Method and system for fast establishing communication link between satellite and ground

Abstract

The present application relates to a kind of star ground fast establishing communication link method and system, when low-orbit satellite communication adopts long code system, communication terminal needs to compensate the Doppler of uplink transmission signal and code phase accurately.The method first obtains the time of low-orbit satellite system from broadcast downlink signal acquisition control data frame header moment, according to the dynamic of low-orbit signal and system design guarantee, the time delay error of this moment can be ignored, the transmission time of the communication uplink signal received by the next whole second of low-orbit satellite is obtained, to calculate code phase;Again, the relationship between the Doppler of broadcast downlink signal and the Doppler of communication uplink transmission signal is used, combined with the characteristics of small atomic clock drift, the Doppler and code phase of uplink transmission signal are compensated.

Description

TECHNICAL FIELD

[0001] The application relates to a method and system for quickly establishing a communication link between a satellite and the ground, and belongs to the technical field of satellite communication. BACKGROUND

[0002] Currently, some low-orbit satellites and communication terminals communicate by using long codes with a period of several days. Under the premise that the Doppler and code phase of the low-orbit long code signal are unknown, it is difficult for the low-orbit satellite and the communication terminal to establish a connection. However, the code phase of the low-orbit long code signal needs to calculate the propagation delay between the two. Currently, the propagation delay is calculated by knowing the low-orbit satellite system time, combining the low-orbit satellite ephemeris and the terminal position, and then calculating the propagation delay. In addition, the Doppler compensation needs to know the clock drift of the communication terminal. Currently, the common means for obtaining the low-orbit navigation system time and the clock drift can be to receive satellite navigation signals and position them. This method requires the communication terminal to have the ability to receive satellite navigation signals and position them, and after obtaining the satellite navigation system time, the satellite navigation system time is converted into the low-orbit satellite system time. Or the communication terminal has the ability to be time-granted and time-kept, but it is still unable to obtain the clock drift of the communication terminal. This leads to a complex design of the communication terminal, which needs to have additional navigation and positioning capabilities. SUMMARY

[0003] The technical problem of the application is to overcome the shortcomings of the prior art and provide a method and system for quickly establishing a communication link between a satellite and the ground. Simple compensation for the low-orbit long code signal can enable the low-orbit satellite and the communication terminal to quickly establish a connection without navigation time-granting and without providing the clock drift of the terminal.

[0004] The technical solution of the application is a method for quickly establishing a communication link between a satellite and the ground, applied to a communication terminal, and the method comprises the following steps:

[0005] S1-1, receiving a broadcast downlink signal sent by a low-orbit satellite, the broadcast downlink signal modulating a control data frame, the start time of the frame header of the control data frame being aligned with the whole-second time of the low-orbit satellite, the control data frame including the whole-second time corresponding to the current frame header, control information and the ephemeris of the low-orbit satellite, the control information including the pseudo-random code initial phase information used by the communication uplink signal and the communication downlink signal of the communication terminal;

[0006] S1-2, tracking the broadcast downlink signal of the low-orbit satellite in real time, recording the low-orbit satellite whole-second time t1 corresponding to the start time of the frame header in the control data frame when the communication terminal starts communication according to the pseudo-random code initial phase information, and restoring the second pulse signal corresponding to the receiving time of the current control data frame header;

[0007] S1-3, calculating the signal propagation delay tau between the low-orbit satellite and the communication terminal corresponding to the next whole-second time t2 of the low-orbit satellite according to the ephemeris of the low-orbit satellite;

[0008] S1-4, taking the second pulse signal corresponding to the receiving time of the current control data frame header restored in step S1-2 as a reference, delaying for a period of time, and obtaining a communication uplink signal transmitting time t3;

[0009] S1-5, recording the broadcast downlink signal tracking Doppler f track corresponding to the communication uplink signal transmitting time t3, and converting it into the Doppler f dop of the communication uplink signal;

[0010] S1-6, configuring the pseudo-random code and initial phase information used by the communication uplink signal according to the control information contained in the control data frame, modulating the carrier of the communication uplink signal according to the Doppler f dop of the communication uplink signal calculated in step S1-5, and starting to send the communication uplink signal to the low-orbit satellite at the communication uplink signal transmitting time t3, so that when the communication uplink signal reaches the low-orbit satellite, it is exactly the next whole second time of the low-orbit satellite, and the motion Doppler is zero, facilitating the low-orbit satellite to quickly capture and track the communication uplink signal at the next whole second time of the low-orbit satellite according to the agreed pseudo-random code and initial phase information;

[0011] S1-7, receiving the communication downlink signal sent by the low-orbit satellite according to the pseudo-random code and initial phase information, and quickly establishing a communication link with the low-orbit satellite.

