmospheric model to obtain a model Doppler caused by satellite motion and a model pseudorange caused by a neutral atmosphere; obtaining interpolation parameters of the model Doppler and model pseudorange by caching n seconds of model Doppler, model pseudorange, and open-loop prediction time of the occultation in a 1 Hz mission using cubic spline interpolation; obtaining the 100 Hz model Doppler and model pseudorange by interpolation using the interpolation parameters; and calculating the signal transmission time using the model pseudorange; calculating the signal transmission time deviation value using the signal transmission time of the 100 Hz mission cycle, and converting the calculated signal transmission time deviation value into a hardware adjustment count for input into the FGPA adjustment, thereby performing open-loop tracking of the atmospheric occultation.

Description

Technical Field

[0001] The present invention relates to a field, and in particular to a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm. Background Art

[0002] Currently, with the continuous increase in human activities, carbon emissions continue to rise, exacerbating global warming. With global warming, extreme weather events such as heat waves, floods, cold snaps, and typhoons pose serious threats to human life and property. Global warming disrupts the balance of ecosystems, leading to serious consequences such as species extinction and biodiversity loss. The climate environment faces multiple and severe challenges, which not only affect the natural environment but also pose a threat to the sustainable development of human society. In the face of significant climate change, we need to strengthen international cooperation and coordination to promote the achievement of emission reduction targets and the implementation of climate adaptation measures. At the same time, we need to strengthen scientific research and technological innovation to enhance humanity's understanding of and ability to respond to climate change. Accurately capturing atmospheric changes and development trends, providing early warning of natural disasters, and providing sufficient time to implement countermeasures can reduce the loss of life and property. Occultation open-loop tracking technology has been widely used in atmospheric sounding. By using low-orbit satellites to receive occultation signals from GPS or GNSS navigation satellites, we can invert atmospheric parameters such as refractive index, temperature, and pressure from the Earth's surface to an altitude of 80 km. These data are of great significance for weather forecasting, scientific research, military applications, etc. During atmospheric occultation observations, the Doppler and amplitude fluctuations of the received occultation GNSS signal model increase dramatically due to the rapid change of the atmospheric refractive index gradient.

[0003] In existing technologies, open-loop occultation tracking is developed based on traditional phase-locked loop technology. Compared with traditional phase-locked loops, the open-loop tracking algorithm uses an orbit prediction model, an atmospheric refractivity climate Doppler model, and a pseudo-range model to track GNSS signals. Therefore, it is not affected by GNSS signal fluctuations and has the ability to track multiple phases and amplitudes.

[0004] However, due to the significant attenuation of GNSS signals and rapid changes in Doppler frequency in open-loop tracking, it is impossible to stably track atmospheric occultation at lower points during the process of tracking ascending occultation. It is impossible to obtain atmospheric parameters such as atmospheric refractive index, temperature, humidity, and pressure in the altitude range of 0 to 5 km, and thus it is impossible to ensure high-precision and stable tracking of atmospheric occultation. Summary of the Invention

[0005] Based on this, it is necessary to provide a GNSS occultation open-loop prediction method based on the cubic spline interpolation algorithm to address the above technical problems.

[0006] The present invention adopts the following technical solutions:

[0007] A GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm comprises the following steps:

[0008] The GNSS occultation event prediction algorithm is used to determine whether the currently tracked navigation star is an atmospheric occultation, and the position and velocity information of the currently tracked atmospheric occultation is calculated using the GNSS navigation ephemeris; and the positioning module of the navigation receiver is used to obtain the position and velocity information of the low-Earth orbit;

[0009] Based on the velocity and position information of atmospheric occultation and the velocity and position information of low Earth orbit, the atmospheric model is used to perform occultation prediction calculations for atmospheric occultation, and the model Doppler caused by satellite motion and the model pseudorange caused by the neutral atmosphere are obtained;

[0010] In a 1 Hz mission cycle, the atmospheric model is used to obtain the model Doppler, model pseudorange, and open-loop prediction time for n seconds of atmospheric occultation. The interpolation parameters of the model Doppler and model pseudorange are obtained using the cubic spline interpolation method. The model Doppler, model pseudorange, and open-loop prediction time for n seconds of atmospheric occultation, as well as the interpolation parameters of the model Doppler and model pseudorange, are cached in an array.

