Method and an Apparatus of Calibrating a Thermal Satellite for Measuring Land Surface Temperature

Abstract

The invention concerns a method for calibrating a thermal satellite used to measure land surface temperature (LST). Based on real-time infrared image data, a preliminary LST product is calculated. For each location point, a reference surface temperature from a weather model is retrieved. A radiance measurement offset is then determined based on the reference temperature. Using this offset, the satellite's radiance measurements are corrected, and a calibrated LST product is generated. The method allows calibration to begin as soon as infrared data is sensed, enabling real-time LST measurement with real-time calibration. This supports dynamic in-orbit calibration throughout the satellite's orbit and improves the reliability and accuracy of LST measurements.

Claims

1. Method of calibrating a thermal satellite for measuring land surface temperature, LST, the method comprising the following method steps for calibration: calculating (S101) a preliminary LST product (T) based on infrared image data of the earth's surface acquired by the thermal satellite, for a location point included in the preliminary LST product (T), obtaining (S102) a reference surface temperature (R.sub.i) of a same time from a weather model, calculating (S103) a radiance measurement offset (d) based on the reference surface temperature (R.sub.i), determining (S104) a calibrated radiance measurements (L.sub.corr) of the thermal satellite based on the radiance measurement offset d, and calculating (S105) a calibrated LST product (T.sub.corr) based on the calibrated radiance measurements (L.sub.corr), wherein calibration starts directly when real-time infrared image data of the earth's surface has been sensed to enable real-time LST measurements with real-time calibration.

2. Method according to claim 1, after obtaining (S102) the reference surface temperature, the method further comprising: for the location point, comparing (S1021) the preliminary LST of the location point (T.sub.i) and the reference surface temperature (R.sub.i), and performing the calibration (S103, S104, S105) when the preliminary LST of the location point (T.sub.i) is different from the reference surface temperature (R.sub.i), or when a difference between them exceeds a threshold.

3. Method according to claim 1, wherein the reference surface temperature (R.sub.i) obtained from the weather model comprises reference surface temperature (R.sub.i) obtained from an atmospheric reanalysis dataset, preferably, the atmospheric reanalysis dataset comprises ERA5 or Global Forecast System, GFS, dataset.

4. Method according to claim 1, before obtaining the reference surface temperature (R.sub.i), the method further comprising: filtering (S1011) the preliminary LST product (T) to remove errors due to clouds, selecting (S1012) the location point of which the preliminary LST (T.sub.i) is between the 25th and 75th percentile of a product distribution of the filtered product, preferably, the location point is selected by randomly sampling.

5. Method according to claim 1, wherein the location point comprising: a location point of a homogeneous surface with known emissivity, which preferably comprises large water bodies, desserts or ice fields.

6. Method according to claim 1, wherein determining (S104) the calibrated radiance measurements (L.sub.corr) comprising: determining (S104) the calibrated radiance measurements by subtracting (S1041) the radiance measurements offset (d) from initial radiance measurements (L) acquired by the thermal satellite.

7. Method according to claim 1, wherein calculating (S103) the radiance measurement offset (d) based on the reference surface temperature (R.sub.i) comprises: for the location point: calculating (S1031) an expected radiance measurement $\left(L_i^{*}\right)$ based on the reference surface temperature (R.sub.i) of the location point, and calculating (S1032) an offset (d.sub.i) between the expected radiance measurement $\left(L_i^{*}\right)$ and an initial radiance measurement (L.sub.i) of the location point.

8. Method according to claim 7, wherein the offset (d.sub.i) is calculated as a Euclidean distance between the expected radiance measurement $\left(L_i^{*}\right)$ and the initial radiance measurement $\left(L_i^{*}\right).$

9. Method according to claim 7, wherein exactly one location point is provided, and wherein calculating (S103) the radiance measurement offset (d) based on the reference surface temperature (R.sub.i) further comprises: determining (S1033) the offset (d.sub.i) of the location point as the radiance measurement offset (d).