[0012] Another technical solution provided by the application is a method for quickly establishing a communication link between a satellite and the ground, applied to a low-orbit satellite, and the method comprises the following steps:

[0013] S2-1, sending a broadcast downlink signal to a communication terminal, the broadcast downlink signal modulating a control data frame, the starting time of the frame header of the control data frame being aligned with the whole second time of the low-orbit satellite, the control data frame including the whole second time corresponding to the current frame header, control information, and the ephemeris of the low-orbit satellite, the control information including the initial phase information of the pseudo-random code used by the uplink signal and the downlink signal when the low-orbit satellite communicates with the communication terminal;

[0014] S2-2, capturing and tracking the communication uplink signal at the next whole second time of the low-orbit satellite according to the agreed pseudo-random code and initial phase information, and quickly establishing a connection with the communication terminal;

[0015] The communication uplink signal reaches the low-orbit satellite exactly at the next whole second time of the low-orbit satellite, and when the communication uplink signal reaches the low-orbit satellite, the motion Doppler is zero;

[0016] S2-3, sending a communication downlink signal according to the agreed pseudo-random code and initial phase information, and quickly establishing a communication link with the communication terminal.

[0017] Preferably, the communication uplink signal is obtained by the communication terminal through the following method:

[0018] S2-2-1, tracking the low-orbit satellite broadcast downlink signal, obtaining the low-orbit satellite whole second time t1 corresponding to the starting time of the frame header in the control data frame, and restoring the second pulse signal corresponding to the receiving time of the frame header of the current control data frame;

[0019] S2-2-2, according to the low-orbit satellite ephemeris, calculating the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole second time t2 of the low-orbit satellite;

[0020] S2-2-3, taking the second pulse signal corresponding to the receiving time of the frame header of the current control data frame restored in step S2-2-1 as the reference, delaying for a period of time to obtain the communication uplink signal transmission time t3, so that the communication uplink signal reaches the low-orbit satellite at the next whole second time of the low-orbit satellite;

[0021] S2-2-4, recording the Doppler f track of the broadcast downlink signal at the communication uplink signal transmission time t3, and converting it into the Doppler f dop of the communication uplink signal, so that the communication uplink signal reaches the low-orbit satellite with zero motion Doppler;

[0022] S2-2-5, configuring the pseudo-random code initial phase information used by the communication uplink signal according to the control information contained in the control data frame, modulating the carrier of the communication uplink signal according to the Doppler f dop of the communication uplink signal calculated in step S2-2-4, and starting to send the communication uplink signal to the low-orbit satellite at the communication uplink signal transmission time t3.

[0023] Preferably, the next whole second time t2 of the low-orbit satellite is:

[0024] t2=t1+1.

[0025] Preferably, the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole second time t2 of the low-orbit satellite is calculated by the following method:

[0026]

[0027] Where (x s ,y s ,z s ) is the satellite position corresponding to the next whole second time t2 of the low-orbit satellite,

[0028] (x u ,y u ,z u ) is the communication terminal position corresponding to the next whole second time t2 of the low-orbit satellite.

[0029] Preferably, the low-orbit satellite time corresponding to the transmission time t3 of the communication uplink signal is t1+1-τ, wherein τ is the time delay of the next whole second.

[0030] Preferably, the communication terminal adopts an atomic clock.

[0031] Preferably, the Doppler f of the communication uplink signal is dop

[0032] f dop =-k×f track

[0033] wherein k is the nominal frequency of the communication uplink signal carrier / nominal frequency of the broadcast downlink signal carrier; f track is the tracking Doppler of the broadcast downlink signal corresponding to the transmission time of the communication uplink signal.

[0034] Preferably, the broadcast downlink signal adopts spread spectrum BPSK modulation.

