Passive calibration method and device for back scattering coefficient of radar altimeter

Abstract

The invention discloses a passive calibration method and device for a back scattering coefficient of a radar altimeter, relates to the technical field of microwave remote sensing, and aims to solve the problems of low calibration refinement degree and high cost of the back scattering coefficient of the radar altimeter in the prior art. The method comprises the following steps: acquiring target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter; determining an operation system gain of a receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; determining a calibration coefficient of an observation backscattering coefficient of the target radar altimeter based on the operation system gain and the measurement system gain; calibrating the backscattering coefficient of the target radar altimeter based on the calibration coefficient of the observation backscattering coefficient of the target radar altimeter; according to the invention, the backscattering coefficient is finely calibrated at a low cost.

Description

Technical Field

[0001] The invention relates to the field of microwave remote sensing technology, in particular to a passive calibration method and device for a backscattering coefficient of a radar altimeter. Background Art

[0002] Satellite radar altimeters are mainly used to measure the distance from the satellite to the earth's surface. This distance information, combined with precise orbit data, can determine the sea surface height in the geocentric coordinate system or relative to the reference ellipsoid, and can also simultaneously measure the sea surface effective wave height, backscatter coefficient and sea surface wind speed at the sub-satellite point; wherein, the sea surface wind speed is obtained by quantitative inversion of the measured backscatter coefficient through an empirical model. Therefore, the backscatter coefficient of the radar altimeter needs to be accurately calibrated, which is a very necessary and indispensable process. In the prior art, the accurate calibration of the backscatter coefficient of the radar altimeter is mainly achieved through the following two methods: the first is to build a special ground active calibrator device, and realize the error correction of the entire observation system through the satellite-ground synchronous calibration observation method; the second is through typical ground objects, the backscatter coefficient characteristics of these objects have been obtained more accurately, and then the error correction of the entire observation system is realized. However, the calibration costs of these two calibration methods are high, and they can only correct the system deviation. They cannot distinguish which component in the radar altimeter causes the system deviation, resulting in the inability to more finely evaluate the measurement accuracy of the radar altimeter backscatter coefficient, and are not conducive to the optimization of payload design.

[0003] In view of this, there is an urgent need to design a more advanced evaluation method to achieve a refined evaluation of the backscatter coefficient of the radar altimeter at a lower cost, so as to solve the problem of low refinement and high cost of the backscatter coefficient calibration of the radar altimeter in the prior art. Summary of the invention

[0004] The purpose of the present invention is to provide a passive calibration method and device for the backscatter coefficient of a radar altimeter. For the target ground object calibration area, the observation data of the radar altimeter in the noise monitoring mode is used, combined with the typical ground object target method, to determine the calibration coefficient of the observed backscatter coefficient of the radar altimeter, and then calibrate the backscatter coefficient of the radar altimeter, so that the measurement accuracy of the backscatter coefficient of the radar altimeter can be evaluated more finely, and there is no need to build expensive specific equipment, which has the advantages of simplicity, ease of operation and low cost; and solves the problem of low degree of refinement and high cost of backscatter coefficient calibration of radar altimeter in the prior art.

[0005] In order to achieve the above object, the present invention provides the following technical solutions: In a first aspect, the present invention provides a passive calibration method for a radar altimeter backscatter coefficient, which may include: Obtaining target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter; the target observation data is data obtained by observing a target ground object calibration area by the target radar altimeter in a noise monitoring mode; Determining the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; Determining a calibration coefficient of an observed backscatter coefficient of the target radar altimeter based on the operating system gain and a measurement system gain of a ground measurement receiver of the target radar altimeter before launch; The backscatter coefficient of the target radar altimeter is calibrated based on the calibration coefficient of the observed backscatter coefficient of the target radar altimeter.

[0006] Preferably, determining the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data may include: Preprocessing the target observation data and the antenna main lobe radiation brightness temperature simulation data respectively to obtain the average output power of the receiver in the target radar altimeter and the average antenna main lobe radiation brightness temperature at the antenna aperture of the target radar altimeter; An operating system gain of a receiver in the target radar altimeter is determined based on the average output power and the average antenna main lobe radiation brightness temperature.

[0007] Preferably, determining the operating system gain of the receiver in the target radar altimeter based on the average output power and the average antenna main lobe radiation brightness temperature may include using the formula: ; The operating system gain of the receiver in the target radar altimeter is calculated; wherein, represents the operating system gain of the receiver in the target radar altimeter, Indicates the average output power, represents the average antenna main lobe radiation brightness temperature, is the setting value of the altimeter automatic gain control AGC, is the main lobe efficiency, The radiation efficiency of the antenna. is the receiver signal bandwidth and k is the Boltzmann constant.

[0008] Preferably, before acquiring the target observation data of the target radar altimeter in the noise monitoring mode and the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter, the method may include: Using a preset surface radiation brightness temperature model to simulate and calculate the environmental parameters of the target object calibration area, and obtain radiation brightness temperature simulation data of the surface of the target object calibration area; The preset atmospheric radiation transmission model is used to simulate and calculate the radiation brightness temperature simulation data of the surface of the target object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter.

[0009] Preferably, the step of using a preset surface radiation brightness temperature model to simulate and calculate the environmental parameters of the target object calibration area to obtain radiation brightness temperature simulation data of the surface of the target object calibration area may include: Constructing the preset surface radiation brightness temperature model; The step of constructing the preset surface radiation brightness temperature model may include: Obtaining the basic surface brightness temperature observation data of the target object calibration area collected by a preset satellite; the preset satellite is a mature and stably operating satellite; Using multiple radiation emissivity models based on ground objects, the environmental parameters of the target ground object calibration area are simulated and calculated to obtain multiple surface brightness temperature simulation data; the multiple radiation emissivity models include at least a physical model, a semi-empirical model and an empirical model; Based on the basic surface brightness temperature observation data and the plurality of surface brightness temperature simulation data, the error accuracy of the plurality of radiation emissivity models is determined, and the radiation emissivity model whose error accuracy is less than a first preset threshold is determined as the preset surface radiation brightness temperature model.

[0010] Preferably, the step of constructing the preset surface radiation brightness temperature model may then include: Acquiring brightness temperature observation data of an airborne microwave radiometer having the same frequency as the target radar altimeter; Based on the brightness temperature observation data of the airborne microwave radiometer and in combination with the initial frequency of the preset surface radiation brightness temperature model, a first frequency scale correction coefficient between the initial frequency of the preset surface radiation brightness temperature model and the target radar altimeter frequency is established; the initial frequency is an applicable working frequency of the preset surface radiation brightness temperature model itself; Based on the first frequency scale correction coefficient, the surface radiation brightness temperature at the initial frequency of the preset surface radiation brightness temperature model is corrected to obtain the surface radiation brightness temperature at the working frequency condition of the target radar altimeter.