10. Method according to claim 7, wherein calculating (S103) the radiance measurement offset (d) based on the reference surface temperature (R.sub.i) further comprises: when multiple location points are provided, determining (S1034) an average value or a weighted average value of the offset (d.sub.i) of the location point as the radiance measurement offset (d).

11. Method according to claim 7, wherein the expected radiance measurement $\left(L_i^{*}\right)$ of each location point is calculated backwards according to radiative transfer equation, based on the reference surface temperature (R.sub.i), an emissivity .sub.i of a surface of the location point and atmospheric correction parameters, and wherein the atmospheric correction parameters include an upwelling radiance (L.sub.up,i), a downwelling radiance (L.sub.down,i), and an atmospheric transmittance (.sub.i).

12. Apparatus (1) for calibrating a thermal satellite for measuring LST, which comprises: a calibration module (11) configured to perform the steps of the methods according to claim 1.

13. Computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to claim 1.

14. Computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 schematically illustrates measurements of LST utilizing thermal satellites,

[0034] FIG. 2 schematically depicts a flow chart of the calibration method according to one embodiment of the present invention,

[0035] FIG. 3 schematically depicts a flow chart of the calibration method according to another embodiment of the present invention, and

[0036] FIG. 4 schematically depicts a block diagram of the calibration apparatus according one embodiment of the present invention

[0037] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. Unless otherwise indicated, when the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Instead, they are merely examples of devices and methods consistent with some aspects of the present invention as detailed in the appended claims.

[0038] FIG. 1 schematically illustrates the measuring of LST utilizing thermal satellites. As depicted in FIG. 1, the Earth's surface emits thermal infrared radiations as a result of its temperature. The wavelength at which maximum emission occurs varies roughly from 8 to 13 m with the LST ranging from typically 250 K to 330 K. With an increase of the land surface temperature, the wavelength peak moves to shorter wavelengths. For example, in extreme cases of wildfire, the LST reaches 800 K, the maximum emission is around 3.6 m.

[0039] To estimate the LST, thermal satellites measure the spectral radiance of the land surface via infrared sensors with appropriate wavelength windows. Usually, the radiance is sensed in the form of infrared images. Then, using radiative transfer equation (RTE) and assuming that the surface is a perfect emitter, i. e., blackbody, the spectral radiance response of the infrared sensors is converted into the brightness temperatures, i. e., radiance measurements, where the emissivity is fixed at 1.0. As further depicted in FIG. 1, since the spectral radiance responses are affected and altered by the surface material and atmospheric effects, it requires correcting for surface emissivity, atmospheric compositions, humidity and other factors. Usually, the atmospheric correction includes the upwelling radiance, the downwelling radiance, the atmospheric transmittance and so on. The LST product derived from the sensor response may contain an array of geolocated surface temperatures. The position of each element in the array corresponds to the geographic location sensed.

[0040] FIG. 2 shows a flow chart of a method for calibrating the LST measurements of the thermal satellite according to an embodiment of the present invention. As depicted in FIG. 2, the method comprises the following steps: [0041] S101: calculating a preliminary LST product T based on data acquired by the thermal satellite, i. e. real-time infrared image data of the earth's surface, [0042] S 102: for at least one location point included in the preliminary LST product T, obtaining a reference surface temperature R.sub.i of a same time from a weather model, [0043] S103: calculating a radiance measurement offset d based on the reference surface temperature R.sub.i, [0044] S104: determining a calibrated radiance measurements L.sub.corr of the thermal satellite based on the radiance measurement offset d, and [0045] S105: calculating a calibrated LST product T.sub.corr based on the calibrated radiance measurements L.sub.corr.

[0046] Calibration starts directly when real-time infrared image data of the earth's surface has been sensed to enable real-time LST measurements with real-time calibration.

[0047] The method can be computer-implemented and can be applied directly to a processor of the thermal satellite or to offline computational devices calibrating the thermal satellite in terms of LST measurements, for example, servers or distributed computational equipment.