[0035] Preferably, the pseudo code of the broadcast downlink signal is Gold code.

[0036] The present application has one technical solution: a system for quickly establishing a communication link between a satellite and a terminal, which comprises a low-orbit satellite and a communication terminal.

[0037] The low-orbit satellite sends a broadcast downlink signal to the communication terminal, the control data frame is modulated in the broadcast downlink signal, the frame header start time of the control data frame is aligned with the whole second time of the low-orbit satellite, the control data frame comprises the whole second time corresponding to the current frame header, control information and the low-orbit satellite ephemeris, the control information comprises the pseudo-random code initial phase information of the uplink signal and the downlink signal used when the low-orbit satellite communicates with the communication terminal; the communication uplink signal is quickly captured and tracked at the next whole second time of the low-orbit satellite system time according to the agreed pseudo-random code and the initial phase information thereof.

[0038] The communication terminal receives the broadcast downlink signal sent by the low-orbit satellite, and tracks the broadcast downlink signal of the low-orbit satellite in real time; when the communication terminal receives the pseudo-random code initial phase information and starts communication, the whole second time t1 of the low-orbit satellite corresponding to the frame header start time in the control data frame is recorded, and the second pulse signal corresponding to the current control data frame header receiving time is recovered; according to the low-orbit satellite ephemeris, the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole second time t2 of the low-orbit satellite is calculated; the transmission time t3 of the communication uplink signal is obtained by delaying a period of time based on the recovered second pulse signal corresponding to the current control data frame header receiving time; the tracking Doppler f track of the broadcast downlink signal corresponding to the transmission time t3 of the communication uplink signal is recorded, and is converted into the Doppler f dop ​; configuring a pseudo-random code and initial phase information thereof used by the communication uplink signal according to control information contained in the control data frame, calculating a Doppler f dop modulating a carrier of the communication uplink signal, and sending the communication uplink signal to the low-orbit satellite at a time t3 of the communication uplink signal transmission, so that the communication uplink signal reaches the low-orbit satellite at a next whole second time of the low-orbit satellite and the motion Doppler is zero; receiving the communication downlink signal sent by the low-orbit satellite according to the pseudo-random code and the initial phase information thereof, and quickly establishing a communication link with the low-orbit satellite.

[0039] The period of time is 1-2τ.

[0040] Compared with the prior art, the present application has the following beneficial effects:

[0041] (1) The method for obtaining the current satellite navigation system time is simple. The method provided by the present application can obtain the low-orbit satellite system time at the current frame header time from the received downlink signal, and calculate the next whole second transmission time of the low-orbit satellite system. The time error is only the propagation time delay of the low-orbit satellite. The Doppler and code phase precision requirements of the low-orbit satellite dynamic are fully met.

[0042] (2) The method for calculating the Doppler is simple without the need of obtaining the clock drift of the communication terminal. Since the communication terminal uses an atomic clock, the clock drift error of the payload and the communication terminal is less than the compensation precision requirement of the uplink signal Doppler, and can be ignored. Only the Doppler obtained through the downlink signal tracking is needed to calculate the compensation Doppler. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The method flowchart of the embodiment of the present application is shown. DETAILED DESCRIPTION

[0044] The specific embodiment of the present application will be further described in detail below with reference to the accompanying drawings.

[0045] The satellite payload requires the precision of the Doppler and code phase of the low-orbit long code signal, and the communication terminal needs to accurately compensate the Doppler and code phase of the uplink transmission signal.

[0046] The present application does not require the communication terminal to have the navigation and timing capability, and does not need to provide the positioning clock drift. The low-orbit satellite system time at the current frame header time is obtained from the downlink signal, and the propagation time delay is ignored. The transmission time of the next whole second is obtained, which is used to calculate the propagation time delay. The relationship between the Doppler of the broadcast downlink signal and the Doppler of the communication uplink signal is utilized, and the small clock drift of the atomic clock is combined to realize the compensation of the Doppler and code phase of the uplink transmission signal. The method is simple and the precision meets the compensation requirement.

[0047] The core idea of the application is to obtain the low-orbit satellite system time at the current frame header moment from the broadcast downlink signal, and the time plus one is the low-orbit satellite system transmission moment of the next whole second, but there is a propagation delay error, as long as the error meets the time service accuracy and the time when the communication terminal receives the pseudo-random code phase in advance of the next whole second is far greater than 10 ms.