[0011] Preferably, the method of using a preset atmospheric radiation transmission model to simulate and calculate the radiation brightness temperature simulation data of the surface of the target object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter may include: Constructing the preset atmospheric radiation transfer model; The step of constructing the preset atmospheric radiation transmission model may include: Obtain station observation data of atmospheric temperature, humidity and pressure sounding; Based on the station observation data, according to the radiation simulation strategy of the preset error disturbance, the error sensitivity of multiple atmospheric radiation transfer models is simulated to obtain the standard deviation of the simulation accuracy error of the multiple atmospheric radiation transfer models under different atmospheric environments; An atmospheric radiation transfer model whose standard deviation of the simulation accuracy error is less than a second preset threshold is used as the preset atmospheric radiation transfer model.

[0012] Preferably, the step of constructing the preset atmospheric radiation transfer model may then include: Acquiring brightness temperature observation data of an airborne microwave radiometer having the same frequency as the target radar altimeter; Based on the brightness temperature observation data of the airborne microwave radiometer and in combination with the initial frequency of the preset atmospheric radiation transmission model, a second frequency scale correction coefficient between the initial frequency of the preset atmospheric radiation transmission model and the target radar altimeter frequency is established; Based on the second frequency scale correction coefficient, the radiation transfer process of the preset atmospheric radiation transfer model is corrected to achieve accurate simulation of the atmospheric radiation transfer process under the working frequency conditions of the target radar altimeter, and obtain the radiation brightness temperature of the surface radiation transmitted to the antenna aperture of the target radar altimeter through atmospheric radiation.

[0013] Preferably, before acquiring the target observation data of the target radar altimeter in the noise monitoring mode and the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter, the method may include: Determining the target object calibration area; Determining the target object calibration area may include: Acquire historical data of surface brightness temperature observed by a target satellite microwave radiometer; the target satellite microwave radiometer is a satellite microwave radiometer that operates stably and has an operating frequency similar to that of the target radar altimeter; Determine a plurality of restriction conditions for delimiting the target ground object calibration area, wherein the plurality of restriction conditions at least include that the target ground object calibration area and the measurement swath of the target radar altimeter have an overlapping area, the target ground object calibration area includes at least a plurality of ground object types, the target ground object calibration area has the conditions for constructing surface radiation brightness temperature models of the plurality of ground object types, the target ground object calibration area has the conditions for constructing an atmospheric radiation transmission model, and the target ground object calibration area covers or is close to an on-site measurement station with operational stable observation capability; Based on the historical data and the plurality of restriction conditions, the target object calibration area is determined.

[0014] In a second aspect, the present invention provides a passive calibration device for a radar altimeter backscatter coefficient, which may include: A data acquisition module, the data acquisition module is used to acquire target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter; the target observation data is data obtained by observing a target ground object calibration area by the target radar altimeter in a noise monitoring mode; A system gain determination module, the system gain determination module is used to determine the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; A calibration coefficient determination module, the calibration coefficient determination module is used to determine the calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before launch; A scattering coefficient calibration module is used to calibrate the backscattering coefficient of the target radar altimeter based on the calibration coefficient of the observed backscattering coefficient of the target radar altimeter.

[0015] Compared with the prior art, the passive calibration method of the backscatter coefficient of a radar altimeter provided by the present invention obtains target observation data of the target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter; wherein the target observation data is data obtained by observing the target ground object calibration area by the target radar altimeter in the noise monitoring mode; based on the target observation data and the antenna main lobe radiation brightness temperature simulation data, the operating system gain of the receiver in the target radar altimeter is determined; further based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before launch, the operating system gain of the target radar altimeter is determined. The invention discloses a calibration coefficient of an observed backscatter coefficient of a target radar altimeter; finally, based on the calibration coefficient of the observed backscatter coefficient of the target radar altimeter, the backscatter coefficient of the target radar altimeter is calibrated; based on this, it can be determined that the deviation of the backscatter coefficient of the radar altimeter is mainly caused by the payload receiver of the radar altimeter, so as to more finely evaluate the measurement accuracy of the backscatter coefficient of the radar altimeter, and the evaluation process mainly uses simulation technology, does not need to build expensive specific equipment, has the advantages of simplicity and low cost; solves the problem of low degree of refinement and high cost of backscatter coefficient calibration of radar altimeter in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 A schematic diagram of the main flow of a passive calibration method for a radar altimeter backscatter coefficient provided by the present invention; Figure 2 A schematic diagram of the data flow logic relationship between the surface radiation brightness temperature and the average output power of the radar altimeter receiver in a passive calibration method for the backscatter coefficient of the radar altimeter provided by the present invention; Figure 3 A schematic diagram of the linear relationship and slope estimation between the average output power and input brightness temperature of a radar altimeter receiver in a passive calibration method for backscatter coefficient of a radar altimeter provided by the present invention; Figure 4 A schematic structural diagram of a passive calibration device for backscatter coefficient of a radar altimeter provided by the present invention.

[0017] Reference numerals: 210 - radiation brightness temperature data node, 220 - antenna main lobe radiation brightness temperature data node, 230 - radar altimeter data node. DETAILED DESCRIPTION

[0018] In order to facilitate the clear description of the technical solutions of the embodiments of the present invention, in the embodiments of the present invention, the words "first", "second" and the like are used to distinguish the same items or similar items with substantially the same functions and effects. For example, the first threshold and the second threshold are only used to distinguish different thresholds, and do not limit their order of precedence. Those skilled in the art can understand that the words "first", "second" and the like do not limit the quantity and execution order, and the words "first", "second" and the like do not necessarily limit them to be different.

[0019] It should be noted that, in the present invention, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the present invention should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0020] In the present invention, "at least one" means one or more, and "plurality" means two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone, where A and B can be singular or plural. The character " / " generally indicates that the associated objects in the previous time are in an "or" relationship. "At least one of the following items" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, at least one of a, b or c can mean: a, b, c, the combination of a and b, the combination of a and c, the combination of b and c, or the combination of a, b and c, where a, b, c can be single or multiple.

[0021] The two commonly used methods for accurate calibration of the backscatter coefficient of the radar altimeter in the prior art require the construction of a special ground active calibrator device, and the accurate calibration of the backscatter coefficient is achieved through field tests, which requires a lot of manpower, material and financial resources; and the existing active calibrator calibration method and the typical ground object calibration method cannot distinguish which specific component causes the measurement deviation in a more detailed way, and can only correct the error of the measurement deviation of the entire system. These factors are not conducive to the optimization and upgrading of the later payload design, and it is also possible that the efficiency of improving the backscatter coefficient with higher accuracy will be lost due to insufficient refinement of the error correction. In addition, the receiver of the satellite-borne radar is a component for realizing the accurate measurement of microwave signals, and the gain deviation of the receiver itself will directly affect the measurement accuracy of the backscatter coefficient of the microwave signal of the radar. According to research, the main components on the satellite involved in the error source causing the measurement of the backscatter coefficient of the radar altimeter include the transmitter, receiver, antenna, etc.

