CN112422166A - Satellite-ground link time-frequency synchronization method and device - Google Patents

Satellite-ground link time-frequency synchronization method and device Download PDF

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CN112422166A
CN112422166A CN202011154646.7A CN202011154646A CN112422166A CN 112422166 A CN112422166 A CN 112422166A CN 202011154646 A CN202011154646 A CN 202011154646A CN 112422166 A CN112422166 A CN 112422166A
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frequency
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dispersion
satellite
refractive index
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CN112422166B (en
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周晨
赵家奇
唐新杰
王誉轩
赵正予
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Wuhan University WHU
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/1851—Systems using a satellite or space-based relay
    • H04B7/18513—Transmission in a satellite or space-based system
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/1851—Systems using a satellite or space-based relay
    • H04B7/18519—Operations control, administration or maintenance
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
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Abstract

The invention relates to a satellite-ground link time-frequency synchronization method and a device, wherein the method comprises the following steps: 1) judging a frequency allocation scheme of satellite-ground link communication; 2) under the condition that the frequency scheme is a three-frequency scheme, calculating the composite refractive index of the atmosphere to the electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves; 3) calculating the dispersion delay of the troposphere and the delay difference corresponding to the two downlink frequencies according to the composite refractive index; 4) respectively measuring time difference values corresponding to the two downlink frequencies by using a receiver, and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies; 5) and calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay, and performing time-frequency correction on the satellite-ground links by using the total delay. By applying the embodiment of the invention, the precision of time-frequency correction of the satellite-ground link is improved.

Description

Satellite-ground link time-frequency synchronization method and device
Technical Field
The invention relates to the technical field of satellite-ground communication, in particular to a satellite-ground link time-frequency synchronization method and a satellite-ground link time-frequency synchronization device.
Background
The earth atmosphere is divided into a neutral atmosphere and an ionized layer, and the neutral atmosphere and the ionized layer have very important influences on radio wave propagation, mainly including absorption, phase change, propagation delay, frequency dispersion, polarization rotation, multipath effect and the like of radio wave signals, and the influences can cause signals received on the ground to lag behind the time stamps of the signals.
In order to solve the above problems, Time and Frequency Synchronization (Time and Frequency Synchronization) is usually used to perform calibration, and Time and Frequency Synchronization is performed by adjusting clock Time values distributed in different places to a certain accuracy or a certain conformity through Time comparison. The former is called absolute time synchronization (also called time synchronization), and the latter is called relative time synchronization. The frequency synchronization is to adjust the frequency values of the frequency sources distributed in different places to a certain accuracy or a certain conformity through frequency comparison. The former is called pair frequency synchronization (also called frequency correction), and the latter is called relative frequency synchronization.
However, in the conventional atmospheric wave refraction correction, a satellite-ground link generally larger than 3GHz only considers the non-dispersion delay of troposphere atmosphere, and for a positioning, navigation and time service system, the correction accuracy obtained by only considering the non-dispersion delay is about thousands of picoseconds, and the accuracy is far from meeting some high-accuracy application scenes such as high-accuracy time service, high-accuracy positioning and the like. Therefore, the prior art has the technical problem that the electromagnetic wave propagation correction precision in the satellite-ground link which is more than 3GHz is low.
Disclosure of Invention
The invention provides a time-frequency synchronization method and a time-frequency synchronization device for a satellite-ground link, which are used for solving or at least partially solving the technical problem of low precision of electromagnetic wave propagation correction in the satellite-ground link which is larger than 3GHz in the prior art.
In order to solve the above technical problem, a first aspect of the present invention provides a satellite-to-ground link time-frequency synchronization method, including:
s1: judging whether a frequency distribution scheme of satellite-ground link communication is a three-frequency scheme or not;
s2: under the condition that the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme, calculating the composite refractive index of an atmosphere to electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves;
s3: calculating the dispersion delay of the troposphere and the delay difference corresponding to the two downlink frequencies according to the composite refractive index;
s4: respectively measuring time difference values corresponding to the two downlink frequencies by using a receiver, and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies;
s5: and calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay, and performing time-frequency correction on the satellite-ground links by using the total delay.