[0011] During the 100 Hz mission cycle, the 100 Hz model Doppler and model pseudorange are interpolated using the cubic spline interpolation method based on the model Doppler, model pseudorange, open-loop prediction time, and interpolation parameters of the atmospheric occultation buffered in the array for n seconds. The 100 Hz model pseudorange is then used to calculate the GNSS signal transmission time.

[0012] Based on the difference between the 100 Hz GNSS signal transmission time and the GNSS signal transmission time calculated by hardware, the deviation value of the GNSS signal transmission time is obtained and converted, and the hardware adjustment count is obtained and transmitted to the FPGA for adjustment. The atmospheric parameters are inverted in combination with the 100 Hz model Doppler and model pseudorange to capture open-loop atmospheric occultation and realize atmospheric forecasting.

[0013] Preferably, the step of predicting whether the currently tracked navigation star is an atmospheric occultation by using a GNSS occultation event prediction algorithm comprises:

[0014] Obtain the altitude angle, azimuth angle and tangent point height of the navigation star through the navigation star position and velocity information and the low earth orbit satellite position and velocity information;

[0015] If the azimuth and tangent point height of the navigation star at the current moment meet the first and second conditions of occultation, then extrapolate for 40 seconds. If the current navigation star still meets the first and second conditions of occultation, then it is determined that the current navigation star is atmospheric occultation.

[0016] Preferably, the satisfaction of the first condition and the second condition for occultation includes:

[0017] The satisfaction of the first condition for occultation includes: (absaz < 35°) || ((180° - absaz) < 35°)); where absaz is the absolute value of the azimuth angle of the low Earth orbit satellite, and || represents the logical OR operation;

[0018] The satisfaction of the second condition for occultation includes: abot < h < atop; where h is the tangent height of the current navigation star, abot is the lowest tangent height, atop is the highest tangent height, and abot = -125 km, atop = 150 km.

[0019] Preferably, the obtaining of the model Doppler caused by satellite motion and the model pseudorange caused by the neutral atmosphere includes: [[ID=...]] [[ID=...]]

[0020] Using the atmospheric occultation and the position and velocity information of the low Earth orbit, and adopting an atmospheric model prediction calculation to obtain the model Doppler and model pseudorange caused by satellite motion. The formulas are: <00...0138>

[0021] OLFrq = c × ((c / 1000.0 - vw2) / (c / 1000.0 - vw1) - 1);

[0022] OLRange = PseNeo + PseDeion + r2abs;

[0023]

[0023] Where OLFrq is the model Doppler caused by satellite motion, c is the speed of light, vw1 is the speed of the atmospheric occultation, vw2 is the speed of the low Earth orbit; OLRange is the model pseudorange caused by the neutral atmosphere, PseNeo is the phase delay that occurs when the GNSS signal passes through the atmosphere, PseDeion is the phase delay that occurs when the GNSS signal passes through the ionosphere, and r2abs is the straight-line distance from the GNSS signal to the GNSS receiver.

[0024] Preferably, the obtaining of the GNSS signal transmission time includes the following steps:

[0025] Using the cached model Doppler, model pseudorange, open-loop prediction time for n seconds of atmospheric occultation, and the interpolation parameters of the model Doppler and model pseudorange, and adopting the cubic spline interpolation method to interpolate and obtain the model Doppler and model pseudorange at 100HZ. The formulas are:

[0026] Cur_OLFrq = y1[n] + c1[0][n] × d + c1[1][n] × d 3 ,

[0027] , 2 , + c1[2][n] × d 3 ;

[0027] Cur_OLRange=y2[n]+c2[0][n]×d+c2[1][n]×d 2 +c2[2][n]×d 3 ;

