Satellite laser footprint position forecasting method considering fixed field

Abstract

The invention discloses a satellite laser footprint position forecasting method considering a fixed field, and relates to the field of satellite laser altimeter in-orbit geometric calibration. The method comprises the steps of setting a fixed field center point coordinate; selecting a nearest sub-satellite point trajectory to forecast a satellite top crossing moment; calculating a side swing angle, a pitch angle or laser delay light emitting time of a satellite platform so as to enable the laser footprint to accurately fall on the fixed field detector array; before the satellite passes through the top, parameters are uploaded, enough execution time is reserved, and meanwhile a ground detector is started up in advance; when the satellite passes the top, laser is emitted to trigger the detector. According to the method, satellite parameters are actively regulated and controlled, the problem of low efficiency of a traditional passive waiting mode is solved, the footprint position forecasting precision is controlled within 20 meters, the calibration success rate and efficiency are remarkably improved, and the operation cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of on-orbit geometric calibration technology for satellite laser altimeters, and in particular relates to a method for predicting the position of satellite laser footprints that takes into account a fixed field. Background Technology

[0002] Spaceborne laser altimetry, as an active remote sensing technology, has unique advantages such as good directionality, narrow beamwidth, and high measurement accuracy. This technology calculates instantaneous altitude by emitting laser pulses towards the Earth's surface and receiving the echo signals reflected from the surface, accurately determining the round-trip time of the laser pulse between the satellite and the Earth's surface.

[0003] However, after experiencing the severe vibrations during launch, the thermal vacuum effects of the space environment, and long-term operation in orbit, the internal optomechanical structure and pointing parameters of a spaceborne laser altimeter inevitably undergo slight changes. This results in a systematic deviation between the actual landing point of the laser probe on the Earth's surface and the theoretical design value. Therefore, on-orbit geometric calibration becomes a crucial step in ensuring and improving the quality of laser altimeter data products.

[0004] Currently, mainstream on-orbit geometric calibration methods rely on ground-based laser detector arrays. When a satellite flies over the calibration field, if its emitted laser pulses can be captured by ground detectors, the precise geographic coordinates of those detectors are directly correlated with the actual location of the laser footprint. However, traditional methods have the following technical drawbacks:

[0005] The "passive waiting" mode is inefficient and costly: the calibration team needs to deploy detectors in advance and then passively wait for the satellite to fly over them, resulting in a low success rate and high cost for a single effective trigger.

[0006] Accumulated overall error in the forecast model: The accuracy of laser foothold position forecasts depends on the fusion of multi-source data, including ephemeris data errors, attitude measurement errors, time synchronization errors, etc. Accumulated errors lead to inaccurate forecasts.

[0007] Adaptability challenges under complex terrain conditions: Existing forecasting models in mountainous and hilly areas lack high-precision prior digital elevation models, resulting in significant deviations between predicted foothold locations and actual locations.

[0008] To overcome these challenges, we proposed the concept of a "fixed field," which involves selecting a few geographically and environmentally advantageous regions globally to construct permanent laser ground-based detector arrays. However, fixed hardware alone is insufficient; the problem of inaccurate forecasts also needs to be addressed. Therefore, a technological solution capable of shifting from "passive waiting" to "active control" is urgently needed. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for predicting the location of satellite laser footprints that takes into account a fixed field. By accurately calculating the satellite side swing angle or laser delay emission time parameters, the method can achieve accurate prediction and control of the landing point of satellite laser footprints.

[0010] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0011] This invention provides a method for predicting the location of satellite laser footprints considering a fixed field, comprising the following steps:

[0012] Step 1: Set the coordinates of the fixed field center point (B, L, H), where B is latitude, L is longitude, and H is elevation; Step 2: Predict the satellite overpass time t: Select the satellite nadir trajectory line closest to the fixed field center point. The satellite overpass time t is the time when this closest nadir trajectory line passes through the latitude of the fixed field center point; Step 3: Calculate the satellite platform side tilt angle σ and elevation angle required for the satellite laser to emit laser light to the fixed field center point. Alternatively, the laser emission time λ can be delayed so that the laser footprint falls onto the fixed field detector array; Step 4: Before the satellite passes overhead, the parameters calculated in Step 3 are uploaded to the satellite system, and sufficient time is reserved for the satellite platform to perform yaw, pitch maneuvers or laser emission delay commands, while the ground detectors are turned on in advance; Step 5: When the satellite passes overhead, the satellite laser emits a laser to the center point of the fixed field, triggering the detector array in the fixed field to capture the laser footprint.

