Space-based passive positioning method and system for very low frequency pulse source

Abstract

The present invention discloses a space-based passive positioning method and system for a very low frequency pulse source. The method includes the following steps: obtaining the receiving time, propagation path in the ionosphere, and propagation duration of the very low frequency signal emitted by the to-be-positioned very low frequency pulse source by a space-based platform receiver; obtaining the time when the very low frequency signal passes through the ionospheric incidence point based on the obtained receiving time, propagation path in the ionosphere, and propagation duration; obtaining the ionospheric incidence point coordinates based on the propagation path in the ionosphere and the preset receiving point coordinates; and obtaining the coordinates of the to-be-positioned very low frequency pulse source according to the time when passing through the ionospheric incidence point, the ionospheric incidence point coordinates, and the preset positioning conditions, so as to achieve passive positioning. The present invention can improve the positioning accuracy of the very low frequency pulse source.

Description

Technical Field

[0001] The invention belongs to the technical field of passive positioning of very low frequency pulse sources, and in particular relates to a space-based passive positioning method and system for very low frequency pulse sources. Background Art

[0002] Positioning very low frequency pulse sources (for example, lightning, aircraft, etc.) that can radiate very low frequency signals is an important technology related to the development of the national economy and national security. Taking lightning as an example, lightning includes cloud-to-ground lightning and ground-to-ground lightning. When it occurs, it will radiate electromagnetic fields with an extremely wide spectrum range. During the initial breakdown and channel establishment process, very high frequency radiation is mainly generated corresponding to the leader and streamer processes. When a strong current is generated in the post-ionization channel, low frequency and very low frequency radiation is mainly generated corresponding to the cloud-to-ground lightning return stroke process and cloud-to-cloud lightning.

[0003] Using the VLF (Very Low Frequency) and VHF (Very High Frequency) electromagnetic waves generated when lightning occurs, the existing mature lightning location methods mainly include ground-based VLF magnetic orientation method, ground-based VLF time arrival difference method, ground-based VLF time difference lateral hybrid positioning, and ground-based VHF interferometric positioning. Based on the analysis of actual positioning results, it can be seen that the magnetic orientation method has a large error and the interferometric method has a short detection distance. Therefore, the time difference method is currently commonly used for lightning location.

[0004] The ground-based time difference method is used because the echo waveform in the very low frequency band will drift and distort due to the influence of the propagation path and propagation distance, and the interference of topography, electromagnetic environment, etc. will cause errors in time measurement, making the actual detection error of the time difference method reach hundreds of meters to several kilometers. In summary, the accuracy of very low frequency pulse source positioning using the ground-based time difference method is not high enough, and improving the accuracy of very low frequency pulse source positioning is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The object of the present invention is to provide a space-based passive positioning method and system for a very low frequency pulse source, so as to improve the positioning accuracy of the very low frequency pulse source.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a space-based passive positioning method for a very low frequency pulse source, comprising the following steps:

[0008] Obtain the reception time of the space-based platform receiver of the very low frequency signal emitted by the very low frequency pulse source to be located, the propagation path in the ionosphere and the propagation duration;

[0009] Based on the receiving time of the space-based platform receiver, the propagation path in the ionosphere, and the propagation duration, obtain the time when the very low frequency (VLF) signal passes through the ionospheric incidence point; based on the propagation path in the ionosphere and the preset receiving point coordinates, obtain the ionospheric incidence point coordinates.

[0010] According to the time when passing through the ionospheric incidence point, the ionospheric incidence point coordinates, and the preset positioning conditions, obtain the coordinates of the VLF pulse source to be located, and realize passive positioning.

[0011] A further improvement of the method of the present invention lies in that the steps of obtaining the receiving time of the space-based platform receiver for the VLF signal emitted by the VLF pulse source to be located, the propagation path in the ionosphere, and the propagation duration include:

[0012] According to the reading of the on-board clock, obtain the time when the space-based platform receiver receives the VLF signal.

[0013] Based on the coordinates of the space-based platform receiver when receiving the VLF signal and the geomagnetic model, obtain the propagation path of the VLF signal in the ionosphere.

[0014] Based on the propagation path in the ionosphere and the International Ionosphere Model, obtain the propagation duration of the VLF signal along the propagation path in the ionosphere.

[0015] A further improvement of the method of the present invention lies in that the step of obtaining the propagation path of the VLF signal in the ionosphere based on the coordinates of the space-based platform receiver when receiving the VLF signal and the geomagnetic model includes:

[0016] Obtain the current coordinates of the space-based platform receiver in the ionosphere at the receiving time of the space-based platform receiver.

[0017] Determine the current trajectory of the Earth's magnetic field line passing through the current coordinates according to the geomagnetic model, and use the current trajectory as the propagation path of the VLF signal in the ionosphere.

[0018] A further improvement of the method of the present invention lies in that the step of obtaining the propagation duration of the VLF signal along the propagation path in the ionosphere based on the propagation path in the ionosphere and the International Ionosphere Model specifically includes:

[0019] Use Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path.

