A satellite navigation message format adaptation method for air-based platforms

Abstract

The present invention proposes a satellite navigation message format adaptation method for air-based platforms, and in particular proposes a message adaptation and optimization scheme for the specific needs of air-based platforms. The method includes the acquisition of the position of the air-based platform and the mapping of coordinates; the simplified adaptation of the 18-parameter ephemeris model in the original navigation message; the calculation of the ephemeris parameters in combination with the motion analysis of the air-based platform; and the position solution. Through analysis and simulation verification, it is proved that the adaptation model can be applied to the air-based platform through parameter design without changing the format of the satellite navigation message, and can effectively reduce the positioning error in static and uniform linear motion states. By applying the second-order derivative for error correction, the accuracy of the prediction speed is significantly optimized.

Description

Technical Field

[0001] The present invention relates to a message adaptation technology in a satellite navigation system, and is about satellite navigation message adaptation and service performance optimization technology on an air-based platform. The invention aims to extend the determined satellite navigation message parameter design to the air-based platform and perform adaptation application, while improving the positioning service accuracy of the air-based platform under complex motion conditions. The present invention particularly relates to a satellite navigation message format adaptation method for an air-based platform. Background Art

[0002] In the development of modern satellite navigation systems, accurate prediction and broadcasting of satellite position information is one of the key technologies to ensure navigation accuracy. At present, the Global Navigation Satellite System (GNSS) such as Beidou, GPS, and GALILEO widely adopt a 16 / 18 parameter model for Kepler motion description to achieve accurate prediction and broadcasting of satellite position information. These parameters include six orbital elements, which are used to describe the position of the satellite in space and its motion state, and other parameters used to correct factors such as orbital perturbations.

[0003] Among them, the Kepler orbit six-element parameter model is as follows Figure 1 As shown, the model includes eccentricity and semi-major axis Describes orbital shape; argument of perigee , orbital inclination and longitude of ascending node Describes the position of the orbit in an inertial reference frame; true anomaly is a time parameter that describes the specific position of the satellite in orbit. In order to fully consider the impact of perturbation on the satellite orbital motion and adapt to the actual characteristics of the non-strict Keplerian elliptical motion of the satellite orbit, it is also necessary to introduce perturbation correction terms including the orbital inclination change rate and the ascending node longitude change rate.

[0004] However, with the development of space-based platforms (such as pseudo-satellites, etc.), in order not to change the user receiving equipment, it is necessary to carry out satellite message model adaptation and optimization for space-based platforms, so as to achieve accurate broadcasting and prediction of position information on space-based platforms. The present invention proposes a navigation satellite message adaptation model for space-based platforms, which aims to effectively adapt the known position and motion state under the ground-fixed system to the original message structure by adjusting and designing a new model.

[0005] The current design of navigation message ephemeris is mainly for satellite-based navigation sources, and the satellites are located in the middle and high orbits of the earth. At present, there is no adaptation technology that can migrate the messages of existing satellite navigation systems to the navigation messages of air-based platforms. Commonly used methods for calculating ephemeris message parameters include traditional perturbation correction, position prediction based on Kepler elements, and real-time trajectory correction using dynamic models. Traditional perturbation correction and Kepler element-based methods essentially rely on the optimization of positioning accuracy for ground users or under low dynamic conditions. These methods introduce various correction terms to compensate for orbital deviations caused by factors such as the non-spherical earth's gravitational field and atmospheric drag. However, these methods face the problems of insufficient adaptability and reduced positioning accuracy when dealing with highly dynamic air-based platforms. The real-time trajectory correction of the dynamic model attempts to adapt to the motion state of the air-based platform through more sophisticated calculations, but is still limited by the accuracy of the model and the complexity of real-time data processing.

[0006] Among the above methods, it is necessary to rely on accurate dynamic models and high-frequency data updates to complete positioning tasks, which not only increases the computational burden of the system, but also increases the requirements for computing power and response speed. Especially when dealing with high-speed moving or maneuvering air-based platforms, traditional methods are often unable to provide accurate and reliable navigation information due to their lack of adaptability. In addition, methods that rely on complex perturbation corrections also mean higher power consumption, which affects the operating efficiency and mission execution cycle of the satellite. And these methods usually need to rely on the cooperation of multiple types of functional payloads, which places higher requirements on satellite system design and performance.