In one embodiment, step S2 includes:
s2.1: using the formula:
Figure BDA0002742307330000021
calculating the oxygen molecular dispersion refractive index, wherein the oxygen molecular dispersion refractive index is used for measuring the influence of oxygen molecules on the electromagnetic wave dispersion;
wherein, N'Oxygen(f) The oxygen molecular dispersion refractive index when the electromagnetic wave frequency is f; sigma is a summation symbol; i is the serial number of the oxygen molecular dispersion refractive index curve, and i belongs to (38, k); siIs the intensity of the ith line, and Si＝a1×10–7pθ3exp[a2(1-θ)]；a1First spectral line data for attenuation of electromagnetic waves in oxygen; p is the dry air pressure; theta is 300/T, and T is the Kelvin temperature; exp () is an exponential function with a natural constant as the base; a is2Second spectral line data for attenuation of electromagnetic waves in oxygen; fi' is a curve shape factor, and
Figure BDA0002742307330000022
fithe oxygen molecule or water vapor dispersion refractive index curve with the sequence number i corresponds toThe frequency of (d); delta is a correction factor, and
Figure BDA0002742307330000023
a5fifth line data for attenuation of electromagnetic waves in oxygen; a is6Sixth spectral line data of the attenuation of the electromagnetic wave in oxygen; e is the pressure of water vapor, and
Figure BDA0002742307330000024
rho is the water vapor density; Δ f is the width of the oxygen molecular dispersion refractive index curve, and
Figure BDA0002742307330000025
a3third spectral line data for attenuation of electromagnetic waves in oxygen; a is4Fourth spectral line data for attenuation of electromagnetic waves in oxygen; n'D(f) Is a continuous absorption of nitrogen gas caused by atmospheric pressure, and
Figure BDA0002742307330000031
s2.2: by means of the formula (I) and (II),
Figure BDA0002742307330000032
calculating the water molecule dispersion refractive index, wherein the water molecule dispersion refractive index is used for measuring the influence of water molecules and water vapor on the electromagnetic wave dispersion;
N'Water Vapour(f) the refractive index of water molecules when the frequency of the electromagnetic wave is f; i 'is the serial number of the oxygen molecular dispersion refractive index curve, and i' belongs to (1, l); si'Is the intensity of the i' th line, and Si'＝b1×10–1eθ3.5exp[b2(1-θ)]；b1The data is the spectral line data of the attenuation of the electromagnetic wave in the water vapor; b2The other spectral line data of the electromagnetic wave attenuated in the water vapor; and is
Figure BDA0002742307330000033
b3The third spectral line data of the electromagnetic wave attenuated in the water vapor; b4Fourth spectral line for attenuation of electromagnetic wave in water vaporData; b5The fifth spectral line data of the electromagnetic wave attenuated in the water vapor; b6The sixth spectral line data of the electromagnetic wave attenuated in the water vapor;
s2.3: using the formula, N ═ N0+ N' (f), calculating the complex refractive index of the atmosphere, wherein,
n is the composite refractive index of the atmosphere; n is a radical of0Is a frequency independent term; n '(f) is a frequency-dependent dispersion term, and N' (f) is the sum of the water molecular dispersion index and the oxygen molecular dispersion refractive index.
In one embodiment, the invention further comprises:
by means of the formula (I) and (II),
Figure BDA0002742307330000034
the width of the oxygen molecular dispersion refractive index curve is corrected.
In one embodiment, step S4 includes:
s4.1: measuring the first downlink frequency f separately by means of a receiver0Corresponding delayed TIC0(G) A second downlink frequency f2Corresponding delayed TIC2(G) And the first downlink frequency f0And a second downlink frequency f2All downlink frequencies are three-frequency schemes;
s4.2: using formulas
Figure BDA0002742307330000035
Calculating the total electron content in the electric wave path, and obtaining the ionized layer first-order dispersion delay of the satellite-ground link communication, wherein,
STEC is the total electron content in the electric wave path; Δ TIC (G) is the first downlink frequency f0And a second downlink frequency f2The difference in time of arrival of the signal at the ground station, i.e. Δ TIC (g) ═ TIC0(G)-TIC2(G) (ii) a Delta tau is the difference of the double-frequency dispersion delay of the troposphere; k1For ionospheric delay first order coefficient, f1For the second uplink frequency, K3Delay second order term coefficient for ionosphere; b is0Is the earth magnetic field strength.
Based on the same inventive concept, the second aspect of the present invention provides a satellite-ground link time-frequency synchronization apparatus, comprising:
the judging module is used for judging whether the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme or not;
the composite refractive index calculation module is used for calculating the composite refractive index of the atmosphere to electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves under the condition that the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme;
the delay difference calculation module is used for calculating dispersion delay of the troposphere and delay difference corresponding to two downlink frequencies according to the composite refractive index;
the ionized layer first-order dispersion delay calculation module is used for measuring time difference values corresponding to the two downlink frequencies by using the receiver and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies;
and the correction module is used for calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay and performing time-frequency correction on the satellite-ground links by using the total delay.