[0020] According to the calculated refractive index distribution, calculate the propagation speed of the electromagnetic wave at each point on the propagation path, and the calculation expression is v = n r c; where v is the propagation speed; n r is the real part of the refractive index n; c is the speed of light in vacuum;

[0021] The propagation time of the VLF signal at each position on the propagation path is calculated based on the calculated propagation speed, and the calculation expression is where θ is the magnetic dip angle at any point on the propagation path, d is the differential operator, dh is the height differential, ds is the propagation path length corresponding to the height differential, and dt is the time corresponding to the propagation distance ds;

[0022] The propagation duration of the VLF signal along the propagation path in the ionosphere is calculated by integrating the propagation time at each position on the propagation path, and the calculation expression is where t is the total propagation duration of the VLF signal on the propagation path; ∫ is the integral symbol; h1 is the height of the bottom of the ionosphere corresponding to the position O1 where the VLF electromagnetic wave passes.

[0023] A further improvement of the method of the present invention lies in that, in the process of using the Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path, the calculation expression is

[0024]

[0025] where n is the refractive index at the position where the electron concentration is N e , the electron temperature is T e , the ion temperature is T i ; the electron density N at any profile position of the ionosphere is determined according to the International Ionosphere Model e ;

[0026] ω p is the plasma frequency and m e is the electron mass; ε0 is the vacuum permittivity; ω is the frequency at the maximum amplitude in the FFT spectrum analysis of the on-board receiver signal;

[0027] j is the imaginary unit;

[0028] γ e is the collision frequency and γ e = 1.82×10 11 e -0.15h ; h is the height of the satellite receiver;

[0029] is the electron magnetic cyclotron frequency;

[0030] Y T = Ysinα; Y L = Ycosα; α is the angle between the wave vector and the magnetic field, α = 0.

[0031] A further improvement of the method of the present invention lies in that the step of obtaining the coordinates of the VLF pulse source to be located and realizing passive positioning according to the time when passing through the ionospheric incidence point, the ionospheric incidence point coordinates, and the preset positioning conditions specifically includes:

[0032] When the number of receivers on the space-based platform is two and the emission time and emission height of the VLF pulse source are known, according to the receiving point coordinates, the propagation path and propagation duration in the ionosphere, the ionospheric incidence point coordinates and time of the VLF signal are obtained; according to the difference between the emission time and the time when the VLF signal passes through the ionospheric incidence point, the propagation distance of the VLF signal in the atmosphere is obtained; the VLF pulse source is located according to the ionospheric incidence point coordinates, the time when the VLF signal passes through the ionospheric incidence point, and the emission height;

[0033] Alternatively, when the number of receivers on the space-based platform is three and the emission height is known, according to the receiving point coordinates, the propagation path and propagation duration in the ionosphere, the ionospheric incidence point coordinates and time of the VLF signal are obtained; the time difference for the VLF signal to reach the bottom of the ionosphere is calculated according to the time when the VLF signal passes through the ionospheric incidence point, and then the coordinate position of the VLF pulse source is calculated by using the time difference positioning method.

[0034] A further improvement of the method of the present invention lies in that in the process of calculating the time difference for the VLF signal to reach the bottom of the ionosphere according to the time when the VLF signal passes through the ionospheric incidence point, and then calculating the coordinate position of the VLF pulse source by using the time difference positioning method,

[0035] First, use the formula Δt2 = t 11 -t 21 to calculate the theoretical time difference in the ionosphere; in the formula, Δt2 is the theoretical time difference for the receiver to receive the VLF signal; t 11 is the propagation duration of the VLF signal corresponding to the first receiver in the ionosphere; t 21 is the propagation duration of the VLF signal corresponding to the second receiver in the ionosphere;

[0036] Then, use the formula Δt1 = Δt - Δt2 to calculate the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere; in the formula, Δt1 is the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere; Δt is the total time difference for the receiver to receive the VLF signal;

[0037] Based on the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere, the coordinate position of the VLF pulse source is located by using the time difference positioning method.

[0038] A further improvement of the method of the present invention lies in that the preset positioning conditions include one or a combination of more of the following: emission time, emission altitude, and the number of receivers.

[0039] A space-based passive positioning system for a very low frequency pulse source provided by the present invention includes:

[0040] A first acquisition module, configured to acquire the reception time, propagation path in the ionosphere, and propagation duration of the very low frequency signal emitted by the to-be-positioned very low frequency pulse source by a space-based platform receiver;

[0041] A second acquisition module, configured to, based on the acquired reception time, propagation path in the ionosphere, and propagation duration of the space-based platform receiver, acquire the time when the very low frequency signal passes through the ionospheric incidence point; and acquire the ionospheric incidence point coordinates based on the propagation path in the ionosphere and the preset receiver point coordinates;

[0042] A positioning module, configured to acquire the coordinates of the to-be-positioned very low frequency pulse source according to the time when passing through the ionospheric incidence point, the ionospheric incidence point coordinates, and the preset positioning conditions, so as to achieve passive positioning.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] In the method of the present invention, the propagation path is determined by using the coordinates of the receiver and the geomagnetic model, and then the propagation path of the very low frequency signal in the ionosphere is determined according to the propagation path, and then the time when the very low frequency signal enters the ionosphere is obtained. Furthermore, a more accurate time and position for positioning the very low frequency pulse source can be obtained; the method of the present invention is based on a space-based platform (such as a satellite) receiver. Compared with the existing ground-based method, it can reduce the influence of the ground electromagnetic environment such as terrain and ground conductivity on the radio wave propagation time, and can improve the positioning accuracy of the pulse source. Especially for electromagnetic environment complex areas such as mountains and the sea-land boundary, compared with ground-based positioning, the positioning accuracy of the VLF pulse source can be greatly improved.

