Effect of Antireflective Surface at the Radiobrightness Observations for the Topsoil Covered with Coniferous Litter

Transcription

1 966 PIERS Proceedings, Moscow, Russia, August 18 21, 2009 Effect of Antireflective Surface at the Radiobrightness Observations for the Topsoil Covered with Coniferous Litter V. L. Mironov 1, P. P. Bobrov 1, 2, A. S. Yashchenko 1, 2, I. V. Savin 1, and A. V. Repin 1, 2 1 Kirensky Institute of Physics, SB RAS, Krasnoyarsk, Russia 2 Omsk State Pedagogical University, Omsk, Russia Abstract In radio thermal remote sensing of the land, a spot on the earth surface emitting the signal received by the microwave radiometer antenna may be partially or completely covered by forest. We developed a theoretical model to calculate radiobrightness of the layered topsoil. Using this model we derived the dielectric constants pertaining to the separate layers of topsoil. For this purpose, we applied the least squares method for fitting the calculated diurnal radiobrightness dependences to those measured. The forest soil covered with coniferous litter was modeled by a three steps dielectric profile, the interfaces between the separate layers being smooth. The theoretical modeling conducted confirmed that, at the moments of maximal radiobrightness, an antireflective layer was formed on the topsoil surface, most probably due to nonisothermal upward moving of soil moisture. At that, the theoretically calculated dielectric constant of the litter antireflecting layer was found to be on the same order as the value measured in laboratory conditions. 1. INTRODUCTION One of the goals of the SMOS program (Soil Moisture and Ocean Salinity) of the European Space Agency is regional monitoring of soil surface moisture with the use of the space born radiometer at the frequency of 1.4 GHz [1]. As forest areas approximately cover the third of the land, a spot on the land surface radiating towards the radiometer antenna may be partly or completely covered with forest. Once the forest canopy is semi-transparent at the frequency of 1.4 GHz, the studies of radiating properties pertaining to the forest soil covered with litter are of significant interest [2]. Nevertheless, only a few papers are available in the literature regarding the problem of radio thermal radiation of the forest soil covered with the organic matter. For instance, moisture estimations through radiobrightness measurements were carried out in [3] in the case of grassy litter. While the authors of [4] founded that the litter in deciduous forest noticeably affects the radiobrightness observed. In this paper, we studied the influence of coniferous forest litter on the radiobrightness diurnal cycles. 2. MEASUREMENT DESCRIPTION The measurements were carried out at the test site Pogorelsky Bor of the Institute of Forest SB RAS located near the city of Krasnoyarsk (Russia). The soil radiation was measured at the look angle of 45 relative to nadir. The bandwidths of the radiometer at 1.4 and 6.9 GHz were equal to 60 MHz and 200 MHz, respectively. The fluctuation sensitivity of radiometers was about 0.3 K. The directional patterns of receiving antennas at the 3 db level were about 22. The metal sheet reflecting the sky radiation and the smooth water surface were used as standards to calibrate the radiometers. The radiobrightnesses of metal sheets were taken of 3 K and 5 K at the frequencies of 1.4 and 6.9 GHz, respectively. The radiobrightness of the smooth water surface was estimated according to the Fresnel reflection coefficient and Debye model for the water permittivity. An absolute error of calibration was estimated to be of about 3 5 K. The measurements of radiobrightness, T b, were carried out with intervals from 20 to 40 minutes. While the meteorological data of the atmosphere and the temperature in the topsoil were acquired with the frequency once per minute, using the Campbell Scientific Inc weather station. In addition, the moisture of topsoil was determined by the thermostat-weight method twice a day. Since measurement of soil radiobrightness inside a forest environment isn t possible due to absence of strong contrast between radio thermal radiations pertaining to the soil and sky, the second being shaded by the forest canopy, the test site was set up outside the forest. In the process of preparing a test site out of forest, the 20 cm topsoil layer was removed from the plot of 25 square meters, located in the forest glade. Intact soil clods of adequate thickness were taken from the adjoining coniferous forest and placed instead of the removed topsoil, to develop an artificial forest topsoil covered with litter.