[0007] The existing ephemeris models of navigation message 16 / 18 parameters have some defects when directly applied to air-based platforms. This is because these models were originally designed for satellite-based navigation source systems and face serious adaptation and accuracy challenges when directly applied to air-based platforms (such as pseudo-satellite platforms). In particular, when dealing with complex factors such as non-determined parameter elliptical orbit motion and orbital perturbations, these models fail to provide the necessary flexibility and accuracy to meet the unique motion characteristics and prediction requirements of air-based platforms.

[0008] Its main technical defects are: limited scope of application: the satellite orbit motion of traditional global satellite navigation systems (GNSS), such as GPS and BeiDou, satisfies the two-body model, and the satellite position calculation and prediction of the ground or near-ground user segment depends on the message model of Kepler orbit elements, and often assumes that the user is in a static or low-dynamic state. However, when these models are applied to air-based platforms with high or variable speed movement, such as high-altitude unmanned aerial vehicles (UAVs) or pseudo-satellite systems, the navigation messages cannot effectively adapt to these dynamic conditions. Especially when dealing with uniform or uniformly accelerated motion in the ground-fixed or even geographic reference system, the prediction accuracy and stability of the existing models cannot meet the requirements of navigation and positioning services. The traditional methods for determining and setting navigation satellite message parameters cannot be used to characterize and adapt the motion of air-based platforms.

[0009] Insufficient high-dynamic adaptability: Although the traditional GNSS message model can handle dynamic changes within a certain range, it is not adaptable enough to the high-speed movement of air-based platforms and rapidly changing dynamic conditions. The high-speed movement of air-based platforms causes their position relative to the satellite to change rapidly, increasing the deviation between the received signal characteristics and the prediction model, affecting positioning accuracy. In addition, high-dynamic movement may cause changes in the Doppler effect, affecting the signal frequency and phase, and the existing technology lacks an effective mechanism to adapt to these changes, further affecting the accuracy and reliability of positioning.

[0010] The minor technical defects are: Limitations of perturbation correction: Perturbation correction aims to introduce additional correction terms into the orbital dynamics model to compensate for orbital deviations caused by factors such as the non-spherical earth's gravitational field, atmospheric drag, and the gravity of the sun and the moon. The perturbation correction of existing technologies is mainly aimed at optimizing the positioning accuracy of ground users, focusing on reducing the impact of small disturbances in satellite orbits. However, for air-based platforms moving at high speed in different atmospheric layers, the perturbation factors are more complex. For example, the impact of changes in atmospheric density on the orbit far exceeds that of ground users. Existing technologies have failed to fully consider these special factors in perturbation correction, limiting their effectiveness in application to air-based platforms. Summary of the invention

[0011] The present invention proposes a satellite navigation message format adaptation method for air-based platforms, and in particular proposes a message adaptation and optimization scheme for the specific needs of air-based platforms. Through analysis and simulation verification, it is proved that the adaptation model can be applied to air-based platforms through parameter design without changing the satellite navigation message format, and the adaptation model can effectively reduce the adaptation error by applying the second-order derivative for error correction under static and uniform linear motion conditions, and significantly optimizes the accuracy of the predicted speed. In addition, the present invention also points out that under high-speed conditions, it is necessary to further improve the model and error correction strategy to maintain navigation accuracy.

[0012] The present invention adopts the following technical scheme, a satellite navigation message format adaptation method for an air-based platform, the method comprising the following steps:

[0013] Step 1, position acquisition and coordinate mapping of the air-based platform; used to convert the real-time position and motion state of the air-based platform through the Earth-centered inertial coordinate system ECI to the Earth-centered Earth-fixed coordinate system ECEF, so as to achieve accurate tracking and prediction of the motion trajectory of the air-based platform;

[0014] Step 2, simplified adaptation of the original 18-parameter ephemeris model: used to adjust and simplify the standard original 18-parameter ephemeris model according to the specific motion characteristics and environmental conditions of the air-based platform to reduce the resource requirements for navigation message transmission and processing while maintaining positioning accuracy;

[0015] Step 3, parameter adaptation of the air-based platform position; used to analyze the motion state of the air-based platform, including speed, acceleration, and direction, and calculate the adapted ephemeris parameters accordingly, so as to adjust the navigation message in real time and improve the navigation accuracy;

[0016] Step 4, position solution, is used to calculate and solve the precise position information of the air-based platform based on the adjusted and optimized ephemeris parameters combined with the navigation signal received by the user receiver.

[0017] The present invention has the following beneficial technical effects:

[0018] 1. Improve positioning accuracy and adaptability

[0019] Technical features: By realizing the precise mapping of the position of the air-based platform from the Earth-centered inertial coordinate system (ECI) to the Earth-centered Earth-fixed coordinate system (ECEF), the message parameters are timely adapted and optimized.

