A multi-target-based comprehensive calibration and verification method for space-borne scatterometer

Abstract

The application discloses a kind of satellite-borne scatterometer comprehensive calibration and verification method based on multi-objective.The method is after satellite-borne scatterometer on-orbit operation, high-precision calibration is realized using on-board inner calibration and star-ground integrated outer calibration means, supplemented with ocean calibration, rain forest calibration, cross calibration and other calibration verification means, to ensure on-orbit observation accuracy.In addition, the method connects on-board real-time link inner calibration, star-ground integrated outer calibration, ocean calibration, rain forest calibration, cross calibration and other multi-target calibration and verification technologies from the angle of business use and information feedback, forms a set of practical complete satellite-borne scatterometer calibration and verification method, greatly improves the completeness of whole process of business system.

Description

A Multi-Target-Based Integrated Calibration and Verification Method for Spaceborne Scattermeters Technical Field

[0001] This invention relates to a multi-target-based method for the integrated calibration and verification of a spaceborne scatterometer, belonging to the field of satellite remote sensing technology. Background Technology

[0002] The Fengyun-3 (FY-3) meteorological satellite is my country's new generation of polar-orbiting meteorological satellites. Launched in July 2021, the FY-3E satellite is the fifth in the series and the world's first operational civilian meteorological satellite operating in a dawn-dusk orbit. The onboard scatterometer (i.e., wind field measurement radar) carried by this satellite is the first active microwave remote sensing instrument on a Chinese meteorological satellite and the world's first dual-frequency, dual-polarization fan-beam conical scanning radar. Its function is to acquire high-precision wind field information (including wind speed and direction) of the global ocean surface through backscattering measurements of the Earth system, providing crucial data for assimilation research and numerical weather prediction.

[0003] With the rapid development of satellite quantitative remote sensing technology and the quantitative application of remote sensing data, the calibration of remote sensing instruments has become increasingly urgent. The accuracy of calibration directly affects the scientific application value of remote sensing instrument observation data; therefore, it is necessary to minimize measurement errors caused by instrument deviations. For spaceborne scatterometers, the main task of calibration is to determine the radar backscattering coefficient of the target based on the measured target echo power.

[0004] Chinese invention patent ZL 201310638139.4 discloses an external calibration method for a spaceborne microwave scatterometer. Addressing the issue of requiring a large scattering cross-section for calibration targets, this calibration method employs the following steps: calculating the satellite's overhead time and the antenna pointing of the active calibrator based on the satellite's orbital parameters, and adjusting the antenna pointing of the active calibrator to a designated position; performing self-calibration on the active calibrator before the satellite's arrival to obtain the gain of the transponder and receiver channels; calculating the backscattering coefficient using the gain of the transponder channel; calculating the backscattering coefficient of the active calibrator using the spaceborne microwave scatterometer, and obtaining the final correction value from the backscattering coefficient. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a comprehensive calibration and verification method for a spaceborne scatterometer based on multiple targets.

[0006] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution:

[0007] A multi-target-based method for the integrated calibration and verification of a spaceborne scatterometer includes the following steps:

[0008] S1: Acquire L0 data and determine whether L0 data has changed; if the result is that L0 data has changed, initiate a near real-time alarm for instrument status change and proceed with subsequent steps; otherwise, proceed with subsequent steps.

[0009] S2: Real-time internal calibration of L0 data;

[0010] S3: Perform other preprocessing on the real-time internal calibration data to obtain L1 data; in the external calibration process, calculate the backscattering coefficient on the real-time internal calibration data;

[0011] S4: Design an external calibration observation plan, carry out preliminary preparations for external calibration observation, and initiate external calibration observation; proceed to step S5.

[0012] Among them, the preliminary work for external calibration observation includes the design of the construction process for the ground-based active calibrator and calibration field, and the design of the space-ground integrated external calibration process;

[0013] S5: Acquire observation data from the active calibrator and perform real-time and / or non-real-time processing;

[0014] S6: Combine the data after calculating the backscattering coefficient in step S3 with the data after real-time and / or non-real-time processing in step S5;

[0015] S7: Perform external calibration data processing on the combined data to obtain the three-dimensional antenna pattern and absolute calibration coefficients;

[0016] S8: Based on the three-dimensional antenna pattern and absolute calibration coefficients, determine whether the data exceeds the calibration accuracy requirements;

[0017] If the calibration accuracy requirement is exceeded, the backscattering coefficient of the data is calculated and resampled to obtain L1 data, and then proceed to step S9; if the calibration accuracy requirement is not exceeded, proceed to step S13.

[0018] S9: Process L1 data using one or more of the following methods: ocean calibration, rainforest calibration, and cross-calibration;

[0019] S10: Perform a comprehensive evaluation of the calibration accuracy of data obtained by any one or more of the following methods: ocean calibration, rainforest calibration, and cross-calibration, and determine whether the calibration accuracy exceeds the calibration accuracy requirements; if so, initiate an observation quality alarm, repeat steps S4 to S8, and perform routine monitoring; if not, proceed to the next steps.

[0020] S11: Correct the data obtained from ocean calibration to obtain new data;

[0021] S12: Perform ocean calibration again on the new data obtained in step S11, and evaluate the stability of ocean calibration. After the evaluation is completed, proceed to step S13.