[0045] In addition, with the more accurate time and coordinates when the very low frequency signal enters the ionosphere, by calculating the time difference of different received very low frequency signals propagating from the pulse source to the bottom of the ionosphere, the very low frequency pulse source can be more accurately positioned by using the time difference positioning method. Description of the Drawings

[0046] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the drawings required for description in the embodiments or the prior art; obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0047] Figure 1 Schematic flow chart of a space - based passive positioning method for a very - low - frequency pulse source provided by an embodiment of the present invention;

[0048] Figure 2 Schematic principle diagram of a space - based passive positioning method for a very - low - frequency pulse source provided by an embodiment of the present invention;

[0049] Figure 3 Schematic diagram of a propagation path of lightning VLF waves in an embodiment of the present invention;

[0050] Figure 4 Another schematic diagram of a propagation path of lightning VLF waves in an embodiment of the present invention. Detailed implementation manners

[0051] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0052] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above - mentioned drawings are used to distinguish similar objects, and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non - exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0053] The present invention will be further described in detail below with reference to the accompanying drawings:

[0054] A space - based passive positioning method for a very - low - frequency pulse source provided by an embodiment of the present invention includes the following steps:

[0055] Based on the coordinates of the space - based platform receiver when receiving the very - low - frequency signal and the geomagnetic model, obtain the propagation path of the very - low - frequency signal in the ionosphere;

[0056] Determine the propagation duration of the very low frequency (VLF) signal in the ionosphere along the propagation path based on the propagation path and the international ionosphere model, and determine the time when the VLF signal passes through the incident point and the coordinates of the incident point according to the time when the receiver receives the VLF signal and the propagation duration;

[0057] Perform VLF pulse positioning according to the time when passing through the incident point, the coordinates of the incident point, and the positioning conditions; wherein, the positioning conditions include one or a combination of the following: transmission time, transmission altitude, and the number of receivers.

[0058] Exemplarily and optionally in an embodiment of the present invention, when the transmission time and the transmission altitude of the VLF pulse source are known, the step of performing VLF pulse positioning according to the time when passing through the incident point, the coordinates of the incident point, and the positioning conditions specifically includes:

[0059] Determine the propagation distance of the VLF signal in the atmosphere according to the difference between the transmission time and the time when the VLF signal passes through the incident point;

[0060] Locate the VLF pulse source according to the time when the VLF signal passes through the incident point and the transmission altitude.

[0061] Exemplarily and optionally in an embodiment of the present invention, when the number of receivers is two and the transmission altitude is known, the performing VLF pulse positioning according to the time when passing through the incident point, the coordinates of the incident point, and the positioning conditions includes:

[0062] Calculate the time difference when the VLF signal reaches the bottom of the ionosphere according to the time when the VLF signal passes through the incident point, and then calculate the position of the VLF pulse source by using the time difference positioning method.

[0063] More specifically, the obtaining the propagation path of the VLF signal in the ionosphere includes:

[0064] Obtain the current coordinates of the receiver in the ionosphere at the current moment, determine the current trajectory of the Earth's magnetic field line passing through the current coordinates according to the geomagnetic model, and use the current trajectory as the propagation path of the VLF signal in the ionosphere, wherein the current moment is the moment when the receiver receives the VLF signal.

[0065] Exemplarily and optionally in an embodiment of the present invention, the determining the propagation duration of the VLF signal in the ionosphere along the propagation path based on the propagation path and the international ionosphere model, and determining the time when the VLF signal passes through the incident point and the coordinates of the incident point according to the time when the receiver receives the VLF signal and the propagation duration includes:

[0066] Read out the time when the satellite receives the VLF signal according to the on-board clock;

[0067] Calculate the propagation duration of the very low frequency (VLF) signal along the propagation path in the ionosphere based on the international ionosphere model;

[0068] According to the time when the satellite receives the VLF signal and the propagation duration, calculate the time when the VLF signal propagating along the propagation path reaches the bottom of the ionosphere.

[0069] Exemplarily and optionally in an embodiment of the present invention, the calculating the propagation duration of the VLF signal along the propagation path in the ionosphere based on the international ionosphere model includes:

[0070] Use the Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path;

[0071] According to the refractive index distribution, use the formula, v = n r c, to calculate the propagation speed of the electromagnetic wave at each point on the propagation path, where

[0072] v is the propagation speed of the electromagnetic wave; n r is the real part of the refractive index n; c is the speed of light in vacuum;

[0073] According to the propagation speed, use the formula, to calculate the propagation time of the VLF signal at a certain position on the propagation path, where θ is the magnetic dip angle at any point in the propagation path;

[0074] According to the formula, calculate the propagation duration of the VLF signal along the propagation path in the ionosphere, where t is the total propagation duration of the VLF signal on the propagation path; ∫ is the integral symbol; h1 is the height of the bottom of the ionosphere corresponding to the O1 position passed by the VLF electromagnetic wave.