[0022] Based on this, the present invention proposes a passive calibration method and device for the backscatter coefficient of a radar altimeter, which utilizes the observation data of the noise monitoring mode of the radar altimeter and combines the typical ground target method to realize the acquisition of the backscatter coefficient deviation of the radar altimeter, and finds that the deviation is mainly caused by the error of the receiver's own gain of the onboard payload, so that the measurement accuracy of the backscatter coefficient of the radar altimeter can be evaluated more finely, and there is no need to build expensive specific equipment, which has the advantages of simplicity and low cost; it solves the problem of low degree of refinement and high cost in the calibration of the backscatter coefficient of the radar altimeter in the prior art.

[0023] Next, the technical solution of the present invention is described in detail with reference to the accompanying drawings: See also Figure 1 , Figure 1The main flow diagram of the passive calibration method of the backscatter coefficient of a radar altimeter provided by the present invention is shown in FIG. The execution subject of the method is a server or terminal device equipped with the passive calibration method of the backscatter coefficient of a radar altimeter provided by the present invention; such as a calibration service platform or a mobile calibration device.

[0024] exist Figure 1 The methods may include: Step 110: acquiring target observation data of the target radar altimeter in the noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter; the target observation data is data obtained by observing the target ground object calibration area by the target radar altimeter in the noise monitoring mode.

[0025] Step 120: Determine the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data.

[0026] In step 110 to step 120, for the predetermined target object calibration area, the target radar altimeter is first set to a noise monitoring mode. At this time, the working mode of the radar altimeter is similar to the observation mode of a passive microwave radiometer, and only the radiation from the receiver, antenna loss and the earth's surface is detected; wherein, the radiation response characteristics of the microwave antenna are related to its size, shape, physical temperature and working wavelength, and its characteristics can be accurately determined on the ground before launch, mainly including the main lobe efficiency, radiation efficiency and the measurement of each specific surface brightness temperature, which can be accurately determined by the actual satellite-ground geometric relationship and the ground measurement characteristics. In the present invention, the antenna radiation response can be considered to be accurate and error-free; therefore, the uncertain influencing factors in the target observation data are the influences generated by the receiver. At the same time, simulation calculations are performed for different surface environmental parameters in the target object calibration area, including simulating the surface environmental parameter data using the sea surface or land surface radiation brightness temperature model, and simulating the sea surface or land surface radiation brightness temperature using the atmospheric radiation transmission model, to obtain the radiation brightness temperature at the satellite antenna aperture, so that the operating system gain of the receiver in the radar altimeter can be determined based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; there is no need to construct typical objects or special ground active calibration equipment to obtain data, saving data acquisition costs.

[0027] Step 130: Determine a calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before transmission.

[0028] Step 140: Calibrate the backscatter coefficient of the target radar altimeter based on the calibration coefficient of the observed backscatter coefficient of the target radar altimeter.

[0029] In step 130 to step 140, the radar altimeter can measure the backscatter coefficient of the surface in the active observation mode. Compared with the passive monitoring mode, the setting value of the automatic gain control AGC at this time is different, but known; the response characteristics of the receiver gain are the same as those in the passive monitoring mode. For radar measurement of the backscatter coefficient, in the radar equation used for the backscatter coefficient calculation, the receiver gain is a certain proportional factor, so the backscatter coefficient and the receiver gain change in proportion. When the backscatter coefficient data is produced, the value of the receiver gain uses the ground measurement value before the satellite is launched; therefore, when the receiver gain estimate is inaccurate, it will cause the backscatter coefficient to produce a measurement deviation, which needs to be corrected by passive calibration technology. Research has found that the calibration coefficient or measurement deviation of the backscatter coefficient can also be converted into the deviation of the receiver gain, that is, the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before launch can be used to determine the calibration coefficient of the observed backscatter coefficient of the target radar altimeter; finally, the calibration coefficient can be used to calibrate the backscatter coefficient of the target radar altimeter.

[0030] Based on this, the present invention provides a passive calibration method for the backscatter coefficient of a radar altimeter. By adopting a passive calibration method based on a target (typical) object, a correlation relationship between the target radiation and the receiver gain in the target object is established, and then the actual operating system gain of the receiver is determined, and the calculation deviation of the receiver gain is obtained, thereby improving the refined calibration of the backscatter coefficient; the refined calibration of the backscatter coefficient is achieved at a relatively low cost, which solves the problem of low refinement and high cost in the prior art of the backscatter coefficient calibration of the radar altimeter.

[0031] Preferably, before step 110, that is, before obtaining the target observation data of the target radar altimeter in the noise monitoring mode and the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter, the process may include: determining the target ground object calibration area.

[0032] Specifically, determining the target object calibration area may include the following steps: S1: Obtain historical data of surface brightness temperature observed by the target satellite microwave radiometer; the target satellite microwave radiometer is a satellite microwave radiometer that operates stably and has an operating frequency similar to that of the target radar altimeter.

[0033] S2: Determine multiple restriction conditions for delimiting the target ground object calibration area, the multiple restriction conditions at least include that the target ground object calibration area overlaps with the measurement swath of the target radar altimeter, the target ground object calibration area includes at least multiple ground object types, the target ground object calibration area has the conditions for constructing surface radiation brightness temperature models of multiple ground object types, the target ground object calibration area has the conditions for constructing an atmospheric radiation transfer model, and the target ground object calibration area covers or is close to an on-site measurement station with operational stable observation capabilities.

[0034] S3: Determine the target object calibration area based on historical data and multiple constraints.

[0035] Specifically, the deviation of the backscatter coefficient of the radar altimeter determined by the technical means provided by the present invention is mainly caused by the gain response error of the payload receiver. In order to ensure the correction effect of the receiver gain response, the selected typical land object has a more reliable surface radiation brightness temperature model; however, due to the technical development limitations of the current microwave radiation and atmospheric radiation transmission models, the existing microwave radiation model does not have the same model accuracy in any geographical region of the world, so it is necessary to accurately select the geographical area of ​​the specific land object. In addition, considering the technical characteristics of the passive calibration of the altimeter, the selected calibration land object should cover various types of surfaces, including oceans, land surfaces, polar ice surfaces, etc., so that the radiation brightness temperature data set used for the subsequent calibration calculation process can cover the measurement dynamic range of the altimeter onboard receiver to the greatest extent. At the same time, the geographical area where the selected land objects are distributed should consider factors such as the radiation brightness temperature optimization test related to the passive calibration of the altimeter, the orbit of the satellite altimeter, and the conditions for obtaining the parameters required for the subsequent passive calibration.

