Satellite on-board clock autonomous correction method using lunar occultation observation

Abstract

The application discloses a kind of satellite on-board clock autonomous correction method using lunar occultation observation, utilize observation record and handle the time sequence of stellar occultation phenomenon of moon, correct on-board clock deviation.First, on-board imaging measurement equipment such as star sensor is used to observe the occultation and recurrence phenomenon of moon to specific star, record the time of phenomenon occurrence, form measurement time sequence, then utilize satellite accurate orbit information to combine lunar ephemeris calculation theoretical occultation and recurrence phenomenon time sequence, compare two groups of time sequence, iteratively correct star clock reference, so that actual observation time sequence and theoretical time sequence accurately coincide, to achieve the purpose of star clock reference correction.This method does not need to increase additional new equipment for satellite, only needs to utilize existing common sensitive equipment to complete observation, low in cost, high in system reliability, less on-board resource occupation.

Description

Technical Field

[0001] This invention relates to a method for autonomous calibration of a satellite's onboard clock using lunar occultation observations, belonging to the field of spacecraft guidance, navigation, and control. Background Technology

[0002] The accuracy of the onboard time reference is crucial for satellite operation and mission execution. For example, stable three-axis Earth tracking requires real-time calculation of the satellite's position using onboard orbital parameters and an onboard clock, which in turn adjusts the satellite's attitude angles for Earth tracking. Solar tracking requires the onboard time reference to calculate the real-time solar azimuth and operational status, ensuring accurate solar panel alignment. Problems with the onboard time reference can lead to significant attitude deviations, affecting onboard power and Earth tracking channels, or even attitude control failure and the risk of overall satellite malfunction. For missions such as Earth imaging, a precise onboard time reference is essential for accurate camera pointing and the development of imaging activation sequences. A problem with the onboard clock reference could result in the camera's field of view not effectively covering the target area, leading to imaging mission failure, or significant deviations in the positioning accuracy of ground targets, impacting mapping results.

[0003] Currently, there are two main mature calibration methods: ground-based time synchronization and GNSS time synchronization. These methods calibrate the satellite clock by receiving time codes from the ground or GNSS satellite. Pulsar time synchronization technology, which has been widely studied in recent years, has limited practicality due to its immaturity and the need for large-aperture pulsar ray receivers. The first two time synchronization methods require external signal input to calibrate the satellite clock. If the external signal source cannot provide effective information, such as a failure in the uplink of the ground station or interference with the GNSS satellite system, the satellite will be unable to calibrate its own clock. For highly intelligent satellites to achieve fully autonomous operation, the satellite needs to be able to operate and carry out missions autonomously even without external information input. Autonomous onboard clock correction is one of the prerequisites for achieving autonomous operation. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a satellite on-board clock autonomous correction method using lunar occultation observation. The satellite uses the detection of lunar occultation phenomena to take the least squares of the measured occultation time series and the theoretical occultation time series to correct the on-board clock reference.

[0005] The technical solution of this invention is:

[0006] A method for autonomous calibration of a satellite's onboard clock using lunar occultation observations includes:

[0007] Using detectors on a satellite, observe several known stars, record the number of each star and the time when the star is occulted by the moon, decompose each theoretical occultation time into the sum of the detected occultation time and the clock reference deviation, and form an occultation observation time series.

[0008] Based on the satellite's precise orbital elements, lunar ephemeris, and stellar ephemeris, the theoretical occultation times of the recorded stars obscured by the moon are calculated. Each theoretical occultation time is decomposed into the sum of the theoretical occultation time, the estimated reference time deviation correction, and the detector detection deviation, and a theoretical occultation time series is formed.

[0009] The time difference between the two time series is calculated and the sum of squares is obtained to obtain the deviation index caused by the clock reference deviation; based on the deviation index, the on-board clock reference is corrected using the least squares method.

[0010] Preferably, based on the satellite's precise orbital elements, lunar ephemeris, and sidereal ephemeris, the theoretical occultation time of the recorded stars being obscured by the moon is calculated, including two methods:

[0011] The first scenario is: It is known that the star numbered i is at t... ri -Δt is theoretically visible at time t ri If time is theoretically obscured by the moon and therefore invisible, then t ri The moment is the theoretical occultation moment;

[0012] The second type is: It is known that the star numbered i is at t ri -Δt is theoretically invisible due to lunar obscuration, at t ri If time t is theoretically reproducible and visible, then... ri The time is the theoretical occultation time, where t ri Expressed in star hours and seconds relative to the onboard time base.

