Microwave water vapor absorption channel cross calibration method based on lunar observation

Abstract

The application discloses a microwave water vapor absorption channel cross-calibration method based on moon observation, and the method comprises the following steps: according to the orbit prediction data of the moon entering the cold space observation area of a microwave radiometer, cold space observation data containing moon influence of five channels near the atmospheric detection water vapor absorption line are extracted from a frame of observation data downloaded from the satellite-borne microwave radiometer; the cold space observation data of a reference channel without moon influence is obtained by using the steady-state constraint principle and the cold space observation data of the preselected reference channel, and the calibration coefficient of the reference channel is calculated; the cold space observation radiation of the reference channel containing moon influence is obtained according to the calibration; the cold space observation radiation of the reference channel containing moon influence is used for calibration of other channels based on the transmission relationship of the cold space observation radiation of the five channels, so that the cross-calibration between the channels is realized, and the calibration coefficients of the channels are calculated; and the target brightness temperature of the five channels for the earth observation near the water vapor absorption line is calculated according to the calibration coefficients of the channels.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of radiometric calibration of spaceborne microwave radiometer, in particular to a microwave water vapor absorption channel cross calibration method based on moon observation, and more particularly to a cross calibration technology between 183.31 GHz frequency band atmospheric detection water vapor absorption channels of spaceborne microwave radiometer. BACKGROUND

[0002] The spaceborne millimeter wave radiometer radiance data can be directly assimilated and applied to numerical weather prediction model. The European Center for Medium-Range Weather Forecasts (ECMWF) has extended the prediction time by 4 days through the assimilation of satellite data, in which the radiance data of spaceborne millimeter wave atmospheric detection payload contributes the most, and the assimilation contribution rate of only one AMSU-A instrument is as high as 17%. However, the numerical weather prediction assimilation system has a very high requirement for the accuracy of satellite observation data. Under normal circumstances, the millimeter wave atmospheric temperature detection data (50-60 GHz) is corrected by observation-background field (O-B) bias, and the root mean square error can reach 0.2 K, which produces a significant positive effect in the numerical prediction model. However, for the Fengyun-3 microwave humidity detector and other detection payloads arranged near the high-frequency water vapor absorption line (183.31 GHz), due to the uncertainty of the radiometer in-orbit radiometric calibration, and the spatial and temporal variability of the atmospheric water vapor field is more complex than the temperature field, the bias before correction often exceeds 5.0 K, and even after correction, the data bias is still greater than 1.5 K, which has become the focus of the application of millimeter wave water vapor absorption channel detection data assimilation.

[0003] Due to the limitation of the technical level of the payload, the nonlinearity of the domestic satellite microwave payload is serious. The peak nonlinearity of the SNPP / ATMS atmospheric humidity detection channel in the United States is 0.4 K, and the nonlinearity of the FY-3D / MWHTS millimeter wave atmospheric detection payload with the same design parameters as SNPP / ATMS to be put into business use in 2017 is more than 3.4 K.

[0004] The existing technology for radiometric calibration of spaceborne microwave radiometer is mainly based on laboratory measurement technology before satellite launch or on-orbit cross calibration technology. Figure 1 A flowchart of an on-orbit calibration method, according to the telemetry data and remote sensing data transmitted by the spaceborne microwave radiometer in on-orbit operation, judges whether the working state of the instrument is normal and the observed data is valid; the brightness temperature value generated by the forward simulation and the brightness temperature value of the microwave radiometer of the same type abroad are used to determine the on-orbit calibration accuracy of the microwave radiometer instrument.

[0005] The calibration of the spaceborne microwave remote sensing instrument mainly includes the radiation calibration in the ground vacuum tank before launch and the on-orbit calibration after launch. The ground calibration before launch provides basic parameters for the on-orbit calibration of the instrument, and determines the main technical indexes such as the calibration accuracy and sensitivity of the instrument under the vacuum condition. However, there is a deviation between the experimental data measured by each instrument component based on the thermal vacuum calibration and the measured data of the instrument in the on-orbit operation. The difference in the uncertainty of the inter-channel radiation calibration of the water vapor absorption channel affects the assimilation application of the data.

