Application of passive remote sensing to investigate transmissivities of trees canopies using ground-Based upward-looking microwave radiometers

Vietnam Journal of Science and Technology 55 (6) (2017) 767-779 DOI: 10.15625/2525-2518/55/6/8774 APPLICATION OF PASSIVE REMOTE SENSING TO INVESTIGATE TRANSMISSIVITIES OF TREES CANOPIES USING GROUND-BASED UPWARD-LOOKING MICROWAVE RADIOMETERS Doan Minh Chung1, K. G. Kostov2, Vo Thi Lan Anh1, Huynh Xuan Quang1, Tran Tuan Anh3, Mai Thi Hong Nguyen1, *, Le Dai Ngoc3 1Space Technology Institute,Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam 2Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko chause- Sofia1784, Bulgaria 3Defense Mapping Agency of VietNam, Ministry of Defence, 2/198 Tran Cung, Co Nhue, Tu Liem, Ha Noi, Viet Nam *Email: mthnguyen@sti.vast.vn Received: 10 October 2016; Accepted for publication: 13 May 2017 Abstract. In Vietnam, in recent years, remote sensing technology has developed very strongly in many different application areas. Besides, remote sensing using optical satellite, radar satellites, passive remote sensing includingmicrowave radiometers has been studied for a long time. This paper presents the results of the study on emission and transmissivity of trees canopyusing ground-based upward-looking microwave radiometers at two bands, L and X which were manufactured and calibrated byour research group at the Space Technology Institute - Vietnam Academy of Science and Technology. In the field measurement campaigns in 2015 and 2016, brightness temperatures were calculated from the output signal of the radiometers that measure trees canopy, transmissivity of the canopy in L-band, and X-band was calculated from the physical temperature of the canopy,the brightness temperature of blue sky and the brightness temperature of the canopy after the radiometers were calibrated using blue sky and the black body at the corresponding frequency base on transmissivity model. These results provide empirical evidence to quantify the characteristics of microwave which is transmitted in the canopy to support the analysis and assessment of corresponding data obtained when the device is set high above and measuring downwards to the canopy. Keywords: ultrahigh frequency spectrometer, passive remote sensing, transmittivity. 1. INTRODUCTION Remote sensing of forest canopies from airborne and spaceborne platforms using active and passive microwave systems is an advanced tool for ecosystem monitoring. Microwave Doan Minh Chung, et al. 768 radiometers could give additional information about denseforest transmissivity and biomass, where other remote sensing systems (opticalradiometers, Synthetic-aperture radar - SAR) suffer the early saturation effect. Ground-based upward-looking radiometers have been applied previously for monitoring the temporal changes of the emission of deciduous and coniferous trees, and estimating the corresponding transmissivity and foliar biomass from the radiometric data. In remote sensing, the temperature and emission values are important parameters. Emission is related to temperature and transmission through foliage. Radiation from the trees can be partly absorbed and partially down to the ground. Although it is difficult to determine the emission from the transmissivity, it still affects to the microwave radiation from the canopy. The transmissivity of the vegetation contributes to determining the observed radiation. Transmissivity is one of the useful parameters for evaluating tree parameters. The results of some studies on seasonal variations of emissivity measured by ground-based upward-looking radiometers have been reported to be rather homogeneous. Recently, Mätzler [1] studied the microwave transmissivities of a large oak tree (Fagus silvatica L.) over a frequency range of 1 GHz to 100 GHz using ground-based upward-looking radiometers. Experiments showed that short wave radiation is sensitive to plant parameters. The purpose of this paper is to investigate the emissivity, transmissivity of trees canopies using ground-based upward-looking microwave radiometersat L-band (center frequency of 1.4 GHz) and X-band (center frequency of 11 GHz). 