[0036] Furthermore, atmospheric radiation transfer is also one of the main error sources of the passive calibration method, and there are few observation points for the relevant atmospheric temperature, humidity and pressure profile data. Therefore, in order to improve the processing accuracy of atmospheric radiation transfer, the selected geographical area should also cover or be close to the atmospheric sounding station as much as possible to obtain the measured profile data of atmospheric temperature, humidity, air pressure, wind speed and other information, so as to ensure the model calculation accuracy of atmospheric radiation transfer of radiation brightness temperature.

[0037] Based on the above requirements, the selection criteria and principles for typical geo-location calibration areas may include the following restrictions: (1) The selected geographic area must overlap with the radar altimeter measurement swath.

[0038] (2) The selected geographical area must include three types of land features: polar ice, ocean, and land surface. The land surface types within each area should be as uniform as possible. Considering that the land surface is easily affected by human life, geographical areas far away from towns such as the Amazon rainforest and the Sahara Desert should be selected as much as possible.

[0039] (3) The selected geographical area must have mature and stable conditions for constructing surface radiation brightness temperature models of land, ocean, polar ice, etc., and the surface environmental parameters required for the model construction stage and calibration implementation stage should have stable and reliable acquisition conditions.

[0040] (4) The selected geographical area must have the conditions for constructing a mature and stable atmospheric radiation transfer model, and the surface environmental parameters required for the model construction stage and the calibration implementation stage should have stable and reliable acquisition conditions.

[0041] (5) The selected geographical area should, as far as possible, cover or be close to field measurement sites with operational and stable observation capabilities, including atmospheric sounding sites, surface environment monitoring sites, and surface radiation brightness temperature sites.

[0042] Preferably, in S3, based on the historical data and multiple constraints, determining the target ground object calibration area may include: first, gridding the historical data to obtain a target geographic grid; based on the target geographic grid, combined with the first constraint, determining the first ground object calibration area; the first constraint includes that the target ground object calibration area includes at least multiple ground object types, the target ground object calibration area has the conditions for constructing the surface radiation brightness temperature model of multiple ground object types, and the target ground object calibration area has the conditions for constructing the atmospheric radiation transfer model; further, based on the first ground object calibration area, combined with the second constraint, the geographical grid points covered by the first ground object calibration area are gridded. The grid points are optimized to obtain the second ground object calibration area; the second restriction condition includes the overlapping area between the target ground object calibration area and the measurement swath of the target radar altimeter, and the restriction condition that the target ground object calibration area covers or is close to the on-site measurement station with operational stable observation capability; finally, it is judged whether the stability and reliability of the surface radiation brightness temperature model constructed based on the second ground object calibration area meet the preset requirements. If so, the second ground object calibration area is used as the target ground object calibration area; if not, the first ground object calibration area and the second ground object calibration area are optimized until the stability and reliability of the obtained surface radiation brightness temperature model meet the preset convergence conditions.

[0043] Specifically, the method for selecting the typical land object calibration geographical area can be as follows: first, select mature and stable historical data of satellite microwave radiometer brightness temperature observations with relatively close frequencies, divide the data into 30-day and 1-degree geographical grids, and select the geographical grid points covered by typical land objects in combination with the selection criteria of Articles (2) to (4) above. Then, according to the standards of Articles (1) and (5), combine, merge, adjust and improve the geographical grid points to determine the preliminary calibration area range. In the subsequent construction of the typical land object surface radiation brightness temperature model, it is also possible to optimize and improve the preliminary geographical area range.

[0044] Preferably, before step 110, that is, before obtaining the target observation data of the target radar altimeter in the noise monitoring mode and the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter, it can also include: using a preset surface radiation brightness temperature model to simulate and calculate the environmental parameters of the target object calibration area to obtain the radiation brightness temperature simulation data of the surface of the target object calibration area.

[0045] Furthermore, a preset atmospheric radiation transmission model is used to simulate and calculate the radiation brightness temperature simulation data of the surface of the target object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter.

[0046] Preferably, using a preset surface radiation brightness temperature model to simulate and calculate environmental parameters of the target object calibration area to obtain radiation brightness temperature simulation data of the surface of the target object calibration area may include: constructing a preset surface radiation brightness temperature model.

[0047] Specifically, the surface radiation characteristics of typical landforms such as oceans, land surfaces, and polar ice surfaces are different, and need to be studied separately. At the same time, the radiation characteristic models for typical landforms such as oceans, land surfaces, and polar ice surfaces have been fully developed and belong to the basic research scientific content. The technical difficulty and time cost of the calibration research of the satellite altimeter backscatter coefficient alone are very large. The present invention will be based on the mature radiation emissivity model of typical landforms, including physical models, semi-empirical models, and empirical models, etc., combined with the existing accurately calibrated satellite microwave radiation observation data, through sufficient error uncertainty evaluation, analysis of model accuracy and supporting input surface environmental parameter error accuracy, stability, frequency sensitivity analysis, to construct a database of landform radiation models and supporting data sets for passive calibration of altimeter backscatter coefficients. Since the operating frequency and bandwidth of the altimeter are generally different from the frequency and bandwidth applicable to the existing model, it is necessary to develop a frequency correction function based on the existing surface radiation brightness temperature model based on the observation data of the same frequency of the preset satellite altimeter to accurately derive and calculate the surface radiation brightness temperature of the preset altimeter frequency.

[0048] Preferably, constructing a preset surface radiation brightness temperature model may include: acquiring basic surface brightness temperature observation data of the target object calibration area collected by a preset satellite; the preset satellite is a mature and stably operating satellite; using multiple radiation emissivity models based on the object to simulate and calculate the environmental parameters of the target object calibration area to obtain multiple surface brightness temperature simulation data; the multiple radiation emissivity models include at least a physical model, a semi-empirical model and an empirical model; based on the basic surface brightness temperature observation data and the multiple surface brightness temperature simulation data, determining the error accuracy of multiple radiation emissivity models, and determining the radiation emissivity model whose error accuracy is less than a first preset threshold as the preset surface radiation brightness temperature model.

[0049] Specifically, since many satellite microwave radiometers have been launched internationally for observing environmental parameters such as sea surface temperature, sea surface salinity, atmospheric cloud liquid water, and land surface soil moisture. Therefore, the radiation brightness temperature model of the ocean and land surface has been fully studied and developed. The radiation brightness temperature model construction and frequency scale correction involved in the present invention mainly include three parts: First, there are certain deviations between the existing various surface brightness temperature radiation models, so it is necessary to combine the existing mature and stable satellite microwave radiometer brightness temperature observation data, and establish the deviation correction coefficients between different models on the basis of the pre-selected calibration area, and then evaluate the error accuracy of different models to determine the radiation brightness temperature model that meets the requirements of the present invention. The first preset threshold can be multiple simulation error thresholds for different surface radiation brightness temperatures, for example: the radiation brightness temperature simulation error of the ocean surface should be less than the threshold 2K, the radiation brightness temperature simulation error of land surfaces such as rainforests and polar ice should be less than the threshold 3K, and the radiation brightness temperature simulation error of land surfaces such as deserts should be less than the threshold 5K.