[0075] Optionally, the using the Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path includes:

[0076] Use the formula, to calculate the refractive index distribution at each point on the propagation path, where n is the refractive index at the position where the electron concentration is N e and the electron temperature is T e and the ion temperature is T i at that point, that is, the electron density N at any profile position of the ionosphere can be determined according to the international ionosphere reference model e ; w P is the plasma frequency, and ω is the frequency at the maximum amplitude in the FFT spectrum analysis of the on-board receiver signal; m e is the electron mass; ε0 is the vacuum permittivity; j is the imaginary unit; γ eis the collision frequency, and γ e = 1.82×10 11 e -0.15h ; h is the height of the satellite; Y T = Ysinα; Y L = Ycosα; α is the angle between the wave vector and the magnetic field. Since VLF waves propagate quasi-longitudinally along the magnetic field lines in the ionosphere, α = 0 and

[0077] Exemplarily and optionally in an embodiment of the present invention, calculating the time difference when the VLF signal reaches the bottom of the ionosphere based on the time when the VLF signal passes through the incident point, and then calculating the position of the VLF pulse source by using the time difference positioning method, includes:

[0078] Using the formula, Δt2 = t 11 -t 21 , to calculate the time difference of propagation in the ionosphere, where

[0079] Δt2 is the theoretical time difference for the receiver to receive the VLF signal; t 11 is the propagation duration of the VLF signal corresponding to the first receiver in the ionosphere; t 21 is the propagation duration of the VLF signal corresponding to the second receiver in the ionosphere;

[0080] Calculating the total time difference for the receiver to receive the VLF signal according to the time when the receiver receives the VLF signal;

[0081] Then using the formula, Δt1 = Δt - Δt2, to calculate the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere, where

[0082] Δt1 is the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere; Δt is the total time difference for the receiver to receive the VLF signal;

[0083] Positioning the position of the VLF pulse source by using the time difference positioning method according to the propagation time difference for the VLF signal received by the receiver to reach the bottom of the ionosphere.

[0084] In summary, the embodiment of the present invention uses the coordinates of the receiver and the geomagnetic model to determine the propagation path, and then determines the propagation path of the VLF signal in the ionosphere according to the propagation path, and then obtains the time when the VLF signal enters the ionosphere, and then can obtain a more accurate time and position for positioning the VLF pulse source, thereby improving the positioning accuracy of the pulse source. In addition, with the more accurate time and coordinates when the VLF signal enters the ionosphere, by calculating the time difference of the VLF signals received at different times during the propagation from the pulse source to the bottom of the ionosphere, the VLF pulse source can be more accurately positioned by using the time difference positioning method.

[0085] Example 1

[0086] Please refer to Figure 1 and Figure 2 , the application scenario of Example 1 of the present invention is that at least three satellites operating in the ionosphere are used as the carrier platform of the receiver to perform cloud-to-ground flash or cloud flash positioning. Figure 1 It is a schematic flow chart of a space-based passive positioning method for a very low frequency pulse source provided by an embodiment of the present invention; Figure 2 It is a schematic principle diagram of a space-based passive positioning method for a very low frequency pulse source provided by an embodiment of the present invention; As Figure 1 and Figure 2 shown, an embodiment of the present invention provides a space-based passive positioning method for a very low frequency pulse source, and the method includes:

[0087] S101: Based on the coordinates of the receiver when receiving the very low frequency signal and the geomagnetic model, obtain the propagation path of the very low frequency signal in the ionosphere.

[0088] In the embodiment of the present invention, lightning is used as the very low frequency pulse source, and the receiver is set on a satellite operating in the ionosphere as an example for principle explanation. It can be understood that the very low frequency pulse source can be a very low frequency pulse with a short existence time in the time domain, such as a very low frequency signal with an existence duration of 1 ms, 1000 ms, 1 minute, 10 minutes, etc., or a signal corresponding to a sudden change in amplitude or a sudden change in frequency in a continuous very low frequency wave. In the embodiment of the present invention, the very low frequency pulse signal and its change characteristics are not specifically limited, and any very low frequency signal that can locate the very low frequency radiation source by using the time difference positioning method based on two or more receivers is within the protection scope of the embodiment of the present invention.

[0089] Regarding the setting of the number of receivers, when general detection of very low frequency signals is required, two receivers can be used. When precise detection is required, three or more receivers can be used. Those skilled in the art can set the number of receivers according to the actual application scenario. It should be emphasized that the detection methods for very low frequency signal sources using two or more receivers are all within the protection scope of the embodiment of the present invention.

[0090] For the setting of the platform carrying the receiver, the receiver can be set on vehicles such as satellites, airplanes, rockets, near-space vehicles, etc.; similarly, the same vehicle can carry at least one very low frequency (VLF) receiver. For example, it can be jointly measured by three satellites carrying VLF receivers distributed in different orbits or at different positions on the same orbit, or detected by a single satellite carrying three VLF receivers, or detected by a satellite carrying one VLF receiver in combination with a satellite carrying two VLF receivers. Similarly, the operating altitude of the carrying platform can be within the ionosphere or outside the ionosphere.

[0091] Lightning occurring at the perigee has its frequency mainly concentrated in the VLF (very low frequency) band. The VLF wave propagates from the near ground through the atmospheric waveguide to the bottom of the ionosphere. In the ionosphere, the VLF lightning signal propagates along the magnetic field lines, penetrates the ionosphere and reaches the satellite altitude, and its signal is received by the on-board receiver.