[0048] The preliminary LST product T is an array of the geolocated surface temperatures which may be calculated via any conventional LST retrieval algorithms, for example, single-channel methods, multi-channel methods, multi-angle methods and so on. The position of each LST element included in the product T corresponds to a location point in the sensed land surface. In other words, the LST product T may comprise LSTs of each location points sensed by the thermal satellite. The unit of the LST and reference surface temperature may be Kelvin (K). Similarly, the calibrated radiance measurements L.sub.corr and calibrated LST product T.sub.corr may also be arrays with calibrated radiance measurements and LSTs for all location points included in the preliminary LST products. In this term, the radiance measurement offset d computed based on certain reference location points are applied globally to all locations included in the resolution of the thermal satellite.

[0049] In a specific embodiment, the preliminary LST product may be calculated via inverse of the Planck's law B as depicted in equation (1):

[00001]$\begin{array}{cc}T=B^{-1}\left(\left(\frac{L-L_{\mathrm{up}}}{}-\left(1-\right).Math.L_{down}\right).Math.\frac{1}{},\right)& \left(1\right)\end{array}$ [0050] wherein: [0051] L represents an initial spectral radiance measurements array sensed by the thermal satellite, [0052] represents the emissivity map of the surface containing emissivity of each location points, [0053] is the atmospheric transmittance, [0054] L.sub.up and L.sub.down are the upwelling radiance and downwelling radiances [0055] is the response wavelength

[0056] The preliminary LST produce may also be calculated by other LST retrieval schemes.

[0057] The reference surface temperature R.sub.i should be of equal location and time as the preliminary LST so as the eliminate the influence of different environmental and equipment conditions, like the temperature of part of the thermal satellite. Preferably, for robust calibration, more than one reference location point is selected, and it is recommended to select more than 5 reference location points.

[0058] FIG. 3 depicts another embodiment according to the present invention. Compared to the embodiments shown in FIG. 2, after obtaining (S102) the reference surface temperature, the method comprises the following step: [0059] S1021: for the at least one location point, comparing whether the preliminary LST of the location point T.sub.i and the reference surface temperature R.sub.i are different.

[0060] Only when it is determined that they are different, the method proceeds further the steps of calculating (S103) the radiance measurement offset d, determining (S104) the calibrated radiance measurements L.sub.corr and calculating (S105) the calibrated LST product T.sub.corr of the thermal satellite.

[0061] It can be understood that, when the preliminary LST of certain locations points is not equal to the reference surface temperature obtained from weather model, the absolute range of the LST measurement of the thermal satellite was disturbed and requires calibration.

[0062] In the alternative, step S1021 may compute a difference between the preliminary LST T.sub.i and reference surface temperature R.sub.i of each location point, and proceed the subsequent steps (S103, S104 and S105) only when the difference exceeds a threshold, for example, 0.1 or 1 K. Of course, when multiple location points are selected, the subsequent steps may be performed only when an average difference or a mean squared error of the differences exceeds a threshold. The comparing of the two temperatures is optional and the calibration can be directly applied to each LST measurements to improve its accuracy and robustness.

[0063] In an embodiment, the reference surface temperature R.sub.i acquired from a weather model may be reference temperature R.sub.i acquired from an atmospheric reanalysis dataset. Preferably, the atmospheric reanalysis dataset comprises a ERA5 or GFS dataset. These consistent and comprehensive datasets are produced with modern numerical weather prediction models for a manifold of processes within the atmosphere. The models include the radiative transfer between different layers of the atmosphere, the interaction of the atmosphere with the surface, and the interaction of the atmosphere with the sun. The lowest layer in the model is used to represent the surface conditions of the Earth, and its temperature (so called skin temperature) is computed to solve the system of radiative transfer equations for the stratified atmospheric model. The surface temperatures obtained from these datasets are thus highly accurate and ideal to be utilized as reference temperatures in back calculation via radiative transfer equations.