[0028] Among them, Cur_OLFrq is the model Doppler of atmospheric occultation in the 100HZ mission period, Cur_OLRange is the model pseudorange of atmospheric occultation in the 100HZ mission period, and y1[n] is the interval [x i , x i+1 ] is a cubic polynomial of the n-second model Doppler, with open-loop prediction time i=1,2,…,n, and y2[n] is the interval [x i +x i+1 ] is a cubic polynomial of the n-second model pseudorange, c1[0], c1[1] and c1[2] are the interpolation parameters of the model Doppler, c2[0], c2[1] and c2[2] are the interpolation parameters of the model pseudorange, d=t tic -x[n],t tic It is a moment in the 100HZ task cycle;

[0029] The GNSS signal transmission time is calculated using the model pseudorange Cur_OLRange corresponding to the 100 Hz GNSS signal and the model pseudorange value corresponding to the 100 Hz GNSS signal. The formula is:

[0030]

[0031] Among them, Cur_S_time is the GNSS signal transmission time, Cur_OLRange is the model pseudorange value corresponding to the 100HZ GNSS signal, t tic is the time of 100HZ task cycle, and c is the speed of light.

[0032] Preferably, obtaining the hardware adjustment count includes:

[0033] The hardware adjustment count includes: a code cycle count, a code chip count, and a code phase count corresponding to the GNSS signal transmission time deviation.

[0034] For GPS atmospheric occultation, the conversion formula for hardware adjustment count is:

[0035]

[0036] Among them, Z_GPS is the time conversion value of global positioning system atmospheric occultation, is the GNSS signal transmission time deviation value, code cycle_GPS is the code week count corresponding to the global positioning system atmospheric occultation, code chip_GPSChip count corresponding to the global positioning system atmospheric occultation, chip Phase_GPS is the code phase count corresponding to the global positioning system atmospheric occultation;

[0037] For atmospheric occultation of the BeiDou satellite system, the conversion formula for hardware adjustment count is:

[0038]

[0039] Among them, Z_BD is the time conversion value corresponding to the atmospheric occultation of the BeiDou satellite system, code cycle_BD is the code week count corresponding to the atmospheric occultation of the BeiDou satellite system, code chip_BD Chip count corresponding to atmospheric occultation of the BeiDou satellite system, chip Phase_BD It is the code phase count corresponding to the atmospheric occultation of the BeiDou satellite system.

[0040] At least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects:

[0041] This specification provides a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm. The interpolation parameters of the model Doppler and model pseudorange of a low-frequency mission cycle are obtained by using the cubic spline interpolation algorithm, and the model Doppler, model pseudorange and their interpolation parameters are cached. The cached interpolation parameters are used in a high-frequency mission cycle to interpolate the model Doppler and model pseudorange of atmospheric occultation in the high-frequency mission. The accurate high-frequency GNSS signal transmission time is calculated through the model pseudorange, and the GNSS signal transmission time deviation value is obtained through the difference between the GNSS signal transmission time and the signal transmission time calculated by the hardware and converted into a hardware adjustment count. The atmospheric parameters are inverted in combination with the model Doppler and model pseudorange of the high-frequency mission cycle, thereby performing stable open-loop tracking of the lower point of the atmospheric occultation during the ascent process.

[0042] In summary, the method proposed in the present invention targets the phenomenon of significant attenuation of GNSS signals and rapid changes in Doppler frequency in open-loop tracking. It optimizes the atmospheric occultation open-loop prediction tracking algorithm through a cubic spline interpolation algorithm, changes the atmospheric occultation model Doppler and model pseudorange of the open-loop prediction from 1 Hz to 100 Hz, and achieves stable tracking of atmospheric occultation at a lower point (0 km-5 km) during the ascent process, effectively increasing the number of captured occultations and improving the open-loop prediction accuracy of atmospheric occultation, providing more accurate and rich data resources for atmospheric detection. In addition, the present invention does not involve hardware changes, is easy to operate, and provides favorable technical support for the acquisition of atmospheric parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0044] Figure 1 A schematic flow chart of a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm provided by the present invention;

[0045] Figure 2 A flowchart of GNSS occultation open-loop tracking adjustment after cubic spline interpolation of a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm provided by the present invention;

[0046] Figure 3 A geometrical plane diagram of GPS radio occultation events for a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm provided by the present invention;

