CN111781594B - Ionosphere tomography method based on satellite-borne ice radar - Google Patents

Ionosphere tomography method based on satellite-borne ice radar Download PDF

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CN111781594B
CN111781594B CN202010582755.2A CN202010582755A CN111781594B CN 111781594 B CN111781594 B CN 111781594B CN 202010582755 A CN202010582755 A CN 202010582755A CN 111781594 B CN111781594 B CN 111781594B
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CN111781594A (en
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王成
刘波
肖鹏
陈亮
刘露
刘敏
眭晓虹
郭午龙
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China Academy of Space Technology CAST
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/9004—SAR image acquisition techniques

Abstract

The invention discloses an ionosphere tomography method based on a satellite-borne ice radar. By utilizing the characteristic of split two-pass transmission of ice radar signal receiving and transmitting, the total electron content TEC values of the ionized layer at different angles can be obtained without arranging a ground receiving station. And then calculating the projection lengths of different grids penetrating through the interior of the ionized layer according to the ray propagation path, and finally obtaining the electron density spatial distribution condition of the ionized layer according to the initial value and a multiplicative algebraic reconstruction method MART inversion. The method carries out electron density reconstruction work according to the working characteristics of the ice radar, and can invert ionosphere information with higher resolution than that of the existing GPS tomography due to the combination of the high resolution characteristics of the ice radar.

Description

Ionosphere tomography method based on satellite-borne ice radar
Technical Field
The invention designs an ionosphere tomography method based on a satellite-borne ice radar, belongs to the fields of synthetic aperture radars and ionosphere radio wave propagation, and is mainly used for ionosphere high-resolution detection in polar regions.
Background
Glaciers, ice covers/ice racks and the like in polar regions are most sensitive to global climate change and respond most intensely, are indicators and amplifiers of climate change and play an important role in maintaining global water, energy and material circulation. Aiming at the requirements, the satellite-borne ice radar is the best means for observing the polar region in a large range. However, since the satellite-borne ice radar works in the P-band, the signal of the satellite-borne ice radar is seriously affected by the ionosphere, so that compensation research has to be carried out, but on the other hand, since the echo contains rich ionosphere information, if ionosphere detection can be carried out by using the echo, a new means can be provided for ionosphere fine observation in polar regions by combining the high-resolution characteristic of the ice radar.
The current study on ionospheric tomography is based on GPS or triple-frequency beacon signals. However, whether it is a GPS or triple-band beacon, the signal is propagated in a single pass, and a receiving station is installed on the ground. Therefore, the density of the receiving stations determines the resolution of tomography, but the cost is increased sharply to increase the resolution and establish more receiving stations in the polar region, and the effect of fine observation of the ionosphere in the polar region is not ideal for the tomography resolution obtained by the existing sparsely distributed receiving stations. The ice radar working system designed by the invention is that a main satellite transmits signals, a plurality of receiving radars are arranged in the direction perpendicular to the flight direction of the satellite, and the span is about 100 kilometers. Therefore, when the ice radar performs ionosphere detection, the two-pass propagation characteristic of ice radar signals is fully utilized, a ground receiving station is not required to be arranged, and the use mode of a satellite-borne satellite system can be expanded by utilizing existing resources.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: the ice radar high-resolution characteristic is fully utilized, the ionosphere tomography research is carried out by utilizing the radar echo data, the ionosphere tomography resolution of the existing GPS/triple-frequency beacon is improved, and the fine structure of the ionosphere in a polar region is better observed.
