CN113708875B - Forward link time delay estimation method of low-orbit satellite TDMA communication-in-motion system - Google Patents
Forward link time delay estimation method of low-orbit satellite TDMA communication-in-motion system Download PDFInfo
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Abstract
The application provides a forward link delay estimation method of a transparent forwarding low-orbit satellite TDMA communication-in-motion system, which comprises the following steps: pre-calculating a function of satellite/master station transmission delay and ephemeris time; respectively estimating the ephemeris time of the forward information reaching the satellite and the ephemeris time reaching the end station; estimating free space transmission delay of a forward uplink according to ephemeris time of the forward information reaching the satellite; determining a forward downlink according to the ephemeris time of the forward information reaching the satellite and the ephemeris time of the forward information reaching the end station, and estimating free space transmission delay of the forward downlink; and adding the free space transmission delays of the forward uplink and the forward downlink to obtain the free space transmission delay of the forward link. The method adopts an extrapolation estimation method based on linear approximation, and is simple and convenient to calculate; under the condition that satellite ephemeris information is accurate, good estimation performance can be obtained.
Description
Technical Field
The invention relates to a time synchronization technology of a TDMA satellite communication system, in particular to a forward link time delay estimation method of a transparent forwarding low-orbit satellite TDMA communication-in-motion system.
Background
In TDMA communication systems, the purpose of forward link time synchronization is to achieve Network Clock Reference (NCR) synchronization, i.e., to establish a mapping relationship between an end station timing system and a master station timing system, so as to achieve the purpose of the end station identifying the position of a master station slot through a local timing system. Studies have shown that implementing forward link time synchronization based on algorithms requires estimation of the transmission delay of the forward link.
The generalized forward link propagation delay includes four parts: the transmit side processing delay of the forward link, the receive side processing delay of the forward link, the on-board processing delay, and the free space transmission delay of the forward link. For transparent forwarding satellites, the satellite-borne transponder only performs simple processing such as filtering, amplifying, frequency conversion and the like on signals, and the time delay is far smaller than the free space transmission time delay of a forward link, so that the satellite-borne transponder can be ignored; and the processing delay of the transmitting side and the processing delay of the receiving side of the forward link can be accurately estimated according to a specific processing mechanism. Accordingly, the present application mainly discusses a method for estimating the free space transmission delay of the forward link.
In a transparent repeating satellite TDMA communication system, as shown in fig. 1, the forward link is a transmission link from the primary station to the satellite and then to the end station, and is comprised of two parts, forward uplink and forward downlink. Where forward uplink refers to the transmission link from the primary station to the satellite and forward downlink refers to the transmission link from the satellite to the end station. The free space transmission delay of the forward link can be obtained by estimating the free space transmission delay of the forward uplink and the forward downlink respectively and then adding the two.
In a low orbit satellite communication system, the position of the satellite changes with time, so the forward link is not a fixed transmission link, but changes with time. The primary premise of accurately estimating the free space transmission delay of the forward link is to correctly find which forward link to estimate. For a communication-in-motion scenario, the position of the master station is fixed, and the positions of the satellite and the end station change with time, so that the forward link is uniquely determined by the positions of the satellite and the end station; the positions of the satellite and the end station are functions of time, so that the time when the forward information arrives at the satellite and the time when the forward information arrives at the end station are determined, the positions of the satellite and the end station at corresponding moments can be determined, and the forward link for transmitting the forward information can be determined uniquely. For example, as shown in FIG. 1, the forward information D [ n ]]At t n From the master station at time t' n Arrive at the satellite at the moment, are then forwarded by the satellite to the end station, and at t " n The end station is reached at the moment. At t' n At time, the satellite position is P (t' n ) The method comprises the steps of carrying out a first treatment on the surface of the At t n At the moment, the position of the end station is R (t n ). Thus, D [ n ] is transmitted]Is H→P (t' n )→R(t″ n ). In summary, determining the time at which forward information arrives at the satellite and end station is critical to accurately estimate the forward link free space transmission delay.
Disclosure of Invention
In order to solve the related prior art problems, the invention provides a forward link time delay estimation method of a transparent forwarding low-orbit satellite TDMA communication-in-motion system, which adopts an extrapolation estimation method based on linear approximation and is simple and convenient to calculate; under the condition that satellite ephemeris information is accurate, good estimation performance can be obtained.
