CN116576974B - Self-calibration method of multichannel microwave radiometer - Google Patents

Abstract

The invention discloses a multi-channel microwave radiometer self-calibration method, which relates to the field of atmospheric microwave remote sensing detection, including using a liquid nitrogen-blackbody calibration method to determine the calibration equation; and obtaining the blackbody brightness temperature value and noise through the above calibration equation. The blackbody brightness temperature value measured when the diode is turned on uses the noise injection calibration method to determine a new calibration equation; the equivalent brightness temperature of the noise power is corrected through the slope curve calibration method. The self-calibration method of the multi-channel microwave radiometer disclosed in the present invention requires only one liquid nitrogen-blackbody calibration. The built-in self-calibration module of microwave radiation automatically corrects the calibration drift, effectively improving the brightness temperature output of the microwave radiometer. The accuracy solves the problem of excessive deviation between the current measured brightness temperature of microwave radiometers and the simulated brightness temperature calculated according to the model, and provides an accurate and reliable data source for microwave radiometer atmospheric remote sensing.

Description

A self-calibration method for multi-channel microwave radiometer

Technical field

The invention relates to the technical field of atmospheric microwave remote sensing detection, in particular to a multi-channel microwave radiometer self-calibration method.

Background technique

The multi-channel microwave radiometer is a passive remote sensing tool. The atmospheric microwave radiation brightness temperature signal received by the antenna is the integral quantity of the atmospheric temperature, humidity, pressure and other parameters at different altitudes as implicit functions. By designing advanced algorithms and regularization conditions Constraints, these characteristic signals can be used to invert atmospheric physical parameters such as temperature and humidity at different altitudes.

The actual output of the microwave radiometer is a voltage value. For this reason, it is necessary to accurately construct the quantitative relationship between the radiometer output voltage signal and the received radiation value. This process is called radiometer calibration. Calibration is an important prerequisite for microwave radiometer measurement. The calibration accuracy directly affects the measurement of atmospheric brightness temperature by the microwave radiometer, which in turn affects its remote sensing of atmospheric environmental parameters.

As equipment and components age, their performance becomes unstable. In order to prevent excessive temperature drift, the microwave radiometer needs to be calibrated once every 2 to 3 months to ensure its measurement accuracy. Common calibration methods include liquid nitrogen-blackbody calibration method, injection noise calibration method, etc. Although the liquid nitrogen-blackbody method has high calibration accuracy, the disadvantages are that liquid nitrogen is expensive, unsafe to operate, and inconvenient to transport and store. Liquid nitrogen calibration is usually only applicable to microwave radiometers with small-diameter antennas. Since the use of liquid nitrogen requires manual operation, it is not suitable for repeated use. During a liquid nitrogen-blackbody calibration period, the injection noise method is generally used. This Although this calibration method is useful, the performance of the noise diode will drift, so it is not suitable as a long-term calibration method.

Contents of the invention

In order to overcome the above-mentioned problems existing in the prior art, the present invention proposes a multi-channel microwave radiometer self-calibration method.

The technical solution adopted by the present invention to solve the technical problem is: a multi-channel microwave radiometer self-calibration method, which includes the following steps: Step 1, during the first calibration, use the liquid nitrogen-blackbody calibration method to determine the calibration equation for:

T＝a·V+b

Among them, T is the temperature, V is the voltage, is the blackbody brightness temperature,/> It is the output voltage when the microwave radiometer antenna points to the black body;/> is the equivalent brightness temperature of liquid nitrogen,/> is the output voltage when the microwave radiometer antenna points to liquid nitrogen;

Step 2: Obtain the blackbody brightness temperature value T _BB and the blackbody brightness temperature value T _BB+ND measured when the noise diode is turned on through the calibration equation in step 1. Use the noise injection calibration method to determine the new calibration equation as:

Among them, T _b is the brightness temperature value of the radiometer when detecting targets; V _out is the output voltage value of the radiometer when detecting targets; V _BB is the output voltage of the radiometer when measuring black bodies when the noise diode is turned off; V _{BB + ND} is the output voltage value of the radiometer aimed at the blackbody target after the noise diode is turned on, T _ND is the equivalent brightness temperature of the noise power, T _ND =T _BB+ND -T _BB , where T _BB+ND is V _BB+ND The value calculated by substituting it into step 1 represents the brightness temperature of the radiometer aimed at a blackbody target after the noise diode is turned on. T _BB is calculated by substituting V _BB into step 1 and represents the brightness temperature of the radiometer pointed at the blackbody target after the noise diode is turned off. value.

