An absolute brightness temperature calibration method for a synthetic aperture radiometer system

Abstract

The present invention discloses an absolute brightness temperature calibration method for a synthetic aperture radiometer system. The traditional two-point calibration method has problems such as large measurement error and poor sensitivity, so its scope of use is limited. In order to address these shortcomings, the present invention proposes a new noise injection method, which combines the measured antenna signal and the calibration signal through a coupler and outputs them as one input signal A of the receiving channel, and uses a matching load signal with a precisely known brightness temperature as another input signal B of the receiving channel. The decision module and the control signal generation module in the acquisition processor control the switching of the front-end switch, so that the brightness temperature values ​​of the two input signals of the receiving channel reach a dynamic balance, thereby accurately measuring the absolute brightness temperature of the measured antenna signal. The present invention has the characteristics of small measurement error and high measurement sensitivity, and can be used in various synthetic aperture radiometer systems to accurately measure the absolute brightness temperature of the antenna input.

Description

Technical Field

[0001] The invention relates to an absolute brightness temperature calibration method for a synthetic aperture radiometer system, and belongs to the technical field of space microwave remote sensing. Background Art

[0002] The aperture synthesis radiometer is the main payload of the ocean salinity detection satellite, and is responsible for the high-precision detection of global ocean salinity. During the operation of the radiometer on orbit, it is necessary to measure the target radiation brightness temperature information in real time and with high precision. Traditional real aperture radiometers usually use cold space reflectors and thermal calibration sources, and input two high-precision cold sources and hot sources with known brightness temperatures from the feed aperture at a certain period, so as to calculate the target radiation brightness temperature information in real time. However, due to the characteristics of the aperture synthesis radiometer, which has a large antenna array and a large number of feed sources, the traditional periodic aperture calibration method cannot be used to measure the absolute brightness temperature of the target radiation. Traditional target radiation absolute brightness temperature measurement methods include real aperture radiometers and aperture synthesis radiometers;

[0003] Real aperture radiometer

[0004] Traditional real-aperture microwave radiometers typically employ a cold reflector and a thermal calibration source. The cold reflector reflects precisely known cold-space brightness temperature information (cold source) back to the feed aperture, while the thermal calibration source (heat source) provides a precisely known high-temperature signal at the feed aperture. While in orbit, the system periodically observes the cold source, heat source, and target. Using a two-point calibration method, the system obtains the absolute brightness temperature of the target's radiation.

[0005] Synthetic Aperture Radiometer

[0006] Due to the large antenna array, numerous feed sources, and the lack of on-orbit scanning requirements of synthetic aperture radiometers, traditional periodic aperture calibration methods cannot be used to measure the absolute brightness temperature of the target's radiated radiation. Therefore, internal noise injection and two-point calibration are commonly used to measure the absolute brightness temperature of the feed aperture input. This method periodically inputs different calibration signals of known brightness temperatures into the receiver input port, then uses a two-point calibration method to obtain the absolute brightness temperature of the target's radiated radiation.

[0007] Therefore, the traditional method has the following disadvantages:

[0008] The linearity error of the receiver channel will cause brightness temperature measurement error;

[0009] The observation time of the target is short (only 1 / 3 of the time), which reduces the observation sensitivity of the target's absolute brightness temperature Summary of the Invention

[0010] The technical problem solved by the present invention is: in view of the shortcomings of the existing absolute brightness temperature measurement method of the synthetic aperture radiometer system, a new absolute brightness temperature measurement method for the synthetic aperture radiometer spaceborne application is proposed.