[0020] Technical solution analysis: Considering the high dynamic motion characteristics of air-based platforms, the 16 / 18 parameter model used by traditional satellite-based navigation sources faces challenges in adaptation and accuracy when directly applied. Through the technical solution of the present invention, the known position and motion state under the ground-fixed system can be effectively adapted to the original message structure, thereby overcoming the flexibility and accuracy lacked by traditional models in dealing with complex factors such as non-fixed parameter elliptical orbit motion and orbital perturbations.

[0021] Beneficial effects: significantly improve positioning accuracy and adaptability in high dynamic environments, so that the navigation messages on the air-based platform more accurately reflect the actual motion state of the platform, thereby broadening the application scope and efficiency of the original satellite navigation system. At the same time, users do not need to modify the existing receiver structure and can still navigate with the original receiver hardware.

[0022] 2. Adapt to complex motion states

[0023] Technical features: The Piece Wise Constant System (PWCS) method is used to analyze the motion of the airborne platform and dynamically adapt the navigation message parameters.

[0024] Technical solution analysis: The actual motion trajectory of an air-based platform may be complex and changeable. Using the PWCS method, the motion of an air-based platform can be regarded as uniform motion in a very short time, and when the platform makes maneuvers, it can be corrected to the user by rebroadcasting telegram parameters, thus overcoming the shortcomings of existing technologies in adapting to the high-speed motion and rapidly changing dynamic conditions of air-based platforms.

[0025] Beneficial effects: It enables the navigation system to adapt to the complex motion state of the air-based platform more flexibly, ensures the real-time and accuracy of navigation information, and further improves the reliability of the system and user experience.

[0026] 3. Optimize broadcast frequency to improve prediction accuracy

[0027] Technical features: Dynamically adjust the broadcast frequency of telegrams according to the dynamic motion state of the air-based platform and the receiving conditions of the user end.

[0028] Technical solution analysis: The error of telegram broadcast will diverge with the speed and time of the air-based platform, and the traditional fixed broadcast frequency cannot adapt well to the changes of the platform. By setting the error threshold reasonably, the broadcast frequency can be adjusted according to actual needs, thereby optimizing resource utilization while ensuring navigation accuracy.

[0029] Beneficial effects: This method significantly improves the prediction accuracy of navigation messages, while reducing the waste of resources that may be caused by frequent broadcasting and improving the overall efficiency of the system.

[0030] 4. Avoid complex perturbation corrections

[0031] Technical features: The present invention directly uses the motion characteristics of the airborne platform (such as speed, acceleration and direction) to calculate its position, rather than relying on the traditional perturbation correction model. This method allows the system to directly and accurately report the position of the airborne platform without complex perturbation correction.

[0032] Technical solution analysis: Traditional perturbation correction methods need to consider multiple factors, such as the non-spherical earth's gravitational field, atmospheric drag, and the gravity of the sun and the moon. These factors make the positioning process complicated, and for air-based platforms moving at high speed in different atmospheric layers, the impact of these perturbation factors is more significant and complex. Existing technologies fail to fully consider these special factors, especially in highly dynamic environments such as air-based platforms. This method avoids the complexity and uncertainty of the traditional perturbation correction process by directly using the platform's motion characteristics for position prediction and broadcasting.

[0033] Beneficial effects: Simplified navigation message generation process: Since there is no need to perform complex perturbation correction calculations, the present invention simplifies the navigation message generation and broadcasting process, reduces the calculation burden, and improves system efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of Kepler's orbital parameters;

[0035] Figure 2 The flowchart of the satellite navigation message format space-based platform adaptation method of the present invention;

[0036] Figure 3 (a) is the error divergence diagram when the message frequency is set to 1 hour / time;

[0037] Figure 3 (b) is the error divergence diagram when the message frequency is set to 30 minutes / time;

[0038] Figure 3 (c) shows the error divergence diagram when the message frequency is set to 15 minutes / time;

[0039] Figure 3 (d) is the error divergence diagram when the message frequency is set to 5 minutes / time. DETAILED DESCRIPTION

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0041] The present invention provides a satellite navigation message format adaptation method for an air-based platform, the flow chart of which is as follows: Figure 2 As shown, the specific steps include:

[0042] Step 1: Position acquisition and coordinate mapping of the airborne platform;

[0043] The position of the air-based platform is mapped from the Earth-centered inertial (ECI) coordinate system to the Earth-centered, Earth-Fixed (ECEF) coordinate system. Considering the motion characteristics of the air-based platform and the fact that the navigation satellite broadcasts the coordinates in the Earth-centered inertial coordinate system in the form of navigation ephemeris parameters, it is undoubtedly more convenient to use the Earth-centered, Earth-Fixed coordinate system coordinates when using the air-based platform as a navigation source for navigation.