[0047] Figure 4 A schematic diagram of the GPS radio occultation tangent point height for a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm provided by the present invention. DETAILED DESCRIPTION

[0048] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention and corresponding drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0049] The technical solutions provided by various embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0050] Figure 1 The flowchart of the GNSS occultation open-loop prediction method based on the cubic spline interpolation algorithm in the present invention specifically includes the following steps:

[0051] S101: Determine whether the currently tracked navigation star is an atmospheric occultation using a GNSS occultation event prediction algorithm, calculate the position and velocity information of the currently tracked atmospheric occultation using the GNSS navigation ephemeris, and obtain the position and velocity information of the low Earth orbit using the positioning module of the navigation receiver. Specifically, the steps include:

[0052] Obtain the altitude angle, azimuth angle and tangent point height of the navigation star through the navigation star position and velocity information and the low earth orbit satellite position and velocity information;

[0053] If the azimuth and tangent height of the navigation star at the current moment satisfy the first condition and the second condition for occultation, extrapolate for 40 s. If the current navigation star still satisfies the first condition and the second condition for occultation, determine that the current navigation star is an atmospheric occultation;

[0054] Among them, the first condition includes: (absaz < 35°) || ((180° - absaz) < 35°)); where absaz is the absolute value of the azimuth of the low-Earth orbit satellite, and || represents the logical OR operation; the second condition includes: abot < h < atop; where h is the tangent height of the current navigation star, abot is the lowest tangent height, atop is the highest tangent height, and abot = -125 km, atop = 150 km.

[0055] S102: Through the atmospheric occultation, the speed and position of the low-Earth orbit, use the atmospheric model to perform occultation prediction calculations on the atmospheric occultation, and obtain the model Doppler caused by satellite motion and the model pseudorange caused by the neutral atmosphere, specifically including:

[0056] Through the atmospheric occultation and the position and velocity information of the low-Earth orbit, use the atmospheric model to predict and calculate the model Doppler and model pseudorange caused by satellite motion. The formula is:

[0057] OLFrq = c × ((c / 1000.0 - vw2) / (c / 1000.0 - vw1) - 1);

[0058] OLRange = PseNeo + PseDeion + r2abs;

[0059] Among them, OLFrq is the model Doppler caused by satellite motion, c is the speed of light, vw1 is the speed of the atmospheric occultation, and vw2 is the speed of the low-Earth orbit; OLRange is the model pseudorange caused by the neutral atmosphere, PseNeo is the phase delay that occurs when the GNSS signal passes through the atmosphere, PseDeion is the phase delay that occurs when the GNSS signal passes through the ionosphere, and r2abs is the straight-line distance from the GNSS signal to the GNSS receiver.

[0060] S103: In the 1HZ mission cycle, obtain the model Doppler, model pseudorange, and open-loop prediction time of the atmospheric occultation for n seconds through the atmospheric model, use the cubic spline interpolation method to obtain the interpolation parameters of the model Doppler and model pseudorange; and cache the model Doppler, model pseudorange, open-loop prediction time of the atmospheric occultation for n seconds, and the interpolation parameters of the model Doppler and model pseudorange into an array, specifically including:

[0061] Cache the model Doppler, model pseudorange, and open-loop prediction time of n seconds of atmospheric occultation. In the one-second task, use the cubic spline interpolation method to interpolate the interpolation parameters c1 of the model Doppler and c2 of the model pseudorange.

[0062] For each interval [x i , x i+1 ], the expression is:

[0063] S i (x) = a i +b i (xx i )+c i (xx i ) 2 +d i (xx i ) 3 ;

[0064] Among them, a i 、b i 、c i and d i is the interpolation parameter, which is determined by the following steps:

[0065] For each i=0,1,…,n, S i (x i )=y i , S i (x i+1 )=y i+1 ; Make the second-order derivative continuous: Si″(x i+1 )=S i+1 ″(x i+1 ); Select the periodic cubic spline S″(x0)=S″(x n ) is the boundary condition, which determines all interpolation parameters;

[0066] To determine all interpolation parameters a i 、b i 、c i and d i , use matrix representation and Gaussian elimination method to solve the equations and obtain all interpolation parameters, specifically:

