Method for determining the nonlinear coefficient of on-orbit calibration of spaceborne microwave radiometer before launch

Abstract

The present invention discloses a method for determining an on-orbit calibration nonlinear coefficient of a satellite-borne microwave radiometer before launch. The method comprises: sequentially observing three calibration blackbodies by the satellite-borne microwave radiometer to be measured within a scanning cycle to obtain observation voltages and brightness temperatures corresponding to three different temperatures; the three calibration blackbodies comprise: a cold calibration blackbody at the boiling point of liquid nitrogen, a hot calibration blackbody at ambient temperature, and a calibration blackbody at an intermediate temperature between the cold and ambient temperatures; obtaining a secondary calibration curve from the observation voltages and brightness temperatures at the three different temperatures; determining virtual cold space data from the secondary calibration curve through virtual cold space observation; if observation data at an intermediate value between the virtual cold space data and the hot calibration blackbody data can be determined from the virtual cold space data and the hot calibration blackbody data, calculating the nonlinear coefficient from the virtual cold space data and the hot calibration blackbody data; otherwise, calculating the nonlinear coefficient from the virtual cold space data, the hot calibration blackbody data, and the intermediate temperature blackbody data.

Description

Technical Field

[0001] The present invention relates to the field of satellite-borne microwave radiometer calibration, and in particular to a method for determining an on-orbit calibration nonlinear coefficient of a satellite-borne microwave radiometer before launch. Background Art

[0002] Spaceborne microwave radiometers (herein referred to as radiometers) are highly sensitive receivers used to measure natural microwave radiation from the Earth's surface and atmosphere. Radiometer calibration is crucial for quantitatively applying the measured data. An ideal radiometer is a linear system, so two-point calibration can be used to obtain this linear equation, which in turn calibrates the observed data (voltage data or voltage counts). However, the response characteristics of a practical radiometer system are unlikely to be perfectly linear; nonlinear components inevitably exist. This causes the actual measured radiation (usually expressed as brightness temperature or radiance) to deviate from the value corresponding to the linear equation. This offset is often referred to as the system's nonlinear error, expressed in radiometric units.

[0003] Real-time determination of the system's nonlinear error requires at least three independent calibration references within the radiometer's dynamic range. Using these three points to generate a calibration equation, the observed scene or target data is calibrated to obtain its radiation level. However, obtaining three independent calibration references is difficult for a radiometer on orbit, making it difficult to measure the system's nonlinearity in real time. On-orbit nonlinearity correction typically involves using a lookup table of nonlinear coefficients at different instrument temperatures, or a function of the nonlinear coefficients as they change with instrument temperature, obtained during pre-launch calibration testing. This is combined with the actual operating temperature of the on-orbit instrument to determine the corresponding nonlinear coefficients through table lookup or interpolation, thereby correcting for nonlinear errors.

[0004] The existing pre-launch calibration method for determining the nonlinear coefficient involves using a calibration source cooled by liquid nitrogen (cold source) in a vacuum tank, a constant-temperature source (hot source) at the operating environment, and a calibration source that can vary between the cold and hot source temperatures (hereinafter referred to as the variable temperature source) to simulate the radiant brightness temperature of the Earth scene. A two-point calibration is performed by observing the measured voltage and brightness temperature of the cold and hot sources. The simultaneously observed voltage of the variable temperature source is converted to the observed brightness temperature. The nonlinear coefficient at the current temperature is calculated by subtracting the observed brightness temperature from the actual brightness temperature of the variable temperature source.

[0005] The applicability of radiometer nonlinear coefficients before launch has long been a widely discussed, yet difficult, issue to resolve and verify. The nonlinear coefficients depend not only on the system's operating conditions, such as receiver gain and instrument temperature, but also on the temperatures of the calibration sources, such as the cold and hot sources. The radiometer's hot source typically operates at ambient temperature, which is similar to the ambient temperature on orbit and on the ground. However, the cold source used for on-orbit calibration of spaceborne microwave radiometers is the cold background temperature of the universe, approximately 2.7 K. The cold source used for pre-launch calibration is typically controlled at a temperature between 80 and 100 K (due to atmospheric influences, cold air cannot be used directly on the ground before launch). Therefore, the nonlinear coefficients obtained from current pre-launch calibration may not be directly applicable to on-orbit calibration, potentially resulting in errors and uncertainties. Summary of the Invention

[0006] The purpose of the present invention is to overcome the defects of the prior art and propose a method for determining the nonlinear coefficient of the on-orbit calibration of a satellite-borne microwave radiometer before launch. The method can be directly used for the nonlinear coefficient under on-orbit calibration conditions, or for verifying the nonlinear coefficient of the pre-launch calibration currently used.