[0092] In the embodiment of the present invention, two satellites are taken as an example to calculate the total propagation duration of the lightning very low frequency electromagnetic wave along the propagation path in the ionosphere. It can be understood that the calculation principle for three or more satellites is the same as that for two satellites. Figure 3 It is a schematic diagram of the propagation path of the lightning VLF wave in the embodiment of the present invention, as Figure 3 shown. Assume that the altitude of the satellites is both h, and the two satellites are S1 and S2 respectively; the two satellites operate in the ionosphere. The times when the two satellites receive the lightning VLF pulse signals are t1 and t2 respectively. The times when the two satellites receive the lightning signals can be divided into two parts. One part is the time for the lightning pulse signal to propagate freely from the near surface through the atmosphere to the bottom of the ionosphere, and the other part is the time for it to propagate from the bottom of the ionosphere along the magnetic field lines in the ionosphere to the satellite.

[0093] First, the magnetic field line direction of the lightning VLF wave propagating in the ionosphere is determined by the position of the spaceborne receiver and the geomagnetic model. In practical applications, the geomagnetic model can be the IGRF (International Geomagnetic Reference Field), which is a digital model used to calculate the Earth's main magnetic field, i.e., the core magnetic field, from 1900 to the present. It is established and maintained by the geomagnetic model team supported and sponsored by the V-MOD working group of the International Association of Geomagnetism and Aeronomy (IAGA). According to the IGRF model, the magnetic field line path around the Earth can be determined. Since the very low frequency electromagnetic wave propagates along the magnetic field line path in the ionosphere, the point where the very low frequency electromagnetic wave enters the bottom of the ionosphere to determine the coordinates has uniqueness in the propagation path in the ionosphere. Therefore, the coordinates of the point where the very low frequency electromagnetic wave enters the bottom of the ionosphere can be calculated based on any point on the propagation path of the very low frequency electromagnetic wave combined with the international geomagnetic model. Therefore, in the embodiments of the present invention, the magnetic field line path passed by the satellite when receiving the very low frequency signal can be determined according to the coordinates of the satellite and the international geomagnetic model, and then this magnetic field line path is used as the propagation path of the very low frequency signal in the ionosphere. Furthermore, the coordinates of the point where the very low frequency signal received by the satellite enters the bottom of the ionosphere are determined according to the intersection point of this propagation path and the bottom of the ionosphere.

[0094] The incident point where the lightning VLF wave received by satellite S1 passes through the bottom of the ionosphere in the propagation path is O1, and the incident point where the lightning VLF wave received by satellite S2 passes through the bottom of the ionosphere in the propagation path is O2.

[0095] S102: Based on the propagation path and the international ionosphere model, determine the propagation duration of the very low frequency signal along the propagation path in the ionosphere, and determine the time when the very low frequency signal passes through the incident point and the coordinates of the incident point according to the time when the receiver receives the very low frequency signal and the propagation duration.

[0096] For each propagation path, after obtaining the incident points where the very low frequency electromagnetic wave passes through the positions of O1 and O2, it is also necessary to obtain the time t when the VLF wave freely propagates through the atmosphere from the near surface to the bottom of the ionosphere 10 and the time t when it propagates from the bottom of the ionosphere along the magnetic field line to the satellite in the ionosphere 11 ; The time t when the VLF wave freely propagates through the atmosphere from the near surface to the bottom of the ionosphere 20 and the time t when it propagates from the bottom of the ionosphere along the magnetic field line to the satellite in the ionosphere 21 .

[0097] The times t1 and t2 when two satellites receive the lightning VLF pulse signal can be read according to the on-board clock.

[0098] Then, calculate the propagation time of the very low frequency electromagnetic wave along each propagation path L in the ionosphere:

[0099] Using the Appleton-Hartree (A-H) formula in magnetoionic theory, that is, the dispersion relation for the propagation of electromagnetic waves in a plasma, calculate the refractive index distribution at each point on the propagation path L, where n is the refractive index at the point where the electron concentration is N e , the electron temperature is T e , and the ion temperature is T i ; that is to say, the electron density N at any profile position in the ionosphere can be determined according to the International Reference Ionosphere Model e ; w P is the plasma frequency, and ω is the frequency at the maximum amplitude in the FFT spectrum analysis of the satellite-borne receiver signal; m e is the electron mass; ε0 is the vacuum permittivity; j is the imaginary unit; γ e is the collision frequency, and γ e = 1.82×10 11 e -0.15h ; h is the height of the satellite; Y T = Ysinα; Y L = Ycosα; α is the angle between the wave vector and the magnetic field. Since the VLF wave propagates quasi-longitudinally along the magnetic field line in the ionosphere, α = 0 and

[0100] The International Reference Ionosphere Model, IGRF model, and collision frequency model can jointly calculate the values of X, Y, and Z in the A-H formula, from which the refractive index n at any point on the propagation path of the lightning VLF wave in the ionosphere can be obtained.

[0101] Then, according to the relationship expression between the refractive index and the wave propagation speed: v = n r c, calculate the propagation speed of the electromagnetic wave at each point on the propagation path, where v is the propagation speed of the electromagnetic wave; n r is the real part of the refractive index n; c is the speed of light in vacuum.

[0102] Then, use the formula, to calculate the propagation time of the very low frequency signal at a certain position on the propagation path, where dt is the propagation time of the very low frequency signal at a certain position on the propagation path; θ is the magnetic dip angle at any point in the propagation path, the magnetic field strength is B; h is the height of the satellite carrying the receiver.

