Time-frequency offset tracking method in Tiantong satellite monitoring low-orbit satellite system

Abstract

The invention discloses a time-frequency offset tracking method in a Tiantong satellite monitoring low-orbit satellite system, which belongs to the field of satellite communication, and comprises the following steps: acquiring satellite orbit information, satellite ephemeris information and positioning information of a low-orbit satellite, and resolving current Doppler frequency offset information; performing frequency pre-compensation according to the Doppler frequency offset information, and performing compensation by using the frequency offset captured by the FCCH; the satellite-borne terminal receiver carries out carrier frequency estimation by using a downlink channel, so that the carrier residual frequency difference of a received signal is estimated, and frequency correction is carried out; starting downlink timing tracking; uplink frequency offset compensation is started, so that the frequency of a signal from the low-orbit satellite to the Tiantong satellite is the expected carrier frequency of the Tiantong satellite; and carrying out transmission timing correction by adopting the spaceborne terminal timing drift rate, and carrying out uplink timing adjustment. According to the method, normal access and stable data transmission of the satellite-borne terminal can be realized.

Description

Technical Field

[0001] This invention relates to the field of satellite communication technology, specifically to a time-frequency offset tracking method in a Tiantong satellite monitoring low-orbit satellite system. Background Technology

[0002] The Tiantong system, also known as the Tiantong-1 satellite mobile communication system, is based on the Tiantong-1 satellite (in geosynchronous orbit). It is equipped with a ground-based telemetry, tracking, and command (TT&C) system and dedicated user terminals to achieve signal coverage. It can provide services such as voice calls, SMS (including emergency calls), and low-speed data transmission even in environments without terrestrial networks. It is widely used in emergency rescue, remote area communication, aviation, and maritime scenarios, and is a crucial "lifeline" especially when natural disasters cause ground communication outages.

[0003] Tiantong satellites are high-orbit satellites with an orbital altitude of approximately 36,000 km. They have 109 spot beams, each with a diameter of about 400 km. Adjacent beams overlap, with the overlap area ranging from tens to hundreds of kilometers.

[0004] In the Tiantong satellite monitoring system for low-Earth orbit satellites, a Tiantong terminal is mounted on the target low-Earth orbit satellite. With the help of the Tiantong high-Earth orbit satellite network, the ground control console can control the target low-Earth orbit satellite and transmit telemetry information back to it.

[0005] In satellite communication systems, the Doppler effect occurs due to the relative movement between the transmitter and receiver, which ultimately causes a frequency shift in the received signal. The frequency shift is proportional to their relative speed.

[0006] As mentioned above, Tiantong satellites are geostationary satellites in high orbit. For ground terminals, the relative speed between them and Tiantong satellites is relatively small, and the Doppler frequency shift caused by drift is also relatively small, with a maximum of less than 200Hz. Their uplink and downlink timing drift is also relatively small, and the terminal can use a relatively simple strategy for time and frequency tracking.

[0007] However, for low-Earth orbit (LEO) satellite-borne terminals, the Doppler frequency offset is also relatively large because LEO satellites move at relatively high speeds. In the Tiantong satellite communication system, Tiantong satellites are geostationary orbit satellites, so the relative speed of transmission and reception is mainly caused by the motion of LEO satellites.

[0008] The Doppler frequency shift caused by relative motion is:

[0009]

[0010] in denoted as carrier frequency, approximately 2.2 GHz for the Tiantong communication system; v is the relative velocity; θ is the angle between the direction of the relative velocity and the line connecting the ground or near-ground terminal and the Tiantong satellite; and c is the speed of light.

[0011] The calculation results of the maximum Doppler frequency shift and maximum Doppler rate of change of the spaceborne terminal are shown in Table 1:

[0012] Table 1

[0013] Maximum Doppler frequency deviation (kHz) Maximum Doppler frequency deviation rate (Hz / s) Spaceborne Terminal - Tiantong Satellite 56kHz 135Hz / s

[0014] Table 1 shows the calculated results for the maximum Doppler shift and the maximum rate of change of Doppler. This table ignores the Doppler shift caused by the drift of geostationary satellites (this value is less than 200Hz). For practical systems, low-Earth orbit satellites can calculate their instantaneous velocity and direction of motion based on ephemeris data, thereby compensating for the Doppler frequency shift in advance.

[0015] In addition, the maximum Doppler frequency shift =56K, corresponding satellite movement speed Its corresponding rate of change That is, approximately 0.4 symbols per second.