[0007] To achieve the above object, the present invention proposes a method for determining the nonlinear coefficient of on-orbit calibration of a spaceborne microwave radiometer before launch, the method comprising:

[0008] The satellite-borne microwave radiometer to be measured sequentially observes three calibration blackbodies within a scanning cycle to obtain observation voltages and brightness temperatures corresponding to three different temperatures; the three calibration blackbodies include: a cold calibration blackbody at the boiling point of liquid nitrogen, a hot calibration blackbody at ambient temperature, and an intermediate temperature calibration blackbody at a temperature between the two.

[0009] The secondary calibration curve is obtained from the observed voltage and brightness temperature at three different temperatures;

[0010] The virtual cold air data is determined by the secondary calibration curve through virtual cold air observation;

[0011] If the observation data at the intermediate voltage value between the virtual cold-space data and the thermally calibrated blackbody data can be determined from the virtual cold-space data and the thermally calibrated blackbody data, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated from the virtual cold-space data and the thermally calibrated blackbody data; otherwise, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated from the virtual cold-space data, the thermally calibrated blackbody data and the intermediate temperature blackbody data.

[0012] As an improvement to the above method, the temperatures of the three calibration blackbodies are all stable.

[0013] As an improvement of the above method, the secondary calibration curve is obtained by observing the voltage and brightness temperature of three different temperatures; specifically, the secondary calibration curve is obtained by observing the voltage and brightness temperature (V C ,T C ), corresponding to the observed voltage and brightness temperature of the thermally calibrated blackbody (V H ,T H ), and the observed voltage and brightness temperature (V M ,T M ), and obtain the quadratic calibration curve.

[0014] As an improvement to the above method, the virtual cold air data is determined by the secondary calibration curve through virtual cold air observation; specifically, the method includes:

[0015] The secondary calibration curve is extrapolated or interpolated in real time to obtain the observation voltage corresponding to any brightness temperature within the dynamic range of the satellite-borne microwave radiometer to be measured. According to the cold sky invariance principle, the corresponding virtual cold sky voltage is obtained from the cold sky brightness temperature, thus completing the virtual cold sky observation.

[0016] As an improvement to the above method, the virtual cold-space data and the thermally calibrated blackbody data can be used to determine observation data at a voltage intermediate between the two. Then, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data and the thermally calibrated blackbody data. Specifically, the method includes:

[0017] By the virtual cold air voltage V COS and the observed voltage V of the thermal calibration blackbody H , select the voltage value Thus, the corresponding target brightness temperature T is obtained M1 According to the following formula, the nonlinear coefficient u of the satellite-borne microwave radiometer to be measured when calibrated with cold space at the current instrument temperature is:

[0018]

[0019] Among them, T H is the brightness temperature of the thermally calibrated blackbody, T COS It is the brightness temperature of the frequency corresponding to the cosmic background temperature of 2.7K.

[0020] As an improvement to the above method, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when cold-space calibrated at the current instrument temperature is calculated from the virtual cold-space data, the thermally calibrated blackbody data, and the intermediate-temperature blackbody data; specifically, the method includes:

[0021] By the virtual cold air voltage V COS , the observed voltage V of the intermediate temperature blackbody M and the observed voltage V of the thermal calibration blackbodyH , combined with the corresponding brightness temperature T COS 、T M and T H According to the following formula, the nonlinear coefficient u of the satellite-borne microwave radiometer to be measured when calibrated with cold space at the current instrument temperature is:

[0022]

[0023] Among them, T COS It is the brightness temperature of the frequency corresponding to the cosmic background temperature of 2.7K.