[0022] S13: Complete calibration verification and perform routine monitoring of the observation data from the spaceborne scatterometer.

[0023] Compared with existing technologies, this invention achieves high-precision calibration after the spaceborne scatterometer is in orbit by combining on-board internal calibration with integrated space-to-ground external calibration. It is further supplemented by various calibration verification methods such as ocean calibration, rainforest calibration, and cross-calibration to ensure the accuracy of on-orbit observations. Furthermore, from the perspective of operational use and information feedback, this invention links together multi-target calibration and verification technologies, including on-board real-time link internal calibration, integrated space-to-ground external calibration, ocean calibration, rainforest calibration, and cross-calibration, forming a practical and complete spaceborne scatterometer calibration and verification method, significantly improving the completeness of the entire operational system process. Attached Figure Description

[0024] Figure 1 is a flowchart of the multi-target spaceborne scatterometer integrated calibration and verification method provided in an embodiment of the present invention;

[0025] Figure 2 is a schematic diagram of the principle of real-time internal calibration in an embodiment of the present invention;

[0026] Figure 3 is a flowchart of the construction of the active calibrator and calibration field in an embodiment of the present invention;

[0027] Figure 4 is a schematic diagram of the switching between the conventional observation mode and the external calibration mode of the spaceborne scatterometer in an embodiment of the present invention;

[0028] Figure 5 is a flowchart of the external calibration and data processing in an embodiment of the present invention. Detailed Implementation

[0029] The technical content of the present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] As shown in Figure 1, this embodiment of the invention discloses a multi-target-based spaceborne scatterometer integrated calibration and verification method, including the following steps, including external calibration:

[0031] S1: Acquire L0 data and determine whether L0 data has changed; if the result is that L0 data has changed, initiate a near real-time alarm for instrument status change and proceed with subsequent steps; otherwise, proceed with subsequent steps.

[0032] The steps of the near real-time alarm scheme for instrument status changes are as follows:

[0033] (1) Read near real-time telemetry data;

[0034] (2) Perform quality control on telemetry data, remove data with checksum anomalies, time anomalies, angle anomalies, speed anomalies, etc., and retain normal data, data with internal calibration quality inspection anomalies, noise quality inspection anomalies, and automatic gain control (AGC) quality inspection anomalies;

[0035] (3) Determine whether the internal calibration value has abruptly changed;

[0036] The method for determining whether the internal calibration value has undergone a sudden change is as follows:

[0037] 1) Each time the radar performs internal calibration measurement, there are several internal calibration points. The first m internal calibration points may have different values ​​from the remaining n internal calibration points because they have not yet entered a steady state. In order to accurately determine the anomaly, the first m internal calibration points and the remaining n internal calibration points are divided into group 1 and group 2, and group 1 and group 2 are processed separately, where m and n are both positive integers.

[0038] 2) For each calibration point within a group, both the signal channel and the noise channel are evaluated;

[0039] 3) For the first internal scaling value of each data block in Group 1 and Group 2, use the last normal internal scaling value of the previous data block in the group as R to make a judgment.

[0040] The specific methods for determining the signal channel and noise channel for each group of calibration points are as follows:

[0041] ① Group 1 signal channel processing method:

[0042] The current internal calibration value (A) is compared with the previous normal internal calibration value (R). If |A - R| > the normal internal calibration threshold set according to the instrument characteristics, the current internal calibration value may be abnormal, and an alarm is issued. The judgment of subsequent internal calibration values ​​continues with the previous normal value as R.

[0043] ② Noise channel processing method for Group 1: Same as ①.

[0044] ③ Group 2 signal channel processing method:

[0045] The current internal calibration value (B) is compared with the previous normal internal calibration value (R). If |B - R| > the normal internal calibration threshold set according to the instrument characteristics, the current internal calibration value may be abnormal, and an alarm is issued. The judgment of subsequent internal calibration values ​​continues with the previous normal value as R.

[0046] ④ Noise channel processing method for group 2: Same as ③.

[0047] (4) Determine whether the noise measurement value has a sudden change;

[0048] The method for determining whether a sudden change has occurred in the noise measurement value is as follows:

[0049] 1) Both the signal channel and the noise channel need to be assessed;

[0050] 2) For the first noise measurement value of each data block, use the last normal noise measurement value of the previous data block as R to make a judgment.

[0051] The specific methods for determining the signal channel and the noise channel are as follows:

[0052] ① For the signal channel: average all noise measurements each time; compare the current measurement (C) with the previous normal measurement (R). If |C - R| > the normal noise threshold set according to the instrument characteristics, the current noise measurement is considered to be abnormal, and an alarm is issued. The judgment of subsequent noise measurements continues with the previous normal value as R.

[0053] ② For the noise channel: Same as ①.

[0054] (5) Determine if the gain value changes abruptly: If the gain value AGC at the observation point is greater than the normal gain threshold set according to the instrument characteristics, an alarm will be issued. This step is only performed during the routine ground measurement phase and is not required for other phases.

[0055] S2: Perform real-time internal calibration on L0 data.

