Spaceborne lidar high-precision timekeeping method and system

Abstract

The application provides a kind of spaceborne laser radar high-precision time keeping method and system, comprising steps S1: setting register in laser radar;The register deposits count;Step S2: according to the receiving state of laser radar, the laser radar is calibrated, and the self-keeping time of laser radar preset accuracy is completed;The receiving state includes receiving satellite platform provided second pulse, cannot receive second pulse but receive BC broadcast time and cannot receive BC broadcast time.The application formulates radar time keeping method for three kinds of conditions of laser radar respectively, ensures that spaceborne laser radar still maintains high-precision time keeping and timely, safe light emission when satellite time system appears abnormal (such as second pulse interval anomaly, broadcast time anomaly etc.), accurately records radar echo signal imaging time in remote sensing data, and lays a foundation for high-precision inversion of laser radar data.

Description

Technical Field

[0001] This invention relates to the field of satellite technology, specifically to a high-precision timekeeping method and system for spaceborne lidar. Background Technology

[0002] Traditional satellite passive optical remote sensing technology is heavily dependent on solar scattered light and can only perform imaging measurements in illuminated areas. However, satellite active detection technology, represented by spaceborne lidar, is not affected by sunlight, can perform measurements around the clock, and is not affected by clouds or aerosols. It has obvious advantages and has therefore been widely used in the field of satellite remote sensing.

[0003] Spaceborne lidar performs active detection by periodically sending laser pulses. High-precision inversion places high demands on the time accuracy of the radar. Furthermore, due to the high energy of the laser, if the timing scheme is poor and the output frequency is too high, it may damage the laser and affect the safety of the lidar.

[0004] Chinese patent document CN107092183A discloses a high-precision time synchronization method based on GPS second pulses, which includes the following steps: Step 1: The onboard computer software performs a validity check on the received GPS time code; if the validity check passes, proceed to Step 2; if the validity check fails, proceed to Step 4; Step 2: The onboard computer software determines whether the received GPS time code is the value of the previous cycle; if the received GPS time code is not the value of the previous cycle, proceed to Step 3; if the received GPS time code is the value of the previous cycle, increment the GPS time code by 1, and then proceed to Step 3. However, this patent document mainly addresses the problem of how satellite time management systems synchronize time when seconds are missed, and does not address the timekeeping problem of payloads that simultaneously receive second pulses and broadcast data from the time management system.

[0005] Chinese patent document CN105890591B discloses a method for calculating the exposure time of a high-precision star sensor using second pulse signals. The method records the time T_AOCC when the onboard computer emits the second pulse signal and the number of emitted second pulses SYNC_AOCC. It reads the attitude quaternion and the corresponding number of second pulses syncCnt from the star sensor, as well as the time interval datation from the nearest second pulse. Based on the time interval datation, it determines the validity of the pulse difference flag ΔSYNC_flag and calculates the pulse difference ΔSYNC. Finally, it calculates the onboard computer time T_ST corresponding to the star sensor exposure time based on the second pulse signal time T_AOCC, the second pulse time interval datation, and the pulse difference ΔSYNC. However, this patent document addresses the problem of using second pulses for high-precision time calibration of star sensors to determine satellite attitude, which is fundamentally different from the problem solved by this invention.

[0006] Chinese patent document CN116073935A discloses a clock synchronization system and method based on the IEEE 1588 protocol, comprising: a timestamp determination module for receiving an externally input second pulse signal and determining the timestamp corresponding to the second pulse signal under the local clock time scale; a difference calculation module connected to the timestamp determination module for calculating a time difference value containing frequency difference information based on the timestamp; an adjustment module connected to the difference calculation module for processing the time difference value under the action of the local clock and outputting an adjustment value when the maximum frequency of the local clock meets a preset frequency difference threshold; and a step value determination module connected to the adjustment module for processing a preset step value based on the adjustment value to obtain a frequency difference ratio step value, and adjusting the step value of the time counter corresponding to the local clock based on the frequency difference ratio step value to synchronize the frequency of the local clock with that of the master clock. The patent document uses a second pulse signal to generate a timestamp, while the present invention is a timekeeping solution tailored for spaceborne lidar to address the problem of time interval discrepancies caused by various anomalies in navigation and data broadcasting. It is fundamentally different from the technology used in the patent document.