[0048] Due to the low orbit height of the low-orbit satellite, the propagation delay is in the order of milliseconds, so the error between the frame header moment when the communication terminal receives the broadcast downlink signal and the low-orbit satellite whole second moment is in the order of milliseconds. The acceleration of the low-orbit satellite is about 400 Hz / s at most, and the pseudo-range change rate is about 6000 m / s at most. The propagation delay is not more than 10 ms at most. Then the pseudo-range change within 10 ms is not more than 60 m / s. The 60 m error meets the phase accuracy requirement of the transmission compensation; in addition, the communication terminal will receive the pseudo-random code phase in advance of the next whole second, and the advance time is far greater than 10 ms. Therefore, even if there is a propagation delay, the next whole second moment obtained from the broadcast downlink signal can be used as the uplink signal transmission moment, and the time service accuracy can be accepted.

[0049] As for the Doppler compensation, since the terminal uses an atomic clock, the clock drift can be ignored, and only the communication uplink signal Doppler needs to be calculated according to the broadcast downlink signal Doppler at the transmission moment.

[0050] Embodiment 1

[0051] As shown in Figure 1 , the application provides a method for quickly establishing a communication link between a satellite and the earth, which is applied to a communication terminal and includes the following steps:

[0052] S1-1, receiving the broadcast downlink signal transmitted by the low-orbit satellite, the broadcast downlink signal modulating a control data frame, the frame header start moment of the control data frame being aligned with the low-orbit satellite whole second moment, the control data frame including the whole second time corresponding to the current frame header, control information and the low-orbit satellite ephemeris, the control information including the pseudo-random code initial phase information used by the communication uplink signal and the communication downlink signal of the communication terminal;

[0053] S1-2, tracking the low-orbit satellite broadcast downlink signal in real time, recording the low-orbit satellite whole second time t1 corresponding to the frame header start moment in the control data frame when the communication terminal starts communication according to the pseudo-random code initial phase information, and restoring the second pulse signal corresponding to the current control data frame header receiving moment;

[0054] S1-3, calculating the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole second moment t2 of the low-orbit satellite according to the low-orbit satellite ephemeris;

[0055] S1-4, taking the second pulse signal corresponding to the receiving time of the current control data frame header restored in step S1-2 as a reference, delaying for a period of time to obtain the communication uplink signal transmitting time t3; the period of time is 1-2τ.

[0056] S1-5, recording the broadcast downlink signal tracking Doppler f track , and converting into the Doppler f dop of the communication uplink signal;

[0057] S1-6, configuring the pseudo-random code and initial phase information used by the communication uplink signal according to the control information contained in the control data frame, modulating the carrier of the communication uplink signal according to the Doppler f dop of the communication uplink signal calculated in step S1-5, and sending the communication uplink signal to the low-orbit satellite at the communication uplink signal transmitting time t3, so that when the communication uplink signal reaches the low-orbit satellite, it is exactly the next whole second time of the low-orbit satellite, and the motion Doppler is zero, facilitating the low-orbit satellite to quickly capture and track the communication uplink signal at the next whole second time of the low-orbit satellite according to the agreed pseudo-random code and initial phase information;

[0058] S1-7, receiving the communication downlink signal sent by the low-orbit satellite according to the pseudo-random code and initial phase information, and quickly establishing a communication link with the low-orbit satellite.

[0059] Embodiment 2:

[0060] A method for quickly establishing a communication link between a satellite and the ground, which is applied to a low-orbit satellite and includes the following steps:

[0061] S2-1, sending a broadcast downlink signal to a communication terminal, the broadcast downlink signal modulating a control data frame, the starting time of the frame header of the control data frame aligning with the whole second time of the low-orbit satellite, the control data frame including the whole second time corresponding to the current frame header, control information, and the ephemeris of the low-orbit satellite, the control information including the initial phase information of the pseudo-random code used by the uplink signal and the downlink signal when the low-orbit satellite communicates with the communication terminal;

[0062] S2-2, capturing and tracking the communication uplink signal at the next whole second time of the low-orbit satellite according to the agreed pseudo-random code and initial phase information, and quickly establishing a connection with the communication terminal;

[0063] The communication uplink signal reaches the low-orbit satellite exactly at the next whole second time of the low-orbit satellite, and when the communication uplink signal reaches the low-orbit satellite, the motion Doppler is zero;

[0064] S2-3, sending a communication downlink signal according to the agreed pseudo-random code and initial phase information, and quickly establishing a communication link with the communication terminal.