[0103] Then, use the formula, Calculate the propagation time of the lightning very low frequency (VLF) electromagnetic wave in the ionosphere along the propagation path, where \(t\) is the propagation time of the lightning VLF electromagnetic wave in the ionosphere along the propagation path; \(\int\) is the integral symbol; \(h_1\) is the height of the bottom of the ionosphere corresponding to the incident point \(O_1\) through which the VLF electromagnetic wave passes. Similarly, there is also a corresponding height of the bottom of the ionosphere when the VLF electromagnetic wave passes through the incident point \(O_2\). Thus, the propagation time \(t\) of the lightning VLF wave received by the two satellites in the ionosphere is calculated. 11 and \(t\) 21 .

[0104] The times when the two satellites receive the lightning VLF pulse signals are \(t_1\) and \(t_2\) respectively, and \(t_1\) and \(t_2\) can be read from the on-board clocks of the satellites. Therefore, the formula can be used to calculate the moment \(t\) when the lightning pulse signal freely propagates through the atmosphere from near the ground to reach the incident point \(O_1\) at the bottom of the ionosphere. 10 the moment \(t\) at the incident point \(O_2\) at the bottom position 20 . Similarly, the above method can also be used to calculate the moment \(t\) when the VLF signal corresponding to satellite S3 passes through the incident point \(O_3\). 30 .

[0105] S103: Perform very low frequency (VLF) pulse positioning according to the moment when passing through the incident point, the coordinates of the incident point, and the positioning conditions, where the positioning conditions include: one or a combination of the emission moment, the emission height, and the number of receivers. For example, the time difference when the VLF signal reaches the bottom of the ionosphere can be calculated according to the moment when the VLF signal passes through the incident point, and then the position of the VLF pulse source can be calculated using the time difference positioning method.

[0106] Exemplarily, Figure 4 is another schematic diagram of the propagation path of the lightning VLF wave in the embodiment of the present invention. As Figure 4 shown, the position where the lightning VLF wave received by satellite S1 passes through the bottom of the ionosphere in the propagation path is \(O_1\), the position where the lightning VLF wave received by satellite S2 passes through the bottom of the ionosphere in the propagation path is \(O_2\), the point where the lightning VLF wave received by satellite S3 passes through the bottom of the ionosphere in the propagation path is \(O_3\). According to the propagation time difference \(\Delta t_1\) between the lightning VLF signals received by satellite S1 and satellite S2 when propagating to between \(O_1\) and \(O_2\) at the bottom of the ionosphere.

[0107] According to \(\Delta t_1\), a hyperboloid about \(O_1\) and \(O_2\) can be obtained, and there is an intersection line \(L_1\) between the surface and the ground; similarly, a hyperboloid about \(O_2\) and \(O_3\) can be obtained, and there is an intersection line \(L_2\) between the surface and the ground. The intersection point of the intersection line \(L_1\) and the intersection line \(L_2\) is the lightning position.

[0108] Similarly, the height at which lightning occurs can be calculated. When the located lightning position has a certain height relative to the ground, the lightning can be determined as cloud lightning. Similarly, the three-dimensional coordinates of an aircraft capable of emitting very low frequency pulses can be located.

[0109] In the embodiment of the present invention, first, according to the IGRF and the international ionosphere reference model, the propagation path of the lightning VLF wave along the magnetic field line in the ionosphere, as well as the magnetic dip angle, magnetic field strength, electron density, and collision frequency on the path, can be obtained. The refractive index on the propagation path can be obtained from the A-H formula. According to the relationship between the wave propagation speed and the refractive index, the propagation time of the very low frequency pulse electromagnetic wave in the ionosphere can be calculated, and then the time difference of the lightning signals received by three satellites propagating in the ionosphere can be obtained. Subtracting the total time difference of the lightning signals received by the three satellites from the time difference of propagation in the ionosphere, the time difference of the lightning signals received by the three satellites propagating from the ground to the bottom of the ionosphere can be obtained. The two hyperboloids obtained intersect at a point on the ground, and thus the position of the very low frequency pulse source can be located.

[0110] Applying the embodiment of the present invention, the receiver is installed on the high-altitude carrying platform in the ionosphere, avoiding errors caused by terrain and conductivity, etc., and improving the positioning accuracy. Moreover, since the position of the carrying platform is higher and its receiving range is larger, the detection distance of the embodiment of the present invention is longer.

[0111] Embodiment 2

[0112] The difference between Embodiment 2 and Embodiment 1 of the present invention lies in the calculation method of the time difference of the incident point O1 where the lightning VLF wave reaches the bottom of the ionosphere:

[0113] The time difference Δt2 between the two lightning VLF waves received by two satellites propagating in the ionosphere can be calculated according to the propagation durations t 11 and t 21 using the formula Δt2 = t 11 -t 21 .

[0114] Then, according to the times t1 and t2 when the two satellites receive the lightning VLF pulse signals, the total time difference Δt between the two satellites can be calculated;

[0115] Then, using the formula Δt1 = Δt - Δt2, the time difference between the lightning VLF signals received by satellite S1 and satellite S2 reaching points O1 and O2 can be calculated.

[0116] Embodiment 3

[0117] In Embodiment 3 of the present invention, based on Embodiment 1, when the number of receivers installed on the carrying platform is two and the emission height of the very low frequency pulse source is known, the specific positioning method can be:

[0118] When the emission height of the very low frequency (VLF) pulse source is at the ground, a hyperboloid about O1 and O2 can be obtained according to Δt1. There is an intersection line L1 between the hyperboloid and the ground, and the intersection line L1 is taken as the positioning result of the VLF pulse source.