[0007] Chinese patent document CN115175297A discloses a satellite payload second pulse autonomous recovery synchronization method, which includes the following steps: Step S1: second pulse synchronization and initialization reference difference calculation; Step S2: anomaly judgment and second pulse correction difference measurement; Step S3: second pulse autonomous compensation recovery. However, this patent document requires the payload to provide a return pulse signal, which is complex to implement, and it does not formulate a timekeeping scheme for the state where there is only broadcast time of several tubes.

[0008] Chinese patent document CN115047748B discloses a timing device and method based on satellite navigation signals. The device includes a local reference source, a signal receiving and source difference measurement module, a controllable frequency division module, a parameter generation module, and a controllable delay module. The signal receiving and source difference measurement module receives satellite navigation signals and measures the source difference of the local reference signal to obtain its source difference value. The local reference signal is also input to the controllable frequency division module to generate a master clock. The parameter generation module uses the source difference value of the local reference signal to generate an output frequency adjustment value, divides the master clock to generate a second pulse signal, and then inputs it to the controllable delay module to obtain a controllable second pulse output relative to satellite time calibration. However, this patent document outputs a second pulse after receiving the navigation second pulse. Since directly synchronizing the navigation second pulse with a lidar poses a safety hazard, this method is not suitable for spaceborne lidar. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the purpose of this invention is to provide a high-precision timekeeping method and system for spaceborne lidar.

[0010] A high-precision timekeeping method for spaceborne lidar according to the present invention includes:

[0011] Step S1: Set up a register in the lidar; the register stores the count;

[0012] Step S2: Calibrate the lidar according to its receiving status to achieve self-timekeeping at the preset accuracy of the lidar; the receiving status includes receiving second pulses provided by the satellite platform, receiving BC broadcast time but not receiving second pulses, and receiving BC broadcast time but not receiving BC broadcast time.

[0013] Preferably, step S1 includes setting a first register and a second register; the first register stores the whole-second count i; the second register stores the 0.1ms count j within a second.

[0014] Preferably, step S2 includes the following sub-steps:

[0015] Step S2.1: When the lidar is in the state of receiving second pulses provided by the satellite platform, it uses the second pulses that have been legally verified to calibrate i and j, and emits a laser pulse when the self-synchronous time reaches the preset accuracy, and marks the timestamp i+j in each laser pulse echo signal remote sensing packet;

[0016] Step S2.2: When the lidar is in a state where it cannot receive second pulses but can receive BC broadcast time, it uses the broadcast time that has been legally verified to calibrate i and j, reaches the preset accuracy of self-keeping time, and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet;

[0017] Step S2.3: When the lidar is unable to receive the BC broadcast time, it keeps time according to its own maintenance time and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet.

[0018] Preferably, step S2.1 includes the following sub-steps:

[0019] Step S2.1.1: The lidar is powered on, and j begins counting for 0.1ms;

[0020] Step S2.1.2: Upon receiving the first second pulse, assign the integer second of the second pulse to i and clear j to zero;

[0021] Step S2.1.3: Upon receiving a subsequent second pulse, determine the value of j:

[0022] If j < 9000, then no time synchronization operation will be performed;

[0023] If 9000≤j≤11000, then assign the whole second of the second pulse to i and clear j to zero;

[0024] If j > 11000, then increment i by 1 and assign j - 10000 to j;

[0025] Step S2.1.4: If j>11000 appears for 5 consecutive seconds, it is determined that the second pulse reception is abnormal, and the BC broadcast data is used to calibrate the time it maintains.

[0026] Preferably, step S2.2 includes the following sub-steps:

[0027] Step S2.2.1: The lidar is powered on, and j starts counting for 0.1ms without emitting light.