Step 3: Correct the T _ND in Step 2 through the slope curve calibration method;

Step 4. For subsequent use, repeat steps 2-3 within the calibration cycle to perform self-calibration.

The above-mentioned multi-channel microwave radiometer self-calibration method, the step 2 is specifically: align the radiometer antenna with the target black body, disconnect the noise diode and close the noise diode respectively to obtain the output voltages V _BB and V _BB+ND , put V _BB and V _BB+ND into the calibration equation obtained in step 1 respectively, and obtain the corresponding brightness temperature values T _BB and T _BB+ND . The equivalent noise injected by the diode is the difference between the two: T _ND = TBB _+ND - _TBB .

The above-mentioned multi-channel microwave radiometer self-calibration method, the step 3 specifically includes:

Step 3.1, determine the calibration equation as:

Among them, T _sky (θ) is the brightness temperature of the detection target when the observation zenith angle is θ, V _sky (θ) is the output voltage of the microwave radiometer detection target when the noise diode is turned on, and V _BB is the radiometer measurement when the noise diode is turned off. The output voltage when the black body is used; V _BB+ND is the output voltage value of the radiometer aimed at the black body target after the noise diode is turned on, and T _BB is the brightness temperature value of the black body;

Step 3.2, obtain the exact T _ND through step 2, let T' _ND =r·T _ND , substitute it into the calibration equation in step 3.1 of the calibration equation, and obtain T _sky (θ ₁ ) and T _sky (θ ₂ );

Step 3.3, express the total attenuation of the atmosphere in terms of brightness temperature and average radiation temperature:

Among them, τ(θ) is the atmospheric opacity, T _m is the average radiation temperature of the atmosphere, and T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ;

Step 3.4, substitute T _sky (θ ₁ ) and T _sky (θ ₂ ) obtained in step 3.2 into step 3.3 to obtain the opacity τ (θ ₁ ) and τ (θ ₂ ), as τ (θ) = τ (0° )sec(θ) relationship, we get

Step 3.5, adjust r so that t ₁ = t ₂ in step 3.4, and the r value at this time is the calculated value;

In step 3.6, substitute the r value obtained in step 3.5 into T' _ND =r·T _ND in step 3.2 to obtain the corrected T' _ND .

In the above-mentioned self-calibration method of a multi-channel microwave radiometer, the value range of the zenith angle θ in step 3.2 is [0°, 80°].

The above-mentioned multi-channel microwave radiometer self-calibration method, the step 3 specifically includes:

Step 3.1, determine the calibration equation as:

Among them, T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ, V _sky (θ) is the voltage value output by the microwave radiometer circuit system when the observation zenith angle is θ, and V _BB is when the noise diode is turned off. The output voltage of the radiometer when measuring a black body; V _BB+ND is the output voltage value of the radiometer aimed at the black body target after the noise diode is turned on, and T _BB is the brightness temperature value of the black body;

Step 3.2, obtain the exact T _ND through step 2, let T' _ND =r·T _ND , substitute it into the calibration equation in step 3.1 of the calibration equation, and obtain T _sky (θ ₁ ) and T _sky (θ ₂ );

Step 3.3, express the total attenuation of the atmosphere in terms of brightness temperature and average radiation temperature:

Among them, τ(θ) is the atmospheric opacity, T _m is the average radiation temperature of the atmosphere, and T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ;

Step 3.4, substitute T _sky (θ ₁ ) and T _sky (θ ₂ ) obtained in step 3.2 into step 3.3 to obtain the opacity τ (θ ₁ ) and τ (θ ₂ ), as τ (θ) = τ (0° )sec(θ) relationship, regression returns τ(0°);

Step 3.5, substitute τ(0°) obtained in step 3.4 into step 3.3 to calculate T _sky (0°), and Bring it into the calibration equation in step 3.1 and calculate T' _ND ,

Step 3.6, repeat steps 3.1-3.5 until T' _ND is stable.