[0119] Furthermore, when the emission height of the VLF pulse source is at a certain height from the ground, the positioning result of the VLF pulse source can also be obtained by using a similar method as above.

[0120] Embodiment 4

[0121] In Embodiment 4 of the present invention, based on Embodiment 1, when the emission time and emission height of the VLF pulse source are known, the VLF pulse is positioned according to the time when passing through the incident point, the coordinates of the incident point, and the positioning conditions.

[0122] For example, according to the difference between the emission time of the VLF pulse source and the time when the VLF pulse signal arrives at the incident point O1, the running distance of the VLF pulse signal in the atmospheric layer between the ground and the bottom of the ionosphere is calculated. Then, a circular ring can be made on the plane where the emission height of the VLF pulse source is located, and thus the positioning result of the VLF pulse source is obtained.

[0123] It can be understood that the positioning results of the VLF pulse source can be obtained in Embodiment 3 and Embodiment 4. Other devices can be used as assistance. For example, the direction finding result of a ground side-looking instrument is used as a reference to obtain the accurate position of the VLF pulse source. That is to say, the positioning of the VLF pulse source can be carried out by combining a space-based receiver and a ground-based direction finding device such as a receiver.

[0124] Embodiment 5

[0125] Based on Embodiment 1, when the carrying platform operates in space outside the ionosphere, a receiver with a side-looking function can be installed on the carrying platform. For example, an electromagnetic vector sensor disclosed in the Chinese patent application with the application number 2020100801132 is used to orient the VLF signal, and the path of the VLF signal from leaving the ionosphere to the receiver is determined according to the position of the receiver and the orientation result. Then, the coordinates of the exit point where the VLF signal leaves the ionosphere are determined. Furthermore, the magnetic field line passing through the exit point is determined according to the coordinates of the exit point, and the trajectory of the magnetic field line in the ionosphere is taken as the propagation path of the VLF signal in the ionosphere. Then, the method based on Embodiment 1 is used to position the VLF pulse source.

[0126] Embodiment 6

[0127] Embodiment 6 provides a space-based passive positioning system for a VLF pulse source, and the system includes:

[0128] An acquisition module, configured to acquire a propagation path of a very low frequency (VLF) signal in the ionosphere based on coordinates at the moment when a receiver receives the VLF signal and a geomagnetic model;

[0129] A calculation module, configured to determine a propagation duration of the VLF signal in the ionosphere along the propagation path based on the propagation path and an international ionosphere model, and determine a moment when the VLF signal passes through an incident point and coordinates of the incident point according to the moment when the receiver receives the VLF signal and the propagation duration;

[0130] A positioning module, configured to perform VLF pulse positioning according to the moment when the VLF signal passes through the incident point, the coordinates of the incident point, and positioning conditions, where the positioning conditions include one or a combination of a transmission moment, a transmission altitude, and the number of receivers.

[0131] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0132] The present application is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or block in the flowcharts and / or block diagrams, and the combination of processes and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in one process Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0133] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured article including instruction means, and the instruction means implements the functions specified in one process Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks.

[0134] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus, so that a series of operation steps are executed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions for implementing the steps of the process Figure 1 one process or a plurality of processes and / or blocks Figure 1 steps for the functions specified in one block or a plurality of blocks.

[0135] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific embodiments of the present invention or make equivalent replacements. Any modification or equivalent replacement without departing from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A space-based passive positioning method for a very low frequency pulse source, characterized in that, The method includes the following steps: Obtaining the receiving time, propagation path in the ionosphere, and propagation duration of the very low frequency (VLF) signal emitted by the VLF pulse source to be located, by a space-based platform receiver; Based on the obtained receiving time, propagation path in the ionosphere, and propagation duration of the space-based platform receiver, obtaining the time when the VLF signal passes through the ionospheric entry point; based on the propagation path in the ionosphere and the preset receiving point coordinates, obtaining the ionospheric entry point coordinates; According to the time when passing through the ionospheric entry point, the ionospheric entry point coordinates, and the preset positioning conditions, obtaining the coordinates of the VLF pulse source to be located, so as to achieve passive positioning; Wherein, The step of obtaining the receiving time, propagation path in the ionosphere, and propagation duration of the VLF signal emitted by the VLF pulse source to be located includes: obtaining the time when the space-based platform receiver receives the VLF pulse signal according to the on-board clock reading; based on the coordinates of the space-based platform receiver when receiving the VLF signal and the geomagnetic model, obtaining the propagation path of the VLF signal in the ionosphere; based on the propagation path in the ionosphere and the international ionosphere model, obtaining the propagation duration of the VLF signal along the propagation path in the ionosphere; The step of obtaining the propagation duration of the VLF signal along the propagation path in the ionosphere based on the propagation path in the ionosphere and the international ionosphere model specifically includes: Using Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path; Calculate the propagation speed of electromagnetic waves at each point on the propagation path according to the refractive index distribution obtained by calculation. The calculation formula is v = n r c; where v is the propagation speed; n r is the real part of the refractive index n; c is the speed of light in vacuum; The propagation time of the VLF signal at each position on the propagation path is calculated based on the calculated propagation speed, and the calculation expression is where θ is the magnetic dip angle at any point in the propagation path, d is the differential operator, dh is the height differential, ds is the propagation path length corresponding to the height differential, and dt is the time corresponding to the propagation distance ds; The propagation duration of the VLF signal along the propagation path in the ionosphere is obtained by integrating the propagation time at each position on the propagation path. The calculation expression is as follows: In the formula, t is the total propagation duration of the VLF signal on the propagation path; ∫ is the integral symbol; h1 is the height of the bottom of the ionosphere corresponding to the O1 position passed by the VLF electromagnetic wave.