The application provides an ionosphere tomography method based on a satellite-borne ice radar, which comprises the following steps:
(1) establishing a radar echo frequency domain model S influenced by an ionized layer according to the working geometry and the tomography principle of the ice radarr(fτ)：
Figure BDA0002553674390000021
Wherein f isτFor the radar spectrum range, B is the bandwidth, KrIs the distance to chirp rate, RTAnd RRPropagation distances of the transmitted and received signals, respectively, c is the speed of light, Δ φiono(fτ) Is the additional phase in the echo caused by ionospheric dispersion;
(2) construction of N sets of reference functions H' (f) comprising time durationsτ) Wherein N is iteration times, and a radar echo frequency domain model S is obtainedr(fτ) Respectively performing range compression with N groups of reference functions to obtain N groups of pulse pressure results, and performing TEC according to parameters corresponding to maximum amplitude values in the N groups of pulse pressure resultsREstimate, wherein, TECRIs the ionospheric size experienced by the receive path;
(3) the ionized layer is divided into a plurality of grids, the electron density value of each grid is assumed to be the same, the grids are divided into M grids, if I ice radar receiving satellites exist, the total ray is I, and the TEC of the ith rayRThe values are expressed as:
Figure BDA0002553674390000022
wherein, the TECR_iTEC for ith rayRValue, AijThe projection length matrix of the ith ray in the jth grid is expressed, when the ith ray passes through the jth grid, the ray and the edge of the grid must have two intersection points, the coordinates of the two intersection points can be obtained according to the geometric expressions of the ray and the edge, the distance between two points calculated according to the coordinates of the two intersection points is the projection length of the ith ray in the jth grid, namely Aij；
(4) Establishing initial value N of electron density of jth grid by utilizing IRI modelej 0Then iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, and the space ionized layer electron density distribution N of the ice radar detection area is finally obtained after K times of iterationej K。
In some embodiments, the establishing a radar echo frequency domain model of ionosphere influence according to ice radar working geometry and tomography principles comprises:
establishing a radar echo frequency domain model S influenced by an ionized layer according to the working geometry and the tomography principle of the ice radarr(fτ)：
Figure BDA0002553674390000031
Wherein f isτFor the radar spectrum range, B is the bandwidth, KrIs the distance to chirp rate, RTAnd RRPropagation distances of the transmitted and received signals, respectively, c is the speed of light, Δ φiono(fτ) Is the additional phase in the echo caused by ionospheric dispersion;
Figure BDA0002553674390000032
wherein A is a constant, f0For a central carrier frequency, TECTAnd TECRIonospheric size, TEC, experienced by the transmit and receive paths, respectivelyT＝TECRCos phi, where phi is the angle between the receiving path and the vertical direction;
tpis a frequency fτ+f0Echo phase velocity of (2):
Figure BDA0002553674390000033
in some embodiments, the constructing includes N sets of reference functions H' (f) of durationτ) The method comprises the following steps:
Figure BDA0002553674390000034
wherein the content of the first and second substances,
Figure BDA0002553674390000035
n is iteration number, N can be set to 1000, Tt+ n.DELTA.T represents the duration of the nth set of reference functions, where TtRepresenting the echo duration without ionospheric dispersion influence, and delta T representing the step length of each iteration, and modeling the radar echo frequency domainr(fτ) Respectively compressing the obtained data with N groups of reference functions in the distance direction to obtain N groups of pulse pressure results RC (T)t+n·ΔT)＝Sr(fτ)·H′(fτ) Searching for RC (T)tN value n corresponding to maximum amplitude value of + n · Δ T)maxThe amount of broadening of the echo can be determined as Δ Tmat＝ΔTiono＝nmaxΔ T, then TECRThe estimated values are:
Figure BDA0002553674390000041
wherein f isstart＝f0-B2 and fstop＝f0+ B2 is the start frequency and cut-off frequency, respectively, A is a constant, f0Is the center carrier frequency.
In some embodiments, the initial value N of the electron density of the jth grid is constructed by using an IRI modelej 0Then iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, and the space ionized layer electron density distribution N of the ice radar detection area is finally obtained after K times of iterationej KThe method comprises the following steps:
establishing initial value N of electron density of jth grid by utilizing IRI modelej 0Then, iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, the iteration frequency is set to be K, and the iteration formula is as follows:
Figure BDA0002553674390000042
the above formula Nej k+1Represented by the electron density N in the jth grideThe (k + 1) th iteration result, wherein, the TECR_iTEC inverted for ith rayRThe value of the one or more of the one,<·>represents the inner product, | | · | | represents the norm, AiIs represented by AijAll values of row i, Ne kRepresents the electron density distribution, λ, of the kth iteration of all the gridskIs an iterative relaxation factor with the value range of 0 to lambdakLess than or equal to 1, and finally obtaining the space ionized layer electron density distribution N of the ice radar detection area through K iterationsej K。
Has the advantages that: according to the method, the total electron content TEC information of the ionized layer obtained by different receiving satellite geometric relations and echoes of the ice radar can be used for obtaining the space electron density distribution situation through inversion. The ionospheric information with higher resolution than that of the existing GPS tomography can be inverted. The method fully utilizes the characteristics of double-pass propagation and high resolution of ice radar signals, and can obtain total electron content TEC values of the ionized layer at different angles without arranging a ground receiving station. The defects that a ground receiving station needs to be arranged and the resolution ratio is low in the traditional ionosphere tomography of the polar region are overcome.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a schematic diagram of ice radar ionospheric tomography;
fig. 3 is a tomographic simulation result, in which the short dashed line is the true electron density distribution assumed by us, the long dashed line represents the initial iteration value, and the solid line is the inversion result obtained after the iteration is performed several times.