[0028] Step S2.2.2: Upon receiving the first time broadcast interruption, assign the whole second to i and clear j to zero;

[0029] Step S2.2.3: Upon receiving a subsequent time broadcast interruption, determine the value of j:

[0030] If j < 8000, then no time synchronization operation will be performed;

[0031] If 8000≤j≤12000, then assign the whole second of the second pulse to i and clear j to zero;

[0032] If j > 12000, then increment i by 1 and assign j - 10000 to j.

[0033] Preferably, the lidar has no time reference before receiving the navigation second pulse for time synchronization or the BC broadcast time, and only performs self-timekeeping without emitting light.

[0034] Preferably, after receiving the navigation second pulse time synchronization or BC broadcast time, the lidar performs light emission operation when i+j reaches the corresponding time point according to the pulse frequency, and transmits the i+j time to the data processor to record the time corresponding to the laser pulse for use in data inversion.

[0035] Preferably, the light emission time of the lidar is recorded in real time. If the time difference between the currently calculated light emission time and the previous light emission time is less than a certain threshold that affects the safety of the laser, then no light is emitted at the current time.

[0036] Preferably, the lidar maintains preset timekeeping accuracy and operates even when the satellite timing system malfunctions, and records the radar echo signal imaging time in the remote sensing data; the malfunctions include the second pulse interval not reaching the preset value and the broadcast time not reaching the preset value.

[0037] A high-precision timekeeping system for spaceborne lidar according to the present invention includes:

[0038] Module M1: A register is set up inside the lidar; the register stores the count;

[0039] Module M2: calibrates the lidar based on its receiving status, achieving self-timekeeping at the lidar's preset accuracy; the receiving status includes receiving second pulses provided by the satellite platform, receiving BC broadcast time but not receiving second pulses, and not receiving BC broadcast time.

[0040] Preferably, the module M1 includes a first register and a second register; the first register stores the whole-second count i; and the second register stores the 0.1ms count j within a second.

[0041] Preferably, module M2 includes the following sub-modules:

[0042] Module M2.1: When the lidar is in the state of receiving second pulses provided by the satellite platform, it uses the second pulses that have been validated for validity to calibrate i and j. When the self-synchronous time reaches the preset accuracy, it emits a laser pulse and marks the timestamp i+j in each laser pulse echo signal remote sensing packet.

[0043] Module M2.2: When the lidar is in a state where it cannot receive second pulses but can receive BC broadcast time, it uses the broadcast time that has been validated to calibrate i and j, reaches the preset accuracy of self-keeping time, and emits a laser pulse, marking the timestamp i+j in each laser pulse echo signal remote sensing packet;

[0044] Module M2.3: When the lidar is unable to receive the BC broadcast time, it keeps time according to its own maintenance time and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet.

[0045] Preferably, module M2.1 includes the following sub-modules:

[0046] Module M2.1.1: The lidar is powered on, and j begins a 0.1ms count;

[0047] Module M2.1.2: Upon receiving the first second pulse, assign the exact second of the second pulse to i and clear j to zero;

[0048] Module M2.1.3: Upon receiving a subsequent second pulse, determine the value of j:

[0049] If j < 9000, then no time synchronization operation will be performed;

[0050] If 9000≤j≤11000, then assign the whole second of the second pulse to i and clear j to zero;

[0051] If j > 11000, then increment i by 1 and assign j - 10000 to j;

[0052] Module M2.1.4: If j>11000 appears for 5 consecutive seconds, the second pulse reception is determined to be abnormal, and the BC broadcast data is used to calibrate the duration maintained by itself.

[0053] Preferably, module M2.2 includes the following sub-modules:

[0054] Module M2.2.1: The lidar is powered on, and j starts counting for 0.1ms without emitting light.

[0055] Module M2.2.2: Upon receiving the first time broadcast interrupt, assign the whole second to i and clear j to zero;

[0056] Module M2.2.3: Upon receiving a subsequent time broadcast interruption, determine the value of j:

[0057] If j < 8000, then no time synchronization operation will be performed;

[0058] If 8000≤j≤12000, then assign the whole second of the second pulse to i and clear j to zero;

[0059] If j > 12000, then increment i by 1 and assign j - 10000 to j.