[0056] The real-time internal calibration is achieved using a ratio method, and its specific implementation process is shown in Figure 2. In normal measurement mode, the power output from the satellite scatterometer transmitter to the antenna transceiver switch is Pt, and the power of the echo signal received by the satellite scatterometer receiver is Pr. In the internal calibration loop, the power output from the transmitter to the antenna transceiver switch is Pt, but the signal does not reach the antenna transceiver switch; instead, it is attenuated by Lc through the internal calibration loop coupler before reaching the receiver. The power of the calibration signal received by the receiver is Prc, which can be expressed by the formula:

[0057]

[0058] Using the internal calibration loop, the expression for calculating the transmit / receive power ratio parameter in the radar equations can be obtained as follows:

[0059]

[0060] The aforementioned Lc can be precisely calibrated before instrument launch and remains essentially unchanged over time after satellite launch. By introducing the transmitted signal into the internal calibration loop, it is unnecessary to know the exact values ​​of the transmitter output power during measurement and calibration; internal calibration can be achieved by directly measuring the ratio of the received power in the two measurements. Ignoring the antenna system of the onboard scatterometer, the signal path during internal calibration only differs from that during measurement by having an additional internal calibration loop coupler; the other measurement paths are the same for both. Therefore, the internal calibration loop can eliminate drift generated within the onboard measurement system of the onboard scatterometer.

[0061] Next, the internal calibration accuracy needs to be evaluated periodically to monitor the observation stability of the spaceborne scatterometer. The specific steps are as follows:

[0062] (1) Read remote sensing data from the spaceborne scatterometer;

[0063] (2) Extract the value of each internal calibration signal in each scan cycle;

[0064] (3) For each set of internal calibration observations, remove the large values ​​at the beginning that have not yet entered a steady state;

[0065] (4) Take the average of the internal calibration values ​​within N seconds before and after each internal calibration value (N is a positive integer; the specific selection of N should be consistent with the way internal calibration data is used in business and is related to the degree of signal fluctuation), and calculate 3 times the standard deviation as the error caused by the leakage signal interfering with the internal calibration signal during internal calibration. This refers to the data error in the coupling loop during radar internal calibration.

[0066] (5) The main factors affecting the internal calibration accuracy K include: insertion loss measurement error at the transmitting front end, insertion loss measurement error at the receiving front end, coupling measurement error of the calibration loop, antenna standing wave measurement error, receiver automatic gain control value error during internal calibration, receiver automatic gain control value error during echo measurement, scanning insertion loss fluctuation, and internal calibration signal accuracy. Therefore, the formula for calculating the internal calibration accuracy is:

[0067]

[0068] in, For the insertion loss measurement error at the launch front end, To receive the insertion loss measurement error at the front end, To calibrate the measurement error of the loop coupling, For antenna standing wave measurement error, This refers to the receiver's automatic gain control value error during internal calibration. This refers to the error in the receiver's automatic gain control value during echo measurement. This represents the insertion loss fluctuation during a 360-degree scan of the rotating joint.

[0069] (6) The average internal calibration accuracy calculation result for one day is taken as the average value to represent the internal calibration accuracy evaluation result for that day.

[0070] (7) Analyze the long-term stability of the internal calibration accuracy based on the daily evaluation results.

[0071] The ratio-based internal calibration method is chosen in this invention because it has fewer chances of generating errors and allows for frequent internal calibration, making it more advantageous.

[0072] S3: Perform other preprocessing on the real-time internal calibration data to obtain L1 data; in the external calibration process, calculate the backscattering coefficient on the real-time internal calibration data.

[0073] Other preprocessing steps include backscattering coefficient calculation and resampling.

[0074] S4: Design an external calibration observation plan, carry out preliminary preparations for external calibration observation, and initiate external calibration observation, then proceed to step S5.

[0075] The preliminary work for external calibration observation includes the design of the construction process for active calibrators and calibration fields, and the design of the integrated space-ground external calibration process.

[0076] As shown in Figure 3, the design process for the construction of the active calibrator and calibration field includes the selection of the external calibration field, the functional design of the active calibrator, the parameter design of the active calibrator, the design of the integrated satellite-ground calibration mode, the construction of the external calibration field, the design of the status monitoring scheme, the design of the data push strategy, the research and development of the external calibration system, and the development of the calibration model.

[0077] Among them, the selection of the external calibration field, the design of the active calibrator function, the design of the status monitoring scheme, and the design of the data push strategy are the more important contents in the early preparation work of external calibration observation. The rest will not be elaborated here.

[0078] In one embodiment of the present invention, the principle for selecting the location of the external calibration field is as follows:

[0079] S411. For the novel spaceborne scatterometer with a fan-beam conical scanning system, the beam footprint is large, the antenna scanning speed is slow, and the pulse repetition frequency is low. To meet the measurement requirements for antenna pattern angular coverage and angular interval, it takes approximately one month to complete an external calibration using a single ground-based active calibrator. If two external calibration fields with a distance between them that is much larger than the observation amplitude of the spaceborne scatterometer are used, the external calibration time can be halved. Therefore, it is recommended to construct two external calibration fields with a large distance between them. Furthermore, if three external calibration fields are constructed, the on-orbit positioning deviation of the spaceborne scatterometer can be calculated, improving the positioning accuracy of the spaceborne scatterometer.

[0080] The primary objective of the S412 spaceborne scatterometer is global sea surface wind field observation, requiring full coverage of all ocean areas. Therefore, the active calibrator should be installed in inland areas far from the ocean.