[0011] The technical solution of the present invention is: a method for calibrating absolute brightness temperature for a synthetic aperture radiometer system, comprising: measuring antenna signal TA and calibration signal T ’ N After being combined through the coupler, the output signal brightness temperature is The coupler output signal is input to the receiving channel after passing through switch 2; the receiving channel amplifies, filters, down-converts and filters the input signal and then outputs an intermediate frequency signal IF, which enters the acquisition processor; the acquisition processor digitally quantizes the intermediate frequency signal IF through AD and then sends it to the FPGA; the FPGA internally includes a multiplier-accumulator a, a multiplier-accumulator b, a judgment module, a switch 1 control signal generation module and a switch 2 control signal generation module; the switch 2 control signal generation module generates a square wave signal with a period of TS and a duty cycle of 50%; during the period when the square wave signal is at a level, switch 2 is controlled to cut off port 2, at which time the receiving channel input matches the load signal, the sampled data of AD enters the multiplier-accumulator a, and the multiplier-accumulator a multiplies and accumulates the sampled data and outputs the result A; during the period when the square wave signal is at a high level, switch 2 is controlled to cut off port 1, at which time the receiving channel input signal is The sampling data of AD enters the multiplication and accumulation device b, which multiplies and accumulates the sampling data and outputs the result B. After one TS period ends, the two multiplication and accumulation results A and B enter the decision module. The decision module adjusts the duration of τ according to the size of A and B until the balance between A and B is achieved, and finally the absolute brightness temperature value T of the antenna input is obtained. A .

[0012] The calibration signal T ’ N The calibration signal generated by the diode noise source is the brightness temperature of the signal after passing through the attenuator and switch 1; the control signal of switch 1 is generated by the acquisition processor and controls whether the calibration signal of the diode noise source is connected to the coupler.

[0013] The control signal of switch 2 is generated by the acquisition processor, and controls the input signal of the subsequent receiving channel to connect to the antenna branch, that is, the coupler output, or to connect to the matching load.

[0014] The switch 1 control signal generating module generates the switch 1 control signal according to the result of the judgment module. When the switch 1 control signal is high, the switch 1 is switched to port 1. At this time, T ’ N The signal is coupled to the antenna branch through the coupler and then enters the receiving channel. When the switch 1 control signal is low, the switch 1 switches the 2 port. At this time, T’ N The signal is disconnected, and only the antenna signal enters the receiving channel;

[0015] During the period when the control signal of switch 1 is at a high level, the duration is τ*TS / 2, and the value of τ is obtained by the real-time output of the acquisition processor.

[0016] The decision module adjusts the duration of τ according to the magnitudes of A and B until the balance between A and B is achieved, and finally obtains the absolute brightness temperature value T of the antenna input A , including: when A > B in the decision module, the control signal generation module of switch 1 controls to increase the duration of τ, thereby increasing the magnitude of B; when A < B in the decision module, the control signal generation module of switch 1 controls to decrease the duration of τ, thereby decreasing the magnitude of B; and so on in a loop until the balance between A and B is achieved, and finally obtains the absolute brightness temperature value T of the antenna input A .

[0017] The absolute brightness temperature value T A = T REF - τT ’ N , where T REF The brightness temperature value is the temperature value at the point where the matched load is located.

[0018] The advantages of the present invention compared with the prior art are as follows:

[0019] 1) The linearity error of the receiver channel will not cause measurement errors; in the traditional two-point calibration method, the selected brightness temperature values of the two calibration signals usually have a large difference, and the receiver channel is at different working points during two-point calibration, resulting in measurement errors caused by the linearity error of the receiver channel. Compared with the traditional two-point calibration method, when A and B are balanced, the receiver channel is always at the same working point in the present invention, avoiding the measurement errors caused by the linearity error.

[0020] 2) Compared with the method of two-point calibration measurement, the observation time of the target in this method is increased, effectively improving the sensitivity of the measurement of the absolute brightness temperature of the target. The traditional two-point calibration method needs to periodically observe the high-temperature calibration signal, the low-temperature calibration signal and the target signal, and the observation time of the target is less (only 1 / 3 of the total observation time). In the present invention, the observation time of the target is increased to 1 / 2 of the total observation time, and the increase in the observation time can effectively improve the sensitivity of the measurement of the absolute brightness temperature of the target. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] <o000085> It is a schematic diagram of the absolute brightness temperature measurement system. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Figure 1The schematic diagram of the solution of this method is shown. The signal of the measured antenna TA and the calibration signal T ’ N are combined through a coupler and then output. The output signal brightness temperature is where the calibration signal T , N is the brightness temperature of the calibration signal generated by the diode noise source after passing through the attenuator and switch 1. The control signal of switch 1 is generated by the acquisition processor. There is a "switch 1 control signal generation module" in its internal FPGA, which controls whether the calibration signal of the diode noise source is connected to the coupler. The output signal of the coupler is input to the receiving channel after passing through switch 2. The control signal of switch 2 is generated by the acquisition processor. There is a "switch 2 control signal generation module" in its internal FPGA, which controls the input signal of the subsequent receiving channel to switch to the antenna branch (i.e., the output of the coupler) or the matching load. The receiving channel amplifies, filters, down-converts and filters the input signal and then outputs the intermediate frequency signal IF. The central signal IF enters the acquisition processor.

