On-board internal calibration method of on-board microwave radiometer

Abstract

The invention provides an onboard internal calibration method for an onboard microwave radiometer. The method comprises the following steps: step 1) determining a calibration equation through liquid nitrogen external calibration; 2) calculating a noise source brightness temperature required by internal calibration of the system according to a calibration equation obtained by external calibration; 3) during flight of the airborne microwave radiometer, establishing an internal calibration equation in airborne flight through a noise source and a matched load; and 4) carrying out liquid nitrogen external calibration cross validation on the accuracy of the determined onboard internal calibration equation. The airborne flight internal calibration method determined by the invention is suitable for an observation environment with dramatic temperature change, and solves the problem that a ground liquid nitrogen external calibration equation is no longer tried out due to the change of system gain caused by the change of environment temperature; the method can be used as a reference method for internal calibration of microwave radiometer foundation, satellite-borne, unmanned aerial vehicle-borne, manned vehicle-borne and other observation platforms, so that a foundation is laid for subsequent application research of microwave remote sensing data such as aviation calibration and satellite-ground comparison.

Description

Technical Field

[0001] The present application belongs to the field of microwave remote sensing observation, and specifically relates to an on-board internal calibration method for an airborne microwave radiometer. This inventive method can be used for internal calibration data processing of microwave radiometers on ground-based, satellite-based, unmanned aerial vehicle-based and manned aircraft-based observation platforms, and is a key step in the quantification of microwave radiometers. Background Art

[0002] Microwave radiometer is a highly sensitive passive observation detector. Due to its all-day and all-weather observation advantages, it is widely used in ground-based (ground, ship-borne), air-based (aircraft, UAV, missile, sounding balloon) and satellite-based (satellite, spacecraft, space shuttle) observation platforms. In recent years, airborne microwave radiometers have been used in microwave remote sensing observation experiments such as aviation calibration and satellite-ground comparison, and have developed rapidly, with high research value and broad application prospects.

[0003] Calibration technology is a key step in the quantification of microwave radiometer radiation observation. Ground calibration is usually carried out by liquid nitrogen external calibration, in which a liquid nitrogen-cooled absorbing black body is used as a low-temperature reference source, and an absorbing black body at ambient temperature is used as a heat source reference source. The calibration equation can be determined by two-point calibration technology. However, when the microwave radiometer is carried on an airborne platform and flies, the instrument temperature of the microwave radiometer drops rapidly due to the rapid effect of high-altitude convection, and the system gain changes accordingly. The calibration coefficient determined by the ground liquid nitrogen external calibration is no longer applicable. The problem of quantitative calibration of radiation observation of airborne microwave radiometers under weather conditions with drastic changes in ambient temperature cannot be solved. Summary of the invention

[0004] The purpose of the present invention is to overcome the above technical defects and provide an onboard internal calibration method for an airborne microwave radiometer. The method uses the matching load and injected noise in the microwave radiometer system as standard sources to achieve real-time system internal calibration during airborne flight.

[0005] In order to achieve the above-mentioned purpose, the present application proposes an on-board internal calibration method of an airborne microwave radiometer, the method comprising:

[0006] Step 1) determining a calibration equation by external calibration with liquid nitrogen;

[0007] Step 2) Calculate the noise source brightness temperature required for the system internal calibration using the calibration equation obtained by the external calibration;

[0008] Step 3) During the flight of the airborne microwave radiometer, an internal calibration equation is established through a noise source and a matching load.

[0009] As an improvement of the above method, the step 1) comprises:

[0010] A liquid nitrogen cooled absorbing black body is used as a low temperature reference source, and an absorbing black body at ambient temperature is used as a heat source reference source;

[0011] The fitting coefficient is obtained according to the linear fitting formula:

[0012]

[0013] Where a and b represent the calibration fitting coefficients respectively; V h Represents the voltage of the high temperature reference source; V c Represents the voltage of the low temperature reference source; T h represents the brightness temperature of the high temperature reference source; T c represents the brightness temperature of the low temperature reference source;

[0014] Determine the liquid nitrogen external calibration equation:

[0015] T=aV+b

[0016] Where T is the brightness temperature and V is the observation voltage.