[0044] The conversion of the position of a space-based platform from the Earth-centered inertial coordinate system to the Earth-centered Earth-fixed coordinate system is a fundamental process in space navigation and ground tracking systems. This conversion involves consideration of the Earth's rotation, because the Earth-centered inertial coordinate system is fixed relative to the stars, while the Earth-centered Earth-fixed coordinate system rotates with the Earth's rotation.

[0045] The conversion process can be described by the following formula:

[0046] [ x E C E F y E C E F z E C E F ] = [ c o s ( θ ) s i n ( θ ) 0 − s i n ( θ ) c o s ( θ ) 0 0 0 1 ] [ x E C I y E C I z E C I ]

[0047] in, is the position coordinate of the air-based platform in the geocentric inertial coordinate system, It is the position coordinate converted to the Earth-centered Earth-fixed coordinate system. is the Earth's rotation angle, which represents the angle of transformation from the Earth-centered inertial coordinate system to the Earth-centered Earth-fixed coordinate system. It can be calculated using the following formula:

[0048]

[0049] is the average rotation speed of the Earth, which is approximately Radians / second. is the current time, is the reference time (usually chosen to be 12 noon Greenwich Mean Time). Here, the time and All should be expressed in a unified time unit (such as seconds, hours, etc.).

[0050] In this way, the position of an airborne platform in an Earth-centered inertial coordinate system can be accurately converted to an Earth-centered Earth-fixed coordinate system, thereby facilitating more convenient navigation and position tracking calculations using Earth-centered Earth-fixed coordinate system coordinates.

[0051] Step 2, simplification of the 18-parameter ephemeris model of the navigation message; the message adaptation of the air-based platform in the Earth-centered Earth-fixed coordinate system at the starting time requires changing the calculation method of the original 18-parameter ephemeris model, and broadcasting the position of the air-based platform without changing the original message format. To simplify the adaptation process, it is assumed that the air-based platform is located in an elliptical orbit with the center of the earth as the focus, and is always at the perigee, that is, the original mean anomaly angle is 0. At the same time, the perigee argument Set to , eccentricity The relevant values ​​of the reference ellipsoid are used.

[0052] The semi-major axis distance of the six elements of the navigation satellite orbit is used , orbital inclination , Longitude of ascending node The spatial position of the airborne platform can be constrained. In order to facilitate the description of the platform's geographic coordinates later, the perigee argument is and mean anomaly The simplified settings of these two give the relationship between the geographic coordinates, orbital inclination, and ascending node longitude as follows:

[0053]

[0054] In the formula are the latitude and longitude of the air-based platform in the ground-fixed system, It is the longitude of the ascending node of the imaginary orbit where the platform is located.

[0055] Step 3, adapt the navigation ephemeris parameters according to the motion mode of the air-based platform. The actual motion trajectory of the air-based platform may be complex and changeable. The Piece Wise Constant System (PWCS) method is used for analysis. The motion of the air-based platform can be regarded as uniform motion in a very short period of time. Once the platform changes, it will be corrected to the user by re-broadcasting the telegram parameters. Step 3 specifically includes the following steps.

[0056] Step 3.1 Parameter adaptation of the airborne platform position at the initial moment: At the initial moment when the airborne platform starts broadcasting, its position needs to be accurately calibrated. Because the original satellite angular velocity needs to be corrected to 0 to represent the coordinates under ECEF, , .

[0057] is the average angular velocity of the satellite at the reference time in the original message parameters, is the difference between the satellite average angular velocity at the reference time and the calculated value, is the rate of change of the difference between the satellite average angular velocity at the reference time and the calculated value, is the gravitational constant in the geocentric inertial coordinate system. , It is the semi-major axis distance of the imaginary orbit at the reference time.