[0081] S413. Because the spaceborne scatterometer receives both the relay signal from the ground-based active calibrator and the background echo from the location of the active calibrator during external calibration, especially given the large beam footprint and high antenna sidelobes of the fan-beam spaceborne scatterometer, strict limitations must be placed on the radar cross-section of the ground surface to suppress the influence of ground echoes and meet the requirements for external calibration accuracy. This means that the external calibration site for the spaceborne scatterometer needs to be far from urban areas and selected in a location with uniform and weak backscattering.

[0082] S414. The terrain changes of the external calibration field should be relatively gradual to avoid a sudden increase in ground backscattering.

[0083] S415. There should be no obstructions from terrain, buildings, vegetation, etc., to ensure normal transmission and reception of active calibrator signals.

[0084] S416. There should be no significant electromagnetic interference within the operating frequency range of the active calibrator.

[0085] S417. In terms of climate conditions, arid and low-rainfall locations are the best choice, as they can reduce the impact of atmospheric attenuation on electromagnetic waves and improve external calibration accuracy.

[0086] S418. Feasibility of power supply, network, infrastructure and long-term maintenance should also be comprehensively considered.

[0087] The specific details of the active calibrator functional design are as follows:

[0088] S421. Transmission calibration function: Calibrates the transmission characteristics of the spaceborne scatterometer, mainly including the transmission pattern and beam pointing.

[0089] S422. Receiver calibration function: Calibrates the receiving characteristics of the spaceborne scatterometer, mainly including the receiving pattern and beam pointing.

[0090] S423, Delayed forwarding function: calibrates the transceiver loop characteristics of the spaceborne scatterometer, mainly referring to the absolute calibration coefficient;

[0091] S424 has the capability to analyze radar transmitted signals, such as signal frequency, signal bandwidth, pulse width, and signal time-frequency characteristics;

[0092] S425 has real-time signal processing and display functions;

[0093] The S426 is equipped with remote control and unattended operation capabilities, and can remotely control the active radar active calibrator at the ground data processing center.

[0094] S427. Monitoring function: It can automatically monitor the key status parameters of the active calibrator and automatically upload them remotely;

[0095] S428, Self-calibration function: It can effectively calibrate the measurement accuracy of the active calibrator itself, including internal calibration, external calibration and field calibration;

[0096] The S429 has the ability to network multiple units and extend communication capabilities.

[0097] The design requirements for the condition monitoring scheme are as follows:

[0098] S431. The active calibrator is designed to report status information including: calibration station code, status information collection time, server connection status, active calibrator connection status, UPS connection status, UPS power supply status, task reception status, and task execution result; status information of other auxiliary devices can be added as needed.

[0099] S432. Status Reporting: The active calibrator collects the current information of the calibration station at configurable fixed time intervals and can flexibly report the system operation status of the calibration station using methods such as FTP; according to user habits, it periodically uploads the system operation status to the user-specified file transfer protocol (FTP) path at configurable fixed time intervals.

[0100] The specific details of the data push strategy design are as follows:

[0101] S441, Calibration Raw Data Acquisition: Transmit the raw data of the active calibrator to a fixed storage location on the data processing server;

[0102] S442, Raw Data Packaging: The raw data is packaged according to the packaging settings, and the packaged data is stored in the specified location;

[0103] S443. Data auxiliary information extraction: Extract auxiliary information from the data packet header and store the auxiliary information;

[0104] S444, Valid Data Extraction Parameter Settings: Set the threshold value for filtering subpackage data;

[0105] S445. Valid Data Filtering: Filter the subpackage data according to the valid data filtering settings, and store the filtering results in the specified path;

[0106] S446. Effective data storage: Store the effective data and auxiliary data of the same original file package in the same folder and manage them for backend access;

[0107] S447. Data Upload: Package the valid data and auxiliary data of the same original file package and transmit them to the file transfer protocol server directory specified by the user for the user to view and further process.

[0108] S448. To prevent problems during the data upload process and improve the reliability of data push, after the first data push, the file information in the file transfer protocol server directory is automatically read after a specified time interval to determine whether the data push was successful; if it is unsuccessful, it is automatically pushed again.

[0109] There are many ways to push calibration station observation data to a user's specified path. The above content only uses the file transfer protocol as an example. Other methods will not be described in detail here.

[0110] The working mode design in the integrated space-ground external calibration process generally includes three modes: receiving mode, transmitting mode, and relaying mode. Since the antenna of the spaceborne scatterometer is often a passive antenna, the transmitting pattern is the same as the receiving pattern. Therefore, it can be simplified to receiving mode and relaying mode. The specific process design is as follows:

[0111] S451. Initiate external calibration plans periodically or irregularly based on the calibration requirements of the spaceborne scatterometer.

[0112] S452. The active calibrator generates a detailed calibration task based on the external calibration plan document, satellite orbit prediction document and the geographical location information of the active calibrator, and calculates the active calibrator power-on time and pointing angle information.

[0113] S453, the active calibrator is turned on and preheated in advance, and the auxiliary equipment is also turned on automatically;

[0114] S454. After the active calibrator completes its self-test, it will periodically point to the predetermined direction to begin receiving signals from the onboard scatterometer, during which auxiliary equipment will continue to observe.

[0115] S455. Depending on the requirements of the calibration task, the working process of satellite and ground instruments differs depending on whether the receiving mode or the relay mode is used.