Detailed Description
The ionospheric tomography method based on satellite-borne ice radar according to the present invention is described in detail below by way of example with reference to the accompanying drawings, where the main radars used are shown in table 1. Ionospheric parameters can be simulated by an IRI model.
TABLE 1 ionospheric parameters
Parameter(s) Configuration of
Carrier frequency (f)0) 300MHz
Bandwidth (B) 17.5MHz
Track height (H) 500km
Maximum range of traffic (L) 120km
Number of strong points on ground 6 are
Number of receiving satellites 25 of
As shown in fig. 1, the invention is a flow chart of an ionospheric tomography method based on a satellite-borne ice radar, and the method comprises the following specific steps:
step 101, establishing a radar echo frequency domain model influenced by an ionosphere.
As shown in FIG. 2, which is a schematic diagram of ice radar working geometry and tomography, a square in the vertical middle direction is a main satellite, other receiving satellites fly along two sides, the length of the whole cross orbit is 120km, and the vertical height is 500 km. According to the geometric relationship, the frequency domain expression of the echo after considering the ionosphere is as follows:
Figure BDA0002553674390000061
wherein f isτFor the radar spectrum range, B is the bandwidth, KrIs the distance direction chirp slope, Δ φiono(fτ) For additional phase, R, caused by ionospheric dispersion in the echoTAnd RRRespectively the propagation distance of the transmitted signal and the propagation distance of the received signal, and c is the speed of light.
The phase velocity of the echo is affected by the ionospheric dispersion characteristics, when the frequency f isτ+f0The echo phase velocity of (c) can be expressed as:
Figure BDA0002553674390000062
wherein A is a constant, fτFor the radar spectrum range, c is the speed of light, f0For a central carrier frequency, TECTAnd TECRIonospheric size, TEC, experienced by the transmit and receive paths, respectivelyT＝TECRCos φ, where φ is the angle of the receive path from the vertical.
Then the additional phase delta phi caused by ionospheric dispersion in the echoiono(fτ) Comprises the following steps:
Figure BDA0002553674390000063
and the ideal reference function at this time is:
Figure BDA0002553674390000064
thus, when the echo matches the pulse pressure with the reference function, the frequency domain expression of the output signal is:
Figure BDA0002553674390000065
due to delta phiiono(fτ) There is distortion in the output signal.
Step 102, group distance direction compression is performed, and ionosphere sizes suffered by the paths are received.
In addition, the slave formula (2)It can be seen that the echo time delays of different frequency points in the echo frequency spectrum are different, and the initial frequency f in the bandwidth B isstart＝f0-B/2 and cut-off frequency fstop＝f0The dispersion time delay generated by + B/2 is respectively
Figure BDA0002553674390000071
Then the broadening amount of the echo is
Figure BDA0002553674390000072
And extracting ionosphere information in the echo based on a self-focusing algorithm. Due to the additional phase error caused by the ionospheric information in the echo, the subsequent and ideal reference function pulse pressure will be distorted, as described in equation (5). Continuously and iteratively changing the duration of the reference function, and matching the pulse pressure with the echo once per change, namely the process of formula (5), until the amplitude is maximum, wherein the duration of the reference function is changed by an amount delta TmatAnd the amount of echo broadening DeltaTiono(i.e., equation (7)) is the same, the ionosphere TEC value can be back-derived by knowing this variable for the duration of the reference function, and its iterative equation can be written as
ΔTmat＝ΔTiono＝max(RC(Tt+n·ΔT,Tp_i)) (8)
Wherein RC refers to a duration of Tp_iIs compressed in the range direction with n sets of reference functions, Tt+ n.DELTA.T denotes the duration of the n sets of reference functions, TtIndicating the echo duration without ionospheric dispersion effects and deltat indicating the step size per iteration. After n groups of matching filtering are finished, finding out the n value corresponding to the maximum amplitude value, and obtaining the broadening quantity of the echo as
ΔTmat＝ΔTiono＝nmax·ΔT (9)
Then, the TEC can be obtained from the following formulaRHas a value of
Figure BDA0002553674390000073
Step 103, calculating the projection lengths of different rays in the ionosphere grid.