The above-mentioned self-calibration method of a multi-channel microwave radiometer, the determination process of the calibration equation in step 1 is specifically: align the radiometer antenna with the liquid nitrogen and the target black body, and obtain the output voltage and/> Liquid nitrogen temperature/> It is known that the black body temperature/> Measured by a thermometer, combine two sets of data/> Substituting into the calibration equation T=a·V+b, the parameters a and b are obtained.

The beneficial effect of the present invention is that the present invention discloses a self-calibration method and process for a multi-channel microwave radiometer, which does not require frequent human intervention and only requires one liquid nitrogen-blackbody calibration. Through the built-in self-calibration of microwave radiation The module automatically corrects the calibration drift, effectively improves the brightness temperature output accuracy of the microwave radiometer, solves the problem of excessive deviation between the current measured brightness temperature of the microwave radiometer and the simulated brightness temperature calculated according to the model, and provides accurate information for microwave radiometer atmospheric remote sensing. Reliable data source.

Description of the drawings

The present invention will be further described below in conjunction with the accompanying drawings and examples.

Figure 1 is a flow chart of the present invention;

Figure 2 is a schematic diagram of the liquid nitrogen-blackbody calibration process of the present invention;

Figure 3 is a schematic diagram of the injection noise calibration process of the present invention;

Figure 4 is a schematic diagram of the tilt curve calibration process of the present invention.

Detailed ways

In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to the drawings and specific implementation modes.

This embodiment discloses a multi-channel microwave radiometer self-calibration method, as shown in Figure 1, which specifically includes the following steps:

Step 1. Liquid nitrogen-blackbody calibration. The specific process is shown in Figure 2. First, it is assumed that there is a linear relationship between the output voltage signal of the radiometer and the received radiation value, that is, the calibration equation is T=a·V+b , aim the radiometer antenna at the liquid nitrogen and the target black body respectively, and obtain the output voltage. and/> Then combine the two sets of data (liquid nitrogen/> and bold/> liquid nitrogen temperature It is known that the black body temperature/> (measured by a thermometer) is substituted into the calibration equation to obtain the values of parameters a and b:

Determine the scaling equation T=a·V+b.

Step 2, noise injection calibration. The specific process is shown in Figure 3. Aim the radiometer antenna at the target blackbody, disconnect the noise diode and close the noise diode respectively to obtain the output voltages V _BB and V _BB+ND , and substitute them into In the calibration equation obtained above, the corresponding brightness temperature values T _BB and T _BB+ND are obtained, then the equivalent noise injected by the diode is the difference between the two: T _ND =T _BB+ND -T _BB , new The scaling equation is:

Among them, T _b is the brightness temperature value of the radiometer when detecting the target; T _BB is the brightness temperature value of the black body; V _out is the output voltage value of the radiometer when detecting the target; V _BB is the black body measured by the radiometer when the noise diode is turned off. The output voltage when; V _BB+ND is the output voltage value of the radiometer aimed at the blackbody target after the noise diode is turned on.

In the formula, since T _ND is unknown, T _ND needs to be initialized before each calibration. The error caused by the fluctuation of T _ND in the short term can be ignored, so liquid nitrogen absolute calibration can be used in each cycle to calculate T _ND , and the noise injection calibration method can be used in this cycle; and the change of T _ND This can be solved by tilt curve scaling.

Step 3, slope curve calibration. The specific process is shown in Figure 4. Assuming that the atmosphere is ideally stratified horizontally, it satisfies τ(θ)=τ(0°)sec(θ). This is the theoretical basis for tilt curve calibration.

The actual output of the microwave radiometer is a voltage value. For this purpose, it is necessary to accurately construct the quantitative relationship between the radiometer output voltage signal and the received radiation value. This process is called radiometer calibration. The output of the microwave radiometer is The voltage signal of the detected target has no actual physical meaning and cannot reflect the physical properties of the detected target. The calibration process is to convert the voltage signal into a radiation amount (brightness temperature) with actual physical meaning. The conversion equation is T=a ·V+b. When the noise diode of the built-in circuit system of the microwave radiometer is turned off and the antenna is pointed at the black body, the system output voltage V _BB is measured. Substituting into the above conversion equation, we get (T _BB , V _BB ); when the diode is turned on and the antenna is pointed at the black body, V is measured _BB+ND , substitute into the above conversion equation, we get (T _BB+ND , V _BB+ND ); treat the above four values as known quantities; turn on the diode, align the antenna with the detection target, and measure V _sky (θ), Substituting into the above conversion equation, we get (T _sky (θ), V _sky (θ)), where V _sky (θ) is known (circuit system output value), T _sky (θ) is unknown, and is the quantity to be determined; because T It has a linear relationship with V. Obviously, these three points are collinear and can be obtained And because T _ND =T _BB+ND -T _BB , it is assumed that the scaling equation