[0006] The window channel of the spaceborne microwave radiometer is less affected by the atmosphere, and the relatively stable earth target can be used to calibrate the relative deviation of the inter-channel radiation calibration, but the absorption channel cannot use the stable earth target to realize the relative calibration between channels. SUMMARY

[0007] The purpose of the present application is to overcome the defects of the prior art, and a microwave water vapor absorption channel cross calibration method based on moon observation is proposed.

[0008] In order to achieve the above purpose, the present application provides a microwave water vapor absorption channel cross calibration method based on moon observation, which comprises:

[0009] According to the orbit prediction data of the moon entering the cold air observation area of the microwave radiometer, the cold air observation data of the moon influence in the five channels near the atmospheric detection water vapor absorption line are extracted from a frame of observation data downloaded by the spaceborne microwave radiometer;

[0010] For the reference channel selected in advance from the five channels, the cold air observation data of the reference channel are used to obtain the cold air observation data of the reference channel without the influence of the moon by using the steady-state constraint principle, and the calibration coefficient of the reference channel is calculated;

[0011] Based on the calibration coefficient of the reference channel, the cold air observation radiation of the reference channel containing the moon influence is calibrated;

[0012] Based on the transfer relationship of the cold air observation radiation of the five channels, the cold air observation radiation of the reference channel containing the moon influence is used for calibration of other channels, so as to realize the cross calibration between channels, and the calibration coefficient of each channel is calculated;

[0013] The target brightness temperature of the five channels near the water vapor absorption line for the earth observation is calculated by the calibration coefficient of each channel.

[0014] As an improvement of the above method, the five channels near the atmospheric detection water vapor absorption line specifically include: the center frequencies are 183±1GHz, 183±1.8GHz, 183±3GHz, 183±4.5GHz and 183±7GHz.

[0015] As an improvement of the above method, the method further comprises a step of selecting a reference channel, specifically comprising:

[0016] Before the satellite is launched, the non-linear factors obtained by the vacuum calibration test of the microwave radiometer are compared and analyzed, and the on-orbit cold space, hot source, instrument temperature, and on-orbit sensitivity and gain state parameters of the channel are combined and analyzed comprehensively, so that a channel with high calibration accuracy and stable on-orbit performance is selected from the five channels as the reference channel.

[0017] As an improvement of the above method, for the reference channel selected in advance from the five channels, the cold space observation data of the reference channel without the influence of the moon is obtained by using the steady-state constraint principle, and the calibration coefficient of the reference channel is calculated; specifically comprising:

[0018] According to the orbit prediction data, the cold space observation data of the reference channel near the current observation data containing the influence of the moon is extracted, and the cold space observation data without the influence of the moon equivalent to the current frame is extrapolated, which is matched with 2.73K, and combined with the hot source observation data and the hot source temperature telemetry data, the calibration coefficient of the reference channel is calculated.

[0019] As an improvement of the above method, the calibration coefficient of the reference channel comprises the intercept a0 and the slope coefficient b0 of the calibration equation of the reference channel, and satisfies the following formula:

[0020]

[0021]

[0022] R w，0 =f plank (T w +dT w，0 )

[0023] R sp，0 =f plank (T sp +dT sp，0 )

[0024] Wherein, C w，0 is the hot source observation count value of the reference channel, C sp，0 is the cold space observation count value of the reference channel, R w，0 is the hot source radiation of the reference channel; R sp，0 is the cold space observation radiation of the reference channel, f plank () represents the Planck function, T w is the on-board hot source temperature obtained by telemetry, T sp is the cold space temperature, dT w，0 , dT sp，0The first and second terms are the cold source bandwidth correction terms of the reference channel and the non-reference channel, respectively, and are calculated according to pre-launch simulation.