One or more technical solutions in the embodiments of the present application have at least one or more of the following technical effects:
when the frequency distribution scheme of satellite-ground link communication is judged to be a three-frequency scheme, firstly, the composite refractive index of an atmosphere to electromagnetic waves is calculated according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves; and calculating troposphere dispersion delay and delay difference corresponding to two downlink frequencies according to the composite refractive index; then, respectively measuring time difference values corresponding to the two downlink frequencies by using a receiver, and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies; and finally, calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay, and performing time-frequency correction on the satellite-ground links by using the total delay. Because the method of the invention brings troposphere dispersion delay error and ionosphere dispersion delay into the error correction scope when time-frequency synchronization is carried out, compared with the prior art, more error factors are considered, and further the time-frequency synchronization precision can be improved.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a diagram of the conditions of a mesoscale meteorological model used in the prior art;
FIG. 2 is a graph of prior art tropospheric non-dispersive delay in both Western and Shanghai locations;
FIG. 3 is a schematic diagram of a satellite-to-ground link according to an embodiment of the present invention;
fig. 4 is a schematic flowchart of a time-frequency synchronization method for a satellite-ground link according to an embodiment of the present invention;
FIG. 5 is a graph illustrating dispersion delay data due to oxygen and water vapor in an embodiment of the present invention;
FIG. 6 is a graph of calculated difference in dual-frequency tropospheric dispersion delay for both Western Ampere and Shanghai, respectively, according to an embodiment of the present invention;
FIG. 7 is a graph of the magnitude of dispersion delay provided by an embodiment of the present invention;
FIG. 8 shows the TEC data of the world over 1 month, 1 day to 3 days 2014;
FIG. 9 is a diagram illustrating typical Shanghai and Western Ann frequency ionospheric delay errors, in accordance with an embodiment of the present invention;
FIG. 10 is a graph of the effect of tropospheric dispersion and ionospheric dispersion on electric waves;
FIG. 11 is a diagram illustrating delay correction residuals applied in a three-frequency scheme according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
To illustrate the prior art more clearly, the mesoscale meteorological model WRF is now used to analyze tropospheric errors:
results generated using a mesoscale meteorological model in the prior art, such as shown in fig. 1, cross-sections of tropospheric pressure, vapor pressure and temperature at typical locations and times, with (a) in fig. 1 being background atmospheric pressure; in FIG. 1, (b) is the background water vapor pressure; in FIG. 1, (c) represents the background temperature.
The non-dispersion delay error of the troposphere is mainly calculated through modeling of atmospheric temperature and wet pressure of the troposphere, and based on a mesoscale meteorological model WRF, the non-dispersion delay of the troposphere in 2019, 1 month, 1 day, 30 days of Seaman and Shanghai two places is calculated. FIG. 2 is a graph of prior art non-dispersive tropospheric delay in both Seaman and Shanghai, as shown in FIG. 2, the upper line being the non-dispersive delay in the Shanghai region, which is between 7000 and 7800 picoseconds; the lower line is the non-dispersive delay of the Western region, which is between 5800 and 6300 picoseconds; it can be seen that this delay is on the order of a few thousand picoseconds (multiplied by the speed of light corresponding to a distance on the order of a few meters).
In the current time-frequency comparison of satellite-to-ground links (satellite-to-ground communication links), a Two-way time-frequency transfer (TWSTFT) mode is often adopted, and the synchronization precision of the method is in the nanosecond level. However, in some special application fields, the synchronization accuracy is required to be less than 1 picosecond, and if the time accuracy required by the satellite-ground link is about 0.5ps (0.15mm millimeter), the requirement cannot be met in the prior art.
First, fig. 3 is a schematic diagram of a satellite-ground link provided by an embodiment of the present invention, as shown in fig. 3, since the synchronization precision of a satellite-ground clock in the prior art is tens of nanoseconds, that means that it is necessary to consider the problem of the difference between uplink and downlink paths in this time scale,this difference has two components, one is the difference in propagation medium and one is the difference in geometric path. Within tens of ns, the difference in propagation medium between the upstream path and the downstream path is completely negligible; in addition, for the difference in geometric path, at a satellite velocity of 8km/s, a space station orbital height of 500km, the difference between the up path and the down path in the zenith direction is 6 x 10-15m, the distance is very short and can be completely ignored, so that the uplink and downlink propagation paths can be regarded as 'simultaneous same path' under the current synchronization precision, that is to say, the uplink and downlink can be regarded as consistent.
Based on the above discussion process, therefore, the following mathematical model can be established:
the downlink frequencies allocated for the space station and the ground system are 20.8GHz and 30.4GHz, respectively, and the frequencies allocated for the uplink are 26.8GHz and 30.4GHz, respectively. For the first uplink frequency f'0(30.4GHz) and a second uplink frequency f1(26.8GHz), and a first downlink frequency f0(30.4GHz) and a second downlink frequency f2(20.8GHz), the following equations can be respectively established for the actual uplink and downlink without considering the factors such as path asymmetry, relativistic effect, etc.:
Figure BDA0002742307330000061
wherein the content of the first and second substances,
tic(s) is the delay recorded for a space station; s is a clock of the space station; g is a clock of the ground station;
Figure BDA0002742307330000062
tropospheric dispersion delay corresponding to the second uplink frequency;
Figure BDA0002742307330000063
is ionospheric dispersion delay;
Figure BDA0002742307330000071
wherein the content of the first and second substances,
TIC (G) recorded for ground stationDelaying;
Figure BDA0002742307330000072
tropospheric dispersion delay corresponding to the second downlink frequency;
Figure BDA0002742307330000073
is the ionospheric dispersion delay.