[0013] As a preferred technical solution of the present invention, step 1 includes: Step 1.1, if the coordinates of the fixed field center point are unknown, the coordinates are measured using a GNSS receiver and set as the coordinates of the fixed field center point; Step 1.2, if the coordinates of the fixed field center point are known, they are directly set as the coordinates of the fixed field center point, and the coordinates need to be monitored and updated regularly by GNSS, with an update frequency of no less than once a month.

[0014] As a preferred embodiment of the present invention, step 2 includes:

[0015] Step 2.1, input the satellite orbit prediction file;

[0016] Step 2.2: Predict the satellite's nadir trajectory without tilting or pitching.

[0017] Step 2.3: Select the nadir trajectory line closest to the center point of the fixed field, and calculate the time when the trajectory line passes through the latitude of the center point of the fixed field, which is taken as the satellite overpass time t.

[0018] As a preferred embodiment of the present invention, step 3 includes:

[0019] Step 3.1: Input the precise satellite orbit prediction file, which is calculated by extrapolation based on precise orbit data closer to the time of satellite transit over a fixed field compared to the satellite orbit prediction file in Step 2.1, and has higher prediction accuracy;

[0020] Step 3.2: Calculate the required side yaw angle σ of the satellite platform based on the satellite laser positioning model;

[0021] Step 3.3: Calculate the required pitch angle for the satellite platform. Alternatively, the laser emission time λ can be used, with the laser emission time λ being the preferred adjustment parameter.

[0022] As a preferred embodiment of the present invention, the matrix form of the satellite laser positioning model is as follows:

[0023]

[0024] In the formula, This is the transformation matrix from the satellite's body coordinate system to its orbital coordinate system. This is the transformation matrix from the orbital coordinate system to the Earth-fixed coordinate system. This is a fixed offset between the laser emission reference point and the satellite's center of mass. The change in offset between the laser emission reference point and the satellite's center of mass. For the coordinates of the fixed field center point, For the satellite's center of mass in the Earth-fixed coordinate system The coordinates below, This is the distance between the satellite and the Earth. This is the laser pointing vector.

[0025] As a preferred embodiment of the present invention, the satellite laser positioning model is simplified as follows:

[0026]

[0027] In the formula, ;

[0028] Furthermore, the transformation matrix from the orbital coordinate system to the Earth-fixed coordinate system Side swing angle Pitch angle and yaw angle The relationship is:

[0029]

[0030] in:

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037]

[0038]

[0039] Side swing angle The calculation formula is as follows:

[0040]

[0041] As a preferred embodiment of the present invention, in step 3.3, the formula for calculating the laser delayed emission time λ is as follows: in, The distance along the track from the star's apex to the center point of the fixed field after the lateral swing. For satellite flight speed; and / or pitch angle The calculation formula is: in, This represents the distance between the satellite and the Earth.

[0042] The present invention has the following beneficial effects:

[0043] It has achieved a technological paradigm shift from "passive waiting" to "active control", and significantly improved the calibration success rate by reverse-engineering satellite control parameters;

[0044] By introducing precise orbit prediction and a refined geometric model, various systematic errors were effectively compensated, and the accuracy of footprint position prediction was controlled within 20 meters.

[0045] By adopting a fixed-field concept combined with an active control strategy, the cost of a single effective trigger has been significantly reduced, laying the foundation for the operational use of satellite laser altimetry technology.

[0046] By prioritizing the laser delayed emission strategy, the high-precision time control capability of the satellite system was fully utilized, further improving the accuracy of footprint position control.

[0047] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0048] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0050] 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.

[0051] This invention provides a satellite laser footprint position prediction method that takes into account a fixed field. Its core idea is to shift from "passive prediction" to "active control," that is, to deduce the required satellite state adjustment amount based on the target position (fixed field detector). The following describes the implementation of this invention in detail using the application of the Gaofen-7 satellite laser altimeter in the Sunite fixed field in Inner Mongolia as an example.

[0052] Example: Footprint position prediction by the laser altimeter on the Gaofen-7 satellite

[0053] like Figure 1 As shown, the construction of the laser altimeter prediction model and the optically rigorous geometric imaging model involve the same process, which involves rotation between various coordinate systems. Through the rotation of each coordinate system, the satellite pointing direction is finally transmitted to the ground point. The satellite laser footprint position prediction method that takes into account the fixed field proposed in this invention specifically includes the following steps:

[0054] Step 1: Set the coordinates of the fixed field center point (112.7, 42.7, 1000). Step 1 specifically includes the following sub-steps:

[0055] Step 1.1: If the coordinates (B, L, H) of the fixed field center point are unknown, use a GNSS receiver to accurately measure the coordinates (112.7, 42.7, 1000) of the center point and set them as the coordinates of the fixed field center point.