[0060] Preferably, the lidar has no time reference before receiving the navigation second pulse for time synchronization or the BC broadcast time, and only performs self-timekeeping without emitting light.

[0061] Preferably, after receiving the navigation second pulse time synchronization or BC broadcast time, the lidar performs light emission operation when i+j reaches the corresponding time point according to the pulse frequency, and transmits the i+j time to the data processor to record the time corresponding to the laser pulse for use in data inversion.

[0062] Preferably, the light emission time of the lidar is recorded in real time. If the time difference between the currently calculated light emission time and the previous light emission time is less than a certain threshold that affects the safety of the laser, then no light is emitted at the current time.

[0063] Preferably, the lidar maintains preset timekeeping accuracy and operates even when the satellite timing system malfunctions, and records the radar echo signal imaging time in the remote sensing data; the malfunctions include the second pulse interval not reaching the preset value and the broadcast time not reaching the preset value.

[0064] Compared with the prior art, the present invention has the following beneficial effects:

[0065] 1. This invention addresses three scenarios for lidar: normal reception of high-precision second pulses provided by the satellite platform, abnormal second pulse reception but normal reception of BC broadcast time, and inability to receive BC broadcast time. It establishes radar timing methods for each scenario, ensuring that the spaceborne lidar maintains high-precision timing and timely, safe light emission even when the satellite timing system malfunctions (e.g., abnormal second pulse intervals, abnormal broadcast times). This accurately records the imaging time of the radar echo signal in the remote sensing data, laying the foundation for high-precision inversion of lidar data.

[0066] 2. This invention is simple to implement and can be widely applied in fields such as on-orbit safe operation of lidar data and high-precision inversion of remote sensing data, and has good practicality.

[0067] Other beneficial effects of the present invention will be explained in detail through the introduction of specific technical features and technical solutions in specific embodiments. Those skilled in the art should be able to understand the beneficial technical effects brought about by these technical features and technical solutions through the introduction of these technical features and technical solutions. Attached Figure Description

[0068] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0069] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0070] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0071] Reference Figure 1 As shown, this invention proposes radar timing methods for three possible time synchronization states that a spaceborne lidar may encounter in orbit (being able to receive high-precision second pulses provided by the satellite platform normally, receiving second pulses abnormally but being able to receive BC broadcast time normally, and being unable to receive BC broadcast time normally). These methods ensure that the spaceborne lidar maintains high-precision time synchronization and normal operation even when the satellite time synchronization system malfunctions (such as abnormal second pulse intervals or abnormal broadcast times).

[0072] The specific steps are as follows:

[0073] The lidar is equipped with two registers: register A is used to store the whole-second count i, and register B is used to store the 0.1ms count j within a second.

[0074] When the high-precision second pulse for navigation can be received normally, the valid second pulse is used to calibrate i and j, thereby achieving high-precision self-timekeeping and transmitting laser pulses. The timestamp i+j is added to each laser pulse echo signal remote sensing packet.

[0075] When the lidar is powered on, j starts counting every 0.1ms; upon receiving the first second pulse, the exact second of the second pulse is assigned to i, and j is cleared to zero.

[0076] Upon receiving subsequent second pulses, the value of j is determined as follows:

[0077] If j < 9000, then no time synchronization operation will be performed;

[0078] If 9000≤j≤11000, then assign the whole second of the second pulse to i and clear j to zero;

[0079] If j > 11000, then increment i by 1 and assign j - 10000 to j;

[0080] If j>11000 appears for 5 consecutive seconds, the second pulse reception is considered abnormal, and the BC broadcast data is used to calibrate the duration of the pulse.

[0081] When the high-precision second pulse for navigation cannot be received normally, the BC broadcast time is received, and the broadcast time that has been validated for validity is used to calibrate i and j, thereby achieving high-precision self-timekeeping and transmitting laser pulses. The timestamp i+j is added to each laser pulse echo signal remote sensing packet.