In order to achieve the purpose of the invention, the invention is realized by the following technical scheme:
a forward link time delay estimation method of a transparent forwarding low-orbit satellite TDMA communication-in-motion system comprises the following steps:
s1, establishing a rectangular coordinate system
With ephemeris time t as abscissa and forward information Dn is transmitted]Ephemeris time t of (2) n Is the origin of the abscissa; with transmission delay tau 'between satellites/main stations' SAT/HUB (t) is the ordinate and takes 0 as the origin of the ordinate;
s2, in a rectangular coordinate system
In (2), two linear equations are established: one is the identity linear equation l 1 :τ′ SAT/HUB (t) =t, the other is a function τ of satellite/master station transmission delay and ephemeris time SAT/HUB (t)＝g 1 Coordinate translated version τ 'of (t)' SAT/HUB (t)＝τ SAT/HUB (t+t n )＝g 1 (t+t n ) In interval [0, T]Local approximate straight-line segment equation l above 2 :/>
T is a time increment, and +.>
Representing forward directionMaximum value of the free space transmission delay of the uplink;
wherein, straight line segment equation l 2 The establishment method of (2) is as follows:
in curve τ' SAT/HUB (t)＝g 1 (t+t n ),t∈[0,T]Two adjacent points (0, g) 1 (t n ) Sum (T, g) 1 (t μ ) And), wherein t μ ＝t n +T；
According to (0, g 1 (t n ) Sum (T, g) 1 (t μ ) A straight line segment equation is established according to the coordinates of the two points, and the obtained
S3, solving a straight line l 1 And straight line segment l 2 Is the intersection point coordinate t of (2) intersect ；
Straight line l 1 And straight line segment l 2 Is the intersection point coordinate t of (2) intersect The solution is obtained by the following formula:
s4, estimating the ephemeris time of the forward information Dn reaching the satellite to obtain
t′ n ＝t n +t intersect ；
S5, estimating forward information Dn]Ephemeris time t' for reaching end station n : when the end station correctly analyzes the forward information D [ n ]]Recording the local NCR time at the moment; mapping the NCR time to the star time according to the mapping relation between the NCR time at the end station side and the star time
Will->
As t' n Is an approximation of (i) i.e.)>
S6, according to the pre-calculated parameters: function tau of satellite/master station transmission delay and ephemeris time SAT/HUB (t), and forward information D [ n ]]Ephemeris time t 'to satellite' n Estimating free space transmission delay tau of forward uplink FL/UL (t′ n )：
τ FL/UL (t′ n )＝τ SAT/HUB (t′ n )；
S7, estimating free space transmission delay of the forward downlink, comprising the following steps:
based on ephemeris and forward information Dn of the satellite]Ephemeris time t 'to satellite' n Estimating the position P (t 'of the satellite in the geocentric earth fixed (ECEF) coordinate system' n )；
Based on the forward information D [ n ]]Ephemeris time t' for reaching end station n The GNSS position of the end station at this time is determined and the position R (t') of the end station in the ECEF coordinate system is further converted n )；
Estimating the distance d between the satellite and the end station in the ECEF coordinate system SAT/RCST (t′ n ,t″ n )＝||P(t′ n )-R(t″ n ) I, wherein i·i represents the euclidean distance between two points;
using distance d between satellite and end station SAT/RCST (t′ n ,t″ n ) Dividing by the propagation velocity c of the electromagnetic wave, i.e. the free space transmission delay of the forward downlink
S8, adding the free space transmission time delay of the forward uplink and the forward downlink to obtain the free space transmission time delay tau of the forward link FL/FS (t′ n ,t″ n )：
τ FL/FS (t′ n ,t″ n )＝τ FL/UL (t′ n )+τ FL/DL (t′ n ,t″ n )。
The invention has the beneficial effects that: an extrapolation estimation method based on linear approximation is adopted, so that the calculation is simple and convenient; under the condition that satellite ephemeris information is accurate, good estimation performance can be obtained.
Drawings
Fig. 1 is a forward link schematic diagram of a transparent repeating low-orbit satellite TDMA communication system.
FIG. 2 is an illustration of estimating forward information Dn in the present invention]Ephemeris time t 'to satellite' n Schematic of the method of (a).
FIG. 3 is a geometrical schematic of steps S1-S3 of the method of the invention.
Detailed Description
For the purpose of making the objects, technical solutions and specific implementation methods of the present application more clear, the present application will be described in further detail with reference to examples of the accompanying drawings.