2 Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18 21, RESULTS OF MEASUREMENT According to our observations of radiobrigtness diurnal cycles for the bare soil, the topsoil radiobrightness and emissivity maxima relating to the bare soils were found to take place from 03:00 to 06:00 pm and from 06:00 to 08:00 pm, respectively. Taking into account the relationship, T b = χt, between the radiobrightness, T b, and absolute thermodynamic temperature, T, where χ is emissivity, the time shift of about from 2 to 3 hours between the two maximums can be explained by continuing topsoil drying, which increases emissivity after its thermodynamic temperature reached maximum and started to decrease. Finally, in spite of thermodynamic temperature decrease, the radiobrightness remains increasing due to increase in emissivity. The radiobrightness diurnal cycles measured for the topsoil covered with coniferous litter are shown in Fig. 1. In contrast to a bare soil radibrightness diurnal cycle, the radiobrightness maxima observed in Fig. 1 are seen in the early morning hours from about 03:00 to 06:00 am. This interval of time is, first, shifted to about 12 hours earlier, relative to the one pertaining to a bare soil, and, second, it corresponds to minimal topsoil temperatures. Taking into account the formula T b = χt, the maximum radiobrightness in the early morning hours for the topsoil covered with litter must occur only due to increase in its emissivity. Tb, 250 K aug aug aug 25 aug 26 aug 27 aug 28 aug 29 Date 12:00 12:00 12:00 12:00 12:00 12:00 12:00 Time (AM) Figure 1: Diurnal cycles of T b of the litter-covered soil at the horizontal polarization and frequencies of 6.9 GHz (1) and 1.4 GHz (2). t,cº; χ 0, ; 0,80 15; 0,75 10; 0,70 5; 0,65 22 aug 23 aug 24 aug 25 aug 26 aug 27 aug 28 aug 29 12:00 12:00 12:00 12:00 12:00 12:00 12:00 Date Time (AM) Figure 2: The time dependence of the litter-covered soils emissivity on the horizontal polarization of at frequencies 6.9 GHz (1) and 1.4 GHz (2); the time dependence of the thermodynamic temperature of the forest litter at the depth of 0.5 cm (3). The respective diurnal cycles of emissivity are shown in Fig. 2 alongside with the thermodynamic temperature cycle. As seen from Fig. 2, the maximums of emissivity coincide with the minimums of thermodynamic temperature, and vice versa, which proves the origin of this maximum due to increasing in emissivity. As follows from Figs. 1 and 2, the emissivity minimum at the frequency

3 968 PIERS Proceedings, Moscow, Russia, August 18 21, 2009 of 6.9 GHz are seen more distinctly, as compared to the radibrightness ones. This fact suggests that only the most upper thin surface layer, which lies within the sensing depth of the radiometer at 6.9 GHz, must mainly affect a total emissivity of the topsoil covered with litter. The nature of radibrightnes diurnal cycle pertaining to the topsoil covered with litter is analyzed in the following section. 4. SIMULATION OF THE TOPSOIL EMISSIVITY We carried out numerical simulation for the radiobrightness diurnal cycle. In the process of simulation, there were used data from [6] on the complex permittivity (CDP) of coniferous litter, as well as the results of CDP measurements conducted by us for the samples of litter collected from the measurement plot. When modeling the emission of soil covered with coniferous litter, the topsoil matter was assumed to be isothermic. This assumption caused the error of 5 7 K in the modeled radiobrightness, which exceeds an absolute calibration error of both radiometers. Despite that, the results of simulations allowed to bring to understanding the nature for the maximum radiobrightness of a topsoil covered with litter to occur in the morning hours. Since the relative frequency bandwidth of both radiometers did not exceed 4%, the radiobrightness temperature simulation was carried out only at the medium frequencies. As showed the analysis carried out, the antenna pattern impact on the simulated radiobrightness at the horizontal polarization did not exceed 3 K, which appeared to be less then the radiometers calibration error. Therefore, the simulation was conducted for the case of plane wave, with the incident angle being of 45 degrees. The radiobrightness temperature of the soil covered with forest litter was calculated by the formula T b = (1 R)T (1) where R = r 2 is the reflection coefficient by power, r is the reflection coefficient by amplitude, T is the absolute temperature of the topsoil at the depth of 1 cm. Comparison of T b measured and that calculated with either isothermal or nonisothermal topsoil proved the thermodynamic temperature in topsoil at the depth of 1 cm be very close to the effective temperature used in the isothermal model. Therefore, the further modeling was performed with the use of the thermodynamic temperature at the depth of 1 cm. The reflection coefficient of multilayer medium was determined by using the following expression [5]: r = r 0 + r 1 exp( 2ik 1 X) 1 + r 0 r 1 exp( 2ik 1 X), (2) where r is a complex reflection coefficient of the topsoil at the air-soil boundary, r 0 complex Fresnel s coefficient at the same boundary, r 1 is the reflection coefficient at boundary between the air-soil layer and the one situated under it, k z1 = k 0 ε1 sin 2 θ is a normal to the boundary projection of the wave number vector pertaining to the first layer, k 0 = 2π/λ 0 is a wave number in the vacuum, ε 1, X are the complex permittivity and thickness of the first upper layer, respectively, i = 1. The topsoil soil CDP dependence on depth, x, is determined by the percentage of organic matter present in the soil. In reality, the real part of the CDP ε (x) changed from 1.2 in the most top layer to 11 at the depth of 4 5 cm. For the permittivity profile, the expression proposed in [4] was applied: ε (x) = ε l +(ε l ε p ) 1 1 ( ) +(ε p ε s ) 1 1 ( ) (3) 1 + exp x xp Q p 1 + exp x x s Q s where ε l, ε p, and ε s are the permittivities (real parts of the CDP) of the non overrotten litter, partly overrotten litter, and mineral soil, respectively; x is the depth coordinate, x p is the average depth at which the non overrotten litter borders the partly overrotten litter, x s the average depth at which the partially overrotten litter borders the mineral soil; Q p, Q s are the parameters to determine the thickness of transitional layers between the basic layers named above. A transition layer has the thickness equal to about 10Q. The typical permittivity profile of topsoil covered with coniferous litter, is shown in Fig. 3.