[0025] As an improvement of the above method, the cross-calibration between channels is realized by using the cold space observation radiation of the reference channel containing the moon influence to calibrate the other channels based on the transfer relationship of the cold space observation radiation of the five channels, and the calibration coefficients of the channels are calculated.

[0026] According to the transfer relationship of the cold space observation radiation of the five channels, the cold space observation radiation T sp，moon，0 of the reference channel containing the moon influence is used to calculate the cold space observation radiation R sp，ch of the non-reference channel.

[0027] R sp，ch = f plank (T sp，moon，0 +dT sp，ch )

[0028] Wherein, f plank () represents the Planck function, ch = 1, 2,..., 4, and dT sp，ch is the cold source bandwidth correction term of the chth non-reference channel, which is calculated according to pre-launch simulation.

[0029] The thermal source radiation R w，ch of the chth non-reference channel is obtained according to the following formula:

[0030] R w，ch = f plank (T w +dT w，ch )

[0031] Wherein, T w is the on-board thermal source temperature, which is obtained by telemetry; and dT w，ch is the thermal source bandwidth correction term of the chth non-reference channel, which is calculated according to pre-launch simulation.

[0032] The intercept a ch and the slope term coefficient b ch of the chth non-reference channel calibration equation are obtained according to the following formula:

[0033]

[0034]

[0035] Wherein, C w，ch is the thermal source observation count value of the chth non-reference channel, and C sp，ch is the cold space observation count value of the chth non-reference channel.

[0036] Compared with the prior art, the application has the advantages that:

[0037] 1、The method can effectively reduce the relative radiation deviation between channels and improve the application effect of remote sensing data;

[0038] 2、The method uses an extraterrestrial target as a medium to complete the relative radiation calibration between atmospheric water vapor absorption channels;

[0039] 3、For the characteristics of the water vapor absorption channel of the satellite-borne microwave atmospheric probe, the application provides a cross-calibration method between channels based on lunar observation data on orbit, and the radiation transfer relationship of each channel is established based on the reference channel, so that the uncertainty of the radiation calibration result of the water vapor absorption channel near 183GHz tends to be consistent, and the data use efficiency and effect of the data in the numerical weather prediction assimilation application are improved. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is the on-orbit calibration process of the prior art;

[0041] Figure 2 is a flowchart of the microwave water vapor absorption channel cross-calibration method based on lunar observation of the application;

[0042] Figure 3 is a schematic diagram of the moon entering the cold air observation domain;

[0043] Figure 4 is the change of the cold air observation count value after the moon enters the cold air observation domain;

[0044] Figure 5 is the calculation of the cold air observation count value under the steady-state constraint condition. DETAILED DESCRIPTION

[0045] According to the application experience of the on-orbit radiation calibration of the microwave atmospheric probe load of the Fengyun 3A / B / C three satellites, around the core technical problem of on-orbit radiation calibration which has plagued the quantitative application of satellite-borne millimeter wave atmospheric humidity detection data, the application proposes to use the characteristics that the on-orbit observation of the moon by the satellite-borne millimeter wave radiometer does not have the influence of atmospheric water vapor, and to develop the relative radiation calibration technology between on-orbit millimeter wave water vapor absorption channels through the on-orbit lunar observation data of the Fengyun satellite millimeter wave atmospheric humidity detector.

[0046] The technical solutions of the application will be described in detail below with reference to the drawings and examples.

[0047] Example 1

[0048] As Figure 2 shown, example 1 of the application provides a microwave water vapor absorption channel cross-calibration method based on lunar observation.

[0049] 1. Orbit prediction technology for moon entering cold space of microwave radiometer

[0050] Based on the Fengyun-3 ground operation system of the National Satellite Meteorological Center, the existing moon orbit prediction operation algorithm is used, the microwave wet and temperature detection instrument of the Fengyun-3C satellite is taken as the benchmark instrument coordinate system, and the research contents such as instrument installation matrix on-orbit calibration, satellite attitude compensation, and scanning drag parameterization are focused on, so as to accurately calculate the time and relative position of the moon appearing in the instantaneous field of view (FOV) of the instrument cold space, and extract the cold space observation data containing the moon information from the historical and real-time data, and establish the moon observation data set.