[0024] Compared with the prior art, the advantages of the present invention are:

[0025] Through the calibration method principle of the present invention, the existing calibration data (data generated by previous thermal vacuum calibration) can be reprocessed to obtain the nonlinear coefficient when on orbit, that is, the three-point calibration equation formed by the cold source, variable temperature source and heat source in the same observation period is extrapolated to form a virtual cold space observation, and then the nonlinear coefficient when using the cold space is calculated by the above formula. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a flow chart of a method for determining the nonlinear coefficient of on-orbit calibration of a satellite-borne microwave radiometer before launch. DETAILED DESCRIPTION

[0027] The present invention proposes a method for determining the nonlinear coefficient of on-orbit calibration of a spaceborne microwave radiometer before launch, which includes: the principle and method for determining system nonlinearity based on three-point calibration; the cold-space invariance principle of radiometer calibration and the concept of virtual cold-space observation; and a simple calibration test scheme and data processing method for determining the nonlinear coefficient of the radiometer on-orbit before launch.

[0028] The data processing method of the present invention is: through virtual cold air observation, the virtual cold air voltage of each calibration cycle is determined; using the virtual cold air observation data, combined with the heat source data and another observation data located in the middle observation range, the nonlinear coefficient when using cold air calibration at the current instrument temperature can be calculated.

[0029] Through the calibration method principle of the present invention, the existing calibration data (data generated by previous thermal vacuum calibration) can be reprocessed to obtain the nonlinear coefficient when on orbit, that is, the three-point calibration equation formed by the cold source, variable temperature source and heat source in the same observation period is extrapolated to form a virtual cold space observation, and then the nonlinear coefficient when using the cold space is calculated by the above formula.

[0030] This invention encompasses three key points: 1) a method for determining the nonlinearity of a radiometer system based on three-point calibration; 2) the cold-space invariance principle, virtual observation, and the concepts of virtual cold space for radiometer calibration; and 3) a simplified calibration test scheme and data processing method for determining the nonlinear coefficient of a radiometer on-orbit before launch. The details are discussed below:

[0031] 1) Principle and method of determining the nonlinearity of radiometer system based on three-point calibration

[0032] We directly express the radiometer system response as a quadratic curve, that is, the calibration equation of the radiometer can be expressed as:

[0033]

[0034] Where a, b, c are calibration coefficients. This is an accurate expression of the response characteristics of the radiometer system, which includes nonlinear components. From (1), it can be seen that accurate calibration of the radiometer requires three effective observation points. We assume that the coordinates of these three points are: (V C ,T C ),(V M ,T M ),(V H ,T H ) represent the voltage and brightness temperature at the low, intermediate, and high temperature points of the calibration, respectively. Using these three points, we can get the quadratic term coefficient as:

[0035]

[0036] The other two coefficients b and c can be obtained by solving the equation, which is relatively complicated and will not be listed here.

[0037] In addition, according to the definition of the two-point calibration equation of the radiometer and the nonlinear coefficient, the coefficient a can also be expressed as:

[0038]

[0039] It can be proved that for the error accuracy requirements of radiometer calibration measurement data, the above two definitions are the same within the same range on one side of the quadratic curve (one side of the symmetry axis). Combining (2) and (3), we get:

[0040]

[0041] Formula (4) provides a direct calculation method for the nonlinear coefficient: the nonlinear coefficient of the radiometer system response range can be calculated through three calibration points. In addition, from (4), it can be seen that if there is a target brightness temperature T M1 , its output voltage V M1 Exactly located at VH ,V C In the middle, there are:

[0042]

[0043] so,

[0044]

[0045] The significance of formula (6) is that when the output voltage is found to be at the midpoint between the cold source and hot source voltages, the nonlinear coefficient is independent of the output voltage and is only related to the temperature of the cold and hot sources and the temperature of the calibration source corresponding to the midpoint voltage. This formula provides a convenient method for determining the nonlinear coefficient of the radiometer system:

[0046] Before launching, the nonlinear coefficient of the system can be directly calculated by using the temperature change of the variable temperature source to find the voltage between the output voltages of the cold and hot sources at the set instrument temperature. The corresponding brightness temperature is used as the input of (6).

[0047] During on-orbit operation, a similar method can be used to find the surface type corresponding to the median voltage in the output voltage of the Earth background, and through model calculations or other synchronous observations, the nonlinear coefficient of the system can be determined or the accuracy of the model brightness temperature can be evaluated.