[0023] The acquisition processor digitally quantifies the sampled signal through AD and then enters the FPGA. Among them, there are a multiply-accumulator, a decision module, a switch 1 control signal generation module and a switch 2 control signal generation module inside the FPGA. The switch 2 on control signal generation module generates a square wave signal with a period of TS and a duty cycle of 50%. During the low level of the control signal, it controls switch 2 to switch to port 2. At this time, the receiving channel inputs the matching load signal, and the sampled data of AD enters the multiply-accumulator a. The multiply-accumulator a multiplies and accumulates the sampled data and then outputs the result as A. During the high level of the control signal, it controls switch 2 to switch to port 1. At this time, the input signal of the receiving channel is The sampled data of AD enters the multiply-accumulator b. The multiply-accumulator b multiplies and accumulates the sampled data and then outputs the result as B. After one TS cycle ends, the two multiply-accumulation results A and B enter the decision module, and the decision module judges the sizes of A and B.

[0024] Meanwhile, the switch 1 control signal generation module generates a switch 1 control signal according to the result of the decision module. During the high level period (with a duration of τ*TS / 2), switch 1 switches to port 1. At this time, T ’ N The signal is coupled to the antenna branch through the coupler and then enters the receiving channel. During the low level period, switch 1 switches to port 2. At this time, T ’ N The signal is disconnected, and only the antenna signal TA enters the receiving channel. When A>B in the decision module, the switch 1 control signal generation module controls to increase the duration of τ, thereby increasing the size of B. When A<B in the decision module, the switch 1 control signal generation module controls to decrease the duration of τ, thereby decreasing the size of B. In this way, it loops until the balance between A and B is reached. At this time, we can obtain the following formula:

[0025]

[0026]

[0027] Thus we get:

[0028] T A +τT' N =T REF

[0029] Then we get:

[0030] T A =T REF -τT' N

[0031] Where T REF The brightness temperature is the temperature value T0 at the point where the matching load is located. T0 can be obtained in real time by placing a high-precision temperature measuring resistor near the matching load, thereby obtaining T REF =T0. The τ value can be obtained through the real-time output of the acquisition processor, thereby finally obtaining the absolute brightness temperature value T input by the antenna. A .

[0032] Taking the absolute brightness temperature measurement method of the ocean salinity satellite synthetic aperture radiometer subsystem as an example, select one channel to measure the absolute brightness temperature value of the noise injection receiver input port. The test conditions are as follows:

[0033] AD sampling rate: 100MHz;

[0034] AD resolution: 10bit;

[0035] T REF Brightness temperature: 305K

[0036] T S Time: 20ms

[0037] T ’ N Brightness temperature: 750K

[0038] Noise injection receiver input signal: liquid nitrogen cooling source + adjustable attenuator

[0039] 1) The method proposed in the present invention has a single integration time of 10ms, and the two ports are switched in turn, with each port taking 500ms.

[0040] Measured using a prototype:

[0041] Ta_std (antenna absolute brightness temperature variance) = 0.2155K

[0042] Ta_mean (mean absolute brightness temperature of antenna) = 172.01K

[0043] 2) Traditional two-point calibration method, single integration 10ms, three ports switched in turn, each port 330ms.

[0044] Measured using a prototype:

[0045] Ta_std (antenna absolute brightness temperature variance) = 0.2754K

[0046] Ta_mean (mean absolute brightness temperature of antenna) = 172.10K

[0047] 3) Traditional two-point calibration method, single integration is 10ms, three ports are switched in turn, each port is 500ms.