[0051] 2. Reference channel screening and radiation fine calibration technology

[0052] The center frequency points of the water vapor absorption channels of the microwave atmospheric detection load of the Fengyun-3 satellite are set as shown in Table 1, which is consistent with the channel setting of the meteorological satellites in the world at present. When the relative calibration is carried out based on the data of the moon entering the cold space, the channel with the best linearity and stability is selected as the reference channel, and then the relative calibration of other channels to the reference channel is completed according to the moon observation brightness temperature. First, the nonlinearity factor of all channels is analyzed and calculated through the vacuum test data before the satellite is launched, and then the optimal reference channel is determined by combining the on-orbit sensitivity and gain change of the cold space, heat source and channel with the response relationship of the engineering telemetry data, establishing a channel quality composite analysis model. After the reference channel is determined, the radiation at the entrance pupil is equal to that of other absorption channels with the same center frequency, and after the coupling correction of the antenna feed system, the moon radiation measurement value of each channel transmitted by the reference channel is obtained.

[0053] Table 1 Atmospheric water vapor detection channel parameters

[0054]

[0055] 3. Reference channel radiation calculation technology

[0056] The reference channel radiation calculation includes two parts of calibration coefficient calculation and target brightness temperature calculation. The moon only affects the cold space observation data in the calibration of the cold space, and has no effect on the heat source data and the ground observation data. Therefore, when determining the scan line calibration coefficient, only the cold space observation data without the influence of the moon is needed. At this time, through the steady-state data constraint, the cold space observation data without the influence of the moon can be obtained, the calibration coefficient is calculated, and then the target brightness temperature of the ground-atmosphere system is calculated according to the business calibration process. The calibration calculation formula is as follows:

[0057] T ch =T 0，ch +ΔT ch

[0058]

[0059]

[0060] R0=a ch (T inst )×C+b ch (T inst )

[0061] where T ch , T 0，ch , ΔT ch are channel brightness temperature, channel linear brightness temperature, channel non-linear brightness temperature correction respectively.

[0062] e n，ch , n = 0, 1, 2, ch = 1,... 5 are channel non-linear brightness temperature correction coefficients; T int is instrument temperature; R0is channel linear calibration radiance value, a ch , b ch are channel calibration coefficients, C is channel observed count value; is the inverse function of Planck function.

[0063] Calibration coefficient calculation formula is as follows:

[0064]

[0065]

[0066] R w，ch = f plank (T w +dT w，ch )

[0067] R sp，ch = f plank (T sp +dT sp，ch )

[0068] where T w is on-board heat source temperature, obtained by telemetry; C w，ch is heat source observed count value (5 per frame); T sp is cold space temperature, generally 2.73K, cold space temperature when the moon has an impact is no longer 2.73K and needs to be calculated by modeling the reference channel and then cross-transferred to other channels; C sp，ch is cold space observed count value (3 per frame); dT w，ch , dT sp，ch are cold and hot source bandwidth correction terms, calculated according to pre-launch simulation; R w，ch is heat source radiation; R sp，ch is cold space radiation.

[0069] Thus, the reference channel calibration coefficient includes the intercept a0 and the slope coefficient b0 of the reference channel calibration equation, and satisfies the following formula:

[0070]

[0071]

[0072] R w，0 =f plank (T w +dT w，0 )

[0073] R sp，0 =f plank (T sp +dT sp，0 )

[0074] Wherein, C w，0 is the heat source observation count value of the reference channel, C sp，0 is the cold space observation count value of the reference channel, R w，0 is the heat source radiation of the reference channel; R sp，0 is the cold space observation radiation of the reference channel, f plank () represents the Planck function, T w is the temperature of the heat source on the satellite, which is obtained by telemetry, T sp is the cold space temperature, dT w，0 and dT sp，0 are the heat source and cold source bandwidth correction terms of the reference channel, respectively, which are calculated according to pre-launch simulation.