[0067] a i =y i ;

[0068]

[0069] Among them, h i =x i+1 -x i , represents an interval of cubic spline polynomial; mi =s” i (x i ), which represents the second-order derivative of each interval expression;

[0070] Furthermore, the occultation model Doppler is interpolated using cubic spline, and the interpolation parameter c1 is specifically: c1[0] = b i , c1[1]=c i , c1[2]=d i ; Perform cubic spline interpolation on the pseudo-range of the occultation model, and the interpolation parameter c2 is specifically c2[0]=b i , c2[1]=c i , c2[2]=d i ;

[0071] The model Doppler, model pseudorange, open-loop prediction time, and interpolation parameters of the model Doppler and model pseudorange of the atmospheric occultation are cached into an array.

[0072] S104: In the 100HZ duty cycle, see Figure 2 Based on the model Doppler, model pseudorange, open-loop prediction time, and interpolation parameters of the atmospheric occultation model and model pseudorange cached in the array, the cubic spline interpolation method is used to interpolate the 100 Hz model Doppler and model pseudorange. The GNSS signal transmission time is calculated using the 100 Hz model pseudorange. Specifically, the following steps are performed:

[0073] The model Doppler and model pseudorange of the 100 Hz mission period are obtained by interpolating the model Doppler and model pseudorange of the cached atmospheric occultation n seconds, the open-loop prediction time, and the interpolation parameters of the model Doppler and model pseudorange using the cubic spline interpolation method. The formula is:

[0074] Cur_OLFrq=y1[n]+c1[0][n]×d+c1[1][n]×d 2 +c1[2][n]×d 3 ;

[0075] Cur_OLRange=y2[n]+c2[0][n]×d+c2[1][n]×d 2 +c2[2][n]×d 3 ;

[0076] Among them, Cur_OLFrq is the model Doppler of the 100HZ mission, Cur_OLRange is the model pseudorange of the 100HZ mission, and y1[n] is the interval [x i , x i+1 ] is a cubic polynomial of the n-second model Doppler, with open-loop prediction time i=1,2,…,n, and y2[n] is the interval [xi +x i+1 ] is a cubic polynomial of the n-second model pseudorange, c1[0], c1[1] and c1[2] are the interpolation parameters of the model Doppler, c2[0], c2[1] and c2[2] are the interpolation parameters of the model pseudorange, d=t tic -x[n],t tic It is a moment in the 100HZ task cycle;

[0077] The GNSS signal transmission time is calculated using the model pseudorange Cur_OLRange corresponding to the GNSS signal with a 100 Hz duty cycle and the model pseudorange value corresponding to the GNSS signal with a 100 Hz duty cycle. The formula is:

[0078]

[0079] Among them, Cur_S_time is the GNSS signal transmission time, Cur_OLRange is the model pseudorange value corresponding to the 100HZ mission cycle GNSS signal, t tic is the time of 100HZ task cycle, and c is the speed of light.

[0080] S105: Based on the difference between the 100 Hz GNSS signal transmission time and the GNSS signal transmission time calculated by hardware, the deviation value of the GNSS signal transmission time is obtained and converted, and the hardware adjustment count is obtained and transmitted to the FPGA for adjustment. The atmospheric parameters are inverted in combination with the 100 Hz model Doppler and model pseudorange to capture open-loop atmospheric occultation and realize atmospheric forecasting. Specifically, the following steps are included:

[0081] The hardware adjustment count includes: code cycle count, code chip count and code phase count corresponding to the GNSS signal transmission time deviation.