[0050] It should be noted that, based on the pre-selected radiation brightness temperature model, the seasonal variation characteristics and stability characteristics of brightness temperature in different calibration areas are analyzed in detail. Combined with the requirements for the stability of the calibration input environmental parameters and the numerical range of brightness temperature variation, the pre-selected calibration area range is improved, that is, the calibration area range preliminarily determined in S3 is improved.

[0051] Furthermore, after constructing the preset surface radiation brightness temperature model, the following may be included: obtaining the brightness temperature observation data of the airborne microwave radiometer with the same frequency as the target radar altimeter; establishing the first frequency scale correction coefficient between the initial frequency of the preset surface radiation brightness temperature model and the frequency of the target radar altimeter based on the brightness temperature observation data of the airborne microwave radiometer and the initial frequency of the preset surface radiation brightness temperature model; the initial frequency is the working frequency applicable to the preset surface radiation brightness temperature model itself; based on the first frequency scale correction coefficient, correcting the surface radiation brightness temperature at the initial frequency of the preset surface radiation brightness temperature model to obtain the target surface radiation brightness temperature under the same frequency condition of the target radar altimeter. Thus, the preset surface radiation brightness temperature model can be used to simulate and calculate the environmental parameters of the target object calibration area to obtain the radiation brightness temperature simulation data of the surface of the target object calibration area.

[0052] Furthermore, the radiation brightness temperature simulation data of the surface of the target object calibration area is simulated and calculated using a preset atmospheric radiation transmission model to obtain the antenna main lobe radiation brightness temperature simulation data at the target radar altimeter antenna aperture. This may also include: constructing a preset atmospheric radiation transmission model.

[0053] Studies have shown that the atmosphere has two effects on satellite microwave radiation measurement: one is that oxygen, water vapor, cloud liquid water, etc. in the atmosphere will absorb, attenuate and scatter the radiation of the surface layer of the earth; the other is that the atmospheric material itself will also generate microwave radiation. Therefore, it is necessary to construct an atmospheric radiation transmission model to fully simulate the influence of the up-radiation and down-radiation of the earth's surface radiation during the atmospheric radiation transmission process, and then deduce the accurate radiation brightness temperature of the earth's surface radiation brightness temperature at the antenna aperture of the onboard remote sensor. Considering that the construction of a preset atmospheric radiation transmission model dedicated to the satellite radar altimeter is a basic scientific research content, the technical difficulty and time consumption are too long. The present invention will be based on the existing mature atmospheric radiation transmission model for microwave radiometer calibration, including physical models, semi-empirical models and empirical models, etc., combined with the existing accurately calibrated satellite microwave radiation observation data, through sufficient error uncertainty evaluation, analysis of model accuracy and the accuracy, stability and frequency sensitivity analysis of the supporting input atmospheric profile environmental parameter errors, to construct an atmospheric radiation transmission model and a database of supporting data sets for passive calibration of the altimeter backscattering coefficient. Since the operating frequency and bandwidth of the altimeter are generally different from the frequency and bandwidth applicable to the existing model, it is necessary to develop a frequency correction function based on the brightness temperature of the existing model based on the observation data of the same frequency of the preset satellite altimeter, so as to accurately derive and calculate the radiation brightness temperature of the preset altimeter frequency.

[0054] Specifically, constructing a preset atmospheric radiation transfer model may include: first, obtaining station observation data of atmospheric temperature, humidity and pressure soundings; secondly, based on the station observation data, according to a radiation simulation strategy of a preset error disturbance, simulating the error sensitivity of multiple atmospheric radiation transfer models, and obtaining the standard deviation of the simulation accuracy errors of multiple atmospheric radiation transfer models under different atmospheric environments; finally, the atmospheric radiation transfer model whose standard deviation of the simulation accuracy error is less than a second preset threshold is used as the preset atmospheric radiation transfer model.

[0055] For example, considering that atmospheric radiation transmission is different under different conditions such as clear sky, cloudy, and rain, and the existing atmospheric radiation model may also have accuracy differences at different temperatures and in different regions due to differences in modeling accuracy. Therefore, the atmospheric radiation transmission model of the present invention is constructed. First, relying on the existing atmospheric radiation transmission model, the station observations of the actual atmospheric temperature, humidity and pressure sounding are selected, and the accuracy sensitivity of different atmospheric radiation transmission models under clear sky, cloudy, and rain conditions is analyzed by the radiation simulation technology of artificially increasing error disturbances, that is, the standard deviation of the simulation accuracy error. The second preset threshold can be a plurality of simulation error thresholds for the atmospheric radiation brightness temperature under different environments. For example, the error sensitivity to the average upward radiation brightness temperature of the atmosphere should be less than the threshold of 2K, and the simulation error of the atmospheric optical thickness under clear sky should be less than the threshold of 0.02dB.

[0056] After constructing the preset atmospheric radiation transmission model, the following may be included: first, obtaining the brightness temperature observation data of the airborne microwave radiometer with the same frequency as the target radar altimeter. Then, based on the brightness temperature observation data of the airborne microwave radiometer and the initial frequency of the preset atmospheric radiation transmission model, a second frequency scale correction coefficient between the initial frequency of the preset atmospheric radiation transmission model and the frequency of the target radar altimeter is established; finally, based on the second frequency scale correction coefficient, the radiation transmission process of the preset atmospheric radiation transmission model is corrected to achieve accurate simulation of the atmospheric radiation transmission process under the working frequency conditions of the target radar altimeter, and obtain the radiation brightness temperature of the surface radiation brightness temperature transmitted to the antenna aperture of the target radar altimeter through atmospheric radiation. Therefore, the preset atmospheric radiation transmission model can be used to simulate and calculate the radiation brightness temperature simulation data of the surface of the target ground object calibration area, and obtain the radiation brightness temperature transmitted to the antenna aperture of the target radar altimeter for the environmental parameters of the target ground object calibration area.

[0057] It should be noted that radar altimeters on other satellites with the same frequency conditions as the target radar altimeter in the present invention can directly use the simulation model disclosed in the present invention for simulation calculation, without the need to rebuild the simulation model.

[0058] Based on this, the airborne microwave radiometer brightness temperature observation data covering the selected applicable frequency of the existing atmospheric radiation transfer model and the preset altimeter frequency are used to establish the frequency scale correction coefficient between the existing atmospheric radiation transfer model frequency and the preset altimeter frequency, so as to realize accurate atmospheric radiation transfer simulation under the preset altimeter operating frequency conditions.