[0056] If the center point coordinates are unknown when a fixed field is first activated, a GNSS receiver must be used to accurately measure the center point coordinates. Taking the fixed field in Sunite, Inner Mongolia as an example, the measured center point coordinates are (112.7, 42.7, 1000).

[0057] Step 1.2: If the coordinates (B, L, H) of the fixed field center point are known, then directly set them as the coordinates of the fixed field center point.

[0058] If this is not the first time using a fixed field, the coordinates of the center point of the fixed field measured previously can be used as input. However, the coordinates of the center point of the fixed field need to be monitored and updated regularly, with an update frequency of no less than once a month.

[0059] Step 2, predict the satellite's overhead transit time t. Step 2 specifically includes the following sub-steps:

[0060] Step 2.1, input the satellite orbit prediction file;

[0061] Taking the laser calibration of the Gaofen-7 satellite in 2025 as an example, the calibration test on August 20 was selected as an example. The satellite orbit prediction file is named: gaofen7_predpv_20250819.g7p. Its main information includes the predicted position and velocity (X, Y, Z, VX, VY, VZ) of the satellite's center of mass from 0:00:00 on August 19 to 23:59:59 on the night of August 20.

[0062] Step 2.2: Predict the satellite's nadir trajectory without tilting or pitching.

[0063] Assuming the satellite does not tilt, calculate the satellite's nadir trajectory. The center point of the fixed field falls on or around this trajectory line. The nadir trajectory line is a continuous line, and there are generally 2-3 lines around the center point of the fixed field.

[0064] Step 2.3: Select the nadir trajectory line closest to the center point of the fixed field, and calculate the time when the nadir trajectory line passes through the latitude of the center point of the fixed field, i.e., the time t when the satellite passes over the top;

[0065] If the center point of the fixed field is on the trajectory line, it is assumed that the satellite does not sway and the requirements are met. If the center point of the fixed field is not on the trajectory line, select the trajectory line closest to the center point and check the satellite laser time near the latitude of the center point of the trajectory line, that is, the satellite overpass time t. The Gaofen-7 satellite time system adopts time code mode, and the satellite overpass time is 367153185.3 seconds.

[0066] Step 3: Accurately predict the satellite laser's emission to the fixed field center point. The satellite platform needs to adjust the tilt angle σ and elevation angle. Alternatively, the laser emission time λ may be delayed. Step 3 specifically includes the following sub-steps:

[0067] Step 3.1: Input the accurate satellite orbit prediction file;

[0068] The input satellite orbit prediction file with higher accuracy is gaofen7_predpv_20250820.g7p. This file is calculated by extrapolating precise orbit data closer to the transit time, and is more accurate in terms of prediction accuracy.

[0069] Step 3.2: Calculate the side swing angle σ required for the satellite laser to emit laser light to the fixed field center point of the satellite platform;

[0070] Based on the closest point between the nadir point trajectory and the center point of the fixed field, and with reference to the satellite laser positioning model, the side swing angle of the satellite platform σ = 0.06 degrees can be calculated.

[0071] The satellite laser positioning model is the core technology of this invention, and its complete form is as follows:

[0072] The model takes into account the relative position offset and rotational geometry of the satellite platform's center of mass, the laser's emission position, the Earth's ellipsoid.

[0073] By simplifying the process, we obtain: ;

[0074] Where the transformation matrix The elements consist of the satellite's attitude angles (yaw angle σ, pitch angle σ). The yaw angle (Φ) is determined by the yaw angle. By solving the simultaneous equations, the formula for calculating the yaw angle σ is finally obtained:

[0075]

[0076] Step 3.3: Satellite platform pitch maneuver and satellite laser emission delay can achieve the same purpose: adjusting the position of the satellite laser along the orbit. Generally, satellite laser emission delay is preferred between the two strategies, but the appropriate strategy should be selected based on the capabilities of the satellite system.

[0077] For adjustments along the orbital direction, the accuracy of laser delayed emission from the Gaofen-7 satellite is far superior to the accuracy of the satellite platform's pitch; therefore, the laser emission delay strategy is preferred. Based on the distance between the nadir point and the center point of the fixed field along the orbital direction after the side swing, the laser emission delay time is calculated to be 0.14 seconds.

[0078] The formula for calculating the laser delayed emission time λ is: ,in denoted as denoted as 'v', and 'v' as 'satellite speed'.

[0079] Step 4: Before the satellite passes overhead, upload the parameters to the satellite system, allowing sufficient time for the satellite platform to perform maneuvers or delay laser emission commands according to the instructions, and power on the ground detectors in advance.

[0080] The calculated command parameters are encoded and uploaded to the satellite system in advance through the ground system and telemetry and control system to ensure that the satellite has enough time to execute the command before passing overhead, the fixed field synchronous detector is activated, and all detectors are checked to be in working condition.