[0048] Another meaning of formula (6) shows that the nonlinear coefficient of the system is related to the temperature of the cold and hot sources, and the nonlinear coefficients of the calibration before launch and the calibration on orbit after launch are different. C The contribution of , (6) can be changed to:

[0049]

[0050] This formula is an approximate expression of the nonlinear coefficient and can be used to estimate the magnitude of the on-orbit nonlinear coefficient. The magnitude of the nonlinear coefficient depends on the 2T M and T H The relative size of , and thus determines the sign of u. M =T H When , the nonlinear coefficient of the system is approximately 0.

[0051] 2) The cold air invariance principle, virtual observation, and the concept of virtual cold air for radiometer calibration

[0052] The cold background temperature of the universe is approximately 2.7K, a constant. According to Planck's law, the brightness temperature of radiation at this temperature, or the cosmic background brightness temperature, depends only on frequency and is independent of observation time. Therefore, for a radiometer with a fixed system specification, its cold-sky brightness temperature is constant, a property we call cold-sky invariance. The cold-sky invariance calibration principle exploits the fact that the theoretical brightness temperature of the cold sky remains constant over time. This temperature is used before launch to determine the system output voltage, and thus the nonlinear coefficients required for on-orbit calibration.

[0053] According to formula (1), within a scanning cycle, observation data at three different brightness temperatures can be used to generate a quadratic calibration curve, which represents the system's true response curve for the current cycle. The real-time three-point calibration curve represents the stable response characteristics within the current scanning cycle and is valid throughout its dynamic range. Therefore, using this curve, the observed voltage corresponding to any brightness temperature within the radiometer's dynamic range can be extrapolated. This process is called radiometer virtual observation.

[0054] Using this virtual observation principle, we can easily generate the virtual observation voltage V of the cold air in the current scanning cycle. COS In the virtual process, the cold air brightness temperature T COS is the actual cold air in orbit, which is constant, and its voltage V COS It is virtual and is the result of real-time extrapolation of the equation obtained by three-point calibration. It is assumed here that the nonlinear coefficient is only related to the instrument operating state parameters (such as instrument temperature, receiver gain, etc.). That is:

[0055]

[0056] By solving equation (8), we can get V COS .

[0057] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0058] Example 1

[0059] like Figure 1 As shown, embodiment 1 of the present invention proposes a method for determining the nonlinear coefficient of on-orbit calibration of a satellite-borne microwave radiometer before launch. The method comprises the following steps:

[0060] The satellite-borne microwave radiometer to be measured observes three calibration black bodies in sequence within one scanning cycle to obtain the observation voltage and brightness temperature corresponding to three different temperatures.

[0061] The secondary calibration curve is obtained from the observed voltage and brightness temperature at three different temperatures;

[0062] The virtual cold air data is determined by the secondary calibration curve through virtual cold air observation;

[0063] If the virtual cold-space data and the thermally calibrated blackbody data can be used to determine the observed data at a voltage intermediate between the two, then the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data and the thermally calibrated blackbody data. Otherwise, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data, the thermally calibrated blackbody data, and the intermediate-temperature blackbody data. The specific process is as follows:

[0064] The present invention uses three independent calibration black bodies at different temperatures, including a cold calibration black body at the boiling point of liquid nitrogen, a hot calibration black body at ambient temperature, and an intermediate temperature calibration black body at a temperature between the two. The temperatures of the three black bodies are stable.

[0065] The test environment is temperature-stable and controllable, enabling the instrument to operate under on-orbit conditions. A conventional vacuum environment or atmospheric pressure laboratory can be used, and the temperature can typically be stabilized within a range of 5-30°C, such as 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C. The specific temperature range depends on the operating conditions on the satellite.

[0066] During the experiment, after the instrument temperature and blackbody temperature stabilized, the radiometer observed three calibration blackbodies in turn to obtain observation data; at the same time, the instrument's operating temperature and the temperature of the calibration blackbody were recorded.

[0067] Finally, the nonlinear coefficient of the low-end calibration using cold air on orbit is calculated by formula (9). It should be noted that if the intermediate temperature calibration blackbody T M In the range of 90 to 210K, this brightness temperature value can be used. If the value error is large, the brightness temperature of the cold calibration black body can be used as the intermediate temperature T M Participate in calculations.