2. METHODOLOGY 2.1. Experimental description Figure 1. Three microwave radiometers mounted on a mechanical support. Three microwave radiometers, namely the L-band radiometer LNIR, the C-band radiometer CRM and the X-band radiometer XRM were used for measuring the microwave emission of the objects under investigation. The microwave units and the antennas of the radiometers LNIR, CRM and XRM were mounted on a mechanical support with antenna angle positioning system (Figure 1). Application of passive remote sensing to investigate transmissivities of trees canopies using 769 The LNIR is a Dicke-type noise-injection radiometer with center frequency 1.41 GHz. The CRM is a total-power radiometer with center frequency tunable in the range 3.5 to 3.7 GHz. The XRM is a Dicke-type radiometer with center frequency tunable in the range 10.95 to 11.25 GHz. The antenna beamwidths (at – 3 dB) of the LNIR, CRM and XRM are 32º, 15º and 17º, respectively. All three radiometers have resolution (sensitivity) ≤ 0.3 K with one second integration time. The radiometers have two measurement modes: LOCAL and REMOTE. When working in LOCAL mode, each radiometer calculates the average value of the output frequency and the variance for any measuring interval between 1 second and 1024 seconds. When working in REMOTE mode, the radiometer should be connected to a personal computer (via the RS 232 output port) for data recording and post-processing. The joint field experiment was carried out in 2015 and 2016 near the Space Technology Institute, VAST campus in Hanoi, Vietnam (18 Hoàng Quốc Việt, coordinates 21º02ʹ48.3ʺ N, 105º48ʹ03.6ʺ E and altitude of 13 meters above sea level). A group of several African mahogany trees (Khaya senegalensis) was selected for this experiment. An infrared thermometer Fluke 63 and an electronic thermometer were used for measuring the temperatures of different tree elements, the temperature of the soil below the trees, the air temperature below the trees, and the temperature of the microwave absorber used for calibration of the radiometers. Before measuring the brightness temperature of an object or the environment, the microwave radiometers should be calibrated. Calibration of the microwave radiometers Figure 2. Scheme for calibration of radiometers. Calibration of the microwave radiometers was performed to determine the exact relationship between the output signal of the microwave radiometers (voltage or frequency) and the brightness temperature of the object. Calibration was done with two special reference materials: the black body (Absorber) represents the "hot standard object", and the blue sky, symbolizing the "cold standard object" (Figure 2). The radiometers were calibrated by measuring the emission of the sky and a high-quality microwave absorber with known physical temperature [2]. Doan Minh Chung, et al. 770 The microwave radiometers output signal was recorded for about 15 minutes and calculated on average. The output signal fi is converted to the brightness temperature TB by the following formula: , , , ( )B sky B absB B sky sky sky abs T T T T f ff f − = + − − (1) where TB,abs is the brightness temperatures of the absorber,TB,sky is the brightness temperatures of the blue sky and f, fsky, fabs are the value of the microwave radiometers signal (frequency) when measuring the canopy, blue sky and the absorber, respectively. After calibrating the radiometers, the radiometers were installed on the ground being directed upwards through the canopy of the investigated African mahogany trees. The tree canopy as seen from the microwave radiometers position for angles 0º, 15º , 30º , 45º and 60º (relative to zenith), in this position downwelling microwave radiation of the canopy was measured. 2.1. Transmission model of trees canopy using ground-based upward-looking microwave radiometers The passive remote sensing method is based on the measurement of the object's brightness temperatures by microwave radiometer, and then application of a physical model to calculate the quantities to be surveyed. 