Figure BDA0002742307330000074
Wherein the content of the first and second substances,
Figure BDA0002742307330000075
to correspond to the second uplink frequency f1Tropospheric delay of (a); τ is the path delay; τ (f)1) For the second uplink frequency f1Corresponding dispersion delay.
According to the formula of ionized layer A-H, there are
Figure BDA0002742307330000076
Where STEC is the total electron content along the path. K1Delaying the first order coefficients for the ionosphere; k2Delay second order term coefficient for ionosphere; b is0Is the geomagnetic field strength;
Figure BDA0002742307330000077
for the second uplink frequency f1Delay of ionospheric dispersion.
Figure BDA0002742307330000078
Wherein the content of the first and second substances,
Figure BDA0002742307330000079
to correspond to the second downlink frequency f2Tropospheric delay of (a); τ is the path delay; τ (f)2) For the second downlink frequency f2Corresponding dispersion delay.
According to the formula of ionized layer A-H, there are
Figure BDA00027423073300000710
Wherein the content of the first and second substances,
Figure BDA00027423073300000711
for the second downlink frequency f2Delay of ionospheric dispersion.
Based on the above formula, the clock error for the space station and the ground station can be expressed as:
Figure BDA00027423073300000712
thus, from the point of view of the clock difference comparison: there are three factors to consider:
(1) tropospheric dispersion delay
Figure BDA00027423073300000713
(2) Ionospheric first order dispersion delay
Figure BDA00027423073300000714
(3) Ionospheric second order dispersion delay
Figure BDA0002742307330000081
It is emphasized that fig. 1-3 only serve a schematic order of magnitude, and thus, there is no need to distinguish to which regions a data point belongs.
Therefore, fig. 4 is a schematic flow chart of a satellite-ground link time-frequency synchronization method provided in the embodiment of the present invention, and as shown in fig. 4, the method includes:
s401: judging a frequency allocation scheme of satellite-ground link communication, wherein the frequency allocation scheme comprises the following steps: the invention only considers the situation that the frequency allocation scheme is a three-frequency scheme.
The main cause of tropospheric dispersion is the relaxation absorption of oxygen molecules, water vapor molecules, and atmospheric settled particles, with the water vapor molecules having the greatest effect. FIG. 5 is a graphical illustration of tropospheric dispersion delay data according to an embodiment of the present invention; as shown in fig. 5, (a) in fig. 5 and (b) in fig. 5 show dispersion delays caused by moisture particles such as humid air and cloud mist, respectively. Tropospheric dispersion must be taken into account due to current accuracy requirements, and furthermore, due to the presence of tropospheric dispersion, cross-interaction with ionospheric dispersion can occur, affecting the correction of ionospheric delay. Therefore, for the calculation of tropospheric dispersion delay, a combination of a microwave radiometer and a corresponding high-precision atmospheric model is required.
S402: and under the condition that the frequency scheme is a three-frequency scheme, calculating the composite refractive index of the atmosphere to the electromagnetic wave according to the influence of oxygen molecules and water vapor on the dispersion of the electromagnetic wave.
By means of the formula (I) and (II),
Figure BDA0002742307330000082
and calculating the oxygen molecular dispersion refractive index, wherein,
N'Oxygen(f) the oxygen molecular dispersion refractive index when the electromagnetic wave frequency is f; sigma is a summation symbol; i is the serial number of the oxygen molecular dispersion refractive index curve, and i belongs to (38, k); siIs the intensity of the ith line, and Si＝a1×10–7pθ3exp[a2(1-θ)]；a1First spectral line data for attenuation of electromagnetic waves in oxygen; p is the dry air pressure; theta is 300/T, and T is the Kelvin temperature; exp () is an exponential function with a natural constant as the base; a is2Second spectral line data for attenuation of electromagnetic waves in oxygen; f'iIs a curve shape factor, and
Figure BDA0002742307330000083
fithe frequency corresponding to the oxygen molecule or water vapor dispersion refractive index curve with the serial number i; delta is a correction factor, and
Figure BDA0002742307330000091
a5fifth line data for attenuation of electromagnetic waves in oxygen; a is6Is electromagneticSixth spectral line data of wave decay in oxygen; e is the pressure of water vapor, and
Figure BDA0002742307330000092
rho is the water vapor density; Δ f is the width of the oxygen molecular dispersion refractive index curve, and
Figure BDA0002742307330000093
a3third spectral line data for attenuation of electromagnetic waves in oxygen; a is4Fourth spectral line data for attenuation of electromagnetic waves in oxygen; n'D(f) Is a continuous absorption of nitrogen gas caused by atmospheric pressure, and
Figure BDA0002742307330000094
table 1 shows the line data for the attenuation in oxygen, as shown in Table 1, Table 1
Figure BDA0002742307330000095
Figure BDA0002742307330000101
Figure BDA0002742307330000111
By means of the formula (I) and (II),
Figure BDA0002742307330000112
and calculating the water molecule dispersion refractive index, wherein,
N'Water Vapour(f) the refractive index of water molecules when the frequency of the electromagnetic wave is f; i' is the serial number of the oxygen molecular dispersion refractive index curve, and i belongs to (1, l); si'Is the intensity of the i' th line, and Si'＝b1×10–1eθ3.5 exp[b2(1-θ)]； b1The data is the spectral line data of the attenuation of the electromagnetic wave in the water vapor; b2Is electromagneticAnother spectral line data of the wave attenuation in water vapor; and is
Figure BDA0002742307330000113
b3The third spectral line data of the electromagnetic wave attenuated in the water vapor; b4The fourth spectral line data of the electromagnetic wave attenuated in the water vapor; b5The fifth spectral line data of the electromagnetic wave attenuated in the water vapor; b6And the sixth spectral line data of the attenuation of the electromagnetic wave in the water vapor.