[0065] The communication uplink signal is obtained by the communication terminal through the following method:

[0066] S2-2-1, tracking the low-orbit satellite broadcast downlink signal, obtaining the low-orbit satellite whole-second time t1 corresponding to the starting time of the frame header in the control data frame, and restoring the second pulse signal corresponding to the receiving time of the frame header of the current control data frame;

[0067] S2-2-2, according to the low-orbit satellite ephemeris, calculating the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole-second time t2 of the low-orbit satellite;

[0068] S2-2-3, taking the second pulse signal corresponding to the receiving time of the frame header of the current control data frame restored in step S2-2-1 as the reference, delaying a period of time 2τ, obtaining the communication uplink signal transmission time t3, so that when the communication uplink signal reaches the low-orbit satellite, it is exactly the next whole-second time of the low-orbit satellite, and the τ is the propagation delay between the low-orbit satellite and the communication terminal; the period of time is 1-2τ.

[0069] S2-2-4, recording the broadcast downlink signal Doppler f track , and converting it into the Doppler f dop of the communication uplink signal, so that;

[0070] S2-2-5, configuring the pseudo-random code initial phase information used by the communication uplink signal according to the control information contained in the control data frame, modulating the carrier of the communication uplink signal according to the Doppler f dop of the communication uplink signal calculated in step S2-2-4, and starting to send the communication uplink signal to the low-orbit satellite at the communication uplink signal transmission time t3.

[0071] In the above two embodiments, according to the low-orbit satellite system time relationship, specifically:

[0072] t2=t1+Δt+1 (1)

[0073] In the formula, t2, t1, and Δt respectively represent the next whole-second low-orbit satellite system time, the low-orbit satellite system time corresponding to the current frame header time, and the propagation delay error.

[0074] According to the dynamic characteristics of the low-orbit satellite, the code phase error influence caused by the propagation delay error Δt can be ignored, so the next whole-second time t2 of the low-orbit satellite is:

[0075] t2=t1+1. (2)

[0076] In the above two embodiments, the signal propagation delay τ between the low-orbit satellite and the communication terminal corresponding to the next whole-second time t2 of the low-orbit satellite is calculated through the following method:

[0077]

[0078] wherein (x s ,y s ,z s ) is the satellite position corresponding to the next whole second time t2 of the low-orbit satellite,

[0079] (x u ,y u ,z u ) is the communication terminal position corresponding to the next whole second time t2 of the low-orbit satellite.

[0080] In the above two embodiments, the low-orbit satellite time corresponding to the transmission time t3 of the communication uplink signal is t1+1-τ, wherein τ is the time delay of the next whole second time.

[0081] In the above two embodiments, the signal Doppler has the following relationship:

[0082] f track = f SV +f D -f R

[0083] f dop =-k×(f track +2×f R -f sv )

[0084] wherein f track is the broadcast downlink signal tracking frequency; f D is the motion-induced Doppler; f SV is the broadcast downlink signal corresponding clock drift, in Hz; f R is the communication terminal broadcast downlink frequency corresponding clock drift; k is the communication uplink signal carrier nominal frequency / broadcast downlink signal carrier nominal frequency; f dop is the communication uplink signal Doppler. Since the communication terminal uses an atomic clock and the payload clock drift is also small, its error is less than the compensation accuracy requirement of the uplink signal Doppler, so f R and f SV can be ignored, so when the communication terminal uses an atomic clock, the communication uplink signal Doppler f dop is:

[0085] f dop =-k×f track

[0086] wherein k is the communication uplink signal carrier nominal frequency / broadcast downlink signal carrier nominal frequency; f track is the broadcast downlink signal tracking Doppler corresponding to the communication uplink signal transmission time.

[0087] In the two embodiments described above, the broadcast downlink signal preferably employs spread spectrum BPSK modulation, and the pseudocode of the broadcast downlink signal is Gold code.