2.1.1. Brightness temperatures This research was applied and developed by the Mätzler model [1], and compared with the model of Vichev et al. [3]. According to Mätzler model, microwave radiometer measured the brightness temperature TB of down welling radiation, which can be expressed by: , 0 (1 )B B sky B VT tT rT r t T= + + − − (2) where t is the transmissivity and r is the reflectivity of the vegetation layer. The factor 1 - r- t is the emissivity of the vegetation layer, TB,sky is the brightness temperatures of the blue sky, TVis physical temperature of the canopy, TB0 is upward brightness temperatures of the ground,T0 is upward physical temperatures of the ground. We can express r and t as: , ( )B V V B skyT T t T T r Tδ − + − = (3) , V B V B sky T r T T t T T δ+ − = − (4) with the definition of: 0B VT T Tδ = − . According to Mätzler [1], we note that the surface emissivity of the ground below the canopy is near 0.95 over the entire frequency range. Therefore, TB0 approaches T0 even if TB is not very close to T0. The reflectivity r of the canopy is close to 0.1. With this value, we can estimate: 00.1( )Vr T T Tδ = − (5) Application of passive remote sensing to investigate transmissivities of trees canopies using 771 Since T0 and TV were always very similar (differences were typically within ± 2°C), we can neglect rdT in (4) and estimate the transmissivity from the physical temperature of the canopy, the brightness temperatures of the canopy, and the blue sky at the corresponding frequencies by the formula (7). Vichev et al. presented the concept of normalized brightness temperatures as calculated by the formula [2], B BN V T T T = (6) where TV is physical temperature of the canopy, TB is brightness temperatures. 2.1.2. Transmissivity Transmissivity was calculated by using two different models of Mätzler [1] and Vichev et al. [3], denoted as t and t2, respectively. With t is calculated by the formula (7), , V B V B sky T T t T T − = − (7) TB,sky is the brightness temperatures of the sky. Brightness temperatures TB were calculated from the output signal of the radiometers that measure trees canopy by the equation (1); where: TB,abs is the brightness temperatures of the absorber and f, fsky, fabs are the value of the microwave radiometers signal (frequency) when measuring the canopy, blue sky and the absorber, respectively. The sky brightness temperature TB,sky was calculated using the model of Pellarin et al. [4], ,( ) =  ( ) ( ) (8) where TB-ATMS-D(θ) and TB-COS-D(θ) are the downward atmospheric and cosmic brightness temperatures, respectively. The downward atmospheric brightness temperature TB-ATMS-D(θ) was calculated as:  ( )= (1-   !" ) (9) #  $ exp (3.9262 ( 0.2211/ ( 0.0036901" (10)  = exp(4.9274 + 0.002195T2m) (11) where τATM and TATMeq are the atmosphere optical thickness and equivalent temperature, Z (km) is the surface altitude and T2m (K) is the air temperature (measured at 2 m above the ground). The downward cosmic emission TB-COS-D(θ) was calculated as: ( ) = 2313   !" (12) where Tcosmos = 2.7 K. Using equations proposed by Vichev at al., the transmissivities of the tree crowns were calculated as follows: t2 ≈ 1- TBN (13) ∆t is the difference between t and t2 and was calculated as: ∆t = t – t2 (14) Doan Minh Chung, et al. 772 The experiment of Vichev et al. showed that the transmissivities obtained through the two methods of calculation gave similar results with differences ∆t ≤ 0.01. 3. EXPERIMENTALLY MEASURED DATA For the purpose of evaluating transmissivity through the trees canopy at microwave frequency band, specifically the L and X bands, the research team conducted experiments in October 2015 and July 2016 at the Vietnam Academy of Science and Technology campus in Hanoi. Measurements were made with the L-band radiometer in both horizontal and vertical polarization, while that of X band radiometer only performed with thevertical polarization. Immediately after the experiment was conducted, the tree and environment parameters were processed. 