[0058] In order to ensure that the coordinates of the Earth-centered Earth-fixed coordinate system after transformation do not change with time, it is also required that the orbital inclination of the hypothetical orbit , the longitude of the ascending node of the hypothetical orbit In the original satellite orbit model, the longitude of the ascending node is Usually expressed as in, Indicates the time difference between the current time and the reference time, also known as the planning time. For reference time The longitude of the ascending node, is the rate of change of the longitude of the ascending node, The angular velocity of the Earth's rotation is , is the time difference between the current time and the reference time. In order to meet the special needs of the air-based platform, we make special values ​​for the expression of the longitude of the ascending node as follows:

[0059]

[0060] In the original satellite orbit model, the orbit inclination Usually expressed as ;

[0061] in, is the orbital inclination of the hypothetical orbit at the reference time, is the first-order rate of change of orbital inclination. Here, in order to meet the special needs of air-based platforms, we make special values ​​for the expression of orbital inclination as follows:

[0062] Because what is needed is the Earth-centered Earth-fixed coordinate system , through spatial position analysis and simplification of message parameters, we can get:

[0063]

[0064] In the formula, Represents the longitude, latitude and curvature radius of the reference ellipsoid of the airborne platform.

[0065] And the latitude and longitude can be converted into coordinates of the Earth-centered Earth-fixed coordinate system by the following formula.

[0066]

[0067] In the formula is the three-dimensional coordinate of the air-based platform in the Earth-centered Earth-fixed coordinate system, is the eccentricity of the reference ellipsoid.

[0068] Step 3.2: Adaptation of airborne platform motion parameters under uniform linear motion;

[0069] According to the relationship between the longitude and latitude coordinates in the geocentric earth-fixed coordinate system and the coordinates in the rectangular coordinate system, there is the following relationship:

[0070]

[0071]

[0072]

[0073] because All about function, and put it in Perform a second-order expansion at:

[0074]

[0075] Indicates that the function is at the point The function value at , which is used here to represent about Function of Represents the reference point for Taylor expansion of the function; Represents the gradient vector of the function at the reference point; Represents the Hessian matrix of the function at the reference point; , Respectively represent the reference point To any point Vector and vector transpose.

[0076] If it is assumed that the airborne platform moves at a constant speed in a piecewise steady system, the vector It can be expressed as ,by For example, the functional relationship between longitude and ascending node longitude has been established in step 3.1 above, so we can deduce:

[0077]

[0078] It represents the first derivative of longitude with respect to time, that is, the rate of change of longitude; represents the velocity vector of the airborne platform in the Earth-centered Earth-fixed coordinate system ECEF; Represents the gradient vector of longitude at the reference point, that is:

[0079]

[0080] Similarly:

[0081] ,

[0082] , They represent the first-order derivative of latitude and the radius of curvature of the reference ellipsoid with respect to time, namely, the latitude change rate and the semi-major axis change rate; , They represent the latitude and the gradient vector of the radius of curvature of the reference ellipsoid at the reference point respectively.

[0083] Step 3.3: Adaptation of airborne platform motion parameters under uniformly accelerated linear motion;

[0084] In step 3.2, it is assumed that the airborne platform moves approximately at a uniform speed during the transmission interval. However, in some special scenarios, it is better to assume that the platform moves at a uniform acceleration. In this case, you can use To adapt to the platform Acceleration in three directions. Under the 3.2 uniform motion model, we express the vector of position increment as , when the uniform acceleration motion model is used, the following relationship exists:

[0085]

[0086] In the formula They are respectively the air-based platform in the geocentric rectangular coordinate system Acceleration in three directions.

[0087] So we can build the following model to adapt to the uniformly accelerated motion of the platform:

[0088]

[0089] In the formula, Represent the longitude, latitude, and first-order derivative of the radius of curvature of the reference ellipsoid with respect to time; Represents the Hessian matrix of longitude, latitude, and radius of curvature of the reference ellipsoid at the reference point.

[0090] Due to the limitation of the number of message parameters, in order to use the existing message parameters to characterize the uniform acceleration motion of the airborne platform, it is necessary to classify the acceleration into middle, , , . Where T is the broadcast period of the message, , , It is the message parameter actually broadcast after correction.

[0091] Step 3.4, second-order correction of message motion parameters;

[0092] Platform The coordinates can be expanded at a certain point in its track, and the gradient matrix is ​​used to approximate the message parameters. , the Hessian matrix can be used to The three parameters are corrected for the second order. Using the first order derivative will result in a first order deviation between the final predicted velocity result and the actual value. If the second order derivative is considered, it can be used to represent the second order changes of the semi-major axis, orbital inclination, and ascending node longitude:

[0093]

[0094] However, since the message has 18 parameters, there is no parameter that can express the second-order derivative. Therefore, it is necessary to consider the broadcast period and include the error caused by the second-order term in the period. , that is, the actual broadcast parameters are: , , . Where T is the broadcasting period of the message.