As shown in fig. 2, suppose that 8 receiving signal satellites are uniformly distributed in a 120km range, the middle satellite is a transmitting signal satellite, the satellites are uniformly spaced by 15km, the ice surface reflection point is located at the origin of coordinates right below the transmitting satellite, and the height of the satellite is 500 km. The geometric orientation of each ray can be calculated at this point. We need to divide the ionosphere into several grids, and assume that the electron density value of each grid is the same, and divide into M grids, as an example, the grid shown in fig. 2 has a vertical and horizontal size of 10 × 10km, a vertical range of 210-400 km, a horizontal range of-60 km, and 8 receiving satellites, so that the total number of rays is 8, and its corresponding TEC isRThe value may be expressed as:
Figure BDA0002553674390000081
wherein, the TECR_iTEC for ith rayRValue, AijAnd representing a projection length matrix of the ith ray on the jth grid, wherein after the size and the range of the grid and the coordinates of the satellite and the ground reflection point are set, a geometric expression of the space position of four sides of each grid in the rectangular coordinate system and a geometric expression of the space position of a connecting line between the satellite and the ground reflection point in the rectangular coordinate system can be calculated and obtained as shown in fig. 2. Then, when the ith ray passes through the jth grid, the ray and the edge of the grid must have two intersection points, the coordinates of the two intersection points can be obtained according to the geometric expressions of the ray and the edge, the distance between two points calculated according to the coordinates of the two intersection points is the projection length of the ith ray on the jth grid, namely Aij。
And step 104, obtaining the electron density distribution of the space ionized layer of the ice radar detection area by using a multiplicative algebraic reconstruction algorithm.
Construction of jth mesh using IRI modelInitial value N of electron density ofej 0Then, iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, the iteration frequency is set to be K, and the iteration formula is as follows:
Figure BDA0002553674390000082
the above formula Nej k+1Represented by the electron density N in the jth grideThe (k + 1) th iteration result, wherein, the TECR_iTEC inverted for ith rayRThe value of the one or more of the one,<·>represents the inner product, | | · | | represents the norm, AiIs represented by AijAll values of row i, Ne kRepresents the electron density distribution, λ, of the kth iteration of all the gridskIs an iterative relaxation factor with the value range of 0 to lambdakLess than or equal to 1, and finally obtaining the space ionized layer electron density distribution N of the ice radar detection area through K iterationsej K。
Fig. 3 shows the iteration result, where the short dashed line is the true electron density distribution assumed by us, the long dashed line represents the initial iteration value, and the solid line is the inversion result obtained after we iterate several times. It can be seen that the spatial electron density inverted by the ice radar can effectively reflect the real spatial distribution.
Those skilled in the art will appreciate that those matters not described in detail in the description of the present invention are well known in the art.