[0064] The thermal satellite starts the calibration directly when an infrared image of the earth's surface has been sensed. Therefore, same environmental and equipment conditions are applied, which reduces the errors resulting from different statuses of the thermal satellite, different orbit positions and different atmospheric or ground conditions.

[0065] In addition, to improve the robustness of calibration method, it is proposed to eliminate the effect of clouds and outlier points. Thus, before obtaining the reference surface temperature, the method may further comprise the following steps: [0066] S1011: filtering the preliminary LST product T to remove errors due to clouds, and [0067] S1012: selecting the at least one location point of which the preliminary LST T.sub.i is between the 25th and 75th percentile of a product distribution of the filtered product.

[0068] Preferably, the at least one location point (T.sub.i, i1, . . . , N) is randomly sampled from the 25th and 75th percentile of the product distribution of the filtered product. The filtering to remove errors due to clouds can be conventional cloud filtering or screening technologies.

[0069] In addition to the filtering and randomly sampling, the locations points may be sampled or selected from location points of a homogeneous surface with known emissivity, which preferably comprises large water bodies, deserts or ice fields. It is because the temperature data from these surfaces has higher accuracy, consistency and reliability and the back computation of spectral radiance based on the temperature data is thus more reliable.

[0070] As discussed in the first embodiment of the present invention, the radiance measurement offset d computed based on certain reference location points are applied globally to all locations included in the resolution of the thermal satellite. Specifically, this is implemented by the method following step: [0071] S1041: subtracting the radiance measurements offset d from initial radiance measurements L acquired by the thermal satellite so as to obtain the calibrated radiance measurements L.sub.corr.

[0072] The initial radiance measurements L and calibrated radiance measurements L.sub.corr may be arrays containing spectral radiance data of all location points included in the remote sensing image. And the offset d is subtracted from each element of the initial radiance measurements L according to equation (2):

[00002]$\begin{array}{cc}{L}_{corr}=L-d& \left(2\right)\end{array}$

[0073] Correspondingly, the calibrated LST product T.sub.corr is then calculated (S105) based on the calibrated spectral radiance measurements L.sub.corr with emissivity and atmospheric corrections with the inverse of Planck's law B as shown in equation (3):

[00003]$\begin{array}{cc}{T}_{corr}=B^{-1}\left(\left(\frac{L_{corr}-L_{up}}{}-\left(1-\right).Math.L_{down}\right).Math.\frac{1}{},\right)& \left(3\right)\end{array}$

[0074] Apart from the calibrated radiance measurements, the same parameters of equation (1) are applied.

[0075] The radiance measurements offset d applied globally in equation (1) is calculated with respect to the reference location points based on the corresponding reference surface temperature R.sub.i. According to an embodiment, the method further comprises the following steps: [0076] S1031: calculating an expected radiance measurement

[00004]$L_i^{*}$ based on the reference surface temperature R.sub.i for each of the at least one location point, and [0077] S1032: calculating an offset d.sub.i between the expected radiance measurement

[00005]$L_i^{*}$ and an initial radiance measurement L; of the location point.

[0078] Specifically, the expected radiance measurement

[00006]$L_i^{*}$

of and the initial radiance measurements L.sub.i of each reference location points may be represented as complex values and the offset (d.sub.i) is calculated as a Euclidean distance between the expected radiance measurement

[00007]$\left(L_i^{*}\right)$

and the initial radiance measurement

[00008]$\left(L_i^{*}\right),$

as depicted in equation (4):

[00009]$\begin{array}{cc}{d}_i={.Math.L_i^{*}-L_i.Math.}_2,i1,2,.Math.N& \left(4\right)\end{array}$

[0079] Particularly, as introduced above, the method may sample or select any number of reference location points. In case that only one location point is selected, the method may comprise the step of: [0080] S1033 determining the offset d.sub.i of the location point as the radiance measurement offset d to be applied globally.