3.1. The sky brightness temperature TB,sky In the experiment, the blue sky brightness temperature often get approximate values: TB,sky ≈ 5 K. However in reality, TB,sky changes with altitude, air temperature, and the viewing angle can be calculated based on the model of Pellarin at al.. In this study, experimental location was fixed so that the height Z was not changed. Tables 1 and 2 below present the results of the calculation of blue sky brightness temperature for two times in situ measurements. Table1. Calculation results of TB,sky with Z = 0.012 km, and T2m = 300 K. θ τATM (K) TATMeq(K) TB-ATMS-D(K) TB-COS-D (K) TB,sky (K) 00 266.64 0.01 1.73 2.68 4.41 150 266.64 0.01 1.79 2.68 4.47 300 266.64 0.01 1.99 2.68 4.67 450 266.64 0.01 2.44 2.68 5.11 600 266.64 0.01 3.44 2.67 6.11 Table 2. Calculation results of TB,sky with Z = 0.012 km, and T2m = 308 K. θ τATM (K) TATMeq (K) TB-ATMS-D(K) TB-COS-D (K) TB,sky (K) 00 271.36 0.01 1.71 2.68 4.39 150 271.36 0.01 1.77 2.68 4.45 300 271.36 0.01 1.97 2.68 4.65 450 271.36 0.01 2.41 2.68 5.09 600 271.36 0.01 3.40 2.67 6.07 3.2.Brightness temperature of the canopy Application of passive remote sensing to investigate transmissivities of trees canopies using 773 Table 3. Brightness temperature of the canopy when the antenna is oriented towards the outside edge of the garden (measurements on 26/10/2015). θ Tabs(K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBLH (K) 00 304.2 3400 8968 303.5 6677 4.41 127.8 150 304.2 3400 8968 303.2 5762 4.47 177.1 300 304.2 3400 8968 302.4 5931 4.67 168.0 450 304.2 3400 8968 301.0 6142 5.12 156.9 600 304.2 3400 8968 300.6 6017 6.11 164.1 θ Tabs (K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBLV (K) 00 304.2 3425 8735 303.5 6658 4.41 121.7 150 304.2 3425 8735 303.2 5812 4.47 169.5 300 304.2 3425 8735 302.4 6082 4.67 154.3 450 304.2 3425 8735 301.0 6175 5.12 149.3 600 304.2 3425 8735 300.6 6224 6.11 147.1 θ Tabs (K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBXV (K) 00 304.2 3581 7255 303.5 4365 4.41 240.2 150 304.2 3581 7255 303.2 4375 4.47 239.4 300 304.2 3581 7255 302.4 4408 4.67 236.8 450 304.2 3581 7255 301.0 4582 5.12 222.7 600 304.2 3581 7255 300.6 4407 6.11 237.2 Figure 3. Brightness temperature of the canopy (measurements on 26/10/2015). Value TBLH in the same condition is often greater than the value TBLVof about 5 K to 10 K, but TBXV usually get the largest value in the range of 210 K and 250 K. The difference of the brightness temperature is mainly due to observation angle θ decision. Doan Minh Chung, et al. 774 Table 4. Brightness temperature of the canopy when the antenna is oriented towards the middle of the garden (measurements on 16/07/2016). θ Tabs (K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBLH (K) 00 305.5 3470 9125 306.0 6595 4.39 139.1 150 305.5 3470 9125 305.8 5776 4.45 182.7 300 305.5 3470 9125 304.5 5592 4.65 192.6 450 305.5 3470 9125 303.6 5486 5.09 198.4 600 305.5 3470 9125 302.8 5264 6.07 210.5 θ Tabs (K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBLV (K) 00 305.5 3395 9110 306.0 6607 4.39 136.3 150 305.5 3395 9110 305.8 5790 4.45 179.3 300 305.5 3395 9110 304.5 5721 4.65 183.1 450 305.5 3395 9110 303.6 5597 5.09 189.8 600 305.5 3395 9110 302.8 5316 6.07 204.9 θ Tabs (K) fabs (Hz) fsky (Hz) TV (K) f (Hz) TB,sky (K) TBXV (K) 00 305.5 2147 7296 306.0 3675 4.39 216.1 150 305.5 2147 7296 305.8 3455 4.45 229.0 300 305.5 2147 7296 304.5 3406 4.65 231.9 450 305.5 2147 7296 303.6 3297 5.09 238.4 600 305.5 2147 7296 302.8 3215 6.07 243.4 Figure 4. Brightness temperature of the canopy (measurements on 16/07/2016). Normally, when the antenna is oriented towards the middle of the garden to measure brightness temperature of trees canopy, the minimum value of brightness temperature will be corresponding to the observed vertical direction. When lowering the observation direction, that means the angle between the antenna axis and the vertical direction θ increases, all of the value TBLH, TBLV, TBXV tend to increase. Application of passive remote sensing to investigate transmissivities of trees canopies using 775 3.3. Transmissivity of the canopy Table 5. Transmissivity of the canopy when the antenna is oriented towards the outside edge of the garden (measurements on 26/10/2015). θ TV (K) TB,sky (K) TB (K) tLH TBN t2LH ∆ tLH 00 303.5 4.4 127.8 0.588 0.421 0.579 0.009 150 303.2 4.5 177.1 0.422 0.584 0.416 0.006 300 302.4 4.7 168.0 0.451 0.556 0.444 0.007 450 301.0 5.1 156.9 0.487 0.521 0.479 0.008 600 300.6 6.1 164.1 0.464 0.546 0.454 0.009 θ TV (K) TB,sky (K) TB (K) tLV TBN t2LV ∆ tLV 00 303.5 4.4 121.7 0.608 0.401 0.599 0.009 150 303.2 4.5 169.5 0.448 0.559 0.441 0.007 300 302.4 4.7 154.3 0.497 0.510 0.490 0.008 450 301.0 5.1 149.3 0.513 0.496 0.504 0.009 600 300.6 6.1 147.1 0.521 0.489 0.511 0.011 θ TV (K) TB,sky (K) TB (K) tXV TBN t2XV ∆ tXV 00 303.5 4.41 