Fig. 1 is a schematic diagram of a forward link of a transparent repeating low-orbit satellite TDMA communication system. The forward link is a transmission link from the primary station to the satellite and then to the end station, and consists of two parts, forward uplink and forward downlink. Where forward uplink refers to the transmission link from the primary station to the satellite and forward downlink refers to the transmission link from the satellite to the end station. Since the position of the low-orbit satellites is time-varying, the forward link is not a fixed transmission link, which is time-varying. For a communication-in-motion scenario, the position of the master station is fixed while the positions of the satellites and end stations change over time, so the forward link is uniquely determined by the positions of the satellites and end stations. In the present figure, the forward information D [ n ]]At t n From the master station at time t' n Arrive at the satellite at the moment, are then forwarded by the satellite to the end station, and at t " n The end station is reached at the moment. At t' n At time, the satellite position is P (t' n ) The method comprises the steps of carrying out a first treatment on the surface of the At t n At the moment, the position of the end station is R (t n ). Thus, D [ n ] is transmitted]Is H→P (t' n )→R(t n )。
The embodiment of the application provides a forward link time delay estimation method of a transparent forwarding low-orbit satellite TDMA communication-in-motion system, which is designed according to the following ideas:
firstly, according to the ephemeris information of the satellite and the GNSS position information of the master station, pre-calculating the function tau of the transmission delay and the ephemeris time of the satellite/master station in the satellite visible window SAT/HUB (t)＝g 1 (t)。
Then, forward information Dn is estimated respectively]Ephemeris time t 'to satellite' n And ephemeris time t' to end station n 。
Then, based on the forward information D [ n ]]Ephemeris time t 'to satellite' n Sum function tau SAT/HUB (t)＝g 1 (t) estimating the free space transmission delay τ of the forward uplink FL/UL (t′ n )＝τ SAT/HUB (t′ n )。
Then, based on the forward information D [ n ]]Ephemeris time t 'to satellite' n And ephemeris time t' to end station n Determining transmission D [ n ]]And estimating the free space transmission delay tau of the forward downlink FL/DL (t′ n ,t″ n )。
And finally, adding the free space transmission delays of the forward uplink and the forward downlink to obtain the free space transmission delay of the forward link.
First, three preconditions are prepared for using the methods described in the embodiments of the present application:
1. and at the master station side, establishing a mapping relation between NCR time and star duration.
2. The master station transmits forward information D [ n ]]And the transmission time (here, the transmission time refers to information D [ n ]]Time of departure from the primary station transmit antenna) corresponding ephemeris time t n And transmitted to the end station.
3. The end station can acquire the GNSS position information of the end station in real time.
Under the precondition, in order to simplify the calculation complexity of the forward link delay estimation, a set of parameters, namely a function of satellite/master station transmission delay and ephemeris time, are prepared in advance. The specific calculation method is as follows:
1) And calculating the function of the position of the satellite in the ECEF coordinate system and the ephemeris time in the satellite visible window according to the ephemeris information of the satellite.
2) And obtaining the position of the master station under the ECEF coordinate system according to the GNSS position conversion of the master station.
3) Under the ECEF coordinate system, calculating the function d of the distance between the satellite and the master station and the ephemeris time according to the position information of the satellite and the master station SAT/HUB (t)＝f 1 (t)。
4) By d SAT/HUB (t)＝f 1 Dividing (t) by the propagation speed c of electromagnetic wave to obtain the function tau of satellite/main station transmission delay and ephemeris time SAT/HUB (t)＝d SAT/HUB (t)/c。
Under the above conditions, the free space transmission delay estimation of the forward link is performed:
s1, establishing a rectangular coordinate system
With ephemeris time t as abscissa and forward information Dn is transmitted]Ephemeris time t of (2) n Is the origin of the abscissa; with transmission delay tau 'between satellites/main stations' SAT/HUB (t) is the ordinate and takes 0 as the origin of the ordinate.
S2, in a rectangular coordinate system
In (2) two linear equations are established. One is the identity linear equation l 1 :τ′ SAT/HUB (t) =t, the other is a function τ of satellite/master station transmission delay and ephemeris time SAT/HUB (t)＝g 1 Coordinate translated version τ 'of (t)' SAT/HUB (t)＝τ SAT/HUB (t+t n )＝g 1 (t+t n ) In interval [0, T]Local approximate straight-line segment equation l above 2 :/>
The latter method of establishment is as follows: first, in curve τ' SAT/HUB (t)＝g 1 (t+t n ),t∈[0,T]Two adjacent points are selected (0,g 1 (t n ) Sum (T, g) 1 (t μ ) And), wherein t μ ＝t n +T, T is a time increment, and +.>
Representing the maximum value of the forward uplink free space transmission delay. Then, a straight line segment equation is established according to the coordinates of the two points to obtain
S3, solving a straight line l 1 And straight line segment l 2 Is obtained by the intersection point coordinates of
As shown in fig. 3, a geometric diagram of steps S1 to S3 of the method of this embodiment represents a scenario in which the distance between the satellite and the master station is gradually reduced, so that the transmission delay of the forward uplink is also gradually reduced.