[0082] For GPS atmospheric occultation, the conversion formula for hardware adjustment count is:

[0083]

[0084] Among them, Z_GPS is the time conversion value of global positioning system atmospheric occultation, is the GNSS signal transmission time deviation value, code cycle_GPS is the code week count corresponding to the global positioning system atmospheric occultation GNSS signal transmission time deviation value, code chip_GPS The chip count corresponding to the time deviation value of the global positioning system atmospheric occultation GNSS signal transmission, chip Phase_GPS The code phase count corresponding to the global positioning system atmospheric occultation GNSS signal transmission time deviation value;

[0085] For atmospheric occultation of the BeiDou satellite system, the conversion formula for hardware adjustment count is:

[0086]

[0087] Among them, Z_BD is the time conversion value corresponding to the atmospheric occultation of the BeiDou satellite system, code cycle_BD is the code week count corresponding to the time deviation value of the atmospheric occultation GNSS signal of the BeiDou satellite system, code chip_BD The chip count corresponding to the time deviation of the atmospheric occultation GNSS signal of the BeiDou satellite system, chip Phase_BD It is the code phase count corresponding to the transmission time deviation value of the atmospheric occultation GNSS signal of the BeiDou satellite system.

[0088] This embodiment takes a GPS occultation open-loop prediction as an example. The low-orbit satellite is operating at an altitude of 500km, with an orbital inclination of 55 degrees, an attitude designed to be oriented toward the Earth, and a three-axis stabilized elliptical orbit. The geometric structure diagram of the GPS radio occultation event can be found in Figure 3 When the GPS is predicted to be an atmospheric occultation, the above steps are used to start predicting the model Doppler and model pseudorange values ​​of the GPS star and cache the data. When the cached model pseudorange and model Doppler exceed 4s, a cubic spline interpolation operation is performed to calculate the cubic spline interpolation coefficient a of the star. i 、b i 、c i and d i , that is, the model Doppler parameters c1[0][3], c1[1][3] and c1[2][3], c2[0][3], c2[1][3], c2[2][3], that is, the cubic spline model Doppler Cur_OLFrq and model pseudorange Cur_OLRange are obtained in each interval [x i ,x i +1] interpolation polynomial:

[0089] Cur_OLFrq=y1[3]+c1[0][3]×d+c1[1][3]×d 2 +c1[2][3]×d 3 ;

[0090] Cur_OLRange=y2[3]+c2[0][3]×d+c2[1][3]×d 2 +c2[2][3]×d 3 ;

[0091] Among them, Cur_OLFrq is the model Doppler of the 100HZ mission, Cur_OLRange is the model pseudorange of the 100HZ mission, and y1[n] is the interval [x i , xi+1 ] is the cubic polynomial of the n-second model Doppler, y2[n] is the interval [x i +x i+1 ] is a cubic polynomial of the n-second model pseudorange, c1[0], c1[1] and c1[2] are the interpolation parameters of the model Doppler, c2[0], c2[1] and c2[2] are the interpolation parameters of the model pseudorange, d=t tic -x[n],t tic It is a moment in the 100HZ task cycle.

[0092] The model Doppler and model pseudorange values ​​of GPS atmospheric occultation at the current moment in the 100HZ mission cycle obtained by the above method and steps are used to further calculate the current TIC signal transmission time Cur_S_time through the model pseudorange value. The signal transmission time t calculated by hardware is subtracted from Cur_S_time. hw The obtained difference Δt is used to convert the model Doppler value corresponding to the current Tic moment and Δt into the code week, code chip, and code phase counts corresponding to the signal transmission time deviation, which are then placed into the FPGA for adjustment and use to accurately complete the open-loop tracking of the GPS atmospheric occultation signal.

[0093] Take a GPS ascending occultation as an example, see Figure 4 OQ is the height from the occultation tangent point to the Earth's center, and the tangent point height h = HR, where R is the Earth's radius. Using this patented occultation prediction method, stable GPS atmospheric occultation tracking can be achieved at a tangent point height of approximately 1 km. This overcomes the drawback of existing methods that prevent stable tracking of atmospheric occultations at lower altitudes, significantly increasing atmospheric parameters such as atmospheric refractive index, temperature, humidity, and pressure within the 0-5 km altitude range. Furthermore, this inventive method does not involve hardware modifications and is easy to implement, providing favorable technical support for the acquisition of atmospheric parameters.