[0088] Example 3:

[0089] This embodiment provides a system for rapidly establishing a communication link between satellite and ground, the system including a low-Earth orbit satellite and a communication terminal;

[0090] A low-Earth orbit (LEO) satellite transmits a broadcast downlink signal to a communication terminal. The broadcast downlink signal includes a modulated control data frame. The start time of the control data frame header is aligned with the LEO satellite's integer second time. The control data frame includes the integer second time corresponding to the current frame header, control information, and the LEO satellite ephemeris. The control information includes the initial phase information of the pseudo-random code used by the LEO satellite for uplink and downlink signals when communicating with the communication terminal. According to the agreed pseudo-random code and its initial phase information, the uplink communication signal is quickly acquired and tracked at the next integer second of the LEO satellite system time.

[0091] The communication terminal receives the broadcast downlink signal transmitted by the low-Earth orbit (LEO) satellite and tracks the LEO satellite broadcast downlink signal in real time. When the communication terminal receives the pseudo-random code initial phase information and starts communication, it records the LEO satellite integer second time t1 corresponding to the start time of the frame header in the control data frame, and recovers the second pulse signal corresponding to the receiving time of the current control data frame header. Based on the LEO satellite ephemeris, it calculates the signal propagation delay τ between the LEO satellite and the communication terminal corresponding to the next integer second time t2 of the LEO satellite. Using the recovered second pulse signal corresponding to the receiving time of the current control data frame header as a reference, it delays for a period of time to obtain the communication uplink signal transmission time t3. It records the broadcast downlink signal tracking Doppler f corresponding to the communication uplink signal transmission time t3. track And converted into Doppler f of the uplink communication signal. dop According to the control information contained in the control data frame, configure the pseudo-random code and its initial phase information used by the communication uplink signal, and according to the calculated Doppler f of the communication uplink signal. dop The carrier wave of the modulated uplink communication signal is transmitted to the low-Earth orbit (LEO) satellite starting at transmission time t3, ensuring that the uplink signal arrives at the LEO satellite at the next full second of its orbit and that motion Doppler is zero. The downlink communication signal transmitted by the LEO satellite is received according to the pseudo-random code and its initial phase information, and a communication link is quickly established. This time period is 1-2τ.

[0092] Example 4:

[0093] Taking actual star search as an example, the broadcast downlink signal frequency is 1176.45MHz, the communication uplink signal frequency is 2020MHz, and k=2020 / 1176.45=1.718730077776361. After receiving the broadcast downlink signal, the second pulse can be obtained through frame synchronization, and the low-orbit satellite time corresponding to the low-orbit satellite time corresponding to the frame header of the control data frame is obtained through the text, and the Doppler f track of the 1176.45MHz frequency point is tracked. It is assumed that the pseudo-random code phase is received to start communication, the low-orbit satellite time t1=52330 seconds corresponding to the current frame header time of the broadcast downlink signal is recorded, the low-orbit satellite time t2=52331 seconds of the next whole second is obtained, and the signal propagation delay τ=0.003329609 seconds between the low-orbit satellite and the communication terminal corresponding to the low-orbit satellite time t2 of the next whole second is calculated according to the low-orbit satellite ephemeris and the terminal position. The low-orbit satellite system time corresponding to the transmission time should be:

[0094] t3=52330+1-0.003329609=52330.996670391

[0095] In addition, the downlink tracking Doppler f track =-436 at this time is recorded, the Doppler f dop =-1.71873×(-436)=749.3663 is compensated.

[0096] That is, the communication uplink signal is transmitted at the low-orbit satellite system time t3=52330.996670391, and 749.3663Hz is directly compensated to the transmission intermediate frequency, so that the compensation index requirement can be met, the satellite payload can receive the communication uplink transmission signal, and the effect is ideal.

[0097] The contents not described in detail in the specification of the application belong to the known technology of the person skilled in the art.