[0048] Measured using a prototype:

[0049] Ta_std (antenna absolute brightness temperature variance) = 0.2127K

[0050] Ta_mean (mean absolute brightness temperature of antenna) = 172.18K

[0051] The test results show that, with the same integration time per port, the traditional two-point calibration method has lower sensitivity than the proposed method. Only when the traditional two-point calibration method uses a 500ms integration time per port (1.5s measurement per cycle) does the calculated brightness temperature sensitivity match that of the dynamic balance method using a 500ms integration time per port (1s measurement per cycle).

[0052] That is to say, when the synthetic aperture radiometer system is working in orbit, within the same measurement time, the absolute brightness temperature measurement method proposed in this paper has a temperature measurement sensitivity 1.23 times higher than that of the traditional two-point calibration method.

[0053] Although the present invention has been disclosed above in terms of preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solutions of the present invention using the technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the scope of protection of the technical solutions of the present invention.

Claims

1. A method for calibrating absolute brightness temperature for a synthetic aperture radiometer system, characterized in that: include: The measured antenna signal TA and the calibration signal T ’ N After being combined through the coupler, the output signal brightness temperature is The coupler output signal is input to the receiving channel after passing through switch 2; the receiving channel amplifies, filters, down-converts and filters the input signal and then outputs an intermediate frequency signal IF, which enters the acquisition processor; the acquisition processor digitally quantizes the intermediate frequency signal IF through AD and then sends it to the FPGA; the FPGA internally includes a multiplier-accumulator a, a multiplier-accumulator b, a judgment module, a switch 1 control signal generation module and a switch 2 control signal generation module; the switch 2 control signal generation module generates a square wave signal with a period of TS and a duty cycle of 50%; during the period when the square wave signal is at a level, switch 2 is controlled to cut off port 2, at which time the receiving channel input matches the load signal, the sampled data of AD enters the multiplier-accumulator a, and the multiplier-accumulator a multiplies and accumulates the sampled data and outputs the result A; during the period when the square wave signal is at a high level, switch 2 is controlled to cut off port 1, at which time the receiving channel input signal is The sampling data of AD enters the multiplication and accumulation device b, which multiplies and accumulates the sampling data and outputs the result B. After one TS period ends, the two multiplication and accumulation results A and B enter the decision module. The decision module adjusts the duration of τ according to the size of A and B until the balance between A and B is achieved, and finally the absolute brightness temperature value T of the antenna input is obtained. A ; The calibration signal T ’ N The signal brightness temperature after the calibration signal generated by the diode noise source passes through the attenuator and switch 1; the control signal of switch 1 is generated by the acquisition processor and controls whether the calibration signal of the diode noise source is connected to the coupler; The control signal of switch 2 is generated by the acquisition processor, and controls the input signal of the subsequent receiving channel to connect to the antenna branch, that is, the coupler output, or to the matching load; The switch 1 control signal generating module generates the switch 1 control signal according to the result of the judgment module. When the switch 1 control signal is high, the switch 1 is switched to port 1. At this time, T ’ N The signal is coupled to the antenna branch through the coupler and then enters the receiving channel. When the switch 1 control signal is low, the switch 1 switches the 2 port. At this time, T ’ N The signal is disconnected and only the antenna signal enters the receiving channel; The duration of the high level period of the switch 1 control signal is τ*TS / 2, and the τ value is obtained through the real-time output of the acquisition processor; The decision module adjusts the duration of τ according to the magnitudes of A and B until the balance between A and B is achieved, and finally obtains the absolute bright temperature value T of the antenna input A , including: when A > B in the decision module, the switch 1 control signal generation module controls to increase the duration of τ, thereby increasing the magnitude of B; when A < B in the decision module, the switch 1 control signal generation module controls to decrease the duration of τ, thereby decreasing the magnitude of B; and so on in a loop until the balance between A and B is achieved, and finally the absolute bright temperature value T of the antenna input is obtained A .

2. The absolute brightness temperature calibration method for a synthetic aperture radiometer system according to claim 1, characterized in that: The absolute brightness temperature T A =T REF -τT ’ N , where T REF The brightness temperature value is the temperature value matching the load point.

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