[0017] As an improvement of the above method, the step 2) comprises:

[0018] Step 2-1) controlling the temperature of the microwave radiometer by a temperature control device, performing external liquid nitrogen calibration at multiple temperatures according to the steps described in step 1), and then obtaining calibration equations at different temperatures in turn;

[0019] Step 2-2) The noise source voltage V obtained at different radiometer temperatures a Substitute into the calibration equation at the corresponding temperature obtained in step 2-1) to obtain the brightness temperature T of the noise source at different temperatures a ;

[0020] Step 2-3) By taking the brightness temperature T of the noise source at different temperatures a The average value of determines the noise source brightness temperature required for system calibration in step 3).

[0021] As an improvement of the above method, the step 3) comprises:

[0022] Step 3-1) using the matching load and injected noise integrated in the microwave radiometer system as reference sources for calibration in the system, the matching load as a low-temperature reference source, and the matching load coupled injected noise as a high-temperature reference source;

[0023] Step 3-2) During the UAV-mounted microwave radiometer flight test, obtain the voltage V of the noise source h , matching load voltage V c And matching load temperature T c ;

[0024] Step 3-3) Obtain the fitting coefficient according to the linear fitting formula:

[0025]

[0026] Where a and b represent the calibration fitting coefficients respectively; V h Represents the voltage of the high temperature reference source; V c Represents the voltage of the low temperature reference source; T h represents the brightness temperature of the high temperature reference source; T c represents the brightness temperature of the low temperature reference source;

[0027] Step 3-4) Determine the internal calibration equation of the drone-mounted microwave radiometer:

[0028] T=aV+b

[0029] Where T is the brightness temperature and V is the observation voltage.

[0030] As an improvement of the above method, the matching load is related to the ambient temperature, and the temperature value measured by the temperature measuring device in the airborne microwave radiometer is used as the accurate temperature of the matching load; the injected noise temperature is determined by the liquid nitrogen external calibration in step 1) and step 2).

[0031] As an improvement of the above method, it also includes:

[0032] Step 4) Use liquid nitrogen external calibration to cross-validate the accuracy of the determined on-machine internal calibration equation.

[0033] As an improvement of the above method, the step 4) comprises:

[0034] Step 4-1) Set the temperature control point of the temperature controller to be consistent with the temperature of the microwave radiometer on board, repeat step 1) to perform liquid nitrogen external calibration to obtain the ground liquid nitrogen external calibration equation under the temperature control point;

[0035] Step 4-2) Use the observation results of the microwave radiometer on the computer using the ground liquid nitrogen external calibration equation determined in step 4-1) to cross-validate the accuracy of the calibration results on the machine.

[0036] Compared with the prior art, the advantages of this application are:

[0037] The on-board internal calibration method of an airborne microwave radiometer involved in the present invention has the significant advantage that it is suitable for observation environments with drastic temperature changes, solves the problem that the system gain changes due to changes in ambient temperature, resulting in the ground liquid nitrogen external calibration equation no longer being applicable, and realizes real-time internal calibration of the airborne microwave radiometer. The microwave remote sensing data calibrated by this inventive method has high accuracy and can be used as a reference for other airborne radiation calibrations, thereby laying a foundation for subsequent data application research such as aviation calibration and satellite-to-ground comparison. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Shown is a flow chart of the onboard internal calibration method of an airborne microwave radiometer. DETAILED DESCRIPTION

[0039] The technical solution of the present application is described in detail below with reference to the accompanying drawings.