[0013] The preferred method for determining whether a star is theoretically visible is:

[0014] t is calculated using a high-precision orbital extrapolation model. ri The satellite position vector with time number i is calculated using the lunar ephemeris, and the direction vector from the satellite to the center of the moon is obtained by subtracting the two.

[0015] Use stellar ephemeris to calculate the direction vector of a star relative to a satellite;

[0016] Calculate the angle between the two vectors and determine if the angle is less than the angle of view of the lunar disk. If it is less, then theoretically the star is blocked by the moon and cannot be seen, and the current moment is the theoretical occultation moment; otherwise, the star is theoretically visible.

[0017] Preferably, each theoretical occultation time is decomposed into the sum of the deviations between the detected occultation time and the clock reference, forming an occultation observation time series, which is A:

[0018] A = {t1 t2 … t} n}

[0019] t i =T0+t si +ΔT

[0020] In the formula, i is a positive integer from 1 to n, and n is the number of stars that have been recorded; the time of the detected occultation is T0+t. si T0 is the satellite time reference epoch, t si It is the occultation time of the star numbered i, that is, the time interval from the epoch time to the occultation detection time, and ΔT is the unknown and uncorrected time base deviation.

[0021] Preferably, each theoretical occultation time is decomposed into the sum of the theoretical occultation time, the estimated reference time deviation correction, and the detector detection deviation, forming a theoretical occultation time series, which is denoted as B.

[0022] B = {t} r1 t r2 … t rn}

[0023] t ri =T0+t rsi +ΔT f +1 / 2t d

[0024] In the formula, the occultation theory states that the occultation occurs at time T0+t. rsi , t rsi ΔT is the time interval from the epoch to the theoretically occurring moment of the occultation. f t is the estimated correction amount for the reference time deviation. d The detection period of the detector is 1 / 2t. d This is to detect the deviation of the detector.

[0025] Preferably, the time difference between the two time series is calculated and the sum of squares is taken to obtain the deviation index caused by the clock reference deviation, including:

[0026] Find the time difference between two time series:

[0027] Δ = AB = {Δ1 Δ2 … Δ n}

[0028] ={t1-t r1 t2-t r2 … tn -t rn}

[0029] ={(ΔT-ΔT) f )+(t s1 -t rs1 -1 / 2t d )…(ΔT-ΔT f )+(t sn -t rsn -1 / 2t d )}

[0030] Sum of squares over all time differences yields the deviation index:

[0031]

[0032] Preferably, the on-board clock reference is corrected using the least squares method. By selecting different clock reference deviation correction amounts, the deviation index is minimized. The on-board time reference is then adjusted based on the clock reference deviation correction amount corresponding to the minimum deviation, thus completing the correction.

[0033] Preferably, the Newton-Raphson iteration method is used to select different clock reference deviation correction amounts to minimize the deviation index.

[0034] Preferably, there are two methods for determining the occultation time:

[0035] The first scenario is: It is known that the star numbered i is at t... i -Δt is visible at time t i If it is constantly obscured by the moon and cannot be seen, then t i The moment is the moment of occultation;

[0036] The second type is: It is known that the star numbered i is at t i -Δt is obscured by the moon and is invisible at time t i If time is reproduced and visible, then t i The moment is the moment of occultation.

[0037] Preferably, the occultation time is obtained by adding the relative star time of the occultation occurrence measured by the occultation time detector to the on-board clock reference.

[0038] The advantages of this invention compared to the prior art are:

[0039] (1) This invention does not rely on external artificial information at all. Whether it is GNSS satellite time synchronization or ground time synchronization, it relies on external artificial signal input. However, the satellite on-board clock autonomous correction method using lunar occultation observation provided by this invention corrects the on-board clock reference only by observing the astronomical phenomenon of lunar occultation. It does not rely on artificial information support at all, has extremely strong anti-interference ability, and is suitable for satellite autonomous operation and maintenance under complex conditions such as ground time station failure or damage, and interference with satellite navigation system. It has high reliability.