Among them, T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ, V _sky (θ) is the voltage value output by the microwave radiometer circuit system when the observation zenith angle is θ, and V _BB is when the noise diode is turned off. The output voltage of the radiometer when measuring a black body; V _BB+ND is the output voltage value of the radiometer aimed at the black body target after the noise diode is turned on, T _BB is the brightness temperature value of the black body, T _ND ,V _BB+ND ,V _BB ,T _BB is known, but since T _ND will offset in a longer time period, and in order to consider the transmission loss caused by the waveguide and radome (located in front of the noise source) at the front end of the radiometer, let T' _ND = r· T _ND , correct r through slope curve calibration, thereby correcting the calibration equation T' _ND .

Using the accurate T _ND obtained by noise injection calibration, let the new T' _ND = r·T _ND and bring it into the calibration equation The value range of zenith angle θ is [0°, 80°]. Take any two values θ ₁ and θ ₂ within the value range to obtain T _sky (θ ₁ ) and T _sky (θ ₂ ). The total atmosphere Attenuation is expressed in terms of brightness temperature and mean radiant temperature:

The value of the average atmospheric radiation temperature T _m is usually regarded as a constant. This value can be regression statistics based on historical sounding data, and the value of T _m can be inferred from the surface atmospheric temperature T _g .

Calculate T _sky (θ ₁ ) and T _sky (θ ₂ ) from the calibration equation, and then use Obtain the opacity τ(θ ₁ ) and τ(θ ₂ ), and from the relationship τ(θ)=τ(0°)sec(θ), we get

Adjust r so that t ₁ =t ₂ , thereby obtaining the value of the correction factor r.

or:

Using τ(θ ₁ ) and τ(θ ₂ ), we have relationship,/> In this way, τ(0°) is regressed, and T _sky (0°) is calculated using τ(0°), which is then brought into the calibration equation to calculate T' _ND , and the cycle continues until T' _ND is stable.

Step 4. For subsequent use, repeat steps 2-3 within the calibration cycle to perform self-calibration.

The above embodiments are only exemplary embodiments of the present invention and are not used to limit the present invention. The protection scope of the present invention is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent substitutions should also be deemed to fall within the protection scope of the present invention.

Claims (6)