[0040] like Figure 1 As shown, the onboard internal calibration method of the airborne microwave radiometer provided by the present application includes:

[0041] Step 1) determining a calibration equation by external calibration with liquid nitrogen;

[0042] Step 1-1) using a liquid nitrogen cooled absorbing blackbody and an absorbing blackbody at ambient temperature as reference sources for liquid nitrogen external calibration of the system, the liquid nitrogen cooled absorbing blackbody as a low temperature reference source, and the absorbing blackbody at ambient temperature as a heat source reference source;

[0043] Step 1-2) In the liquid nitrogen external calibration test of the microwave radiometer system, obtain the voltage V of the high temperature reference source h , the voltage of the low temperature reference source V c and the voltage V of the noise source in the system a ;

[0044] Step 1-3) Since the microwave radiometer is a linear system, the input and output of the system are determined to be in a linear relationship, and the fitting coefficient can be obtained according to the linear fitting formula, where the linear fitting formula is:

[0045] T=aV+b (1)

[0046] Where T is the brightness temperature, V represents the observed voltage, and a and b represent the calibration fitting coefficients respectively;

[0047] Step 1-4) The fitting coefficients a and b are calculated according to formula (1) and step 1-3), where the fitting coefficient formula is:

[0048]

[0049] Among them, V h Represents the voltage of the high temperature reference source; V c Represents the voltage of the low temperature reference source; T h represents the brightness temperature of the high temperature reference source; T c Represents the brightness temperature of the low-temperature reference source.

[0050] According to the fitting coefficients a and b determined by formulas (2) and (3), the liquid nitrogen external calibration equation (1) of the drone-mounted microwave radiometer can be determined.

[0051] Step 2) Calculate the noise source brightness temperature required for the system internal calibration using the calibration equation obtained by the external calibration;

[0052] Step 2-1) controlling the temperature of the microwave radiometer by a temperature control device, performing external liquid nitrogen calibration at multiple temperatures according to the steps described in step 1), and then obtaining calibration equations at different temperatures in turn;

[0053] Step 2-2) The noise source voltage V obtained at different radiometer temperatures a Substitute into the calibration equation at the corresponding temperature obtained in step 2-1) to obtain the brightness temperature T of the noise source at different temperatures a ;

[0054] In step 2-3), due to the constant temperature control of the noise source inside the microwave radiometer system, the temperature of the noise source varies within a small range of ±0.1K. The brightness temperature of the noise source required for the calibration in the system in step 3) is determined by taking the average value.

[0055] Step 3) During the flight of the airborne microwave radiometer, an internal calibration equation during the airborne flight is obtained through a noise source and a matching load;

[0056] Step 3-1) The matching load and injected noise integrated in the microwave radiometer system are used as reference sources for calibration within the system, the matching load is used as a low-temperature reference source, and the matching load coupled injected noise is used as a high-temperature reference source. Among them, the matching load is related to the ambient temperature, and the real-time temperature value measured by the platinum resistance thermometer (Platinum Resistance Thermometer, referred to as PRT, attached to different positions of the instrument to obtain the temperature of the microwave radiometer; in other embodiments, other temperature measurement devices with similar accuracy can also be used to measure the temperature) is used as the accurate temperature of the matching load. The noise injection consists of a directional coupler, a noise source, and a switch for injecting noise signals. The noise source system is internally set to constant temperature control, and its noise temperature T h Determined by liquid nitrogen external calibration in step 1) and step 2);

[0057] Step 3-2) During the UAV-mounted microwave radiometer flight test, obtain the voltage V of the noise source h , matching load voltage V c And matching load temperature T c ;

[0058] Step 3-3) Since the microwave radiometer is a linear system, the input and output of the onboard microwave radiometer are still linearly related, and the fitting coefficients a and b can be obtained according to the linear fitting formulas (1)-(3);

[0059] Step 3-4) Based on the fitting coefficients obtained in step 3-3), the internal calibration equation (1) of the drone-borne microwave radiometer can be preliminarily determined.

[0060] Step 4) Liquid nitrogen external calibration is used to cross-validate the accuracy of the on-board internal calibration equation.

[0061] Step 4-1) Set the temperature control point of the temperature controller to be consistent with the temperature of the microwave radiometer on board, repeat step 1) to perform liquid nitrogen external calibration to obtain the ground liquid nitrogen external calibration equation under the temperature control point;

[0062] Step 4-2) using the observation results of the microwave radiometer on the computer using the ground liquid nitrogen external calibration equation determined in step 4-1), cross-validating the accuracy of the calibration results on the computer;

[0063] Step 4-3) Periodically check and maintain the on-board characteristic parameters based on the verification results in step 4-2), and input the new results into the on-board calibration operation software of the microwave radiometer.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present application.