2. The space-based passive positioning method for a very low frequency pulse source according to claim 1, characterized in that, The step of obtaining the propagation path of the VLF signal in the ionosphere based on the coordinates of the space-based platform receiver when receiving the VLF signal and the geomagnetic model includes: Obtaining the current coordinates of the space-based platform receiver in the ionosphere when the space-based platform receiver receives the VLF pulse signal; Determining the current trajectory of the Earth's magnetic field line passing through the current coordinates according to the geomagnetic model, and taking the current trajectory as the propagation path of the VLF pulse signal in the ionosphere.

3. The space-based passive positioning method for a very low frequency pulse source according to claim 1, characterized in that, During the process of using Appleton-Hartley to calculate the refractive index distribution at each point on the propagation path, the calculation expression is where n is the refractive index at the position where the electron concentration in the propagation path is N e , the electron temperature is T e , and the ion temperature is T i ; the electron density N at any profile position of the ionosphere is determined according to the international ionosphere model e ; ω p is the plasma frequency and m e is the electron mass; ε0 is the vacuum permittivity; ω is the frequency at the maximum amplitude in the FFT spectrum analysis of the on-board receiver signal; j is the imaginary unit; γ e is the collision frequency and γ e = 1.82×10 11 e -0.15h ; h is the height of the satellite receiver; is the electron cyclotron frequency; Y T = Y sinα; Y L = Y cosα; α is the angle between the wave vector and the magnetic field, α = 0.

4. The space-based passive positioning method of a very low frequency pulse source according to claim 1, characterized in that, The step of obtaining the coordinates of the VLF pulse source to be located according to the time when passing through the ionospheric entry point, the ionospheric entry point coordinates, and the preset positioning conditions, so as to achieve passive positioning specifically includes: When the number of space-based platform receivers is two and the emission time and emission altitude of the VLF pulse source are known, according to the receiving point coordinates, the propagation path in the ionosphere, and the propagation duration, obtaining the ionospheric entry point coordinates and time of the VLF signal; according to the difference between the emission time and the time when the VLF signal passes through the ionospheric entry point, obtaining the propagation distance of the VLF signal in the atmosphere; positioning the VLF pulse source according to the ionospheric entry point coordinates, the time when the VLF signal passes through the ionospheric entry point, and the emission altitude; Alternatively, when the number of space-based platform receivers is three and the launch altitude is known, based on the receiving point coordinates, the propagation path and propagation duration in the ionosphere, obtain the ionospheric entry point coordinates and time of the VLF signal; calculate the time difference of the VLF signal reaching the bottom of the ionosphere according to the time when the VLF signal passes through the ionospheric entry point, and then use the time difference positioning method to calculate the coordinate position of the VLF pulse source.

5. The space-based passive positioning method of a very low frequency pulse source according to claim 4, characterized in that, In the process of calculating the time difference of the VLF signal reaching the bottom of the ionosphere according to the time when the VLF signal passes through the ionospheric entry point, and then using the time difference positioning method to calculate the coordinate position of the VLF pulse source, First, use the formula Δt2 = t 11 -t 21 to calculate the theoretical time difference of propagation in the ionosphere; where Δt2 is the theoretical time difference of the receiver receiving the VLF signal; t 11 is the propagation duration of the VLF signal corresponding to the first receiver in the ionosphere; t 21 is the propagation duration of the very low frequency signal corresponding to the second receiver in the ionosphere; calculate the propagation time difference of the VLF signal received by the receiver reaching the bottom of the ionosphere by using the formula Δt1 = Δt - Δt2; where Δt1 is the propagation time difference of the VLF signal received by the receiver reaching the bottom of the ionosphere; Δt is the total time difference of the VLF signal received by the receiver. Based on the propagation time difference of the VLF signal received by the receiver reaching the bottom of the ionosphere, use the time difference positioning method to locate the coordinate position of the VLF pulse source.

6. The space-based passive positioning method of a very low frequency pulse source according to claim 1, characterized in that, The preset positioning conditions include one or more combinations of the launch time, launch altitude, and the number of receivers.

7. A passive positioning system for a space-based very low frequency pulse source, characterized in that, A space-based passive positioning method for the VLF pulse source according to claim 1, comprising: A first acquisition module, configured to acquire the receiving time, propagation path, and propagation duration in the ionosphere of the VLF signal emitted by the VLF pulse source to be located by a space-based platform receiver; A second acquisition module, configured to obtain the time when the VLF signal passes through the ionospheric entry point based on the acquired receiving time, propagation path, and propagation duration of the space-based platform receiver in the ionosphere; obtain the ionospheric entry point coordinates based on the propagation path in the ionosphere and the preset receiving point coordinates; A positioning module, configured to obtain the coordinates of the VLF pulse source to be located according to the time when passing through the ionospheric entry point, the ionospheric entry point coordinates, and the preset positioning conditions, so as to achieve passive positioning.

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