[0016] In summary, compared to ground terminals, spaceborne Tiantong terminals have larger Doppler frequency offset, maximum Doppler frequency offset change rate, and timing change rate. Therefore, traditional time and frequency tracking methods are no longer suitable for spaceborne Tiantong terminals. Summary of the Invention

[0017] The purpose of this invention is to provide a time-frequency offset tracking method in a Tiantong satellite monitoring low-orbit satellite system, which can realize normal access and stable data transmission of the onboard terminal.

[0018] To achieve the above objectives, the present invention provides a technical solution:

[0019] A time-frequency offset tracking method for a Tiantong satellite monitoring low-Earth orbit satellite system includes:

[0020] S1: Obtain the satellite orbit information, satellite ephemeris information and positioning information of the low-Earth orbit satellite, and perform calculations to obtain the velocity and direction of the low-Earth orbit satellite;

[0021] S2, calculate the current Doppler frequency offset information based on the speed and direction of the low-orbit satellite and the speed and direction of the Tiantong satellite;

[0022] S3, perform frequency pre-compensation based on the calculated Doppler frequency offset information, use the Tiantong frequency calibration channel FCCH to perform frequency sweep within a preset range, and use the frequency offset captured by the FCCH for compensation;

[0023] S4. After compensating for the frequency offset captured by FCCH, downlink frequency offset tracking is initiated, including: the satellite terminal receiver uses the downlink channel to estimate the carrier frequency, realizes the estimation of the carrier residual frequency difference of the received signal, and then uses the frequency estimation result to perform frequency correction.

[0024] S5, Initiate downlink timing tracking, including: the onboard terminal receiver uses the downlink channel to perform time offset estimation, and then corrects the downlink timing based on the time offset estimation result;

[0025] S6, initiate uplink frequency offset compensation to ensure that the frequency of the signal arriving at the Tiantong satellite from the low-Earth orbit satellite is the desired carrier frequency of the Tiantong satellite. ;

[0026] S7, initiate uplink timing tracking, including: using the onboard terminal timing drift rate Perform transmission timing correction, the timing drift rate for:

[0027]

[0028] in, For Doppler frequency shift, The drift factor;

[0029] Uplink timing is adjusted based on the timing drift rate R to ensure that the time when the uplink signal arrives at the Tiantong satellite remains unchanged.

[0030] Furthermore, in S1, the onboard terminal calculates the satellite orbit information, satellite ephemeris information and positioning information of the low-orbit satellite, and further obtains the angle information between the low-orbit satellite and the Tiantong satellite.

[0031] Furthermore, in S2, the current Doppler frequency offset information is calculated. for:

[0032]

[0033] in, , These are the longitude, latitude, and altitude of the Tiantong satellite, respectively. , These are the longitude, latitude, and altitude of the low-Earth orbit satellite, respectively. , These represent the speeds of the Tiantong satellite in the longitude, latitude, and altitude directions, respectively. , These represent the speeds of the low-Earth orbit satellite in the longitude, latitude, and altitude directions, respectively. For carrier frequency, It is the speed of light.

[0034] Furthermore, in S3, frequency pre-compensation is performed based on the calculated Doppler frequency offset information, including: calculating the current Doppler frequency offset as... During physical layer initialization, the RF center frequency of the transmitter and receiver is set as follows: based on the desired center frequency + Doppler frequency offset. .

[0035] Furthermore, in S3, the Tiantong frequency calibration channel FCCH is chirp modulated with a transmission symbol rate of 16K, and the frequency range captured by the Tiantong terminal baseband processing chip for the FCCH channel is [-4K, 4K].

[0036] Furthermore, in S3, the preset range is within 10KHz.

[0037] Furthermore, after compensating for the frequency offset captured by FCCH, the residual frequency offset is controlled within 1KHz.

[0038] Furthermore, in S6, let the carrier frequency of the downlink signal be... The carrier frequency of the uplink signal is , Due to Doppler frequency shift The frequency of the signal reaching the low-Earth orbit satellite terminal is ;

[0039] Initiate uplink frequency offset compensation; the carrier frequency transmitted from the low-Earth orbit satellite terminal is... After Doppler frequency shift The signal frequency reaching the Tiantong satellite is the desired frequency. .

[0040] Furthermore, in S7, the drift factor It can be one of the following values:

[0041] (1) Current transmission frequency value expressed in GHz;

[0042] (2) =1.995.