[0040] (2) This invention does not require the addition of special payloads. It can observe the occultation phenomenon by relying on existing star sensors or typical on-board imaging equipment such as cameras. The on-board time reference can be corrected simply by analyzing and calculating the observation results. The cost is small, and this function can also be achieved by software upgrades for satellites already in orbit.

[0041] (3) The present invention can achieve high clock correction accuracy through a large number of observations. The lunar occultation phenomenon is a common astronomical phenomenon that occurs dozens of times every day. By accumulating observations, the satellite can obtain a sufficient number of observation time series, which can effectively eliminate the measurement error of the detector in the correction calculation process and achieve high clock correction accuracy. Attached Figure Description

[0042] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0043] Figure 1 This is a flowchart of the satellite on-board clock autonomous calibration method according to an embodiment of the present invention;

[0044] Figure 2 This is a flowchart of the occultation observation and recording process according to an embodiment of the present invention;

[0045] Figure 3 This is a schematic diagram of the constituent elements of the occultation time series according to an embodiment of the present invention;

[0046] Figure 4 This is a flowchart illustrating the theoretical occultation time series calculation in an embodiment of the present invention. Detailed Implementation

[0047] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] The most direct method for on-orbit autonomous time calibration of small satellites is to rely on astronomical observation data. Stars are extremely far from Earth, and their positions on the celestial sphere can be considered fixed over a short period, thus serving as a spatial reference to provide spacecraft with sufficiently accurate directional information. Star sensors detect stars at different positions on the celestial sphere and perform calculations to provide accurate spatial attitude parameters for the spacecraft. The Moon, as Earth's natural satellite, has been orbiting Earth stably for billions of years, and its orbital parameters have been precisely determined by humans, allowing for accurate prediction of its precise location at any time within several years. Furthermore, the Moon's radius reaches 1738 km, which is relatively far from Earth's orbit. From nearby observations, the Moon's angular radius reaches approximately 0.25° or more. Such a large celestial body, during its orbit around the celestial sphere, will inevitably occult many stars. That is, at a certain point in time, a star will be obscured by the Moon or will escape its occultation. This phenomenon is highly correlated with the positions of the observer, the Moon, and the stars. If the observer's position is known, the Moon's orbital parameters are precisely known, and the star's position information is known and constant, then the time when the Moon occults a star is a value that can be accurately predicted. Conversely, if the observer successfully observes a lunar occultation under the above conditions, then their own clock can be calibrated.

[0049] This invention proposes a method for autonomous calibration of a satellite's onboard clock using lunar occultation observations, such as... Figure 1 As shown, the satellite utilizes lunar occultation detection to take the least squares method between the measured occultation time series and the theoretical occultation time series, thereby correcting the onboard clock reference. When external time calibration is unavailable, the satellite has the ability to correct its own onboard clock deviation through fully autonomous astronomical observations, providing a backup means for onboard clock calibration and improving the reliability of the satellite's autonomous operation.

[0050] The method specifically includes:

[0051] S1, Observation and recording of stellar occultation phenomena.

[0052] Observational recording of stellar occultation phenomena, such as Figure 2As shown, the sensors used by the satellite to detect occultations include: star sensors or imaging cameras, and other sensitive devices capable of identifying the Moon and stars. The satellite's onboard software has the ability to identify stars in the captured images. There are two ways to record occultations: 1) The possible occultation time can be roughly estimated using the satellite's onboard time. Considering clock deviation, the satellite's pointing should be adjusted in advance so that the sensor can image the lunar occultation location for a period of time. The imaging time should ensure that it covers the onboard clock deviation, and the occultation phenomenon should be recorded; 2) During periods when the satellite is not in orbit for imaging or other tasks, the satellite's sensor pointing should be adjusted and the dark side of the Moon should be continuously observed to identify any occultation phenomena occurring therein.

[0053] Occult Time T i There are two ways to confirm this: 1) a known star stari in t i -Δt is visible at time t i 1) The star is constantly obscured by the moon and cannot be seen; 2) A known star is at t i -Δt is obscured by the moon and is invisible at time t i The moment is reproduced and visible, t i Time and star number stari record, where t i Expressed in star hours and seconds relative to the onboard time base.