[0059] For further information, please refer to Figure 2 , Figure 2 A schematic diagram of the data flow logic relationship between the surface radiation brightness temperature and the average power output by the radar altimeter receiver in a passive calibration method for the backscatter coefficient of a radar altimeter provided by the present invention.

[0060] exist Figure 2 In the embodiment, for the typical (target) ground object calibration area, that is, the surface environmental parameters, the surface radiation brightness temperature model in the radiation brightness temperature data node 210 can be used to simulate and calculate the environmental parameters of the target ground object calibration area to obtain the radiation brightness temperature simulation data of the surface of the target ground object calibration area; and then the atmospheric radiation transmission model in the antenna main lobe radiation brightness temperature data node 220 can be used to simulate and calculate the radiation brightness temperature simulation data of the surface of the target ground object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter; finally, the antenna main lobe radiation brightness temperature simulation data is transmitted to the radar altimeter data node 230, that is, transmitted to the radar altimeter on the satellite.

[0061] Specifically, in the calibration stage using typical objects, the radar altimeter adopts the noise monitoring mode. The working mode of the altimeter is similar to the observation mode of the passive microwave radiometer, which only detects the radiation from the receiver, antenna loss and the earth's surface. The surface radiation brightness temperature is consistent with the average power P output of the radar altimeter receiver. n The logical relationship between the data flows is as follows Figure 2 As shown. The radiation response characteristics of the microwave antenna are related to its size, shape, physical temperature, and operating wavelength. Its characteristics can be accurately determined on the ground before transmission, mainly including the main lobe efficiency, radiation efficiency, and the measurement of each specific surface brightness temperature, which can be accurately determined by the actual satellite-ground geometric relationship and ground measurement characteristics. In the data processing of the present invention, the antenna radiation response can be considered to be accurate and error-free. The receiver mainly realizes the expected average power ( ), the characteristics directly related to the measurement mainly include the specific value of the automatic gain control setting ( ), receiver gain ( ), the receiver's own thermal noise (noise equivalent temperature).

[0062] According to the microwave radiation measurement principle, after considering the receiver's own thermal noise, the theoretical calculation formula for the average power measured by the target radar altimeter receiver is as follows: is the noise equivalent temperature of the receiver's own thermal noise, and the average power available at the receiver input is , so the average power output by the receiver is Given by: (1) In formula (1), Indicates the receiver system gain, AGC, and pass Indicates the setting value of the altimeter automatic gain control AGC (read directly from the altimeter line data), represents the main lobe efficiency, represents the radiation efficiency of the antenna, B represents the receiver signal bandwidth, k represents the Boltzmann constant (1.3806E-23J / K), T BML Indicates the average brightness temperature affected by the antenna main lobe (antenna main lobe radiation brightness temperature), represents the average brightness temperature from the external response of the main lobe (antenna side lobe brightness temperature), Indicates the physical temperature of the antenna. It is the noise equivalent temperature of the receiver's own thermal noise. It should be noted that formula (1) is the basic equation for the relationship between the antenna aperture radiation brightness temperature and the receiver gain in the passive calibration of typical ground objects. The antenna main lobe radiation brightness temperature is also called the radiation brightness temperature of the input receiver.

[0063] When designing a microwave antenna, a key design factor is to minimize the side lobe effect of the microwave antenna and enhance the radiation response of the main lobe of the antenna as much as possible. About 0.7 (accurately measured on the ground before launch). While the satellite is in orbit, the microwave antenna has been subjected to certain temperature control, and its physical temperature variation range is not large. The receiver itself has also been designed for temperature control, and its noise equivalent temperature variation range is small.

[0064] Therefore, for formula (1), the brightness temperature of the antenna main lobe radiation compared to the surface radiation is The contribution of the second part to the average power is very small and can be ignored. Considering the linear response of the receiver to the average power measurement (the power response of general radar receivers is designed to be linear), it can be assumed that and The relationship between is linear. Therefore, it can be seen from formula (1) that the altimeter output power increases with the brightness temperature of the altimeter antenna main lobe radiation in different scenes. Changes with the value.

[0065] Therefore, in step 120, determining the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data can include: performing data preprocessing on the target observation data and the antenna main lobe radiation brightness temperature simulation data, respectively, to obtain the average output power of the receiver in the target radar altimeter and the average antenna main lobe radiation brightness temperature at the antenna aperture of the target radar altimeter; determining the operating system gain of the receiver in the target radar altimeter based on the average output power and the average antenna main lobe radiation brightness temperature.

[0066] For examples, see Figure 3 , Figure 3 A schematic diagram of the linear relationship and slope estimation between the average output power and input brightness temperature of a radar altimeter receiver in a passive calibration method for a radar altimeter backscatter coefficient provided by the present invention.

[0067] exist Figure 3 In the The interpolation value is combined with the corresponding average power output of the receiver By interpolating the values ​​of , we can estimate the slope of the linear response of the power. We can get the following ratio formula: (2) Further, preferably, determining the operating system gain of the receiver in the target radar altimeter based on the average output power and the average antenna main lobe radiation brightness temperature may include using the formula: (3) Calculate the operating system gain of the receiver in the target radar altimeter; in formula (2) to formula (3), represents the operating system gain of the receiver in the target radar altimeter, Indicates the average output power, represents the average antenna main lobe radiation brightness temperature, Indicates the setting value of the altimeter automatic gain control AGC, represents the main lobe efficiency, The radiation efficiency of the antenna. represents the receiver signal bandwidth and k is the Boltzmann constant.

[0068] It should be noted that is the setting value of the altimeter automatic gain control AGC, is the main lobe efficiency, The radiation efficiency of the antenna. is the receiver signal bandwidth, these four parameters are instrument parameters and are all known; k is the Boltzmann constant (1.3806E-23J / K).

[0069] Research has found that in order to obtain the actual system gain of the accurate receiver on the satellite To obtain the calculation result of , we only need to improve the estimation accuracy of the slope of the linear response line in the above figure. This can be achieved in the following two ways: (1) Select as many surface scenes as possible, that is, add more differentiated The observed values ​​make the estimation of the slope of the straight line more accurate, which is why we selected different typical landform types in the early stage.

[0070] (2) Improve the accuracy of the surface radiation brightness temperature model and the atmospheric radiation transmission model, and obtain the antenna main lobe radiation brightness temperature at the antenna aperture more accurately. , which is mainly ensured by the construction of surface radiation brightness temperature model and atmospheric radiation transfer model.

[0071] Furthermore, the calibration coefficients of the backscatter coefficients can be accurately estimated.