[0068]

[0069] If there is a target brightness temperature T M1 , its output voltage V M1 Exactly located at V H ,V COS In the middle, the nonlinear coefficient u under the cold air condition on orbit can be directly calculated by (6), T COS It is the brightness temperature of the frequency corresponding to the cosmic background temperature of 2.7K.

[0070]

[0071] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and are intended to be encompassed by the claims of the present invention.

Claims

1. A method for determining a nonlinear coefficient of on-orbit calibration of a spaceborne microwave radiometer before launch, the method comprising: The satellite-borne microwave radiometer to be measured observes three calibration black bodies in sequence within one scanning cycle to obtain the observation voltage and brightness temperature corresponding to three different temperatures. The three calibration blackbodies include: a cold calibration blackbody at the boiling point of liquid nitrogen, a hot calibration blackbody at ambient temperature, and an intermediate temperature calibration blackbody at a temperature between the two. The secondary calibration curve is obtained from the observed voltage and brightness temperature at three different temperatures; The virtual cold air data is determined by the secondary calibration curve through virtual cold air observation; If the virtual cold-space data and the thermally calibrated blackbody data can be used to determine the observed data at an intermediate voltage between the two, then the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data and the thermally calibrated blackbody data; otherwise, the nonlinear coefficient of the spaceborne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data, the thermally calibrated blackbody data, and the intermediate temperature blackbody data; The method of determining virtual cold air data by using the secondary calibration curve and virtual cold air observation specifically includes: The secondary calibration curve is extrapolated or interpolated in real time to obtain the observed voltage corresponding to any brightness temperature within the dynamic range of the satellite-borne microwave radiometer to be measured. Based on the cold sky invariance principle, the corresponding virtual cold sky voltage is obtained from the cold sky brightness temperature, thus completing the virtual cold sky observation. The virtual cold-space data and the thermally calibrated blackbody data can be used to determine observation data at a voltage intermediate between the two. The nonlinear coefficient of the satellite-borne microwave radiometer to be measured when using the cold-space calibration at the current instrument temperature is calculated using the virtual cold-space data and the thermally calibrated blackbody data. This specifically includes: By the virtual cold air voltage V COS and the observed voltage V of the thermal calibration blackbody H , select the voltage value Thus, the corresponding target brightness temperature T is obtained M1 According to the following formula, the nonlinear coefficient u of the satellite-borne microwave radiometer to be measured when calibrated with cold space at the current instrument temperature is: Among them, T H is the brightness temperature of the thermally calibrated blackbody, T COS is the brightness temperature of the frequency corresponding to the cosmic background temperature of 2.7K; The method of calculating the nonlinear coefficient of the satellite-borne microwave radiometer to be measured when using cold-space calibration at the current instrument temperature based on the virtual cold-space data, the thermally calibrated blackbody data, and the intermediate-temperature blackbody data specifically includes: By the virtual cold air voltage V COS , the observed voltage V of the intermediate temperature blackbody M and the observed voltage V of the thermal calibration blackbody H , combined with the corresponding brightness temperature T COS 、T M and T H According to the following formula, the nonlinear coefficient u of the satellite-borne microwave radiometer to be measured when calibrated with cold space at the current instrument temperature is: Among them, T COS It is the brightness temperature of the frequency corresponding to the cosmic background temperature of 2.7K.

2. The method for determining the nonlinear coefficient of on-orbit calibration of a spaceborne microwave radiometer before launch according to claim 1, characterized in that: The temperatures of the three calibration blackbodies are all stable.

3. The method for determining the nonlinear coefficient of on-orbit calibration of a spaceborne microwave radiometer before launch according to claim 2, characterized in that: The secondary calibration curve is obtained by observing the voltage and brightness temperature of three different temperatures; specifically, the secondary calibration curve is obtained by observing the voltage and brightness temperature (V C ,T C ), corresponding to the observed voltage and brightness temperature of the thermally calibrated blackbody (V H ,T H ), and the observed voltage and brightness temperature (V M ,T M ), and obtain the quadratic calibration curve.

Images