240.2 0.212 0.792 0.208 0.003 150 303.2 4.47 239.4 0.213 0.790 0.210 0.003 300 302.4 4.67 236.8 0.220 0.783 0.217 0.003 450 301.0 5.12 222.7 0.265 0.740 0.260 0.005 600 300.6 6.11 237.2 0.215 0.789 0.211 0.004 where TBN, tXV, t2XV and ∆ tXV are a non-unit coefficient. Figure 5. Transmissivity of the canopy (measurements on 26/10/2015). In all experimental phases, tLH and tLV at θ = 00 get the maximum value ranges from 0.55 to 0.6. When the antenna of the radiometers is oriented towards the outside edge of the garden, Doan Minh Chung, et al. 776 changing of the angle θ does not significally influence the transmissivity of L-band at both polarization. But when the antenna is oriented towards the middle of the garden, transmissivity of L-band at both polarization significantly reduced while increasing the angle θ. tLH and tLV value at the angle θ = 600 only between 0.31 to 0.33 showed that the leaves have little impact to transmissivity of L-Band. Table 6. Transmissivity of the canopy when the antenna is oriented towards the middle of the garden (measurements on 16/07/2016). θ TV (K) TB,sky (K) TB (K) tLH TBN t2LH ∆ tLH 00 306.0 4.39 139.1 0.553 0.455 0.545 0.008 150 305.8 4.45 182.7 0.408 0.598 0.402 0.006 300 304.5 4.65 192.6 0.373 0.633 0.367 0.006 450 303.6 5.09 198.4 0.352 0.654 0.346 0.006 600 302.8 6.07 210.5 0.311 0.695 0.305 0.006 θ TV (K) TB,sky (K) TB (K) tLV TBN t2LV ∆ tLV 00 306.0 4.39 136.3 0.563 0.445 0.555 0.008 150 305.8 4.45 179.3 0.420 0.586 0.414 0.006 300 304.5 4.65 183.1 0.405 0.601 0.399 0.006 450 303.6 5.09 189.8 0.381 0.625 0.375 0.006 600 302.8 6.07 204.9 0.330 0.677 0.323 0.007 θ TV (K) TB,sky (K) TB (K) tXV TBN t2XV ∆ tXV 00 306.0 4.39 216.1 0.298 0.706 0.294 0.004 150 305.8 4.45 229.0 0.255 0.749 0.251 0.004 300 304.5 4.65 231.9 0.242 0.762 0.238 0.004 450 303.6 5.09 238.4 0.218 0.785 0.215 0.004 600 302.8 6.07 243.4 0.200 0.804 0.196 0.004 Figure 6. Transmissivity of the canopy (measurements on 16/07/2016). Application of passive remote sensing to investigate transmissivities of trees canopies using 777 4. DISCUSSION In this experiment, the brightness temperatures TB and TBN corresponding to L-band measurements, do not depend much on the angle of view. According to Vichev et al. [3], the standard brightness temperature TBN and its variations in space and time depend on: branch mass and volume, water content, leaf biomass andwater content and temperature. The transmissivity of L-band, as shown in Table 1, tLH and tLV at angles θ = 00 are values ranging from 0.55 to 0.6, has similar results with those by some authors [3, 5, 6] and [7] and confirms that the canopy is semi-transparent. The error of transmissivity is estimated to be less than 0.01 for the values of TB, which is quite reasonable as compared with reported data. The transmissivity is defined by two different methods. Mätzler's model estimates the transmissivity from the physical temperature of the canopy, the brightness temperature of the canopy, and the blue sky at the corresponding frequencies through equation (6). The model by Vichev et al. used equations (13) to estimate the transmissivity from the physical temperature of the canopy and the brightness temperature of the canopy. Although the two methods have many differences, the results obtained by their application do not differ much each from the other. Experimentation has shown that the difference of the transmissivity obtained through two calculation methods is usually of ∆t ≤ 0.01. Transmissivity of L band with both horizontal and vertical polarization at 00 get value bigger at than that at 600. The brightness temperature of the canopy measured by the L band at the angles varies markedly in both polarizations, which demonstrates that L band is relatively large polarization. Transmissivity in the vertical direction is larger than that in the horizontal direction, which is easy to explain because transmissivity depends on the characteristics of vertical growth trees. The quantitative observations showed that the trunk was a dependent source of L-band radiation, which is consistent with the predictiion by the model of microwave radiation transmitted through trees canopy. The comparison of transmissivity shows that the transmissivity L band is less affected by the leaves of