[0075] Based on the transfer relationship of the cold space observation radiation of the five channels, the cold space observation radiation R sp，moon，0 of the ch-th non-reference channel is obtained according to the cold space observation radiation T sp，ch of the reference channel containing the influence of the moon as follows:

[0076] R sp，ch =f plank (T sp，moon，0 +dT sp，ch )

[0077] Wherein, f plank () represents the Planck function, ch=1, 2,..., 4, dT sp，ch is the cold source bandwidth correction term of the ch-th non-reference channel, which is calculated according to pre-launch simulation;

[0078] The heat source radiation R w，ch of the ch-th non-reference channel is obtained according to the following formula:

[0079] R w，ch =f plank (Tw + dT w，ch )

[0080] where T w is the temperature of the on-board heat source, obtained by telemetry; dT w，ch is the heat source bandwidth correction term of the ch-th non-reference channel, calculated according to pre-launch simulation;

[0081] The intercept a ch and the slope term coefficient b ch of the ch-th non-reference channel calibration equation are obtained according to the following formula:

[0082]

[0083]

[0084] where C w，ch is the heat source observation count value of the ch-th non-reference channel, and C sp，ch is the cold space observation count value of the ch-th non-reference channel

[0085] 4. Radiometric calibration between water vapor absorption channels

[0086] Radiometric calibration is to determine the function relationship of converting the observation count value of the water vapor absorption channel of the spaceborne microwave radiometer into the target radiance brightness temperature. The contemporary international millimeter wave atmospheric detection load is realized in-orbit radiometric calibration by observing the cosmic background (2.72548 K) and the internal black body (280 K), but the two points cannot solve the non-linear calibration. The spectral hemispheric radiation of the moon is constant (Kieffer, 1997), the positional relationship among the sun, the satellite and the moon, and the factors such as libration caused by the 6-7 degree intersection angle between the moon's orbital eccentricity, the rotation axis and the orbital plane normal of the earth rotation determine the difference of the moon radiation flux reaching the entrance pupil of the radiometer antenna. In the extreme case, there is no 2.72548 K cold space from the moon, and the strongest moon (the surface temperature can be as high as 400 K) of 0.5° field of view angle completely enters the cold space observation domain 1.1° beam width, and the antenna main beam with more than 95% main beam efficiency, and the maximum moon radiation measurement value can be close to 200 K, which is more than 150 K. After coupling correction, the radiation of the reference channel is transferred to other channels, so that the matching data of the channel moon observation count value and the moon radiation amount are obtained, and the response function relationship from the count value to the radiation amount is obtained according to the statistical analysis of the matching data, and the radiometric calibration based on the moon observation is realized, but at this time the dynamic range of the data is concentrated in the low end below 200 K.

[0087] Technical effects

[0088] 1. Reduce the relative deviation of inter-channel radiation

[0089] Through steps 1-4, a set of 5 water vapor absorption channels near 183 GHz can obtain the calibration coefficients. Since the channel radiation calibration coefficients are obtained based on the reference channel, the radiation deviation between channels is greatly reduced, and the O-B can be used to monitor and analyze.

[0090] 2. Improve the effectiveness of water vapor absorption channel assimilation data to improve assimilation effect

[0091] The reduction of the radiation deviation between channels improves the efficiency and effect of the 183 GHz water vapor absorption channel assimilation application. More data can be controlled by quality control and entered into the numerical prediction system, improving the assimilation data volume and improving the assimilation effect.

[0092] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the examples, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application do not deviate from the spirit and scope of the present application, and they should be covered in the scope of the claims of the present application.