Table 2 shows the line data for the attenuation in water vapor, as shown in Table 2, Table 2
Figure BDA0002742307330000114
Figure BDA0002742307330000121
According to the dispersion refractive index of the water molecule spectral line in the cloud and mist, by using a formula,
Figure BDA0002742307330000122
and calculating the water molecule dispersion index, wherein,
N'Water Vapour(f) the refractive index of water molecules when the frequency of the electromagnetic wave is f; i is the serial number of the oxygen molecular dispersion refractive index curve, and i belongs to (1, l);
the complex refractive index N varies with frequency due to the absorption effect of atmospheric molecules in the atmosphere, and can be generally decomposed into a frequency-independent term and a dispersion term, so that the formula N ═ N can be used0+ N' (f), calculating the complex refractive index of the atmosphere, wherein,
n is the composite refractive index of the atmosphere; n is a radical of0Is a frequency independent term and N' (f) is a frequency dependent dispersion term.
In a further implementation of an embodiment of the present invention, to account for the doppler effect, a formula may be utilized,
Figure BDA0002742307330000131
the width of the oxygen molecular dispersion refractive index curve is corrected.
S403: and calculating the difference of the double-frequency differential dispersion delay of the troposphere according to the composite refractive index.
The propagation path of the electromagnetic wave ray can be determined by using the obtained composite refractive index based on a ray tracing technology, so that the tropospheric dispersion delay is calculated, the process is the prior art, and the embodiment of the invention is not repeated herein, and the paper "liuyaxu, atmospheric millimeter wave radiation characteristics and application research [ D ]. Nanjing university of Physician sciences ] can be referred to specifically.
Because the troposphere comprises a dispersion term and a non-dispersion term (independent of frequency), the delay of the troposphere path of two downlink frequencies is subtracted, so that the non-dispersion term can be counteracted, and further the dispersion delay difference of the convection process, namely delta tau, is obtained.
According to the given tropospheric dispersion-refraction index, fig. 6 shows the calculated dispersion delay difference of the dual-frequency differential tropospheric dispersion for west ampere and shanghai respectively, the elevation angle of the dispersion delay difference is 30 °, as shown in fig. 6, (a) in fig. 6 is a typical value of the dual-frequency differential tropospheric dispersion delay for west ampere, and the background atmospheric parameters of the dispersion delay difference are calculated by WRF to obtain data of 10 months and 1 days in 2019; fig. 6 (b) is a typical value of dispersion delay of a dual-frequency differential troposphere in the shanghai region, and data of 2019, 10 months and 1 days are obtained by calculating background atmospheric parameters by WRF; as can be seen from fig. 6, the dual-frequency tropospheric dispersion delay difference is between 2-7 picoseconds.
FIG. 7 is a graph of the magnitude of dispersion delay provided by an embodiment of the present invention, tropospheric dispersion delay errors mainly due to relaxation-induced rotational absorption of water vapor molecules, settled particles and oxygen molecules in the tropospheric atmosphere, and FIG. 6 is a graph of the magnitude of dispersion delay provided by an embodiment of the present invention; the delay error, i.e. the dispersion delay, of this part is of the approximate order: on the order of (-11, -7) picoseconds, in agreement with the measured values.
S404: and respectively measuring the delays corresponding to the two downlink frequencies by using a receiver, and calculating the ionized layer first-order dispersion delay of the satellite-ground link communication according to the delays corresponding to the two downlink frequencies.
Measuring the first downlink frequency f separately by means of a receiver0Corresponding delayed TIC0(G) A second downlink frequency f2Corresponding delayed TIC2(G) And the first downlink frequency f0And a second downlink frequency f2All downlink frequencies are three-frequency schemes;
in practical applications, the two measured delays can be characterized using the following formula:
Figure BDA0002742307330000132
Figure BDA0002742307330000133
ionosphere propagation to the satellite-to-ground link radio wave is affected by the Total Electron density Content (TEC) along the radio path. The global TEC distribution can be computed using global, ground-based GNSS networks, such as the international satellite navigation service (IGS) provides such data. As a typical example, fig. 8 shows the domestic overhead TEC data in TECU units from 1 month 1 to 3 days 2014. It should be noted that the total ionospheric electron density content refers to the GNSS satellite signal path (from 20200km to ground), and not to the path of the spatial station to ground.