Claims

1. An onboard internal calibration method for an airborne microwave radiometer, the method comprising: Step 1) determining a calibration equation by external calibration with liquid nitrogen; Step 2) Calculate the noise source brightness temperature required for the system internal calibration using the calibration equation obtained by the external calibration; Step 3) During the flight of the airborne microwave radiometer, an internal calibration equation is established through a noise source and a matching load.

2. The onboard internal calibration method of an airborne microwave radiometer according to claim 1, characterized in that: The step 1) comprises: A liquid nitrogen cooled absorbing black body is used as a low temperature reference source, and an absorbing black body at ambient temperature is used as a heat source reference source; The fitting coefficient is obtained according to the linear fitting formula: Where a and b represent the calibration fitting coefficients respectively; V h Represents the voltage of the high temperature reference source; V c Represents the voltage of the low temperature reference source; T h represents the brightness temperature of the high temperature reference source; T c represents the brightness temperature of the low temperature reference source; Determine the liquid nitrogen external calibration equation: T=aV+b Where T is the brightness temperature and V is the observation voltage.

3. The onboard internal calibration method of an airborne microwave radiometer according to claim 1, characterized in that: The step 2) comprises: Step 2-1) controlling the temperature of the microwave radiometer by a temperature control device, performing external liquid nitrogen calibration at multiple temperatures according to the steps described in step 1), and then obtaining calibration equations at different temperatures in turn; Step 2-2) The noise source voltage V obtained at different radiometer temperatures a Substitute into the calibration equation at the corresponding temperature obtained in step 2-1) to obtain the brightness temperature T of the noise source at different temperatures a ; Step 2-3) By taking the brightness temperature T of the noise source at different temperatures a The average value of determines the noise source brightness temperature required for system calibration in step 3).

4. The on-board internal calibration method of an airborne microwave radiometer according to claim 1, characterized in that: The step 3) comprises: Step 3-1) using the matching load and injected noise integrated in the microwave radiometer system as reference sources for calibration in the system, the matching load as a low-temperature reference source, and the matching load coupled injected noise as a high-temperature reference source; Step 3-2) During the UAV-mounted microwave radiometer flight test, obtain the voltage V of the noise source h , matching load voltage V c And matching load temperature T c ; Step 3-3) Obtain the fitting coefficient according to the linear fitting formula: Where a and b represent the calibration fitting coefficients respectively; V h Represents the voltage of the high temperature reference source; V c Represents the voltage of the low temperature reference source; T h represents the brightness temperature of the high temperature reference source; T c represents the brightness temperature of the low temperature reference source; Step 3-4) Determine the internal calibration equation of the drone-mounted microwave radiometer: T=aV+b Where T is the brightness temperature and V is the observation voltage.

5. The on-board internal calibration method of an airborne microwave radiometer according to claim 4, characterized in that: The matching load is related to the ambient temperature, and the temperature value measured by the temperature measuring device in the airborne microwave radiometer is used as the accurate temperature of the matching load; the injected noise temperature is determined by the liquid nitrogen external calibration in step 1) and step 2).

6. The onboard internal calibration method of an airborne microwave radiometer according to claim 1, characterized in that: Also includes: Step 4) Use liquid nitrogen external calibration to cross-validate the accuracy of the determined on-machine internal calibration equation.

7. The onboard internal calibration method of an airborne microwave radiometer according to claim 6, characterized in that: The step 4) comprises: Step 4-1) Set the temperature control point of the temperature controller to be consistent with the temperature of the microwave radiometer on board, repeat step 1) to perform liquid nitrogen external calibration to obtain the ground liquid nitrogen external calibration equation under the temperature control point; Step 4-2) Use the observation results of the microwave radiometer on the computer using the ground liquid nitrogen external calibration equation determined in step 4-1) to cross-validate the accuracy of the calibration results on the machine.

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