[0043] Furthermore, in S7, based on the dynamic change of the timing drift rate R over time, the onboard terminal acquires the Doppler frequency offset value in real time to achieve real-time updating of the timing drift rate R.

[0044] Compared with the prior art, the beneficial effects of the present invention are:

[0045] The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system of the present invention can be applied to scenarios involving Doppler frequency offset, Doppler frequency offset change rate, and timing change rate of the onboard terminal, thereby enabling normal access and stable data transmission of the onboard terminal.

[0046] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0047] Figure 1 This is a flowchart of the time-frequency offset tracking method in the Tiantong satellite monitoring low-Earth orbit satellite system of the present invention.

[0048] Figure 2 This is a schematic diagram of the downlink frequency offset tracking of the present invention;

[0049] Figure 3 This is a schematic diagram of the downlink timing tracking of the present invention;

[0050] Figure 4 This is a schematic diagram of the uplink frequency offset compensation of the present invention. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0053] Reference Figure 1 As shown, this disclosure provides a time-frequency offset tracking method in a Tiantong satellite monitoring low-Earth orbit satellite system, including:

[0054] S1 acquires the satellite orbit information, satellite ephemeris information, and positioning information of the low-Earth orbit satellite, and performs calculations to obtain the velocity and direction of the low-Earth orbit satellite.

[0055] Among them, the low-Earth orbit satellite-borne terminal calculates the satellite orbit information, satellite ephemeris information and positioning information of the low-Earth orbit satellite, and further obtains the angle information between the low-Earth orbit satellite and the Tiantong satellite.

[0056] S2 calculates the current Doppler frequency offset information based on the speed and direction of the low-orbit satellite and the speed and direction of the Tiantong satellite.

[0057] Calculate the current Doppler frequency offset information for:

[0058] (1)

[0059] in,

[0060]

[0061]

[0062]

[0063]

[0064]

[0065]

[0066] , These are the longitude, latitude, and altitude of the Tiantong satellite, respectively. , These are the longitude, latitude, and altitude of the low-Earth orbit satellite, respectively. , These represent the speeds of the Tiantong satellite in the longitude, latitude, and altitude directions, respectively. , These represent the speeds of the low-Earth orbit satellite in the longitude, latitude, and altitude directions, respectively. For carrier frequency, It is the speed of light.

[0067] S3. Based on the calculated Doppler frequency offset information, frequency pre-compensation is performed. The frequency is swept within a preset range using the Tiantong frequency calibration channel FCCH, and the frequency offset captured by the FCCH is used for compensation.

[0068] The preset range is within 10kHz. That is, the FCCH sweeps within a 10kHz range. It should be noted that the 10kHz range mentioned above is for illustrative purposes only and can be set as needed.

[0069] In this invention, the Tiantong frequency calibration channel FCCH is chirp modulated with a transmission symbol rate of 16K, and the frequency range captured by the Tiantong terminal baseband processing chip for the FCCH channel is [-4K, 4K].

[0070] Specifically, frequency pre-compensation is performed based on the calculated Doppler frequency offset information, including: calculating the current Doppler frequency offset as... During physical layer initialization, the RF center frequency of the transmitter (TX) and receiver (RX) is set as follows: based on the desired center frequency + Doppler frequency offset. .

[0071] In summary, before searching the network, the spaceborne terminal uses ephemeris information to calculate Doppler for pre-compensation, thus keeping the residual frequency offset within a small range. Then, it uses the Tiantong Frequency Calibration Channel (FCCH) for frequency sweeping. The Tiantong FCCH uses chirp modulation with a symbol rate of 16kHz. The Tiantong terminal's baseband processing chip captures frequencies within the range of [-4kHz, 4kHz] of the FCCH channel. The frequency offset captured by the FCCH is then used for compensation.

[0072] By using this step to compensate for the frequency offset captured by FCCH, the residual frequency offset is controlled within 1KHz.

[0073] In summary, Doppler acquisition can be achieved rapidly under high dynamic range by using satellite orbit and positioning information. The calculation of satellite orbit, ephemeris, and positioning information reveals that due to the high satellite speed, the Doppler frequency offset can reach a maximum of 56 kHz. Therefore, the calculated satellite information is converted into current velocity, direction, and angle with the Tiantong satellite, thus obtaining the current Doppler information for Doppler pre-compensation. Frequency pre-compensation is then performed using the calculated frequency information, followed by a small-range frequency sweep to complete pilot channel acquisition.