Claims

1. A method for rapidly establishing a satellite-to-ground communication link, characterized in that, Applied to communication terminals, the method includes the following steps: S1-1. Receive the broadcast downlink signal sent by the low-orbit satellite. The broadcast downlink signal modulates control data frames. The start time of the frame header of the control data frame is aligned with the whole second time of the low-orbit satellite. The control data frame includes the whole second time corresponding to the current frame header, control information, and the ephemeris of the low-orbit satellite. The control information includes the initial phase information of the pseudo-random code used by the communication uplink signal and the communication downlink signal of the communication terminal. S1-2. Real-time tracking of the downlink signal broadcast by the low-orbit satellite. When the communication terminal receives the pseudo-random code initial phase information and starts communication, record the low-orbit satellite whole second time t1 corresponding to the start time of the frame header in the control data frame, and recover the second pulse signal corresponding to the current control data frame frame header reception time. S1-3. Based on the ephemeris of the low-Earth orbit (LEO) satellite, calculate the signal propagation delay τ between the LEO satellite and the communication terminal corresponding to the next full second t2 of the LEO satellite; the next full second t2 of the LEO satellite is: t2 = t1 + 1; S1-4. Using the second pulse signal corresponding to the current control data frame header reception time recovered in step S1-2 as a reference, delay by 1-2τ to obtain the communication uplink signal transmission time t3. S1-5, Record the Doppler f of the broadcast downlink signal corresponding to the uplink signal transmission time t3. track And converted into Doppler f of the uplink communication signal. dop ; S1-6. According to the control information contained in the control data frame, configure the pseudo-random code and its initial phase information used by the communication uplink signal, and calculate the Doppler f of the communication uplink signal according to step S1-5. dop The carrier wave of the uplink communication signal is modulated. The uplink communication signal is sent to the low-Earth orbit satellite starting at the transmission time t3. When the uplink communication signal arrives at the low-Earth orbit satellite, it is the next full second of the low-Earth orbit satellite and the motion Doppler is zero. The low-Earth orbit satellite quickly acquires and tracks the uplink communication signal at the next full second of the low-Earth orbit satellite according to the agreed pseudo-random code and its initial phase information. S1-7. Receive downlink communication signals sent by low-Earth orbit satellites according to the pseudo-random code and its initial phase information, and quickly establish a communication link with the low-Earth orbit satellites.

2. A method for rapidly establishing a satellite-to-ground communication link, characterized in that, Applied to low-Earth orbit satellites, the method includes the following steps: S2-1. Send a broadcast downlink signal to the communication terminal. The broadcast downlink signal modulates a control data frame. The start time of the frame header of the control data frame is aligned with the whole second time of the low-orbit satellite. The control data frame includes the whole second time corresponding to the current frame header, control information, and the ephemeris of the low-orbit satellite. The control information includes the initial phase information of the pseudo-random code used by the uplink and downlink signals when the low-orbit satellite communicates with the communication terminal. S2-2. According to the agreed pseudo-random code and its initial phase information, capture and track the communication uplink signal at the next full second of the low-orbit satellite, and quickly establish a connection with the communication terminal. The communication uplink signal arrives at the low-Earth orbit satellite at the next full second, and the motion Doppler is zero when the communication uplink signal arrives at the low-Earth orbit satellite. S2-3. According to the agreed pseudo-random code and its initial phase information, send downlink communication signals to quickly establish a communication link with the communication terminal; The uplink communication signal is obtained by the communication terminal through the following method: S2-2-1. Track the downlink signal broadcast by the low-orbit satellite, obtain the low-orbit satellite whole second time t1 corresponding to the start time of the frame header in the control data frame, and recover the second pulse signal corresponding to the receiving time of the current control data frame header. S2-2-2. Based on the ephemeris of the low-Earth orbit (LEO) satellite, calculate the signal propagation delay τ between the LEO satellite and the communication terminal corresponding to the next whole second t2 of the LEO satellite; the next whole second t2 of the LEO satellite is: t2 = t1 + 1; S2-2-3. Using the second pulse signal corresponding to the current control data frame header reception time recovered in step S2-2-1 as a reference, delay by 1-2τ to obtain the communication uplink signal transmission time t3, so that when the communication uplink signal arrives at the low-orbit satellite, it is the next whole second of the low-orbit satellite. S2-2-4, Record the uplink signal transmission time t3 of the broadcast downlink signal tracking Doppler f track And converted into Doppler f of the uplink communication signal. dop This ensures that when the uplink communication signal reaches the low-Earth orbit satellite, the motion Doppler is zero. S2-2-5. Configure the initial phase information of the pseudo-random code used for the communication uplink signal according to the control information contained in the control data frame, and calculate the Doppler f of the communication uplink signal according to step S2-2-4. dop The carrier wave of the modulated communication uplink signal is used to start transmitting the communication uplink signal to the low-Earth orbit satellite at the communication uplink signal transmission time t3.