Claims

1. A cross-calibration method for microwave water vapor absorption channels based on lunar observations, the method comprising: Based on the lunar orbit prediction data of the Moon entering the cold space observation area of ​​the microwave radiometer, cold space observation data containing lunar influence were extracted from a frame of observation data transmitted from the spaceborne microwave radiometer in five channels near the atmospheric sounding water vapor absorption line. For a pre-selected reference channel from five channels, using the cold-space observation data of the reference channel, the cold-space observation data of the reference channel without lunar influence is obtained through the steady-state constraint principle, and the calibration coefficient of the reference channel is calculated. Specifically, this includes: extracting the cold-space observation data of the reference channel with lunar influence near the observation data of this frame based on the orbit prediction data, extrapolating to obtain the equivalent cold-space observation data without lunar influence of this frame, matching it with 2.73K, and combining the heat source observation data and heat source temperature telemetry data to calculate the calibration coefficient of the reference channel. Based on the calibration coefficients of this reference channel, the cold space observation radiation containing lunar influences in the reference channel was calibrated. Based on the transmission relationship of cold-space observation radiation from the five channels, the cold-space observation radiation from the reference channel containing lunar influence is used for calibration of other channels, thereby achieving cross-calibration between channels and calculating the calibration coefficients of each channel. The target brightness temperature of the Earth observations near the water vapor absorption line was calculated using the calibration coefficients of each channel.

2. The cross-calibration method for microwave water vapor absorption channels based on lunar observations according to claim 1, characterized in that, The five channels near the atmospheric sounding water vapor absorption line specifically include those with center frequencies of 183±1GHz, 183±1.8GHz, 183±3GHz, 183±4.5GHz, and 183±7GHz.

3. The cross-calibration method for microwave water vapor absorption channels based on lunar observations according to claim 2, characterized in that, The method also includes a reference channel selection step, specifically including: Before satellite launch, the nonlinear factors obtained from the vacuum calibration test of the microwave radiometer were compared and analyzed. Combined with the on-orbit cold air, heat source, instrument temperature, and on-orbit sensitivity and gain status parameters of the channel, the channel with high calibration accuracy and stable on-orbit performance was selected as the reference channel from the five channels after comprehensive analysis.

4. The cross-calibration method for microwave water vapor absorption channels based on lunar observations according to claim 3, characterized in that, The reference channel calibration coefficients include the intercept of the reference channel calibration equation. and slope coefficient The following equation is satisfied: ， ， ， ， in, The heat source observation count value of the reference channel, The cold air observation count value for the reference channel. Heat source radiation for the reference channel; Cold air observation radiation for the reference channel, Denotes the Planck function. The temperature of the onboard heat source is obtained through telemetry. The temperature of the cold air. These are the bandwidth correction terms for the heat source and cold source of the reference channel, respectively, obtained from pre-launch simulation calculations.

5. The cross-calibration method for microwave water vapor absorption channels based on lunar observations according to claim 4, characterized in that, The transmission relationship of cold-space observation radiation based on five channels involves using the cold-space observation radiation from the reference channel, which includes lunar influence, for calibration of other channels, thereby achieving cross-calibration between channels and calculating the calibration coefficients for each channel; specifically including: Based on the transmission relationship of cold-space observation radiation from five channels, and according to the cold-space observation radiation from the baseline channel including lunar influence... Get the first Cold air observation radiation in one non-reference channel for: ， in, Denotes the Planck function. , For the first ch The cold source bandwidth correction term for each non-reference channel was obtained based on pre-launch simulation calculations. The following formula yields the... ch Heat source radiation from each non-reference channel for: ， in, The temperature of the onboard heat source is obtained through telemetry. For the first ch The thermal source bandwidth correction term for each non-reference channel is obtained based on pre-launch simulation calculations. The following formula yields the... ch Intercept of each non-reference channel calibration equation and slope coefficient : ， ， in, For the first ch Heat source observation counts for each non-reference channel, For the first ch Cold air observation counts for each non-baseline channel.

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