[0074] S4. After compensating for the frequency offset captured by FCCH, downlink frequency offset tracking is initiated, including: the satellite terminal receiver uses the downlink channel to estimate the carrier frequency, realizes the estimation of the carrier residual frequency difference of the received signal, and then uses the frequency estimation result to perform frequency correction.

[0075] Specifically, after compensation using the frequency offset captured by FCCH, the residual frequency offset is already within a small range (e.g., within 1 kHz). However, the maximum Doppler frequency offset change rate of the onboard terminal is 135 Hz / s, so the onboard terminal needs to start downlink frequency offset tracking to ensure that the downlink residual frequency offset remains within a relatively small range.

[0076] refer to Figure 2 As shown, the satellite terminal receiver uses the downlink channel to estimate the carrier frequency, which is actually to estimate the carrier residual frequency difference of the received signal, and then use the frequency estimation result to perform frequency correction.

[0077] S5, Initiate downlink timing tracking, including: the onboard terminal receiver uses the downlink channel to perform time offset estimation, and then corrects the downlink timing based on the time offset estimation result.

[0078] The onboard terminal receiver uses FCCH for downlink initial timing, but because the low-Earth orbit satellite moves at a relatively high speed, the timing variation rate is also relatively large, with a maximum timing variation rate of [missing value]. This also necessitates downlink timing tracking to ensure that the downlink residual time offset remains within a relatively small range.

[0079] refer to Figure 3 As shown, the satellite terminal receiver uses the downlink channel to estimate the time offset, and then corrects the downlink timing based on the time offset estimation result.

[0080] S6, initiate uplink frequency offset compensation to ensure that the frequency of the signal arriving at the Tiantong satellite from the low-Earth orbit satellite is the desired carrier frequency of the Tiantong satellite. .

[0081] Onboard satellites can obtain real-time Doppler frequency offset through ephemeris information. In general, the uplink frequency offset compensation strategy needs to ensure that the frequency of the signal arriving at the Tiantong satellite from the low-Earth orbit satellite is the carrier frequency desired by the Tiantong satellite.

[0082] refer to Figure 4 As shown, let the carrier frequency of the downlink signal be... The carrier frequency of the uplink signal is , Because of Doppler frequency deviation The frequency of the signal reaching the low-Earth orbit satellite terminal is In the uplink frequency offset compensation strategy of this invention, the carrier frequency transmitted from the low-Earth orbit satellite terminal is... Thus, after Doppler frequency shift The signal frequency reaching the Tiantong satellite is the desired frequency. .

[0083] S7, initiate uplink timing tracking, including: using the onboard terminal timing drift rate Perform transmission timing correction.

[0084] Because low-Earth orbit satellites move at relatively high speeds, the signal propagation time is constantly changing, so the time when the uplink signal arrives at the Tiantong satellite is also constantly changing, i.e., transmission timing drift.

[0085] The timing drift rate R of a low-Earth orbit satellite terminal has the following characteristics:

[0086] The impact is more significant: Low Earth orbit (LEO) satellites operate at high speeds and low altitudes (generally around 500-2000 kilometers), causing rapid changes in signal transmission paths. This makes timing drift more pronounced in communications than in terrestrial communications; even minor timing drift can lead to inaccuracies in signal reception and transmission, affecting the accuracy of data transmission and the stability of communication links.

[0087] Dynamic variability: Due to the rapid and continuous changes in the relative positions between the satellite, ground station, and terminal, the timing drift rate R also changes dynamically over time, unlike the relatively stable ground communication environment, which requires real-time monitoring and adjustment.

[0088] To ensure that the arrival time of the uplink signal on the Tiantong satellite remains constant, this invention proposes an uplink timing strategy. During the call, to improve the timing accuracy and stability of the onboard terminal transmission, the onboard terminal timing drift rate should be used for transmission timing correction. This timing drift rate R should be based on the Doppler frequency correction information obtained from the onboard terminal. for:

[0089] (2)

[0090] in, For Doppler frequency shift, Let be the drift factor. Assume the onboard satellite obtains the real-time Doppler frequency offset from ephemeris information as . ,but It is the round-trip Doppler shift experienced by the spaceborne terminal.

[0091] Uplink timing is adjusted based on the timing drift rate R to ensure that the time when the uplink signal arrives at the Tiantong satellite remains unchanged.

[0092] In this invention, the timing drift rate depends on the precise carrier frequency used. The onboard terminal assumes a drift factor. It can be one of the following values:

[0093] (1) Current transmission frequency value expressed in GHz;

[0094] (2) =1.995.