Fig. 8 (a) shows the total concentration distribution of electron density in the ionized layer at 00:00UT on 1 month and 1 day 2014; FIG. 8 (b) shows the total concentration distribution of 06:00UT ionized layer electron density on 1 month and 1 day 2014; fig. 8 (c) shows the total electron density distribution of 12:00UT ionosphere on 1 month and 1 day 2014; fig. 8 (d) shows the total 18:00UT ionospheric electron density distribution on 1 month and 1 day 2014. Fig. 8 (e) is a schematic diagram showing a 3-day continuous change in the zenith direction VTEC from shanghai to sienna, where the upper line indicates the TEC data of shanghai and the lower line indicates the TEC data of sienna.
Then, using the formula,
Figure BDA0002742307330000141
the total electron content in the electric wave path is calculated, and further the ionized layer first-order dispersion delay of the satellite-ground link communication can be obtained according to the calculation method in the prior art, wherein,
STEC is ionospheric first-order dispersion delay of satellite-ground link communication; Δ TIC (G) is the first downlink frequency f0And a second downlink frequency f2A difference in delay therebetween, and Δ TIC (g) ═ TIC0(G)-TIC2(G) (ii) a The STEC is the ionospheric first-order dispersion delay of the satellite-ground link communication. Delta tau is the difference of the double-frequency dispersion delay of the troposphere; k1For ionospheric delay first order coefficient, f1Is a second uplink frequency; k3Delay second order term coefficient for ionosphere; b is0Is the earth magnetic field strength.
It should be noted that the arithmetic derivation before S401 indicates that tropospheric dispersion delay is related to temperature θ, air pressure P and water vapor e on the satellite-ground link, so that although the computation of tropospheric dispersion delay is complex, the accuracy measurement based on atmospheric parameter temperature, humidity and pressure can meet the index requirement.
Moreover, the temperature typical measurement accuracy of the current microwave radiometer is 0.5K, the atmospheric humidity typical measurement accuracy is 0.1g/m3, and the corresponding atmospheric refractive index accuracy is 10-8-10-9. According to the above formula, it can be deduced that the corresponding tropospheric dispersion delay accuracy is 0.1ps, so that the microwave radiometer is used for real-time temperature and pressure measurement
In practical applications, the ACES (European Telecommunications Standards Institute) protocol may also be used: based on a European mesoscale weather model, combining the ambient temperature and humidity pressure data of each survey station, and calculating according to the orbit fitting of the space station to obtain the troposphere dispersion delay empirical formula of each survey station, however, the method needs to carry out massive calculation and needs to verify the empirical formula.
S405: and calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay, and performing time-frequency correction on the satellite-ground links by using the total delay.
In the specific implementation process, the delay of the shanghai and 3 frequencies of west safety (30.4GHz, 20.8GHz, 26.8GHz) in the zenith direction is calculated by using the formula, and fig. 9 is a schematic diagram of the ionospheric delay errors of the shanghai and the west safety in the embodiment of the invention; as shown in fig. 9, (a) in fig. 9 is an ionospheric dispersion delay error; fig. 9 (b) shows ionospheric second-order delay errors. The second order delay error is related to the background TEC, the magnetic field, and the polarization of the electric wave. Fig. 10 shows the effect of tropospheric dispersion and ionospheric dispersion on electric waves, and as shown in fig. 10, (a) in fig. 10 is the tropospheric delay difference, and the upper curve is the tropospheric delay difference of 30.4GHz relative to 20.8GHz frequency in the shanghai region; the lower curve is the difference in tropospheric delay at 30.4GHz versus 20.8GHz frequency for the west ampere region; fig. 10 (b) is ionospheric first-order dispersion delay difference; as can be seen from fig. 9 and 10, tropospheric dispersion delay and ionospheric first-order dispersion delay have a great influence on time-frequency synchronization, which is much larger than 1ps, and therefore, it is still necessary to use the above-mentioned influencing factors as the basis for time-frequency synchronization. It should be emphasized that fig. 9 and 10 are merely illustrative of orders of magnitude and that it is not necessary to know the quantitative differences between the degrees of influence from place to place.
Ionospheric second-order dispersion is determined by the earth's magnetic field and the background electron density. For the years of solar activity, the typical ionospheric STEC is 200TECU, with the ionospheric second-order dispersion effect being about 0.6ps (0.5 mm). In addition, in the event of solar activity or ionospheric storm, the ionospheric STEC may be larger, and the ionospheric second-order dispersion effect may also affect the calculation of the ionospheric first-order dispersion effect. Therefore, in practical application, a GNSS-TEC space environment real-time monitoring and early warning system can be placed at a ground survey station to calculate the second-order dispersion effect, and the specific process refers to a mathematical derivation formula before the step S401.