[0054] S2, Establish the occultation observation time series

[0055] The detected occultation data is stored in tabular form in the onboard computer, forming a time series, such as... Figure 3 As shown, the elements of this sequence include star number, occultation time, etc., where the occultation time is obtained by adding the on-board clock reference to the relative star time at which the occultation occurs as measured by the detector.

[0056] S3, Calculating theoretical occultation time series

[0057] The parameters used by the satellite include: precise orbital elements, lunar ephemeris, and sidereal ephemeris. The satellite uses a high-precision orbital extrapolation model, and the calculation steps are as follows: Figure 4 As shown:

[0058] 1) Calculate t using a high-precision orbital extrapolation model ri The satellite position vector at time; here t ri The range of values ​​for t should be determined according to step S1. i The range of values ​​is expanded, for example, by extending the time by several hours, to ensure that the extended time can cover possible clock errors.

[0059] 2) Calculate the lunar position vector using the lunar ephemeris, and subtract the two to obtain the direction vector r of the satellite from the lunar center. s-m ;

[0060] 3) Use stellar ephemeris to calculate the direction vector of the star relative to the satellite;

[0061] 4) Calculate the angle between the two vectors and determine if it is less than the lunar disk angle. The method for calculating the lunar disk angle is as follows:

[0062] α FOV =arctan(R m / r s-m )

[0063] Where R m r is the radius of the moon. s-m The modulus of the vector from the satellite to the lunar center.

[0064] 5) If the included angle is less than α FOV If the star is obscured by the moon, and the angle is greater than α, then the star will be blocked by the moon. FOV Then the star will be visible;

[0065] 6) Theoretical occultation time t ri There are two ways to confirm this, consistent with step S2. If a known star stari is in t ri -Δt is visible at time t ri It is invisible because it is constantly obscured by the moon; if a known star is at t ri -Δt is obscured by the moon and is invisible at time t ri The moment is reproduced and visible, t ri Time and star number stari record, where t ri Expressed in star hours and seconds relative to the onboard time base.

[0066] S4, Solve for the difference between the theoretical and measured time series and take the sum of squares.

[0067] First, add the occultation time from step S2 to the on-board clock reference to form the occultation observation time series A:

[0068] A = {t1 t2 … t} n} (1)

[0069] Among them, each observation t i Both can be written as:

[0070] t i =T0+ΔT+t si (2)

[0071] Here, T0 is the satellite time reference epoch, ΔT is the unknown and uncorrected time reference deviation, and t siThis refers to the stellar time at which the occultation is detected, i.e., the time interval from the epoch time to the occultation detection time. It's worth noting that because the onboard detection equipment has a certain periodicity, even if ΔT = 0, the detected occultation time will almost never coincide with the actual time. The maximum time deviation is equal to the sensor's detection period. However, through multiple detections, this time deviation can be averaged, making its mean approach the detection period t. d 1 / 2 of.

[0072] Then, according to step S3, the theoretical occultation time series B can be formed:

[0073] B = {t} r1 t r2 …t rn} (3)

[0074] Here, each point in time can be written as:

[0075] t ri =T0+t rsi +ΔT f +1 / 2t d (4)

[0076] Among them, t rsi ΔT is the time interval from the epoch to the theoretically occurring moment of the occultation. f This is the estimated correction amount for the reference time deviation.

[0077] For time bases with bias, there will be discrepancies between the actual detected time series and the theoretically accurate time series. These discrepancies are then summarized and calculated.

[0078]

[0079] Sum of the squares of all time differences yields the index of occultation time series deviation caused by time base bias, which can be used to correct the time base.

[0080]

[0081] S5. Correct the on-board clock reference using the least squares method. Repeat steps S2 and S4 to minimize the time difference obtained in S4 using the least squares method.

[0082] Using the least squares method, for ΔT f An estimation is performed to minimize the occultation time series bias, corresponding to ΔT. f This refers to the deviation of the current satellite's onboard time reference, which will be adjusted by -ΔT. f That is, the correction is completed for ΔT. fThe estimation algorithm is simple because there is only one optimization variable. It can be Newton's iteration method or simple traversal method, etc., and it consumes very little on-board computing resources.