[0082] When the lidar is powered on, j starts counting 0.1ms without emitting light; upon receiving the first time broadcast interruption, the whole second is assigned to i, and j is cleared to zero.

[0083] Upon receiving a subsequent time broadcast interruption, the value of j is determined as follows:

[0084] If j < 8000, then no time synchronization operation will be performed;

[0085] If 8000≤j≤12000, then assign the whole second of the second pulse to i and clear j to zero;

[0086] If j > 12000, then increment i by 1 and assign j - 10000 to j.

[0087] When unable to receive the BC broadcast time normally, it maintains its own time and emits laser pulses, and adds the timestamp i+j to each laser pulse echo signal remote sensing packet.

[0088] In the above steps, before receiving the navigation second pulse time synchronization or BC broadcast time, the lidar has no time reference and only performs self-timekeeping without emitting light. After receiving the navigation second pulse time synchronization or BC broadcast time, the lidar emits light when i+j reaches the corresponding time point according to the pulse frequency, and transmits the i+j time to the data processor to record the time corresponding to the laser pulse for use in data inversion.

[0089] The entire process requires recording the last time the lidar emitted light. If the time difference between the currently calculated emission time and the last emission time is less than a certain threshold that affects the safety of the laser, then no light will be emitted at the current time.

[0090] This invention addresses three scenarios for lidar: normal reception of high-precision second pulses from the satellite platform, abnormal second pulse reception but normal reception of BC broadcast time, and inability to receive BC broadcast time. It establishes radar timing methods for each scenario, ensuring that the spaceborne lidar maintains high-precision timing and timely, safe light emission even when the satellite timing system malfunctions (such as abnormal second pulse intervals or broadcast time). This accurately records the imaging time of the radar echo signal in the remote sensing data, laying the foundation for high-precision inversion of lidar data.

[0091] The above are basic embodiments of the present invention. The technical solution of the present invention will be further described below through a preferred embodiment.

[0092] Example 1

[0093] For a satellite with a navigation pulse accuracy of 1 μs, a lidar laser emission frequency of 20 Hz, and a lidar crystal oscillator frequency accuracy of 50 ppm, the maximum cumulative timing deviation within 1 second will not exceed 50 μs. Using the timing method described in this invention:

[0094] 1) When the lidar can receive the second pulse signal normally, the maximum time error between the lidar and the GPS time does not exceed 51μs. Since the time resolution of the lidar is 0.1ms, the time error is within 0.1ms, and the time accuracy is high.

[0095] 2) When the lidar cannot receive the second pulse signal normally but can receive the BC broadcast time normally, since the time difference between the satellite measurement and GPS is within 0.1ms, the maximum error of the lidar time and GPS time does not exceed 150μs. Since the lidar time resolution is 0.1ms, the time error is within 0.2ms, and the time accuracy is high.

[0096] 3) The lidar will emit light when the count of 0.1ms reaches an integer multiple of 50ms. Since the emission time is determined by the radar maintenance time and is not synchronized with the navigation second pulse, BC broadcast time, etc., the lidar is guaranteed to operate safely on orbit.

[0097] 4) Assume that the laser has the following characteristics: the light emission interval must be greater than 10ms; the last light emission time is ti, and the current light emission time is calculated to be tj. If tj-ti<10ms, then no light emission operation will be performed this time, and light emission will be performed again at tj+50ms.

[0098] The present invention also provides a high-precision timekeeping system for spaceborne lidar, which can be implemented by executing the process steps of the high-precision timekeeping method for spaceborne lidar. That is, those skilled in the art can understand the high-precision timekeeping method for spaceborne lidar as a preferred embodiment of the high-precision timekeeping system for spaceborne lidar.