[0081] It is particularly important to emphasize that sufficient time must be reserved for the satellite platform to perform maneuvers. This time window needs to be determined based on the specific performance and maneuverability of the satellite platform, and usually needs to be several minutes to tens of minutes in advance.

[0082] Step 5: As the satellite passes overhead, its laser emits a laser beam to the center point of the fixed field, triggering the detector array within the fixed field to capture the satellite's laser footprint.

[0083] After the satellite passes overhead, the laser altimeter will emit a laser pulse to a fixed field, which will trigger the ground detector array. After some ground detectors sense the laser pulse, the detectors will display the energy value.

[0084] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," 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, 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.

[0085] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for predicting the location of satellite laser footprints considering a fixed field, characterized in that, Includes the following steps: Step 1: Set the coordinates of the fixed field center point (B, L, H), where B is latitude, L is longitude, and H is elevation. Step 2: Predict the satellite overpass time t: Select the satellite nadir trajectory line closest to the fixed field center point. The satellite overpass time t is the time when the trajectory line of the closest nadir point passes through the latitude of the fixed field center point. Step 3: Calculate the satellite platform side-swing angle σ, pitch angle б, or laser delay emission time λ required for the satellite laser to emit laser light to the fixed field center point, so that the laser footprint falls on the fixed field detector array. Step 4: Before the satellite overpass, upload the parameters calculated in Step 3 to the satellite system, and reserve sufficient time for the satellite platform to perform side-swing, pitch maneuvers, or laser delay emission commands, while simultaneously powering on the ground detectors in advance. Step 5: When the satellite overpasses, the satellite laser emits laser light to the fixed field center point, triggering the detector array within the fixed field to capture the laser footprint.

2. The satellite laser footprint position prediction method considering a fixed field according to claim 1, characterized in that, Step 1 includes: Step 1.1, if the coordinates of the fixed field center point are unknown, use a GNSS receiver to measure the coordinates and set them as the coordinates of the fixed field center point; Step 1.2, if the coordinates of the fixed field center point are known, directly set them as the coordinates of the fixed field center point, and regularly monitor and update the coordinates using GNSS, with an update frequency of no less than once a month.

3. The method for predicting the location of satellite laser footprints considering a fixed field according to claim 1, characterized in that, Step 2 includes: Step 2.1, input the satellite orbit prediction file; Step 2.2: Predict the satellite's nadir trajectory without tilting or pitching. Step 2.3: Select the nadir trajectory line closest to the center point of the fixed field, and calculate the time when the trajectory line passes through the latitude of the center point of the fixed field, which is taken as the satellite overpass time t.

4. The satellite laser footprint position prediction method considering a fixed field according to claim 3, characterized in that, Step 3 includes: Step 3.1: Input the precise satellite orbit prediction file, which is calculated by extrapolation based on precise orbit data closer to the time of satellite transit over a fixed field compared to the satellite orbit prediction file in Step 2.1, and has higher prediction accuracy; Step 3.2: Calculate the required side yaw angle σ of the satellite platform based on the satellite laser positioning model; Step 3.3: Calculate the required pitch angle for the satellite platform. Alternatively, the laser emission time λ can be used, with the laser emission time λ being the preferred adjustment parameter.

5. The satellite laser footprint position prediction method considering a fixed field according to claim 4, characterized in that, The matrix form of the satellite laser positioning model is as follows: In the formula, This is the transformation matrix from the satellite's body coordinate system to its orbital coordinate system. This is the transformation matrix from the orbital coordinate system to the Earth-fixed coordinate system. This is a fixed offset between the laser emission reference point and the satellite's center of mass. The change in offset between the laser emission reference point and the satellite's center of mass. For the coordinates of the fixed field center point, For the satellite's center of mass in the Earth-fixed coordinate system The coordinates below, This is the distance between the satellite and the Earth. This is the laser pointing vector.

6. The method for predicting the location of satellite laser footprints considering a fixed field according to claim 5, characterized in that, The satellite laser positioning model is simplified as follows: In the formula, ; Furthermore, the transformation matrix from the orbital coordinate system to the Earth-fixed coordinate system Side swing angle Pitch angle and yaw angle The relationship is: in: Side swing angle The calculation formula is as follows: 。 7. The method for predicting the location of satellite laser footprints considering a fixed field according to claim 4, characterized in that, In step 3.3, the formula for calculating the laser delayed emission time λ is as follows: in, The distance along the track from the star's apex to the center point of the fixed field after the lateral swing. For satellite flight speed; pitch angle The calculation formula is: in, This represents the distance between the satellite and the Earth.

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