[0083] The embodiments described above are merely preferred embodiments of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for autonomous calibration of a satellite's onboard clock using lunar occultation observations, characterized in that, include: Using detectors on a satellite, observe several known stars, record the number of each star and the time when the star is occulted by the moon, decompose each occultation time into the sum of the detected occultation time and the clock reference deviation, and form an occultation observation time series. Based on the satellite's precise orbital elements, lunar ephemeris, and stellar ephemeris, the theoretical occultation times of the recorded stars obscured by the moon are calculated. Each theoretical occultation time is decomposed into the sum of the theoretical occultation time, the estimated reference time deviation correction, and the detector detection deviation, and a theoretical occultation time series is formed. The time difference between the two time series is calculated and the sum of squares is obtained to obtain the deviation index caused by the clock reference deviation; based on the deviation index, the on-board clock reference is corrected using the least squares method.

2. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 1, characterized in that, Based on the satellite's precise orbital elements, lunar ephemeris, and sidereal ephemeris, the theoretical occultation times of recorded stars obscured by the Moon are calculated, including two methods: The first type is: The known number is... The star in In theory, it is possible to see that, at time Theoretically, if the time is obscured by the moon and therefore invisible, then... The moment is the theoretical occultation moment; The second type is: The known number is... The star in Theoretically, it is invisible because it is obscured by the moon. If the moment is theoretically reproducible and visible, then... The time is the theoretical occultation time, where Expressed in star hours and seconds relative to the onboard time base.

3. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 2, characterized in that, The method for determining whether a star is theoretically visible is as follows: Calculations using a high-precision orbital extrapolation model Time number is The satellite's position vector is obtained by calculating the moon's position vector using the lunar ephemeris, and the two are subtracted to obtain the direction vector from the satellite to the center of the moon. Use stellar ephemeris to calculate the direction vector of a star relative to a satellite; Calculate the angle between the two vectors and determine if the angle is less than the angle of view of the lunar disk. If it is less, then theoretically the star is blocked by the moon and cannot be seen, and the current moment is the theoretical occultation moment; otherwise, the star is theoretically visible.

4. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 1, characterized in that, Each occultation time is decomposed into the sum of the deviations between the detected occultation time and the clock reference, forming an occultation observation time series, which is A: In the formula, 1~ positive integer, The number of stars already recorded; the time of the detected occultation is... , The epoch time of the satellite time reference. It was detected that the number was The stellar time of a star's occultation is the time interval from the epoch time to the time of occultation detection. This represents an unknown and yet-to-be-corrected time base deviation.

5. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 4, characterized in that, Each theoretical occultation time is decomposed into the sum of the theoretical occultation time, the estimated reference time deviation correction, and the detector detection deviation, forming a theoretical occultation time series, which is denoted as B: In the formula, the time of occurrence according to the occultation theory is... , It is the time interval from the epoch time to the time when the occultation theory occurs. This is the estimated correction amount for the reference time deviation. This refers to the detector's detection period. This is to detect the deviation of the detector.

6. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 5, characterized in that, By calculating the time difference between two time series and taking the sum of squares, we obtain the deviation index caused by clock reference skew, including: Find the time difference between two time series: Sum the squares of all time differences to obtain the deviation index: 。 7. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 1, characterized in that, The onboard clock reference is corrected using the least squares method. By selecting different clock reference deviation correction amounts, the deviation index is minimized. The onboard time reference is then adjusted based on the clock reference deviation correction amount corresponding to the minimum deviation, thus completing the correction.

8. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 7, characterized in that, By employing the Newton-Raphson iteration method, different clock reference deviation correction amounts are selected to minimize the deviation index.

9. The method for autonomous calibration of a satellite's onboard clock using lunar occultation observations as described in claim 1, characterized in that, There are two methods for determining the occultation time: The first type is: The known number is... The star in It is always visible that, If it is constantly obscured by the moon and cannot be seen, then The moment is the moment of occultation; The second type is: The known number is... The star in Constantly obscured by the moon and therefore invisible, If the moment is reproduced and visible, then The moment is the moment of occultation.

10. A method for autonomous calibration of a satellite's onboard clock using lunar occultation observations, as described in claim 1 or 9, characterized in that... The occultation time is obtained by adding the relative star time of the occultation, measured by the occultation time detector, to the onboard clock reference.

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