Prelaunch Radiometric Characterization and Calibration for Long Wave Infrared Band of FY-4B GHI

Objective Calibration is the process of describing the parameters needed to understand and quantify the expected application performance of a sensor. In the prelaunch radiometric characterization and calibration, the on-orbit environment should be simulated as much as possible, and the tests are performed in a controlled environment using a standard source of known radiation. The measurements made during the prelaunch radiometric calibration are used to verify whether the instrument status is correct, quantify the calibration equation and radiometric measurement model parameters, and estimate the measurement uncertainty. The measurement performance and limitations are determined, and whether the sensor meets the mission requirements are verified. By identifying and characterizing unique sensor performance characteristics, the influence of sensor behavior on expected measurement is minimized. The Geostationary High- speed Imager (GHI) is one of the main payloads of Fengyun-4B (FY-4B) satellite, the research in this paper is an important basis of the on orbit quantitative application for long wave infrared (LWIR) band of GHI. Methods The commonly used radiometric calibration methods in the laboratory are the distant small source combined with the collimator method and the near extended source method. The distant small source combined with collimator method is to place the point source blackbody at the focal plane of the collimator to realize the beam expansion and collimation of the calibration beam, so that the calibration beam can fill the instrument's entrance pupil and field of view. The near extended source method can provide a standard calibration source with good stability, uniformity, Lambertian and large aperture, and does not require expensive low-temperature collimators and accurate collimator calibration parameters. In this paper, the large- area and near extended source blackbody calibration method is used, which can realize the full field of view and full aperture radiometric calibration of GHI. GHI, external blackbody, deep low temperature cold screen and other equipment are placed in the vacuum tank. The cold screen and external blackbody are placed along the direction of the sub-satellite point, directly above the optical aperture of the instrument, and the blackbody is placed behind the cold screen (Fig. 2). The vacuum in the tank is better than 1. 3x10-3 Pa, the heat sink temperature is lower than 100 K, and the effective emissivity of the tank wall surface is better than 0. 9. The effective emissivity of the external blackbody is about 0. 99, and the temperature control range is 180- 330 K. Within this range, the radiance levels of 16 temperature points are set (Table 2). During the experiment, the on-board blackbody observation will be performed simultaneously. The cold screen is used to simulate the 4K space view in orbit, as the "zero radiation" standard for instrument observation, the temperature of the cold screen is lower than 85 K, and the effective emissivity of the surface is better than 0. 9. Results and Discussions The quadratic polynomial can be used to describe the radiometric response model of GHI LWIR, and the calibration equation has good goodness of fit (Fig. 4 and Table 3). At the typical temperature of 300 K, the fitting accuracy of radiometric response model is better than 0. 23 K (Table 4). The analysis and evaluation of various error values in the calibration process show that the prelaunch calibration uncertainty is better than 0. 670 K ( coverage factor is 2) at 300 K (Table 5). The spatial noise characteristics are measured and studied, and the relationship between the fixed pattern noise ( original signal, dark current and response signal) and the radiance is analyzed (Fig. 5). For the problem of unavailable detector on LWIR focal plane, GHI adopts 256x4 to realize mutual backup of 4 line arrays and select the best imaging detector ( Fig. 6). Through the research and analysis of the experimental data, two kinds of the best imaging detector selection criteria are proposed, namely, the principle of maximizing the signal-to- noise ratio and the principle of minimizing the fixed pattern noise of response signal. Thus, two kinds of the best imaging detector combination line arrays are given ( Fig. 7). The detection sensitivity at a typical temperature of 300 K is measured ( Fig. 10). The detection sensitivity of the two new line arrays mentioned above meets the mission requirements (better than 0. 2 K). In particular, for the new line array based on maximizing the signal-to- noise ratio, the overall detection sensitivity at 300 K is better than 0. 1 K, and the average level is about 50 mK. The on- board blackbody reference standard is validated according to the full dynamic range (180- 330 K) radiometric calibration equation obtained from the external blackbody (Fig. 11). Linear fitting between the true brightness temperature and the nominal brightness temperature at the five temperature points ( 290, 295, 300, 305, 310 K) of the on- board blackbody is performed ( Fig. 12 and Table 6). The nominal brightness temperature of the on- board blackbody is slightly lower than the true brightness temperature. At the typical temperature of 300 K, the nominal brightness temperature is about 0. 42 K lower than the true brightness temperature (the average value of the line array). Through the preliminary analysis and judgment of the structural components around the on-board blackbody of GHI and the calibration correction algorithm of the on-board blackbody of similar remote sensing instruments, the main reason why the nominal brightness temperature of the on-board blackbody is lower than the true brightness temperature is that the blackbody is not an ideal blackbody (emissivity is less than 1). When the instrument observes the non-ideal on-board blackbody, the radiation received is not only from the blackbody, but also includes the thermal radiation from the surrounding structural components reflected by the blackbody. Conclusions In view of the mission requirements and design characteristics of the LWIR band of the FY- 4B GHI, the prelaunch calibration and radiometric characteristic measurement methods based on the large-area and near extended blackbody radiation source are studied, and the corresponding calibration device is built to achieve calibration parameter measurement and radiometric characteristic characterization. Through the effective control of the error source of the prelaunch calibration process, the calibration uncertainty is better than 0. 67 K@ 300 K. The spatial noise characteristics and detection sensitivity are measured and studied. According to the characteristics of spatial noise and temporal noise, two methods to select the best imaging detector are proposed, which are based on maximizing the signal- to-noise ratio and minimizing the fixed pattern noise of response signal. Through the selection of the best imaging detector, the problem of no non-effective detector imaging on the satellite image of the long linear focal plane array can be solved. At the same time, the temperature detection sensitivity (average) can reach 50- 60 mK@ 300 K. According to the full dynamic range radiometric calibration equation obtained from the external blackbody measurement, the accuracy of the on-board blackbody reference standard is evaluated. The nominal brightness temperature of the on-board blackbody is slightly lower than the true brightness temperature, approximately 0. 42 K lower at the typical temperature of 300 K. It is preliminarily analyzed and judged that this phenomenon may be caused by the reflection of thermal radiation from the surrounding structural components by the on-board blackbody. Subsequently, the research on the calibration correction algorithm of the on- board blackbody will be performed according to the on-orbit data.