[0116] S456. After the observation mission is completed, the active calibrator and auxiliary equipment shall stop observation on time.

[0117] S457. Automatically store and push data.

[0118] In the receiving mode, the satellite-to-ground instrument's workflow is relatively simple. The onboard scatterometer works normally, and the active calibrator is set according to the satellite orbit. The antenna direction of the active calibrator is adjusted so that the active calibrator can receive signals from the onboard scatterometer.

[0119] As shown in Figure 4, the satellite-to-ground instrument workflow is relatively complex in relay mode. It requires remote control commands from the ground to the satellite to switch the satellite-to-ground instrument mode of the onboard scatterometer, specifically from conventional observation mode to external calibration mode, and vice versa. The satellite-to-ground system workflow in relay mode is as follows: Based on the calibration requirements of the onboard scatterometer, when external calibration is needed, a remote control command is sent from the ground to the satellite to switch the scatterometer to external calibration mode. Simultaneously, the active calibrator is configured according to the satellite orbit, adjusting its antenna direction to receive signals from the scatterometer. The active calibrator amplifies and forwards the received signals back to the scatterometer, which then processes them. After external calibration is completed, a remote control command is sent again to switch the scatterometer back to conventional observation mode.

[0120] S5: Acquire observation data from the active calibrator and perform real-time and / or non-real-time processing.

[0121] S6: Combine the real-time internal calibration data after backscattering coefficient calculation in step S3 with the data after real-time and / or non-real-time processing in step S5.

[0122] S7: Perform external calibration data processing on the combined data to obtain the three-dimensional antenna pattern and absolute calibration coefficients.

[0123] The entire process and data processing of external calibration are shown in Figure 5, specifically including:

[0124] S71: Reads the active calibrator data file to obtain the measured signal waveform, i.e., the change of the voltage quantization value over time.

[0125] S72: Demodulate the data read in step S71 using IQ to obtain a complex signal.

[0126] The IQ demodulation calculation formula is as follows:

[0127]

[0128] Where SigRF is the voltage value obtained in step S71, pi is pi, fc is the intermediate frequency for acquisition, and t is the time for each sampling point.

[0129] S73: The active scaler uses intermediate frequency sampling to down-convert the demodulated signal to obtain the baseband waveform, and constructs a filter window based on the number of sampling points and the signal bandwidth.

[0130] S74: After conversion to baseband, downsampling processing is performed.

[0131] S75: Perform matched filtering on the complex signal to obtain the waveform after matched filtering.

[0132] In this process, pulse compression performs matched filtering on each received LFM (linear frequency modulation) pulse signal. The matched filter is H(t) = Conj(Sig(-t)), and the matched function is constructed as follows:

[0133]

[0134] Where Sig(t) is the LFM signal and Kr is the frequency modulation slope.

[0135] The pulse compression method is as follows:

[0136]

[0137] S76: Calculate the signal amplitude of the complex signal after pulse compression.

[0138] S77: Based on the set threshold value, extract the effective pulse signal, thus completing the signal processing for a single pass of the spaceborne scatterometer and obtaining the antenna gain Gt for this operation.

[0139] S78: Based on active calibrator observation data at different times, azimuth angles, and elevation angles, the antenna gain Gt of the spaceborne scatterometer is:

[0140]

[0141] in, It is the azimuth angle. The pitch angle.

[0142] Based on all valid data collected by the active calibrator in a complete external calibration mission, a three-dimensional antenna pattern is obtained by fitting.

[0143] S79: After accumulating sufficient observation data, calculate the absolute calibration coefficient of the spaceborne scatterometer by combining the backscattering coefficient of the active calibrator calculated by the spaceborne scatterometer.

[0144] In the external calibration data processing, there are many details to consider in order to reduce the amount of data and improve the calibration accuracy of the spaceborne scatterometer, including:

[0145] 1. After converting the active scaler signal to baseband, downsampling is performed. This step can greatly reduce the amount of data.

[0146] 2. During the calculation process, the actual antenna pattern of the active calibrator is used to accurately obtain the antenna gain at different azimuth and elevation angles, rather than using a fixed antenna gain value;

[0147] 3. During the on-site installation of the active calibrator, it is necessary to accurately measure the active calibrator mounting matrix. The measured mounting matrix is ​​used to correct the pointing angle of the active calibrator.

[0148] 4. Calculating the satellite-to-ground geometry using the satellite's actual attitude and orbit can achieve higher positioning accuracy, thereby reducing calibration deviations caused by satellite-to-ground distance and observation geometry during external calibration.

[0149] It should be noted that external calibration can be implemented using passive and active methods. In this embodiment of the invention, the active method is preferred. Passive methods include the passive point target method and the ground-based distributed target method. The passive point target method uses a standard reference reflector for calibration, requiring the reflector to be over 10 meters in size, which is difficult to manufacture with high precision. The ground-based distributed target method utilizes large-area, stable scattering targets on the ground for calibration. Tropical rainforests are relatively ideal calibration targets. Domestic and international research shows that the calibration accuracy using a spaceborne microwave scatterometer in the Amazon rainforest can reach approximately 1 dB. However, the backscattering in tropical rainforests varies with the seasons. Therefore, neither of these passive methods can meet the high-precision external calibration requirements of a spaceborne scatterometer, and thus will not be elaborated upon here.