3. A method for rapidly establishing a satellite-to-ground communication link according to any one of claims 1 or 2, characterized in that... The signal propagation delay τ between the low-Earth orbit satellite and the communication terminal corresponding to the next full second t2 of the low-Earth orbit satellite is calculated using the following method: Among them, (x s ,y s ,z s (x) represents the satellite position at the next full second t2 in low Earth orbit, and (x) represents the satellite position. u ,y u ,z u ) represents the location of the communication terminal at the next full second t2 of the low-orbit satellite.

4. A method for rapidly establishing a satellite-to-ground communication link according to any one of claims 1 or 2, characterized in that... The low-orbit satellite time corresponding to the transmission time t3 of the communication uplink signal is t1+1-τ, where τ is the delay of the next whole second.

5. A method for rapidly establishing a satellite-to-ground communication link according to any one of claims 1 or 2, characterized in that... The communication terminal uses an atomic clock.

6. The method for rapidly establishing a satellite-to-ground communication link according to claim 2, characterized in that... The communication uplink signal Doppler f dop for: f dop =-k×f track Where k is the nominal frequency of the uplink carrier signal / the nominal frequency of the downlink carrier signal; f track This is the Doppler signal tracking signal for the broadcast downlink signal corresponding to the transmission time of the uplink signal in communication.

7. A method for rapidly establishing a satellite-to-ground communication link according to any one of claims 1 or 2, characterized in that... The broadcast downlink signal uses spread spectrum BPSK modulation.

8. A method for rapidly establishing a satellite-to-ground communication link according to any one of claims 1 or 2, characterized in that... The pseudocode for the broadcast downlink signal is Gold code.

9. A system for rapidly establishing a satellite-to-ground communication link, characterized in that... Including low-Earth orbit satellites and communication terminals; A low-Earth orbit (LEO) satellite transmits a broadcast downlink signal to a communication terminal. The broadcast downlink signal includes a modulated control data frame. The start time of the control data frame header is aligned with the LEO satellite's integer second time. The control data frame includes the integer second time corresponding to the current frame header, control information, and the LEO satellite ephemeris. The control information includes the initial phase information of the pseudo-random code used by the LEO satellite for uplink and downlink signals when communicating with the communication terminal. According to the agreed pseudo-random code and its initial phase information, the uplink communication signal is quickly acquired and tracked at the next integer second of the LEO satellite system time. The communication terminal receives the broadcast downlink signal transmitted by the low-Earth orbit (LEO) satellite and tracks the LEO satellite broadcast downlink signal in real time. When the communication terminal receives the pseudo-random code initial phase information and starts communication, it records the LEO satellite integer second time t1 corresponding to the start time of the frame header in the control data frame, and recovers the second pulse signal corresponding to the receiving time of the current control data frame header. Based on the LEO satellite ephemeris, it calculates the signal propagation delay τ between the LEO satellite and the communication terminal corresponding to the next integer second time t2 of the LEO satellite. The next integer second time t2 of the LEO satellite is: t2 = t1 + 1. Using the recovered second pulse signal corresponding to the receiving time of the current control data frame header as a reference, it delays by 1 - 2τ to obtain the communication uplink signal transmission time t3. It records the broadcast downlink signal tracking Doppler f corresponding to the communication uplink signal transmission time t3. track And converted into Doppler f of the uplink communication signal. dop According to the control information contained in the control data frame, configure the pseudo-random code and its initial phase information used by the communication uplink signal, and according to the calculated Doppler f of the communication uplink signal. dop The carrier wave of the uplink communication signal is modulated, and the uplink communication signal is sent to the low-Earth orbit satellite starting at the transmission time t3, so that when the uplink communication signal arrives at the low-Earth orbit satellite, it is the next full second of the low-Earth orbit satellite and the motion Doppler is zero; according to the pseudo-random code and its initial phase information, the downlink communication signal sent by the low-Earth orbit satellite is received, and a communication link is quickly established with the low-Earth orbit satellite.

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