[0095] Step 3.5 Calculate the frequency of message broadcasting;

[0096] Considering that the error of message broadcast will diverge with the speed and time of the airborne platform, which will eventually lead to a decrease in the prediction accuracy of the message, it is necessary to consider the relationship between the frequency of the airborne platform broadcasting messages and the message prediction accuracy. According to the above analysis, the message parameters are obtained. It is related to the flight speed of the flight platform under the ground fixed system. In order to ensure that the final adaptation accuracy can meet the requirements.

[0097] If the correction method of 3.3 is used to correct the calculation of the message, the error within the interval between the two broadcasting times is enveloped by the error curve. At this time, the broadcasting frequency of the message will further affect the accuracy of the final adaptation. The following simulation example sets the airborne platform speed to 255km / h. The broadcast frequencies of 1 hour / time, 30 minutes / time, 15 minutes / time, and 5 minutes / time will be analyzed respectively. Among them, Figure 3 (a) shows the position adaptation error divergence of the platform when the message is broadcast once an hour; Figure 3 (b) shows the position adaptation error divergence of the platform when the message is broadcast once every 30 minutes; Figure 3 (c) shows the position adaptation error divergence of the platform when the message is broadcast once every 15 minutes; Figure 3 (d) shows the position adaptation error divergence of the platform when the message is broadcast once every 5 minutes.

[0098] Assume that the tolerable positioning error is (unit: m), then the period of telegram broadcasting to be set is When only the second-order error is considered, the following relationship exists:

[0099]

[0100] Step 4: Position calculation: the user receiver calculates the coordinates of the airborne platform under the ground fixed system;

[0101] After receiving the navigation ephemeris message calculated by the above adaptation scheme, the user receiver can solve the position of the airborne platform in the ECEF coordinate system. The specific steps are as follows:

[0102] Step 4.1: Calculate the regularization time ;

[0103]

[0104] in, is the present moment, It is the reference time of the message.

[0105] Step 4.2: Calculate the radius of curvature of the reference ellipsoid at the reference time ;

[0106]

[0107] in, The reference semi-major axis of the earth ellipsoid is 6378137.0m. is the deviation of the radius of curvature of the umbo ring of the reference ellipsoid at the reference time relative to the reference value;

[0108] Step 4.3: Calculate the regularization time The radius of curvature of the umbral circle of the reference ellipsoid ;

[0109]

[0110] Step 4.4: Calculate the corrected satellite average angular velocity ;

[0111]

[0112] Step 4.5: Calculate the regularization time The mean anomaly ;

[0113]

[0114] Step 4.6: Calculate the latitude argument ;

[0115]

[0116] Step 4.7: Calculate the corrected semi-major axis distance;

[0117]

[0118] in, It is the correction term of radial distance. The specific calculation method is the same as the original 18-parameter model;

[0119] Step 4.8: Calculate the corrected orbital inclination;

[0120]

[0121] in, It is the correction term for orbital inclination. The specific calculation method is the same as the original 18-parameter model.

[0122] Step 4.9: Calculate the corrected longitude of the ascending node;

[0123]

[0124] Step 4.10: Calculate the coordinates of the airborne platform in the ECEF coordinate system;

[0125] .

[0126] Dynamic adjustment of message parameters: When performing message adaptation under uniform linear motion, the current solution is to adapt the motion state of the platform by adjusting the message parameters. As an alternative, a filtering algorithm-based method such as the Extended Kalman Filter (EKF) can be used to dynamically adjust the message parameters. EKF can take into account the nonlinear characteristics of platform motion and adjust the message parameters in real time through prediction and update steps to adapt to the actual motion state of the platform, thereby improving the accuracy of the navigation message.

[0127] Optimization of broadcast frequency: Regarding the matching design of message broadcast frequency, the current solution uses simulation to determine the relationship between broadcast frequency and navigation accuracy. As an alternative, adaptive broadcast frequency technology can be used to dynamically adjust the message broadcast frequency according to the dynamic motion state of the air-based platform and the user-side receiving conditions. For example, when the air-based platform is maneuvering at high speed, the broadcast frequency is increased to provide more accurate navigation information; while in the stable flight phase, the broadcast frequency can be appropriately reduced to save resources.