[0094] As shown in the table below, the atmospheric occultation events of GPS-26 were captured and tracked through open-loop forecasting within approximately 24 hours. The lowest altitude of atmospheric occultation detection obtained was about 1 km, which greatly improved the detection capability of the occultation receiver and solved the problem of difficulty in obtaining ascending occultation events. The above examples fully demonstrate that the invented method can be widely used in the field of atmospheric detection. By receiving GPS or GNSS navigation satellite signals from low-orbit satellites for occultation observation, atmospheric parameters such as refractive index, temperature, and pressure from the surface to an altitude of 80 km can be inverted. These data are of great significance for weather forecasting, scientific research, early warning of natural disasters, and military applications.

[0095]

[0096] In summary, this embodiment provides a GNSS occultation open-loop prediction method based on a cubic spline interpolation algorithm. This method addresses the phenomena of significant GNSS signal attenuation and rapid Doppler frequency changes during open-loop tracking by optimizing the open-loop prediction and tracking algorithm using a cubic spline interpolation algorithm. This optimizes the atmospheric model Doppler and atmospheric model pseudorange used in the open-loop prediction from 1 Hz to 100 Hz. This ensures stable tracking of atmospheric occultations at lower points during tracking of ascending occultations, thereby acquiring atmospheric parameters such as atmospheric refractive index, temperature, humidity, and pressure within an altitude range of 0 km to 5 km. This effectively increases the number of captured occultations, improves the accuracy of open-loop predictions of atmospheric occultations, and provides more accurate and rich data resources for atmospheric exploration.

[0097] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. Any technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A GNSS occultation open-loop prediction method, characterized in that: The method includes the following steps: Judge whether the currently tracked navigation star is an atmospheric occultation through the GNSS occultation event prediction algorithm, and calculate the position and velocity information of the currently tracked atmospheric occultation through the GNSS navigation ephemeris; and use the positioning module of the navigation receiver to obtain the position and velocity information of the low Earth orbit; Through the velocity and position information of the atmospheric occultation and the velocity and position information of the low Earth orbit, use the atmospheric model to perform occultation prediction calculations on the atmospheric occultation to obtain the model Doppler caused by satellite motion and the model pseudorange caused by the neutral atmosphere; In the 1HZ task cycle, obtain the model Doppler, model pseudorange, and open-loop prediction time of the atmospheric occultation for n seconds through the atmospheric model, and use the cubic spline interpolation method to obtain the interpolation parameters of the model Doppler and model pseudorange; and cache the model Doppler, model pseudorange, and open-loop prediction time of the atmospheric occultation for n seconds, as well as the interpolation parameters of the model Doppler and model pseudorange, into an array; In the 100HZ task cycle, according to the model Doppler, model pseudorange, open-loop prediction time of the atmospheric occultation for n seconds, and the interpolation parameters of the model Doppler and model pseudorange cached in the array, use the cubic spline interpolation method to interpolate and obtain the model Doppler and model pseudorange at 100HZ; and calculate the GNSS signal emission time through the model pseudorange at 100HZ; According to the difference between the GNSS signal emission time at 100HZ and the GNSS signal emission time calculated by hardware, obtain the deviation value of the GNSS signal emission time and convert it, obtain the hardware adjustment count and pass it into the FPGA for adjustment, and combine the model Doppler and model pseudorange at 100HZ to invert the atmospheric parameters to capture the open-loop atmospheric occultation and achieve atmospheric prediction.

2. A GNSS occultation open-loop prediction method according to claim 1, characterized in that: The prediction of whether the currently tracked navigation star is an atmospheric occultation through the GNSS occultation event prediction algorithm includes: Obtain the elevation angle, azimuth angle, and tangent height of the navigation star through the position and velocity information of the navigation star and the position and velocity information of the low Earth orbit satellite; If the azimuth angle and tangent height of the navigation star at the current moment satisfy the first condition and the second condition of occultation, extrapolate for 40s. If the current navigation star still satisfies the first condition and the second condition of occultation, determine that the current navigation star is an atmospheric occultation.

3. A GNSS occultation open-loop prediction method according to claim 2, characterized in that: The satisfaction of the first condition and the second condition of occultation includes: The satisfaction of the first condition of occultation includes: (absaz < 35°) || ((180° - absaz) < 35°)); where absaz is the absolute value of the azimuth angle of the low Earth orbit satellite, and || represents the logical OR operation; The satisfaction of the second condition of occultation includes: abot < h < atop; where h is the tangent height of the current navigation star, abot is the lowest tangent height, atop is the highest tangent height, and abot = -125km, atop = 150km.