the tree. The transmissivity comparison between H and V polarization reveals the small contribution of the trunk to the total absorption of the canopy. From these observations, it can be concluded that tree branches play a major role in the absorption of L-band radiation propagated through the canopy. Experimental results showed that the brightness temperature of a canopy at a fixed position does not differ much at different times of measurement, exept for significant changes in weather. Because the research object is the foliage of the khaya senegalensis canopies that has matured, the state of foliage changed not much during the study period. TBLH mainly reached values in the range of 120 K to 200 K. The value of TBLH at the same test conditions are usually larger than TBLV values of about 5 K to 10 K but the values of TBXV are usually greatest and range from 210 K to 250 K. The difference of brightness temperatures is mainly due to the angle of observation θ decision. Typically, the antenna of the radiometer is directed towards the center of the the khaya senegalensis canopy to measure the brightness temperatures of the canopy, the minimum value of the brightness temperatures will correspond to the vertical upward direction. Lowering the viewing angle means that the angle between the axis of the antenna and the vertical axis θ increases, the values of TBLH, TBLV, TBXVtend to increase. This is understandable because when θ = 00, the radiometer is directed towards the "cold standard object" i.e.the blue sky. The vertical direction is also through the thinnest canopy if the distance from the test site to the edge of the canopy is much larger than the height of the canopy. During the experiment on October 26, 2015, the antenna of the radiometer was pointed to the outer edge of the khaya senegalensis Doan Minh Chung, et al. 778 canopy, at this measurement direction, the radiometer were disturbed by other objects outside the canopy so the results were not as accurate as in the case of the antenna of the radiometer is directed towards the center of the the khaya senegalensis canopy. Although the opening angle of the radiometers antenna was relatively large (the L band is 320 and the X band is 170), but due to the large differences in the viewing angles of 00, 150, 300, 450, 600, the results of the measurements differed significantly. In all experiments, tLH and tLVat angles θ = 00 have the greatest values from 0.55 to 0.6. When the antenna of the radiometer is directed to the outermost edge of the khaya senegalensis canopies, changing the angle θ does not significantly affect the transmissivity of the L-band radiometer at any polarizations. But as the antenna of the radiometer is pointed toward the center of the khaya senegalensis canopies, the transmissivity of the L-band radiometer at both polarizations decreases markedly by increasing the angle θ. The value of tLH and tLVat the angle θ = 600 is only 0.31 to 0.33. This suggests that the leaves have little effect on the transmissivity of the L-band radiometer. Furthermore, there is a difference in the transmissivity between the two polarizations of the L-band when lowering the viewing angle to show that the branches and trunks play a major role in L-band radiation absorption. The X-band transmissivity always get a low value of 0.2 to 0.3 and there is no noticeable change in changing the viewing angle θ. So it can be deduced that the leaves have a role of absorbing X_band radiation much larger than those of L-band. From the above quantitative results it can be shown that the canopy is an opaque medium with X-band frequency but semi-transparent medium for L_band. This finding is consistent with the results of other studies reported e.g. in [5 - 10]. This preliminary study has demonstrated the characteristics of L-band and X-band transmission in tree canopy based on both modelling and empirical data. 5. CONCLUSION Based on the field measurement campaign in 2015 and 2016, performed by using ground- based upward-looking microwave radiometers in L band and X band microwave transmissivities of khaya senegalensis canopies the brightness temperatures and the transmissivity of the canopy have been calculated and discussed. 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