[0095] Choosing the current transmit frequency value expressed in GHz is preferable, but 1.995 is also an acceptable compromise. It's important to note that transmit timing is limited by hardware quantization and errors in the estimator and tracking loop. Therefore, a longer duration should be used to test whether the timing meets requirements.

[0096] Because the timing drift rate R changes dynamically over time, the onboard terminal needs to continuously obtain the real-time Doppler frequency offset value to continuously update the timing drift rate R. Then, uplink timing adjustments are made based on R to ensure that the arrival time of the uplink signal on the Tiantong satellite remains unchanged.

[0097] In the description of this invention, it should be understood that the indicated orientation or positional relationship is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description, and is not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0098] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0099] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A time-frequency offset tracking method in a Tiantong satellite monitoring low-Earth orbit satellite system, characterized in that, include: S1: Obtain the satellite orbit information, satellite ephemeris information and positioning information of the low-Earth orbit satellite, and perform calculations to obtain the velocity and direction of the low-Earth orbit satellite; S2, calculate the current Doppler frequency offset information based on the speed and direction of the low-orbit satellite and the speed and direction of the Tiantong satellite; S3, perform frequency pre-compensation based on the calculated Doppler frequency offset information, use the Tiantong frequency calibration channel FCCH to perform frequency sweep within a preset range, and use the frequency offset captured by the FCCH for compensation; S4. After compensating for the frequency offset captured by FCCH, downlink frequency offset tracking is initiated, including: the satellite terminal receiver uses the downlink channel to estimate the carrier frequency, realizes the estimation of the carrier residual frequency difference of the received signal, and then uses the frequency estimation result to perform frequency correction. S5, Initiate downlink timing tracking, including: the onboard terminal receiver uses the downlink channel to perform time offset estimation, and then corrects the downlink timing based on the time offset estimation result; S6, initiate uplink frequency offset compensation to ensure that the frequency of the signal arriving at the Tiantong satellite from the low-Earth orbit satellite is the desired carrier frequency of the Tiantong satellite. ; S7, initiate uplink timing tracking, including: using the onboard terminal timing drift rate Perform transmission timing correction, the timing drift rate for: ;in, For Doppler frequency shift, The drift factor; Uplink timing is adjusted based on the timing drift rate R to ensure that the time when the uplink signal arrives at the Tiantong satellite remains unchanged.

2. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S1, the onboard terminal calculates the satellite orbit information, satellite ephemeris information and positioning information of the low-orbit satellite, and further obtains the angle information between the low-orbit satellite and the Tiantong satellite.

3. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S2, the current Doppler frequency offset information is calculated. for: ;in, , These are the longitude, latitude, and altitude of the Tiantong satellite, respectively. , These are the longitude, latitude, and altitude of the low-Earth orbit satellite, respectively. , These represent the speeds of the Tiantong satellite in the longitude, latitude, and altitude directions, respectively. , These represent the speeds of the low-Earth orbit satellite in the longitude, latitude, and altitude directions, respectively. For carrier frequency, It is the speed of light.

4. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S3, frequency pre-compensation is performed based on the calculated Doppler frequency offset information, including: calculating the current Doppler frequency offset as... During physical layer initialization, the RF center frequency of the transmitter and receiver is set as follows: based on the desired center frequency + Doppler frequency offset. .

5. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S3, the Tiantong frequency calibration channel FCCH is chirp modulated with a transmission symbol rate of 16K, and the frequency range captured by the Tiantong terminal baseband processing chip for the FCCH channel is [-4K, 4K].

6. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S3, the preset range is within 10KHz.

7. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, After compensating for the frequency offset captured by FCCH, the residual frequency offset is controlled within 1KHz.

8. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S6, let the carrier frequency of the downlink signal be... The carrier frequency of the uplink signal is , Due to Doppler frequency deviation The frequency of the signal reaching the low-Earth orbit satellite terminal is ; Initiate uplink frequency offset compensation; the carrier frequency transmitted from the low-Earth orbit satellite terminal is... After Doppler frequency shift The signal frequency reaching the Tiantong satellite is the desired frequency. .

9. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S7, the drift factor It can be one of the following values: (1) Current transmission frequency value expressed in GHz; （2） =1.995。 10. The time-frequency offset tracking method in the Tiantong satellite monitoring low-orbit satellite system according to claim 1, characterized in that, In S7, based on the dynamic change of the timing drift rate R over time, the onboard terminal acquires the Doppler frequency offset value in real time to achieve real-time updating of the timing drift rate R.

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