[0099] Specifically, a high-precision timekeeping system for spaceborne lidar includes:

[0100] Module M1: A register is set up inside the lidar; the register stores the count;

[0101] Module M2: calibrates the lidar based on its receiving status, achieving self-timekeeping at the lidar's preset accuracy; the receiving status includes receiving second pulses provided by the satellite platform, receiving BC broadcast time but not receiving second pulses, and not receiving BC broadcast time.

[0102] The module M1 includes a first register and a second register; the first register stores the whole second count i; the second register stores the 0.1ms count j within a second.

[0103] Module M2 includes the following sub-modules:

[0104] Module M2.1: When the lidar is in the state of receiving second pulses provided by the satellite platform, it uses the second pulses that have been validated for validity to calibrate i and j. When the self-synchronous time reaches the preset accuracy, it emits a laser pulse and marks the timestamp i+j in each laser pulse echo signal remote sensing packet.

[0105] Module M2.2: When the lidar is in a state where it cannot receive second pulses but can receive BC broadcast time, it uses the broadcast time that has been validated to calibrate i and j, reaches the preset accuracy of self-keeping time, and emits a laser pulse, marking the timestamp i+j in each laser pulse echo signal remote sensing packet;

[0106] Module M2.3: When the lidar is unable to receive the BC broadcast time, it keeps time according to its own maintenance time and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet.

[0107] Module M2.1 includes the following sub-modules:

[0108] Module M2.1.1: The lidar is powered on, and j begins a 0.1ms count;

[0109] Module M2.1.2: Upon receiving the first second pulse, assign the exact second of the second pulse to i and clear j to zero;

[0110] Module M2.1.3: Upon receiving a subsequent second pulse, determine the value of j:

[0111] If j < 9000, then no time synchronization operation will be performed;

[0112] If 9000≤j≤11000, then assign the whole second of the second pulse to i and clear j to zero;

[0113] If j > 11000, then increment i by 1 and assign j - 10000 to j;

[0114] Module M2.1.4: If j>11000 appears for 5 consecutive seconds, the second pulse reception is determined to be abnormal, and the BC broadcast data is used to calibrate the duration maintained by itself.

[0115] Module M2.2 includes the following sub-modules:

[0116] Module M2.2.1: The lidar is powered on, and j starts counting for 0.1ms without emitting light.

[0117] Module M2.2.2: Upon receiving the first time broadcast interrupt, assign the whole second to i and clear j to zero;

[0118] Module M2.2.3: Upon receiving a subsequent time broadcast interruption, determine the value of j:

[0119] If j < 8000, then no time synchronization operation will be performed;

[0120] If 8000≤j≤12000, then assign the whole second of the second pulse to i and clear j to zero;

[0121] If j > 12000, then increment i by 1 and assign j - 10000 to j.

[0122] The lidar has no time reference before receiving the navigation second pulse for time synchronization or the BC broadcast time. It only performs self-timekeeping and does not perform light emission operation.

[0123] After receiving the navigation second pulse for time synchronization or the BC broadcast time, the lidar performs a light emission operation when i+j reaches the corresponding time point according to the pulse frequency, and transmits the i+j time to the data processor to record the time corresponding to the laser pulse for use in data inversion.

[0124] The laser radar emits light at a time that is recorded in real time. If the time difference between the current calculated emitter time and the previous emitter time is less than a threshold that affects the safety of the laser, then no light will be emitted at the current time.

[0125] Even when the satellite timing system malfunctions, the lidar will still maintain preset timekeeping accuracy and continue operating, and the imaging time of the radar echo signal will be recorded in the remote sensing data; the malfunctions include the second pulse interval not reaching the preset value and the broadcast time not reaching the preset value.

[0126] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.