S4, estimating the ephemeris time of the forward information Dn reaching the satellite to obtain
t′ n ＝t n +t intersect 。
Specifically, forward information Dn is estimated]Ephemeris time t 'to satellite' n The method of (2) is as follows: as shown in fig. 1, at t n At the moment, the master station sends forward information Dn]At this time, the satellite is located at P (t n ) Is a position of (c). Thereafter, D [ n ]]In the forward uplink, the satellite continues to orbit in the direction pointed by the arrow. At t' n At this point the satellite moves to a new position P (t' n ) At this time, D [ n ]]Also just reaching the satellite, i.e. with a time delay t' n -t n Rear D [ n ]]Meets the satellite. In the above-described encounter problem, the satellite is in orbit P (t n )→P(t′ n ) Time of movement and Dn]In the forward uplink H→P (t' n ) The transmission delays being exactly equal, i.e. straight line τ SAT/HUB (t)＝t-t n And curve τ SAT/HUB (t)＝g 1 (t) at t n Then intersect necessarily, and the intersection point is t' n (As shown in FIG. 2, the scenario represented is that the satellite distance from the primary station and hence the forward uplink propagation delay decreases gradually. In this figure g 1 (t′ n )＝t′ n -t n ). Thus, solving a system of nonlinear equations
Forward information dn can be obtained]Ephemeris time t 'to satellite' n 。
In general, in a low-orbit satellite communication system, the free space transmission delay of the forward uplink is small, the moving distance of the satellite during the time is short, and the motion track can be approximately a straight line segment. Thus, curve τ SAT/HUB (t)＝g 1 (t) at t n The nearby area may also be approximated as a straight line segment. Let the equation of this straight line segment be
Where T is a time increment. Thus, the above problem can be reduced to a straight line τ SAT/HUB (t)＝t-t n And straight line segment
Is a cross-over problem of (a). To sum up, solve the linear equation set
Forward information dn can be obtained]Ephemeris time t 'to satellite' n 。
S5, estimating forward information Dn]Ephemeris time t to end station″ n 。
Forward information D [ n ]]The time of arrival at the end station is specifically referred to as Dn]Time of arrival at the end station receive antenna. However, since the delay of the reception process of the end station is small, the displacement of the end station is negligible during this time. Therefore, it can be considered that the forward information D [ n ]]Time to end station receiving antenna and end station correctly resolve Dn]Is approximately equal in time. Therefore, the forward information D [ n ] can be estimated by the following method]Ephemeris time t' for reaching end station n : when the end station correctly analyzes the forward information D [ n ]]At that time, the local NCR time at that time was recorded. Then, according to the mapping relation between the NCR time at the end station side and the star time, the NCR time is mapped into the star time
Finally, will->
As t' n Is an approximation of (i) i.e.)>
S6, according to the pre-calculated parameters: function tau of satellite/master station transmission delay and ephemeris time SAT/HUB (t), and forward information D [ n ]]Ephemeris time t 'to satellite' n Estimating free space transmission delay of forward uplink to obtain
τ FL/UL (t′ n )＝τ SAT/HUB (t′ n )。
S7, estimating the free space transmission delay of the forward downlink. The specific method comprises the following steps: first, according to ephemeris information and forward information D [ n ] of satellite]Ephemeris time t 'to satellite' n Estimating the position P (t 'of the satellite in the ECEF coordinate system' n ). Then, based on the forward information D [ n ]]Ephemeris time t' for reaching end station n The GNSS position of the end station at this time is determined and the position R (t') of the end station in the ECEF coordinate system is further converted n ). Then, the distance d between the satellite and the end station in the ECEF coordinate system is estimated SAT/RCST (t′ n ,t″ n )＝||P(t′ n )-R(t″ n ) I. Finally, using the distance d between the satellite and the end station SAT/RCST (t′ n ,t″ n ) Dividing by the propagation velocity c of the electromagnetic wave, i.e. the free space transmission delay of the forward downlink
S8, adding the free space transmission time delay of the forward uplink and the forward downlink to obtain the free space transmission time delay tau of the forward link FL/FS (t′ n ,t″ n ) I.e.