1. A multi-channel microwave radiometer self-calibration method, characterized in that it includes the following steps: Step 1. During the first calibration, use the liquid nitrogen-blackbody calibration method to determine the calibration equation as: T＝a·V+b Among them, T is the temperature, V is the voltage, is the blackbody brightness temperature,/> It is the output voltage when the microwave radiometer antenna points to the black body;/> is the equivalent brightness temperature of liquid nitrogen,/> is the output voltage when the microwave radiometer antenna points to liquid nitrogen; Step 2: Obtain the blackbody brightness temperature value T _BB and the blackbody brightness temperature value T _BB+ND measured when the noise diode is turned on through the calibration equation in step 1. Use the noise injection calibration method to determine the new calibration equation as: Among them, T _b is the brightness temperature value of the radiometer when detecting targets; V _out is the output voltage value of the radiometer when detecting targets; V _BB is the output voltage of the radiometer when measuring black bodies when the noise diode is turned off; V _{BB + ND} is the output voltage value of the radiometer aimed at the blackbody target after the noise diode is turned on, T _ND is the equivalent brightness temperature of the noise power, T _ND =T _BB+ND -T _BB , where T _BB+ND is V _BB+ND The value calculated by substituting it into step 1 represents the brightness temperature of the radiometer aimed at a blackbody target after the noise diode is turned on. T _BB is calculated by substituting V _BB into step 1 and represents the brightness temperature of the radiometer pointed at the blackbody target after the noise diode is turned off. value; Step 3: Correct the T _ND in Step 2 through the slope curve calibration method; Step 4. For subsequent use, repeat steps 2-3 within the calibration cycle to perform self-calibration. 2. A multi-channel microwave radiometer self-calibration method according to claim 1, characterized in that the step 2 is specifically: align the radiometer antenna with the target black body, disconnect the noise diode and close the noise diode respectively. , obtain the output voltages V _BB and V _BB+ND . Put V _BB and V _BB+ND into the calibration equation obtained in step 1 respectively to obtain the corresponding brightness temperature values T _BB and T _BB+ND . The equivalent of diode injection The noise is the difference between the two: T _ND =T _BB+ND -T _BB . 3. A kind of multi-channel microwave radiometer self-calibration method according to claim 1, characterized in that, the step 3 specifically includes: Step 3.1, determine the calibration equation as: Among them, T _sky (θ) is the brightness temperature of the detection target when the observation zenith angle is θ, V _sky (θ) is the output voltage of the microwave radiometer detection target when the noise diode is turned on, and V _BB is the radiometer measurement when the noise diode is turned off. The output voltage when the black body is used; V _BB+ND is the output voltage value of the radiometer aimed at the black body target after the noise diode is turned on, and T _BB is the brightness temperature value of the black body; Step 3.2, obtain the exact T _ND through step 2, let T' _ND =r·T _ND , substitute it into the calibration equation in step 3.1 of the calibration equation, and obtain T _sky (θ ₁ ) and T _sky (θ ₂ ); Step 3.3, express the total attenuation of the atmosphere in terms of brightness temperature and average radiation temperature: Among them, τ(θ) is the atmospheric opacity, T _m is the average radiation temperature of the atmosphere, and T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ; Step 3.4, substitute T _sky (θ ₁ ) and T _sky (θ ₂ ) obtained in step 3.2 into step 3.3 to obtain the opacity τ (θ ₁ ) and τ (θ ₂ ), as τ (θ) = τ (0° )sec(θ) relationship, we get Step 3.5, adjust r so that t ₁ = t ₂ in step 3.4, and the r value at this time is the calculated value; In step 3.6, substitute the r value obtained in step 3.5 into T' _ND =r·T _ND in step 3.2 to obtain the corrected T' _ND . 4. A multi-channel microwave radiometer self-calibration method according to claim 3, characterized in that the zenith angle θ in step 3.2 ranges from [0°, 80°]. 5. A kind of multi-channel microwave radiometer self-calibration method according to claim 1, characterized in that, the step 3 specifically includes: Step 3.1, determine the calibration equation as: Among them, T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ, V _sky (θ) is the voltage value output by the microwave radiometer circuit system when the observation zenith angle is θ, and V _BB is when the noise diode is turned off. The output voltage of the radiometer when measuring a black body; V _{BB + ND} is the output voltage value of the radiometer aimed at the black body target after the noise diode is turned on, and T _BB is the brightness temperature value of the black body; Step 3.2, obtain the exact T _ND through step 2, let T' _ND =r·T _ND , substitute it into the calibration equation in step 3.1 of the calibration equation, and obtain T _sky (θ ₁ ) and T _sky (θ ₂ ); Step 3.3, express the total attenuation of the atmosphere in terms of brightness temperature and average radiation temperature: Among them, τ(θ) is the atmospheric opacity, T _m is the average radiation temperature of the atmosphere, and T _sky (θ) is the atmospheric brightness temperature when the observation zenith angle is θ; Step 3.4, substitute T _sky (θ ₁ ) and T _sky (θ ₂ ) obtained in step 3.2 into step 3.3 to obtain the opacity τ (θ ₁ ) and τ (θ ₂ ), as τ (θ) = τ (0° )sec(θ) relationship, regression returns τ(0°); Step 3.5, substitute τ(0°) obtained in step 3.4 into step 3.3 to calculate T _sky (0°), and Bring it into the calibration equation in step 3.1 and calculate T'_ND; Step 3.6, repeat steps 3.1-3.5 until T' _ND is stable. 6. A multi-channel microwave radiometer self-calibration method according to claim 1, characterized in that the determination process of the calibration equation in step 1 is specifically: aligning the radiometer antenna with the liquid nitrogen and the target black body , get the output voltage and Liquid nitrogen temperature/> It is known that the black body temperature/> Measured by a thermometer, combine two sets of data/> Substituting into the calibration equation T=a·V+b, the parameters a and b are obtained.