Two-way Satellite Time and Frequency Transfer (TWSTFT) is one of the most accurate remote Time comparison techniques in the world at present, and its basic principle is to transmit a local Time signal to a Satellite at a ground station, transmit a Time signal of a Satellite atomic clock to the ground at the Satellite, compare the Two Time signals, and measure the signal Transfer delay, thereby obtaining a high-precision clock difference between the Satellite and the ground station. In the process of satellite two-way time comparison, because the propagation signal is the approximate symmetry of the path, in principle, the error caused by the propagation path is greatly offset, but a part of asymmetric factors can influence the precision of the two-way time comparison. Therefore, the embodiment of the invention considers more detailed errors on the signal propagation path, mainly dispersion delay errors caused by different uplink and downlink frequencies: the experimental example realizes the great improvement of the satellite-to-ground link time comparison precision by considering the calculation of the delay factors, namely troposphere dispersion delay, ionosphere first-order dispersion delay and ionosphere second-order dispersion delay (the second-order dispersion delay is small through calculation, the specific precision is not influenced, and therefore the delay factors are not considered during actual calculation). In practical application, the non-dispersion delay value should be added into the correction data when performing time-frequency correction.
The chromatic dispersion delay calculated in the embodiment S405 of the invention and the non-chromatic dispersion delay calculated in the prior art are used for time-frequency synchronization correction: fig. 11 is a diagram illustrating the total delay between the corrected satellite-ground links according to the embodiment of the present invention, as shown in fig. 11, the upper line in fig. 11 corresponds to the delay error in the shanghai region, and the lower line corresponds to the delay error in the west ampere region, and it can be seen that the delay error is between 0.7 picosecond and 1.3 picoseconds.
FIG. 11 is a graph of the delay correction residual error for the three-frequency scheme of the present invention, taking into account the temperature accuracy of 0.5 ° and the 0.1g/cm of the microwave radiometer3Under the water vapor precision condition, the residual error after troposphere delay correction is about 0.01 picosecond, which shows that the calibration result of the embodiment of the invention has good stability.
In addition, when the co-frequency correction of the satellite-ground link is carried out, because the uplink and the downlink can be equal to those of the same path at the same time, in this case, the ionosphere and troposphere errors can be completely offset according to the mathematical derivation before S401, so the atmospheric transmission factor of the scheme can be eliminated.
Finally, in the near co-frequency scheme, that is, the frequencies of the uplink and the downlink are not completely the same and differ by 200MHz or 400MHz, for example, the uplink is 30.4GHz, and the downlink is 30GHz, and the calibration process is the same as the three-frequency scheme, which is not described herein again.
Example two
Based on the same inventive concept, corresponding to the embodiment shown in fig. 4, the invention further provides a satellite-ground link time-frequency synchronization device, which includes:
the judging module is used for judging whether the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme or not;
the composite refractive index calculation module is used for calculating the composite refractive index of the atmosphere to electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves under the condition that the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme;
the delay difference calculation module is used for calculating dispersion delay of the troposphere and delay difference corresponding to two downlink frequencies according to the composite refractive index;
the ionized layer first-order dispersion delay calculation module is used for measuring time difference values corresponding to the two downlink frequencies by using the receiver and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies;
and the correction module is used for calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay and performing time-frequency correction on the satellite-ground links by using the total delay.
Since the apparatus described in the second embodiment of the present invention is a system for implementing the time-frequency synchronization method for a satellite-ground link in the first embodiment of the present invention, a person skilled in the art can understand the specific structure and deformation of the apparatus based on the method described in the first embodiment of the present invention, and thus details are not described herein again. All the devices adopted in the method of the first embodiment of the present invention belong to the protection scope of the present invention.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (5)

1. A satellite-ground link time-frequency synchronization method is characterized by comprising the following steps:
s1: judging whether a frequency distribution scheme of satellite-ground link communication is a three-frequency scheme or not;
s2: under the condition that the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme, calculating the composite refractive index of an atmosphere to electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves;
s3: calculating the dispersion delay of the troposphere and the delay difference corresponding to the two downlink frequencies according to the composite refractive index;
s4: respectively measuring time difference values corresponding to the two downlink frequencies by using a receiver, and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies;
s5: and calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay, and performing time-frequency correction on the satellite-ground links by using the total delay.