τ FL/FS (t′ n ,t″ n )＝τ FL/UL (t′ n )+τ FL/DL (t′ n ,t″ n )。
Claims (4)
1. A forward link delay estimation method for a transparent-forward low-orbit satellite TDMA communication-with-mobile system, comprising the steps of:
s1, establishing a rectangular coordinate system
With ephemeris time t as abscissa and forward information Dn is transmitted]Ephemeris time t of (2) n Is the origin of the abscissa; with transmission delay tau 'between satellites/main stations' SAT/HUB (t) is the ordinate and takes 0 as the origin of the ordinate;
s2, in a rectangular coordinate system
In (2), two linear equations are established: one is the identity linear equation l 1 :τ′ SAT/HUB (t) =t, the other is a function τ of satellite/master station transmission delay and ephemeris time SAT/HUB (t)＝g 1 Coordinate translated version τ 'of (t)' SAT/HUB (t)＝τ SAT/HUB (t+t n )＝g 1 (t+t n ) In interval [0, T]Upper part of the cylinderLocal approximation of straight-line segment equation
T is a time increment, and +.>
Representing a maximum value of forward uplink free space transmission delay;
s3, solving a straight line l 1 And straight line segment l 2 Is the intersection point coordinate t of (2) intersect ；
S4, estimating the ephemeris time of the forward information Dn reaching the satellite to obtain
t′ n ＝t n +t intersect ；
S5, estimating forward information Dn]Ephemeris time t' for reaching end station n : when the end station correctly analyzes the forward information D [ n ]]Recording the local NCR time at the moment; mapping the NCR time to the star time according to the mapping relation between the NCR time at the end station side and the star time
Will->
As t' n Is an approximation of (i) i.e.)>
S6, according to the pre-calculated parameters: function tau of satellite/master station transmission delay and ephemeris time SAT/HUB (t), and forward information D [ n ]]Ephemeris time t 'to satellite' n Estimating free space transmission delay tau of forward uplink FL/UL (t′ n )：
τ FL/UL (t′ n )＝τ SAT/HUB (t′ n )；
S7, estimating free space transmission delay of the forward downlink:
wherein d SAT/RCST (t′ n ,t″ n ) Is the distance between the satellite and the end station, c is the propagation velocity of the electromagnetic wave;
s8, adding the free space transmission time delay of the forward uplink and the forward downlink to obtain the free space transmission time delay tau of the forward link FL/FS (t′ n ,t″ n )：
τ FL/FS (t′ n ,t″ n )＝τ FL/UL (t′ n )+τ FL/DL (t′ n ,t″ n )。
2. The method for forward link delay estimation of transparent-forward low-orbit satellite TDMA communication-with-mobile system according to claim 1 wherein straight-line segment equation l 2 The establishment method of (2) is as follows:
in curve τ' SAT/HUB (t)＝g1(t+t n ),t∈[0,T]Two adjacent points (0, g1 (t n ) Sum (T, g) 1 (t μ ) And), wherein t μ ＝t n +T；
According to (0, g 1 (t n ) Sum (T, g) 1 (t μ ) A straight line segment equation is established according to the coordinates of the two points, and the obtained
3. The method for forward link delay estimation of transparent-forward low-orbit satellite TDMA communication-with-mobile system according to claim 2, wherein straight line l 1 And straight line segment l 2 Is the intersection point coordinate t of (2) intersect The solution is obtained by the following formula:
4. the method for estimating the forward link delay of a transparent-forward low-orbit satellite TDMA communication-with-mobile system according to claim 1, wherein said estimating the free-space transmission delay of the forward downlink comprises the steps of:
based on ephemeris and forward information Dn of the satellite]Ephemeris time t 'to satellite' n Estimating the position P (t 'of the satellite in the geocentric earth fixed (ECEF) coordinate system' n )；
Based on the forward information D [ n ]]Ephemeris time t' for reaching end station n The GNSS position of the end station at this time is determined and the position R (t') of the end station in the ECEF coordinate system is further converted n )；
Estimating the distance d between the satellite and the end station in the ECEF coordinate system SAT/RCST (t′ n ,t″ n )＝||P(t′ n )-R(t″ n ) I, wherein i·i represents the euclidean distance between two points;
using distance d between satellite and end station SAT/RCST (t′ n ,t″ n ) Dividing by the propagation velocity c of the electromagnetic wave, i.e. the free space transmission delay of the forward downlink
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