[0072] It should be noted that the radar altimeter can measure the backscatter coefficient of the surface in the active observation mode. Compared with the passive monitoring mode, the setting value of the automatic gain control AGC at this time is different, but known; the response characteristics of the receiver gain are the same as those in the passive monitoring mode. For radar measurement of the backscatter coefficient, the receiver gain is a certain proportional factor in the radar equation used in the backscatter coefficient calculation, so the backscatter coefficient changes in proportion to the receiver gain. When the backscatter coefficient data is generated, the value of the receiver gain uses the ground measurement value before the satellite is launched; therefore, when the receiver gain estimate is inaccurate, it will cause the backscatter coefficient to produce a measurement deviation, which needs to be corrected through passive calibration technology.

[0073] Backscatter coefficient Usually expressed in dB, so its measurement deviation or calibration factor Defined as the measurement of the calibrated satellite Compared with the actual measured value The difference between the two is calculated using the following formula: (4) in, Indicates calibration factor or measurement deviation, Indicates the measurement value of the calibrated satellite, express Accurate calibration value (true value); It can be obtained through simulation calculation or actual measurement.

[0074] Preferably, in step 130, determining a calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before launch includes using the formula: (5) Determine the calibration coefficient of the observed backscatter coefficient of the target radar altimeter; wherein, represents the calibration coefficient of the observed backscatter coefficient of the target radar altimeter, represents the operating system gain of the receiver in the target radar altimeter, Represents the measurement system gain of the ground measurement receiver of the target radar altimeter before launch.

[0075] Furthermore, the backscatter coefficient of the target radar altimeter can be accurately calibrated by using the calibration coefficient of the observed backscatter coefficient of the target radar altimeter determined by formula (5).

[0076] In summary, the passive calibration method of the backscatter coefficient of a radar altimeter provided by the present invention achieves the following beneficial effects: (1) The required typical (target) ground object radiation characteristic model was constructed. Based on the accuracy, uncertainty, stability and frequency sensitivity analysis of the existing mature models, this project established a database of radiation models and their supporting data suitable for passive calibration of the backscatter coefficient of the preset radar altimeter, as well as a database of atmospheric radiation transmission models and their supporting data. At the same time, a frequency scale correction function based on the preset altimeter frequency was developed. This technical idea can greatly reduce the time and cost of passive calibration model development.

[0077] (2) Receiver gain is one of the input parameters of the radar equation for calculating the backscatter coefficient of the radar altimeter, and its value can only be determined in a ground laboratory before the satellite is launched. Due to the technical limitations of ground measurements and possible performance degradation on the satellite, performance monitoring and deviation evaluation after orbit are necessary and sufficient. The passive calibration method of the present invention is based on typical ground objects. It can establish a correlation between the radiation of typical ground object targets and the receiver gain, and then determine the actual operating system gain of the receiver, obtain the calculated deviation of the receiver gain, and improve the refined calibration of the backscatter coefficient. It is also very helpful for the design and optimization of subsequent satellite payloads.

[0078] Therefore, the passive calibration method of the backscatter coefficient of the radar altimeter provided by the present invention can be used to identify the observation error caused by the radar altimeter payload receiver. Different from the existing "end-to-end" calibration mode for the entire observation system, it has a more refined and quantitative remote sensing payload calibration capability, and can also assist in the optimization and improvement of subsequent payload design, and promote the improvement of the remote sensing payload measurement technology. Moreover, it does not require the use of expensive active calibrator specific equipment, and can achieve the acquisition and correction of the radar altimeter backscatter coefficient deviation, which has the advantages of simplicity, ease of use and low cost. The problem of low degree of refinement and high cost of the backscatter coefficient calibration of the radar altimeter in the prior art is solved.

[0079] In a second aspect, the present invention provides a passive calibration device for a radar altimeter backscatter coefficient, see Figure 4 , Figure 4 A schematic structural diagram of a passive calibration device for backscatter coefficient of a radar altimeter provided by the present invention.

[0080] exist Figure 4 In the embodiment, the device may include: Data acquisition module 410, data acquisition module 410 is used to obtain target observation data of the target radar altimeter in the noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter; the target observation data is the data obtained by the target radar altimeter observing the target ground object calibration area in the noise monitoring mode.

[0081] The system gain determination module 420 is used to determine the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data.

[0082] The calibration coefficient determination module 430 is used to determine the calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before transmission.

[0083] The scattering coefficient calibration module 440 is used to calibrate the backscattering coefficient of the target radar altimeter based on the calibration coefficient of the observed backscattering coefficient of the target radar altimeter.

[0084] Based on this, the present invention provides a passive calibration device for the backscatter coefficient of a radar altimeter. First, a data acquisition module 410 is used to acquire target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter; the target observation data is data obtained by observing a calibration area of ​​a target object by the target radar altimeter in a noise monitoring mode; then a system gain determination module 420 is used to determine an operating system gain of a receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; further, a calibration coefficient determination module 430 is used to determine a calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and a measurement system gain of a ground measurement receiver of the target radar altimeter before launch; finally, a scattering coefficient calibration module 440 is used to calibrate the backscatter coefficient of the target radar altimeter based on the calibration coefficient of the observed backscatter coefficient of the target radar altimeter; and refined calibration of the backscatter coefficient is achieved at a relatively low cost.

[0085] Although the present invention is described herein in conjunction with various embodiments, in the process of implementing the claimed invention, those skilled in the art may understand and implement other variations of the disclosed embodiments by viewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "one" or "an" does not exclude multiple situations. A single processor or other unit may implement several functions listed in a claim. Certain measures are recorded in different dependent claims, but this does not mean that these measures cannot be combined to produce good results.

[0086] Although the present invention has been described in conjunction with specific features and embodiments thereof, it is apparent that various modifications and combinations may be made thereto without departing from the spirit and scope of the present invention. Accordingly, this specification and the accompanying drawings are merely exemplary illustrations of the present invention as defined by the appended claims and are deemed to cover any and all modifications, variations, combinations or equivalents within the scope of the present invention. Obviously, those skilled in the art may make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, the present invention is intended to include such modifications and variations if they fall within the scope of the claims of the present invention and their equivalents.

Claims

1. A passive calibration method for backscatter coefficient of a radar altimeter, characterized in that: include: Obtaining target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter; The target observation data is data obtained by observing the target object calibration area by the target radar altimeter in the noise monitoring mode; Determining the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; Determining a calibration coefficient of an observed backscatter coefficient of the target radar altimeter based on the operating system gain and a measurement system gain of a ground measurement receiver of the target radar altimeter before launch; The backscatter coefficient of the target radar altimeter is calibrated based on the calibration coefficient of the observed backscatter coefficient of the target radar altimeter.

2. The passive calibration method of the radar altimeter backscatter coefficient as claimed in claim 1, characterized in that: The step of determining the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data comprises: Preprocessing the target observation data and the antenna main lobe radiation brightness temperature simulation data respectively to obtain the average output power of the receiver in the target radar altimeter and the average antenna main lobe radiation brightness temperature at the antenna aperture of the target radar altimeter; An operating system gain of a receiver in the target radar altimeter is determined based on the average output power and the average antenna main lobe radiation brightness temperature.