[0150] S8: Based on the three-dimensional antenna pattern and absolute calibration coefficients, determine whether the data exceeds the calibration accuracy requirements.

[0151] If the calibration accuracy requirement is exceeded, the backscattering coefficient of the data is calculated and resampled to obtain L1 data, and then proceed to step S9; if the calibration accuracy requirement is not exceeded, proceed to step S13.

[0152] S9: Process L1 data using one or more of the following methods: ocean calibration, rainforest calibration, and cross-calibration;

[0153] Ocean calibration, in particular, determines the calibration bias of a spaceborne scatterometer by comparing backscattering data obtained from measurements using a spaceborne scatterometer with simulated backscattering data based on numerical weather prediction and geophysical model functions. In the multi-target integrated calibration and verification method for spaceborne scatterometers, which includes external calibration, after absolute calibration using an active calibrator, ocean calibration is used as a means of calibration verification. Its role is to monitor the residual bias and long-term stability of the data after spaceborne scatterometer calibration processing.

[0154] Rainforest calibration is an effective means of verifying the stability of calibration data from spaceborne scatterometers.

[0155] Selecting the calibration region for the rainforest is a crucial foundational task in rainforest calibration. Long-term, in-orbit measurement data from spaceborne scatterometers operating in the same frequency band and polarization can be used to optimize the calibration region for the scatterometer to be calibrated. This involves considering isotropic characteristics, spatial range, spatial variation, seasonal variation, and diurnal variation, using the stability of the backscattering coefficient of the target region as a screening threshold. High-resolution data is optimal for this process.

[0156] Rainforest calibration modeling requires establishing a model of the relationship between the backscattering coefficient and the incident angle based on long-term on-orbit scattering meter measurement data, distinguishing between different frequency bands and polarizations. If analyzing azimuth dependence or the effects introduced by antenna rotation joints, an azimuth model can be added. If the satellite's orbit is non-polar, a diurnal variation model of the rainforest also needs to be constructed to meet the calibration verification requirements at different observation times.

[0157] Rainforest calibration involves analyzing the histogram distribution of backscattering coefficients measured by a spaceborne scatterometer in the calibration area, calculating the calibration bias, and using long-term series analysis to evaluate the stability of the calibration data. It is important to note that differences in sunlight conditions and dew on vegetation surfaces between morning and afternoon observations result in different backscattering coefficients for ascending and descending orbits, leading to larger standard deviations in the measurement results. This is detrimental to the stability verification of instrument measurement performance; therefore, data analysis should be performed separately for ascending and descending orbits. Furthermore, analyzing the common trends in backscattering across multiple different rainforest regions can eliminate or reduce regional influences.

[0158] Cross-calibration uses similar instruments on domestic and international satellites operating in orbit during the same period as comparison targets to evaluate the data quality of spaceborne scatterometers. The key to cross-calibration is generating matching datasets for the two spaceborne scatterometers, and then using the data from these matching datasets for cross-comparison. Detailed processing steps are as follows:

[0159] 1. For the two spaceborne scatterometers, distinguish between wavebands and polarizations, and perform matching separately;

[0160] 2. Select observation points on the ocean and remove pixels affected by sea ice or precipitation;

[0161] 3. Based on the quality label of the backscattering coefficient, discard data with poor quality;

[0162] 4. Perform atmospheric correction on the spaceborne scatterometer observations using an atmospheric correction lookup table or by using synchronous observation data from a microwave radiometer;

[0163] 5. Set a suitable background area, calculate the uniformity of the background area, and remove pixels with large fluctuations;

[0164] 6. Perform time matching; the time difference threshold can be set to 10–60 minutes.

[0165] 7. Conduct space matching. Considering the grid resolution of the on-orbit spaceborne scatterometer, the distance difference threshold can be set to 10–50 km.

[0166] 8. Conduct observation geometry matching; the threshold for incident angle difference can be set to 1°, and the threshold for azimuth angle difference can be set to 5°.

[0167] 9. Using the matching dataset, conduct cross-comparison of observation data from two spaceborne scatterometers, calculate the backscattering coefficient deviation and standard deviation, and analyze the dependence of the deviation on polarization, incident angle, different projection grid resolutions, relative wind direction and wind speed.

[0168] Building upon this, cross-calibration, i.e., double-difference analysis, can be performed by combining ocean calibration methods. This method can, to some extent, eliminate the influence of spatiotemporal differences, observational geometric differences, and frequency differences.

[0169] S10: Perform a comprehensive evaluation of the calibration accuracy of data obtained from any one or more of the following methods: ocean calibration, rainforest calibration, and cross-calibration, and determine whether the positioning accuracy exceeds the calibration accuracy requirements. If yes, initiate an observation quality alarm, repeat steps S4 to S8, and perform routine monitoring; if no, proceed to the next steps.