[0081] Alternatively, when multiple locations points are selected or randomly sampled, the method may calculate an average value or weighted average value of the offsets d.sub.i, i2, . . . , N and further comprises the step of: [0082] S1034: determining the average value or the weighted average value of the offset of the least one location point as the radiance measurement offset d.

[0083] For the sake of robust, it is recommended to select more than 5 reference location points. In addition, it is also possible to select the offset of one location point as the global radiance measurement offset d.

[0084] In another embodiment, the expected radiance measurement

[00010]$L_i^{*}$

of each location point may be calculated backwards according to Radiative Transfer Equation used for the calculation of LST. The expected radiance measurement

[00011]$L_i^{*}$

may be calculated based on the reference surface temperature R.sub.i, an emissivity .sub.i of a surface of the location point and atmospheric correction parameters applied to this location points which includes: the upwelling radiance L.sub.up,i, the downwelling radiance L.sub.down,i, and the atmospheric transmittance .sub.i.

[0085] Specifically, to calculate the expected radiance measurement

[00012]$L_i^{*}$

of the reference location points, the method applies Planck's law B to estimate the spectral radiance

[00013]$L_{bb,i}^{*}$

of a black body with the given reference temperature R.sub.i as shown in equation (5):

[00014]$\begin{array}{cc}{L}_{bb,i}^{*}=\frac{B\left(R_i,\right)f\left(\right)d}{f\left(\right)d}& \left(5\right)\end{array}$

[0086] Then, the effect of emissivity and atmospheric conditions are further taken into account to calculate the expected radiance measurements

[00015]$L_i^{*}$

the sensor according to equation (6):

[00016]$\begin{array}{cc}{L}_i^{*}={}_i.Math.\left({}_i.Math.L_{bb,i}^{*}+\left(1-{}_i\right).Math.L_{\mathrm{down},i}\right)+L_{\mathrm{up},i}& \left(6\right)\end{array}$

[0087] Then, the expected radiance measurements are applied to equation (4) for the calculation of the global radiance measurement offset d which is subsequently applied to equations (2) and (3) to calculate the calibrated radiance measurements L.sub.corr and calibrated LST T.sub.corr.

[0088] FIG. 4 is a block diagram of an apparatus 1 for calibrating a thermal satellite for measuring LST, the apparatus comprising a calibration module 11 configured to perform the steps of any of the methods as disclosed above. The apparatus can be any device which is capable of computation. Preferably, the apparatus is capable of communicating with satellites and/or control the satellite. For example, the apparatus can be applied to a processor/microprocessor of the thermal satellite. The apparatus may also be computational devices for offline calibration of the thermal satellite in terms of LST measurements, for example, servers or distributed computational equipment.

[0089] In addition, the present invention further provides a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to any of the method above.

[0090] The present invention also provides a computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the steps of the method according to any of the method above.

REFERENCE SYMBOL LIST

[0091] 1 calibration apparatus [0092] 11 calibration module [0093] S101 method step [0094] S102 method step [0095] S103 method step [0096] S104 method step [0097] S105 method step [0098] S1021 method step [0099] S1011 method step [0100] S1012 method step [0101] S1041 method step [0102] S1031 method step [0103] S1032 method step [0104] S1033 method step [0105] S1034 method step [0106] T preliminary LST product [0107] R reference surface temperature [0108] d radiance measurement offset [0109] L.sub.corr calibrated radiance measurements [0110] T.sub.corr calibrated LST product [0111] T.sub.i preliminary LST product of ith location point

[00017]$L_i^{*}$ expected radiance measurement of ith location point [0112] L.sub.i initial radiance measurement of ith location point [0113] d.sub.i offset of ith location point

[00018]$L_{bb,i}^{*}$ spectral radiance estimation of ith location point [0114] emissivity [0115] atmospheric transmittance [0116] L.sub.up upwelling radiance [0117] L.sub.down downwelling radiance [0118] B Planck's law