Claims (3)

1. An ionospheric tomography method based on satellite-borne ice radar, characterized by comprising the following steps:
(1) establishing a radar echo frequency domain model S influenced by an ionized layer according to the working geometry and the tomography principle of the ice radarr(fτ)：
Figure FDA0003394092280000011
Wherein f isτAs radar frequencySpectral range, B is the bandwidth, KrIs the distance to chirp rate, RTAnd RRPropagation distances of the transmitted and received signals, respectively, c is the speed of light, Δ φiono(fτ) Is the additional phase in the echo caused by ionospheric dispersion;
(2) construction of N sets of reference functions H' (f) comprising time durationsτ) Modeling the radar echo frequency domain Sr(fτ) Respectively performing range compression with N groups of reference functions to obtain N groups of pulse pressure results, and performing TEC according to parameters corresponding to maximum amplitude values in the N groups of pulse pressure resultsREstimate, wherein, TECRIs the ionospheric size experienced by the receive path;
(3) the ionized layer is divided into a plurality of grids, the electron density value of each grid is assumed to be the same, the grids are divided into M grids, if I ice radar receiving satellites exist, the total ray is I, and the TEC of the ith rayRThe values are expressed as:
Figure FDA0003394092280000012
wherein, the TECR_iTEC for ith rayRValue, AijThe projection length matrix of the ith ray in the jth grid is expressed, when the ith ray passes through the jth grid, the ray and the edge of the grid must have two intersection points, the coordinates of the two intersection points can be obtained according to the geometric expressions of the ray and the edge, the distance between two points calculated according to the coordinates of the two intersection points is the projection length of the ith ray in the jth grid, namely Aij；
(4) Establishing initial value N of electron density of jth grid by utilizing IRI modelej 0Then iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, and the space ionized layer electron density distribution N of the ice radar detection area is finally obtained after K times of iterationej KSpecifically, an initial value N of the electron density of the jth grid is constructed by utilizing an IRI modelej 0Then, iteration is carried out by utilizing a multiplication algebra reconstruction algorithm, the iteration frequency is set to be K, and the iteration formula is as follows:
Figure FDA0003394092280000021
the above formula Nej k+1Represented by the electron density N in the jth grideThe (k + 1) th iteration result, wherein, the TECR_iTEC inverted for ith rayRThe value of the one or more of the one,<·>represents the inner product, | | · | | represents the norm, AiIs represented by AijAll values of row i, Ne kRepresents the electron density distribution, λ, of the kth iteration of all the gridskIs an iterative relaxation factor with the value range of 0 to lambdakLess than or equal to 1, and finally obtaining the space ionized layer electron density distribution N of the ice radar detection area through K iterationsej K。
2. The method of claim 1, wherein the establishing of the ionosphere-influenced radar echo frequency domain model according to ice radar operating geometry and tomography principles comprises:
establishing a radar echo frequency domain model S influenced by an ionized layer according to the working geometry and the tomography principle of the ice radarr(fτ)：
Figure FDA0003394092280000022
Wherein f isτFor the radar spectrum range, B is the bandwidth, KrIs the distance to chirp rate, RTAnd RRPropagation distances of the transmitted and received signals, respectively, c is the speed of light, Δ φiono(fτ) Is the additional phase in the echo caused by ionospheric dispersion;
Figure FDA0003394092280000023
wherein A is a constant, f0For a central carrier frequency, TECTAnd TECRIonospheric size, TEC, experienced by the transmit and receive paths, respectivelyT＝TECRCos phi, where phi is the angle between the receiving path and the vertical direction;
tpis a frequency fτ+f0Echo phase velocity of (2):
Figure FDA0003394092280000024
3. the method according to claim 2, characterized in that said construction comprises N sets of reference functions H' (f) of durationτ) Modeling the radar echo frequency domain Sr(fτ) Respectively performing range compression with N groups of reference functions to obtain N groups of pulse pressure results, and performing TEC according to parameters corresponding to maximum amplitude values in the N groups of pulse pressure resultsREstimating, including:
Figure FDA0003394092280000031
wherein the content of the first and second substances,
Figure FDA0003394092280000032
n is iteration number, N is set to 1000, Tt+ n.DELTA.T represents the duration of the nth set of reference functions, where TtRepresenting the echo duration without ionospheric dispersion influence, and delta T representing the step length of each iteration, and modeling the radar echo frequency domainr(fτ) Respectively compressing the obtained data with N groups of reference functions in the distance direction to obtain N groups of pulse pressure results RC (T)t+n·ΔT)＝Sr(fτ)·H′(fτ) Searching for RC (T)tN value n corresponding to maximum amplitude value of + n · Δ T)maxThe amount of broadening of the echo can be determined as Δ Tmat＝ΔTiono＝nmaxΔ T, then TECRThe estimated values are:
Figure FDA0003394092280000033
wherein f isstart＝f0-B/2 and fstop＝f0+ B/2 being the starting and cut-off frequencies, respectively, A being a constant, f0Is the center carrier frequency.
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