[0128] Improved position solution: When the user receiver performs position solution, in addition to the traditional method of calculating the position of the airborne platform in the ECEF coordinate system, the Satellite Measurement Processing (SMP) technology can be used as an alternative. SMP technology can more accurately estimate the position of the airborne platform by comprehensively utilizing the measurement data of multiple satellites, especially in complex environments such as urban canyons or mountainous areas. SMP provides an improved solution method to improve the accuracy and reliability of positioning.

[0129] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A satellite navigation message format adaptation method for an air-based platform, characterized in that: The method comprises the following steps: Step 1, position acquisition and coordinate mapping of the air-based platform; used to convert the real-time position and motion state of the air-based platform into the earth-centered earth-fixed coordinate system through the earth-centered inertial coordinate system, so as to realize the real-time determination of the position of the air-based platform and the accurate prediction of the motion trajectory; Step 2, space-based platform adaptation of the satellite navigation message 18-parameter ephemeris model; used to adjust and simplify the standard 18-parameter ephemeris model according to the motion characteristics and environmental conditions of the space-based platform, so as to realize the application of satellite navigation message parameters used to characterize satellite orbits in the space-based platform, while reducing the resource requirements for navigation message transmission and processing, while maintaining positioning accuracy; Step 3, parameter adaptation of the air-based platform position: analyzing the motion state of the air-based platform, obtaining motion parameters including speed, acceleration, direction, etc., and calculating the adapted ephemeris parameters accordingly, and adjusting the navigation message parameters through a certain broadcast frequency design and modulating the navigation signal to the air-based platform, so as to improve the accuracy of calculating the air-based platform position based on the ephemeris parameters; Step 4: Position calculation: the user receiver receives the navigation signal broadcast by the space-based platform, calculates the adjusted and optimized ephemeris parameters, and calculates and calculates the precise position information of the space-based platform; Step 3 specifically includes the following steps: Step 3.1: Parameter adaptation of the airborne platform position at the initial moment; In the satellite orbit model, the longitude of the ascending node Expressed as ,in, Indicates the time difference between the current time and the reference time, also known as the planning time. For reference time The longitude of the ascending node, is the rate of change of the longitude of the ascending node, is the angular velocity of the Earth's rotation, which is ; The expressions for the longitude of the ascending node are evaluated as follows: ， In the satellite orbit model, the orbit inclination It is expressed as: ， in, is the orbital inclination of the hypothetical orbit at the reference time, is the first-order rate of change of the orbital inclination; , after spatial position analysis and simplification of message parameters, we get: ， In the formula, is the longitude, latitude of the space-based platform and the radius of curvature of the umbral circle of the reference ellipsoid; is the semi-major axis distance; And the longitude and latitude are converted into the coordinates of the Earth-centered Earth-fixed coordinate system by the following formula; ， In the formula is the three-dimensional coordinate of the air-based platform in the Earth-centered Earth-fixed coordinate system, is the eccentricity of the reference ellipsoid; Step 3.2: Adaptation of airborne platform motion parameters in uniform linear motion mode; According to the coordinates in the Earth-centered Earth-fixed coordinate system, the longitude and latitude, and the radius of curvature of the reference ellipsoid are as follows: ， ， ， because All about function, which is at the reference point Perform a second-order expansion at: ， Indicates that the function is at the point The function value at , which is used here to represent about Function of Represents the reference point for Taylor expansion of the function; Indicates that the function is at the reference point The function value at ; Indicates that the function is at the reference point The gradient vector at ; Represents the Hessian matrix of the function at the reference point; represents the coordinates of the hollow base platform in the geocentric inertial rectangular coordinate system; where, Indicates that the point is The coordinate values ​​on the axis, Indicates that the point is The coordinate values ​​on the axis, Indicates that the point is The coordinate values ​​on the axis, , From the reference point To any point The vector and the transpose of the vector; If the airborne platform moves at a constant speed in a piecewise steady-state system, the vector Expressed as , Air-based platform The speed of the axis; For example, derive: ， It represents the first derivative of longitude with respect to time, that is, the rate of change of longitude; represents the velocity vector of the airborne platform in the Earth-centered Earth-fixed coordinate system; Represents the gradient vector of longitude at the reference point, that is: ， Similarly: ， ， , They represent the first-order derivative of latitude and the radius of curvature of the reference ellipsoid with respect to time, namely, the latitude change rate and the semi-major axis change rate. represents the rate of change of the semi-major axis distance; , Respectively represent the latitude and the gradient vector of the radius of curvature of the reference ellipsoid at the reference point; Step 3.3: Adaptation of airborne platform motion parameters in uniformly accelerated linear motion mode; use To adapt to air-based platforms The accelerations in the three directions have the following relationship when using the uniformly accelerated linear motion model: ， In the formula They are respectively the air-based platform in the geocentric rectangular coordinate system Acceleration in three directions; The following model is constructed to adapt to the uniformly accelerated linear motion of the air-based platform: ， In the formula Represent the second derivative of longitude, latitude, and the radius of curvature of the reference ellipsoid with respect to time; , , They represent the second-order rates of change of the ascending node longitude, orbital inclination, and semi-major axis distance respectively; Represent the Hessian matrix of longitude, latitude, and radius of curvature of the reference ellipsoid at the reference point; Attribute the acceleration to middle, ， ， ； Among them, T is the broadcast period of the message, , , It is the message parameters actually broadcasted after correction; Step 3.4, second-order correction of message motion parameters; The second-order derivative is used to represent the second-order changes of the semi-major axis distance, orbital inclination, and ascending node longitude: ， The parameters advertised are: , , ; Among them, T is the broadcast period of the message; Step 3.5 Calculate the frequency of message broadcasting; Assume that the tolerable positioning error is , in m, the period of telegram broadcasting to be set When only the second-order error is considered, the following relationship exists: 。 2. The method according to claim 1, characterized in that Step 1 is specifically implemented as follows: converting the position of the airborne platform from the Earth-centered inertial coordinate system to the Earth-centered Earth-fixed coordinate system; The mapping process is described by the following formula: ， in, is the position coordinate of the air-based platform in the Earth-centered inertial coordinate system, It is the position coordinate converted to the Earth-centered Earth-fixed coordinate system; is the Earth's rotation angle, which represents the angle of transformation from the Earth-centered inertial coordinate system to the Earth-centered Earth-fixed coordinate system. It is calculated by the following formula: ， is the average rotation speed of the Earth, is the current time, is the reference time, which is 12 noon Greenwich Mean Time. and , expressed in a unified time unit.