2. The satellite-to-ground link time-frequency synchronization method according to claim 1, wherein the step S2 includes:
s2.1: using the formula:
Figure FDA0002742307320000011
calculating the oxygen molecular dispersion refractive index, wherein the oxygen molecular dispersion refractive index is used for measuring the influence of oxygen molecules on the electromagnetic wave dispersion;
wherein, N'Oxygen(f) The oxygen molecular dispersion refractive index when the electromagnetic wave frequency is f; sigma is a summation symbol; i is the serial number of the oxygen molecular dispersion refractive index curve, and i belongs to (38, k); siIs the intensity of the ith line, and Si＝a1×10–7pθ3exp[a2(1-θ)]；a1First spectral line data for attenuation of electromagnetic waves in oxygen; p is the dry air pressure; theta is 300/T, and T is the Kelvin temperature; exp () is an exponential function with a natural constant as the base; a is2Second spectral line data for attenuation of electromagnetic waves in oxygen; fi' is a curve shape factor, and
Figure FDA0002742307320000012
fithe frequency corresponding to the oxygen molecule or water vapor dispersion refractive index curve with the serial number i; delta is a correction factor, and
Figure FDA0002742307320000013
a5fifth line data for attenuation of electromagnetic waves in oxygen; a is6Sixth spectral line data of the attenuation of the electromagnetic wave in oxygen; e is the pressure of water vapor, and
Figure FDA0002742307320000014
rho is the water vapor density; Δ f is the width of the oxygen molecular dispersion refractive index curve, and
Figure FDA0002742307320000021
a3third spectral line data for attenuation of electromagnetic waves in oxygen; a is4Fourth spectral line data for attenuation of electromagnetic waves in oxygen; n'D(f) Is a continuous absorption of nitrogen gas caused by atmospheric pressure, and
Figure FDA0002742307320000022
d＝5.6×10-4(p+e)θ0.8；
s2.2: by means of the formula (I) and (II),
Figure FDA0002742307320000023
calculating the water molecule dispersion refractive index, wherein the water molecule dispersion refractive index is used for measuring water molecules and water vaporInfluence on electromagnetic wave dispersion;
N'Water Vapour(f) the refractive index of water molecules when the frequency of the electromagnetic wave is f; i 'is the serial number of the oxygen molecular dispersion refractive index curve, and i' belongs to (1, l); si'Is the intensity of the i' th line, and Si'＝b1×10–1eθ3.5exp[b2(1-θ)]；b1The data is the spectral line data of the attenuation of the electromagnetic wave in the water vapor; b2The other spectral line data of the electromagnetic wave attenuated in the water vapor; and is
Figure FDA0002742307320000025
b3The third spectral line data of the electromagnetic wave attenuated in the water vapor; b4The fourth spectral line data of the electromagnetic wave attenuated in the water vapor; b5The fifth spectral line data of the electromagnetic wave attenuated in the water vapor; b6The sixth spectral line data of the electromagnetic wave attenuated in the water vapor;
s2.3: using the formula, N ═ N0+ N' (f), calculating the complex refractive index of the atmosphere, wherein,
n is the composite refractive index of the atmosphere; n is a radical of0Is a frequency independent term; n '(f) is a frequency-dependent dispersion term, and N' (f) is the sum of the water molecular dispersion index and the oxygen molecular dispersion refractive index.
3. The satellite-to-ground link time-frequency synchronization method according to claim 2,
by means of the formula (I) and (II),
Figure FDA0002742307320000024
the width of the oxygen molecular dispersion refractive index curve is corrected.
4. The satellite-to-ground link time-frequency synchronization method according to claim 1, wherein the step S4 includes:
s4.1: measuring the first downlink frequency f separately by means of a receiver0Corresponding delayed TIC0(G) A second downlink frequency f2Corresponding delayed TIC2(G) And the first downlink frequency f0And a second downlink frequency f2All downlink frequencies are three-frequency schemes;
s4.2: using formulas
Figure FDA0002742307320000031
Calculating the total electron content in the electric wave path, and obtaining the ionized layer first-order dispersion delay of the satellite-ground link communication, wherein,
STEC is the total electron content in the electric wave path; Δ TIC (G) is the first downlink frequency f0And a second downlink frequency f2The difference in time of arrival of the signal at the ground station, i.e. Δ TIC (g) ═ TIC0(G)-TIC2(G) (ii) a Delta tau is the difference of the double-frequency dispersion delay of the troposphere; k1Delaying the first order coefficients for the ionosphere; f. of1For the second uplink frequency, K3Delay second order term coefficient for ionosphere; b is0Is the earth magnetic field strength.
5. A satellite-ground link time-frequency synchronization device is characterized by comprising:
the judging module is used for judging whether the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme or not;
the composite refractive index calculation module is used for calculating the composite refractive index of the atmosphere to electromagnetic waves according to the influence of oxygen molecules, water molecules and water vapor on the dispersion of the electromagnetic waves under the condition that the frequency distribution scheme of the satellite-ground link communication is a three-frequency scheme;
the delay difference calculation module is used for calculating dispersion delay of the troposphere and delay difference corresponding to two downlink frequencies according to the composite refractive index;
the ionized layer first-order dispersion delay calculation module is used for measuring time difference values corresponding to the two downlink frequencies by using the receiver and calculating ionized layer first-order dispersion delay of satellite-ground link communication according to the delay difference corresponding to the two downlink frequencies;
and the correction module is used for calculating the total delay between the satellite-ground links according to the sum of the troposphere dispersion delay and the ionosphere first-order dispersion delay and performing time-frequency correction on the satellite-ground links by using the total delay.
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