[0127] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A high-precision timekeeping method for spaceborne lidar, characterized in that, include: Step S1: Set up a register in the lidar; the register stores the count; Step S2: Calibrate the lidar according to its receiving status to achieve self-timekeeping at the preset accuracy of the lidar; the receiving status includes receiving second pulses provided by the satellite platform, receiving BC broadcast time but not receiving second pulses, and receiving BC broadcast time but not receiving BC broadcast time. Step S1 includes setting a first register and a second register; the first register stores the integer second count i; the second register stores the 0.1ms count j within a second; Step S2 includes the following sub-steps: Step S2.1: When the lidar is in the state of receiving second pulses provided by the satellite platform, it uses the second pulses that have been legally verified to calibrate i and j, and emits a laser pulse when the self-synchronous time reaches the preset accuracy, and marks the timestamp i+j in each laser pulse echo signal remote sensing packet; Step S2.2: When the lidar is in a state where it cannot receive second pulses but can receive BC broadcast time, it uses the broadcast time that has been legally verified to calibrate i and j, reaches the preset accuracy of self-keeping time, and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet; Step S2.3: When the lidar is unable to receive the BC broadcast time, it keeps time according to its own maintenance time and emits laser pulses, marking the timestamp i+j in each laser pulse echo signal remote sensing packet.

2. The high-precision timekeeping method for spaceborne lidar according to claim 1, characterized in that, Step S2.1 includes the following sub-steps: Step S2.1.1: The lidar is powered on, and j begins counting for 0.1ms; Step S2.1.2: Upon receiving the first second pulse, assign the integer second of the second pulse to i and clear j to zero; Step S2.1.3: Upon receiving a subsequent second pulse, determine the value of j: If j < 9000, then no time synchronization operation will be performed; If 9000≤j≤11000, then assign the whole second of the second pulse to i and clear j to zero; If j > 11000, then increment i by 1 and assign j - 10000 to j; Step S2.1.4: If j>11000 appears for 5 consecutive seconds, it is determined that the second pulse reception is abnormal, and the BC broadcast data is used to calibrate the time it maintains.

3. The high-precision timekeeping method for spaceborne lidar according to claim 1, characterized in that, Step S2.2 includes the following sub-steps: Step S2.2.1: The lidar is powered on, and j starts counting for 0.1ms without emitting light. Step S2.2.2: Upon receiving the first time broadcast interruption, assign the whole second to i and clear j to zero; Step S2.2.3: Upon receiving a subsequent time broadcast interruption, determine the value of j: If j < 8000, then no time synchronization operation will be performed; If 8000≤j≤12000, then assign the whole second of the second pulse to i and clear j to zero; If j > 12000, then increment i by 1 and assign j - 10000 to j.

4. The high-precision timekeeping method for spaceborne lidar according to claim 1, characterized in that, The lidar has no time reference before receiving the navigation second pulse for time synchronization or the BC broadcast time. It only performs self-timekeeping and does not perform light emission operation.

5. The high-precision timekeeping method for spaceborne lidar according to claim 1, characterized in that, After receiving the navigation second pulse for time synchronization or the BC broadcast time, the lidar performs a light emission operation when i+j reaches the corresponding time point according to the pulse frequency, and transmits the i+j time to the data processor to record the time corresponding to the laser pulse for use in data inversion.

6. The high-precision timekeeping method for spaceborne lidar according to claim 5, characterized in that, The laser radar emits light at a time that is recorded in real time. If the time difference between the current calculated emitter time and the previous emitter time is less than a threshold that affects the safety of the laser, then no light will be emitted at the current time.

7. The high-precision timekeeping method for spaceborne lidar according to claim 1, characterized in that, Even when the satellite timing system malfunctions, the lidar will still maintain preset timekeeping accuracy and continue operating, and the imaging time of the radar echo signal will be recorded in the remote sensing data; the malfunctions include the second pulse interval not reaching the preset value and the broadcast time not reaching the preset value.

8. A high-precision timekeeping system for spaceborne lidar, characterized in that, The high-precision timekeeping method for spaceborne lidar according to any one of claims 1-7 includes: Module M1: A register is set up inside the lidar; the register stores the count; Module M2: calibrates the lidar based on its receiving status, achieving self-timekeeping at the lidar's preset accuracy; the receiving status includes receiving second pulses provided by the satellite platform, receiving BC broadcast time but not receiving second pulses, and not receiving BC broadcast time.

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