[0170] The specific steps for issuing an observation quality alarm are as follows:

[0171] S101: Set the calibration and verification cycle according to the revisit cycle of the spaceborne scatterometer;

[0172] S102: Set the calibration deviation threshold BThres according to the calibration accuracy requirements of the spaceborne scatterometer;

[0173] S103: The average deviation of marine calibration inspection within each calibration inspection cycle is denoted as BNOC;

[0174] S104: The average deviation of the rainforest calibration test within each calibration test cycle is denoted as BRFC;

[0175] S105: The average deviation of the cross-calibration test within each calibration test cycle is denoted as BSNO;

[0176] S106: When the corresponding conditions are met, it is determined that the calibration deviation exceeds the threshold and the observation quality alarm is automatically triggered. The specific corresponding conditions are: BNOC > BThres or BRFC > BThres or BSNO > BThres.

[0177] S11: Correct the data obtained from ocean calibration to obtain new data.

[0178] S12: Perform ocean calibration again on the new data obtained in step S11, and evaluate the stability of the ocean calibration. After the evaluation is completed, proceed to step S13.

[0179] S13: Complete calibration verification and perform routine monitoring of the observation data from the spaceborne scatterometer.

[0180] Compared with existing technologies, this invention achieves high-precision calibration after the spaceborne scatterometer is in orbit by combining on-board internal calibration with integrated space-to-ground external calibration. It is further supplemented by various calibration verification methods such as ocean calibration, rainforest calibration, and cross-calibration to ensure the accuracy of on-orbit observations. Furthermore, from the perspective of operational use and information feedback, this invention links together multi-target calibration and verification technologies, including on-board real-time link internal calibration, integrated space-to-ground external calibration, ocean calibration, rainforest calibration, and cross-calibration, forming a practical and complete spaceborne scatterometer calibration and verification method, significantly improving the completeness of the entire operational system process.

[0181] The above provides a detailed description of the multi-target-based spaceborne scatterometer integrated calibration and verification method provided by this invention. Any obvious modifications made by those skilled in the art without departing from the essential content of this invention will constitute an infringement of the patent rights of this invention and will incur corresponding legal liability.

Claims

1. A method for comprehensive calibration and verification of a spaceborne scatterometer based on multiple targets, characterized in that... The process includes the following steps: S1: Acquire L0 data and determine if a jump has occurred in the L0 data; if the result indicates a jump in the L0 data, initiate a near real-time alarm for the instrument status jump and proceed with subsequent steps; otherwise, proceed with subsequent steps; S2: Perform real-time internal calibration on the L0 data; S3: Perform other preprocessing on the real-time internally calibrated data to obtain L1 data; in the external calibration process, calculate the backscattering coefficient on the real-time internally calibrated data; S4: Design an external calibration observation plan, prepare for the external calibration observation, initiate the external calibration observation, and proceed to step S5; S5: Acquire observation data from the active calibrator and perform real-time and / or non-real-time processing; S6: Combine the real-time internal calibration data calculated in step S3 with the data processed in real-time and / or non-real-time in step S5; S7: Perform external calibration data processing on the combined data to obtain the three-dimensional antenna pattern and absolute calibration coefficients; S8: Based on the three-dimensional antenna pattern and absolute calibration coefficients, determine whether the data exceeds the calibration accuracy requirements; If the calibration accuracy requirement is exceeded, the backscattering coefficient of the data is calculated and resampled to obtain L1 data, and then proceed to step S9; if the calibration accuracy requirement is not exceeded, proceed to step S13. S9: Perform ocean calibration, rainforest calibration, and cross-calibration on the L1 data; S10: Perform a comprehensive evaluation of the calibration accuracy of the data obtained in step S9 and determine whether the positioning accuracy exceeds the calibration accuracy requirements; if so, initiate an observation quality alarm, repeat steps S4 to S8, and perform routine monitoring; if not, proceed to the next step; S11: Correct the data obtained from ocean calibration to obtain new data; S12: Perform ocean calibration again on the new data obtained in step S11 and evaluate the stability of ocean calibration. After the evaluation is completed, proceed to step S13; S13: Complete the calibration verification and perform routine monitoring on the observation data of the spaceborne scatterometer.

2. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, In step S1, the following sub-steps are used to determine whether the internal calibration value has abruptly changed: Each time the radar performs an internal calibration measurement, the first m internal calibration points and the remaining n internal calibration points are divided into group 1 and group 2; for the signal channel in group 1, the current internal calibration value A is compared with the previous normal internal calibration value R. If |A - R| > the normal internal calibration threshold set according to the instrument characteristics, the current internal calibration value may be abnormal, and an alarm is issued; for the signal channel in group 2, the current internal calibration value B is compared with the previous normal internal calibration value R. If |B - R| > the normal internal calibration threshold set according to the instrument characteristics, the current internal calibration value may be abnormal, and an alarm is issued; where m and n are positive integers.

3. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, In step S2, real-time internal calibration is performed using a ratio method, including: In normal measurement mode, the power output from the satellite scatterometer transmitter to the antenna transceiver switch is Pt, and the power of the echo signal received by the satellite scatterometer receiver is Pr; In the internal calibration loop condition, the power output from the transmitter to the antenna transceiver switch is Pt, and the signal does not reach the antenna transceiver switch but is attenuated by Lc through the internal calibration loop coupler before reaching the receiver. The power of the calibration signal received by the receiver is Prc, which can be expressed by the formula: Using the internal calibration loop, the expression for calculating the transmit / receive power ratio parameter in the radar equations is obtained as follows: 。 4. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, In step S2, the internal calibration accuracy is periodically evaluated through the following sub-steps to monitor the observation stability of the spaceborne scatterometer: (1) Read the remote sensing data from the spaceborne scatterometer; (2) Extract each internal calibration signal value in each scan cycle; (3) For each set of internal calibration observations, remove the large values ​​at the beginning that have not yet entered a steady state; (4) Take the average of the internal calibration values ​​within N seconds before and after each internal calibration value, calculate 3 times the standard deviation, and use it as the data error of the coupling loop during radar internal calibration. , where N is a positive integer; (5) The formula for calculating the internal calibration accuracy is: in, For the insertion loss measurement error at the launch front end, To receive the insertion loss measurement error at the front end, To calibrate the measurement error of the loop coupling, For antenna standing wave measurement error, This refers to the receiver's automatic gain control value error during internal calibration. This refers to the error in the receiver's automatic gain control value during echo measurement. (6) The insertion loss fluctuation of the rotating joint in 360-degree scanning; (7) The average internal calibration accuracy calculation result for one day, and the average value is taken to represent the internal calibration accuracy evaluation result for that day; (8) Based on the daily evaluation results, the long-term stability of the internal calibration accuracy is analyzed.

5. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, The preliminary work for external calibration observation in step S4 includes the design of the construction process for the active calibrator and calibration field; among which, the design of the construction process for the active calibrator and calibration field includes the selection of the external calibration field, the functional design of the active calibrator, the parameter design of the active calibrator, the design of the integrated satellite-ground calibration mode, the construction of the external calibration field, the design of the status monitoring scheme, the design of the data push strategy, the research and development of the external calibration system, and the development of the calibration model.

6. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, The preliminary work for external calibration observation in step S4 includes the design of an integrated satellite-ground external calibration process, which includes the following sub-steps: S451, according to the calibration requirements of the onboard scatterometer, initiate an external calibration plan periodically or irregularly; S452, the active calibrator generates a detailed calibration task based on the external calibration plan document, satellite orbit prediction document and the geographical location information of the active calibrator, and calculates the active calibrator power-on time and pointing angle information; S453. The active calibrator is powered on and warmed up in advance, and the auxiliary equipment is also automatically powered on; S454. After the active calibrator completes its self-test, it is periodically pointed in the predetermined direction to begin receiving signals from the spaceborne scatterometer, during which the auxiliary equipment continues to observe; S455. Depending on the requirements of the calibration mission, the working process of the satellite-to-ground instruments differs depending on whether it is in the receiving mode or the relay mode; S456. After the observation mission is completed, the active calibrator and auxiliary equipment stop observing on time; S457. Data storage and data push are performed.

7. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 6, characterized in that, In step S4, the workflow of the space-ground system in the relay mode is as follows: According to the calibration requirements of the onboard scatterometer, when the onboard scatterometer needs to perform relay external calibration, a remote control command is sent from the ground to the satellite to switch the onboard scatterometer to the external calibration mode. At the same time, the active calibrator is set according to the satellite orbit, and the antenna pointing of the active calibrator is adjusted so that the active calibrator receives the signal from the onboard scatterometer. The active calibrator amplifies the received signal and forwards it back to the onboard scatterometer, which then processes it. After the external calibration is completed, a remote control command is sent to switch the onboard scatterometer to the conventional observation mode.

8. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, Step S7 further includes the following sub-steps: S71: Read the data file of the active scaler to obtain the change of the voltage quantization value over time; S72: Demodulate the data read in step S71 using IQ to obtain a complex signal; S73: The active scaler uses intermediate frequency sampling to downconvert the demodulated signal to obtain a baseband waveform, and constructs a filtering window based on the number of sampling points and signal bandwidth; S74: After conversion to baseband, downsampling is performed; S75: Matched filtering is performed on the complex signal to obtain the matched-filtered waveform; S76: Calculate the signal amplitude of the complex signal after pulse compression; S77: Based on the set threshold value, complete the signal processing for a single pass of the spaceborne scatterometer to obtain the antenna gain Gt; S78: Based on the observation data of the active calibrator at different times, azimuth angles, and elevation angles, the antenna gain Gt of the spaceborne scatterometer is: in It is the azimuth angle. The elevation angle is used; based on all valid data collected by the active calibrator in a complete external calibration mission, a three-dimensional antenna pattern is fitted to obtain the antenna radiation pattern. S79: After accumulating sufficient observation data, the absolute calibration coefficient of the spaceborne scatterometer is calculated by combining the backscattering coefficient of the active calibrator calculated by the spaceborne scatterometer.

9. The method for comprehensive calibration and verification of a spaceborne scatterometer as described in claim 1, characterized in that, Step S10 further includes the following sub-steps: S101: Set the calibration verification period according to the revisit period of the spaceborne scatterometer; S102: Set the calibration deviation threshold BThres according to the calibration accuracy requirements of the spaceborne scatterometer; S103: Average the deviation of the ocean calibration verification in each calibration verification period, and record it as BNOC; S104: Average the deviation of the rainforest calibration verification in each calibration verification period, and record it as BRFC; S105: Average the deviation of the cross-calibration verification in each calibration verification period, and record it as BSNO; S106: When the corresponding conditions are met, it is determined that the calibration deviation exceeds the threshold, and the observation quality alarm is automatically triggered. The specific corresponding conditions are: BNOC > BThres or BRFC > BThres or BSNO > BThres.

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