3. The passive calibration method of the radar altimeter backscatter coefficient as claimed in claim 2, characterized in that: Determining the operating system gain of the receiver in the target radar altimeter based on the average output power and the average antenna main lobe radiation brightness temperature includes using the formula: ； The operating system gain of the receiver in the target radar altimeter is calculated; wherein, represents the operating system gain of the receiver in the target radar altimeter, Indicates the average output power, represents the average antenna main lobe radiation brightness temperature, is the setting value of the altimeter automatic gain control AGC, is the main lobe efficiency, The radiation efficiency of the antenna. is the receiver signal bandwidth and k is the Boltzmann constant.

4. The passive calibration method of the radar altimeter backscatter coefficient as claimed in claim 1, characterized in that: The step of obtaining target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter comprises: Using a preset surface radiation brightness temperature model to simulate and calculate the environmental parameters of the target object calibration area, and obtain radiation brightness temperature simulation data of the surface of the target object calibration area; The preset atmospheric radiation transmission model is used to simulate and calculate the radiation brightness temperature simulation data of the surface of the target object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter.

5. The passive calibration method of the radar altimeter backscatter coefficient as claimed in claim 4, characterized in that: The method of using a preset surface radiation brightness temperature model to simulate and calculate the environmental parameters of the target object calibration area to obtain radiation brightness temperature simulation data of the surface of the target object calibration area includes: Constructing the preset surface radiation brightness temperature model; The step of constructing the preset surface radiation brightness temperature model comprises: Obtaining the basic surface brightness temperature observation data of the target object calibration area collected by a preset satellite; the preset satellite is a mature and stably operating satellite; Using multiple radiation emissivity models based on ground objects, the environmental parameters of the target ground object calibration area are simulated and calculated to obtain multiple surface brightness temperature simulation data; the multiple radiation emissivity models include at least a physical model, a semi-empirical model and an empirical model; Based on the basic surface brightness temperature observation data and the plurality of surface brightness temperature simulation data, the error accuracy of the plurality of radiation emissivity models is determined, and the radiation emissivity model whose error accuracy is less than a first preset threshold is determined as the preset surface radiation brightness temperature model.

6. The passive calibration method of the backscatter coefficient of the radar altimeter according to claim 5, characterized in that: The step of constructing the preset surface radiation brightness temperature model comprises: Acquiring brightness temperature observation data of an airborne microwave radiometer having the same frequency as the target radar altimeter; Based on the brightness temperature observation data of the airborne microwave radiometer and in combination with the initial frequency of the preset surface radiation brightness temperature model, a first frequency scale correction coefficient between the initial frequency of the preset surface radiation brightness temperature model and the target radar altimeter frequency is established; the initial frequency is an applicable working frequency of the preset surface radiation brightness temperature model itself; Based on the first frequency scale correction coefficient, the surface radiation brightness temperature at the initial frequency of the preset surface radiation brightness temperature model is corrected to obtain the surface radiation brightness temperature at the working frequency condition of the target radar altimeter.

7. The passive calibration method of the backscatter coefficient of the radar altimeter according to claim 4, characterized in that: The method of using a preset atmospheric radiation transmission model to simulate and calculate the radiation brightness temperature simulation data of the surface of the target object calibration area to obtain the antenna main lobe radiation brightness temperature simulation data at the antenna aperture of the target radar altimeter includes: Constructing the preset atmospheric radiation transfer model; The step of constructing the preset atmospheric radiation transfer model comprises: Obtain station observation data of atmospheric temperature, humidity and pressure sounding; Based on the station observation data, according to the radiation simulation strategy of the preset error disturbance, the error sensitivity of multiple atmospheric radiation transfer models is simulated to obtain the standard deviation of the simulation accuracy error of the multiple atmospheric radiation transfer models under different atmospheric environments; An atmospheric radiation transfer model whose standard deviation of the simulation accuracy error is less than a second preset threshold is used as the preset atmospheric radiation transfer model.

8. The passive calibration method of the backscatter coefficient of the radar altimeter according to claim 7, characterized in that: The step of constructing the preset atmospheric radiation transfer model comprises: Acquiring brightness temperature observation data of an airborne microwave radiometer having the same frequency as the target radar altimeter; Based on the brightness temperature observation data of the airborne microwave radiometer and in combination with the initial frequency of the preset atmospheric radiation transmission model, a second frequency scale correction coefficient between the initial frequency of the preset atmospheric radiation transmission model and the target radar altimeter frequency is established; Based on the second frequency scale correction coefficient, the radiation transfer process of the preset atmospheric radiation transfer model is corrected to achieve accurate simulation of the atmospheric radiation transfer process under the working frequency conditions of the target radar altimeter, and the radiation brightness temperature of the surface radiation transmitted to the antenna aperture of the target radar altimeter through atmospheric radiation is obtained.

9. The passive calibration method of the backscatter coefficient of a radar altimeter according to claim 1, characterized in that: The step of obtaining target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter comprises: Determining the target object calibration area; The step of determining the target object calibration area includes: Acquire historical data of surface brightness temperature observed by a target satellite microwave radiometer; the target satellite microwave radiometer is a satellite microwave radiometer that operates stably and has an operating frequency similar to that of the target radar altimeter; Determine a plurality of restriction conditions for delimiting the target ground object calibration area, wherein the plurality of restriction conditions at least include that the target ground object calibration area and the measurement swath of the target radar altimeter have an overlapping area, the target ground object calibration area includes at least a plurality of ground object types, the target ground object calibration area has the conditions for constructing surface radiation brightness temperature models of the plurality of ground object types, the target ground object calibration area has the conditions for constructing an atmospheric radiation transmission model, and the target ground object calibration area covers or is close to an on-site measurement station with operational stable observation capability; Based on the historical data and the plurality of restriction conditions, the target object calibration area is determined.

10. A passive calibration device for backscatter coefficient of a radar altimeter, characterized in that: include: A data acquisition module, the data acquisition module is used to acquire target observation data of a target radar altimeter in a noise monitoring mode and antenna main lobe radiation brightness temperature simulation data at an antenna aperture of the target radar altimeter; the target observation data is data obtained by observing a target ground object calibration area by the target radar altimeter in a noise monitoring mode; A system gain determination module, the system gain determination module is used to determine the operating system gain of the receiver in the target radar altimeter based on the target observation data and the antenna main lobe radiation brightness temperature simulation data; A calibration coefficient determination module, the calibration coefficient determination module is used to determine the calibration coefficient of the observed backscatter coefficient of the target radar altimeter based on the operating system gain and the measurement system gain of the ground measurement receiver of the target radar altimeter before launch; A scattering coefficient calibration module is used to calibrate the backscattering coefficient of the target radar altimeter based on the calibration coefficient of the observed backscattering coefficient of the target radar altimeter.

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