3. The method according to claim 2, characterized in that Step 2 is specifically implemented as follows: the relationship between the latitude and longitude of the space-based platform in the Earth-centered Earth-fixed coordinate system and the orbital inclination and the longitude of the ascending node is: ， In the formula, are the latitude and longitude of the space-based platform in the Earth-centered Earth-fixed coordinate system, is the longitude of the ascending node of the hypothetical orbit of the space-based platform, is the orbital inclination.

4. The method according to claim 3, characterized in that Step 4 specifically includes the following steps: Step 4.1: Calculate the regularization time ; ， in, is the present moment, is the reference moment; Step 4.2: Calculate the radius of curvature of the reference ellipsoid at the reference time ; ， in, The reference semi-major axis of the earth ellipsoid is 6378137.0m. is the deviation of the radius of curvature of the umbo ring of the reference ellipsoid at the reference time relative to the reference value; Step 4.3: Calculate the regularization time The radius of curvature of the umbral circle of the reference ellipsoid ; ， in, It is the first derivative of the radius of curvature of the reference ellipsoid with respect to time, that is, the rate of change of the semi-major axis; Step 4.4: Calculate the corrected satellite average angular velocity ; ， in, It is the difference between the average angular velocity of the satellite at the reference time in the original message parameters and the calculated value. It is the average angular velocity of the satellite at the reference time in the original message parameters; Step 4.5: Calculate the regularization time The mean anomaly ; ， in, It is the mean anomaly of the reference time in the original message parameters; Step 4.6: Calculate the latitude argument ; ， Where, the argument of perigee is ω, is the eccentric anomaly angle derived from the mean anomaly angle, set to 0°; Step 4.7: Calculate the corrected semi-major axis distance ; ， in, is the correction term for the radial distance, and the specific calculation formula is: ， In the above formula Respectively represent the amplitude of the sine harmonic correction term of the orbit radius and the amplitude of the cosine harmonic correction term of the orbit radius in the original message parameters, which can be simplified as ; Step 4.8: Calculate the corrected orbital inclination; ， In the above formula, is the orbit inclination correction term, and the specific calculation formula is: ， In the above formula Respectively represent the amplitude of the sine harmonic correction term of the orbital inclination and the amplitude of the cosine harmonic correction term of the orbital inclination in the original message parameters, which can be simplified as ; Step 4.9: Calculate the corrected longitude of the ascending node; ， Step 4.10: Calculate the coordinates of the space-based platform in the Earth-centered Earth-fixed coordinate system; 。

Images