4. A GNSS occultation open-loop prediction method according to claim 1, characterized in that: The obtaining of the model Doppler caused by satellite motion and the model pseudorange caused by the neutral atmosphere includes: Through the position and velocity information of the atmospheric occultation and the low Earth orbit, use the atmospheric model to predict and calculate the model Doppler and model pseudorange caused by satellite motion. The formula is: OLFrq=c×((c / 1000.0-vw2) / (c / 1000.0-vw1)-1); OLRange=PseNeo+PseDeion+r2abs; Where OLFrq is the model Doppler caused by satellite motion, c is the speed of light, vw1 is the velocity of atmospheric occultation, and vw2 is the velocity of low Earth orbit. OLRange is the model pseudorange caused by the neutral atmosphere, PseNeo is the phase delay of the GNSS signal passing through the atmosphere, PseDeion is the phase delay of the GNSS signal passing through the ionosphere, and r2abs is the straight-line distance from the GNSS signal to the GNSS receiver.

5. A GNSS occultation open-loop prediction method according to claim 1, characterized in that: The acquisition of the GNSS signal transmission time includes the following steps: The 100 Hz model Doppler and model pseudorange are obtained by interpolating the cached atmospheric occultation model Doppler, model pseudorange, open-loop prediction time, and interpolation parameters of the model Doppler and model pseudorange using the cubic spline interpolation method. The formula is: Cur_OLFrq=y1[n]+c1[0][n]×d+c1[1][n]×d 2 +c1[2][n]×d 3 ; Cur_OLRange=y2[n]+c2[0][n]×d+c2[1][n]×d 2 +c2[2][n]×d 3 ; Among them, Cur_OLFrq is the model Doppler of atmospheric occultation in the 100HZ mission period, Cur_OLRange is the model pseudorange of atmospheric occultation in the 100HZ mission period, and y1[n] is the interval [x i , x i+1 ] is a cubic polynomial of the n-second model Doppler, with open-loop prediction time i=1,2,…,n, and y2[n] is the interval [x i +x i+1 ] is a cubic polynomial of the n-second model pseudorange, c1[0], c1[1] and c1[2] are the interpolation parameters of the model Doppler, c2[0], c2[1] and c2[2] are the interpolation parameters of the model pseudorange, d=t tic -x[n],t tic It is a moment in the 100HZ task cycle; The GNSS signal transmission time is calculated using the model pseudorange Cur_OLRange corresponding to the 100 Hz GNSS signal and the model pseudorange value corresponding to the 100 Hz GNSS signal. The formula is: Among them, Cur_S_time is the GNSS signal transmission time, Cur_OLRange is the model pseudorange value corresponding to the 100HZ GNSS signal, t tic is the time of 100HZ task cycle, and c is the speed of light.

6. A GNSS occultation open-loop prediction method according to claim 1, characterized in that: The obtaining of the hardware adjustment count includes: The hardware adjustment count includes: code cycle count, code chip count and code phase count corresponding to the GNSS signal transmission time deviation; For GPS atmospheric occultation, the conversion formula for hardware adjustment count is: Among them, Z_GPS is the time conversion value of global positioning system atmospheric occultation, is the GNSS signal transmission time deviation value, code cycle_GPS is the code week count corresponding to the global positioning system atmospheric occultation, code chip_GPS Chip count corresponding to the global positioning system atmospheric occultation, chip Phase_GPS is the code phase count corresponding to the global positioning system atmospheric occultation; For atmospheric occultation of the BeiDou satellite system, the conversion formula for hardware adjustment count is: Among them, Z_BD is the time conversion value corresponding to the atmospheric occultation of the BeiDou satellite system, code cycle_BD is the code week count corresponding to the atmospheric occultation of the BeiDou satellite system, code chip_BD Chip count corresponding to atmospheric occultation of the BeiDou satellite system, chip Phase_BD It is the code phase count corresponding to the atmospheric occultation of the BeiDou satellite system.

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