4 Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18 21, To find the relationship between the real and the imaginary parts of litter CDP we used the CDPs of coniferous litter from [6], as well as the CDP values for the topsoil samples picked up at the measurement site and obtained by us in the laboratory conditions. Figure 4 shows the results of simulation for one of the emissivity diurnal cycles measured. For the data in Fig. 4, the difference between emissivities modeled and measured did not exceed 0.01, or 2 3 K in the respective radiobrightnesess. The maximum difference of the overall data did not exceed 0.03 or 7 K. To ensure, when fitting, good agreement between the modeled and measured radiobrightnesses, with the data measured increasing, we had to increase the litter permittivity, as well as its thickness. In the case of wave frequency of 1.4 GHz, at the moments of emissivity maxima observed at 03:30 and 10:00 am (see Fig. 2), the CDPs derived through fitting were found to be of 1.2 i0.05 and 4.0 i0.4 for the non overrotten and partly overrotten litter layers, respectively. These values were found to be close to the ones measured in the laboratory conditions for the samples collected. At the same time, the thickness of the overrotten litter layer was found to be close to a quarter wavelength, as estimated for the overrotten medium. These facts make feasible to suggest that the partly overrotten layer may act as an antireflective one, thus increasing the emmissivity of the forest soil in the morning hours. Apparently, the variations of permittivity in the partly overrotten layer could occur because of dew fallout or as a result of nonisothermal transport of soil moisture from underlying space. ε / (x) Q p 10Q s x p 0,01 0,02 x s 0,03 x, m,04 Figure 3: The depth dependence of the real part of the dielectric permittivity of soil, covered with forest litter. The boundaries of transitional layers (10Q p, 10Q s ) are designated with pointers. χ 0,90 0,85 0,80 0,75 0,70 0,65 12:00 9:36 7:12 4:48 12:00 21:36 7:12 16:48 PM AM PM Time Figure 4: A diurnal emissivity cycle for horizontal polarization. The emissivities measured and modeled are given by symbols 1, 2 and 3, 4, respectively, with 1, 3, and 2, 4 pertaining to the frequencies of 1.4 and 6.9 GHz, respectively.

5 970 PIERS Proceedings, Moscow, Russia, August 18 21, CONCLUSIONS The carried out studies showed that the diurnal radiobrightness cycles of the forest soil covered with litter is substantially different from that of bared soils, with the radiobrightness maxima occuring when the soil temperature is minimal. It was found that the layer of partially overrotten litter induces an antireflective effect, making both the radibrightness and emissivity to rise in the early morning hours. This effect should be taken into account in the data processing algorithms relating to radio thermal remote sensing of soil moisture over the forest territories. There was also proved that the permittivity of the topsoil layered due to presence of forest litter can be retrieved from emissivities measured at two frequencies, using both polarizations. ACKNOWLEDGMENT The work was supported by the RFBR-CNRS grant No REFERENCES 1. Kerr, Y., P. Waldteufel, J.-P. Wigneron, et al., Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission, IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No 8, , Grant, J. P., J.-P. Wigneron, A. A. Van de Griend, et al., A field experiment on microwave forest radiometry: L-band signal behaviour for varying conditions of surface wetness, Remote Sens. Environ., Vol. 109, No. 1, 10 19, Schwank, M., C. Mätzler, M. Guglielmetti, and H. Flühler, L-band radiometer measurements of soil water under growing clover grass, IEEE Transactions on Geoscience and Remote Sensing, Vol. 43, No. 10, , Schwank, M., M. Guglielmetti, C. Mätzler, and H. Flühler, Testing a new model for the l-band radiation of moist leaf litter, IEEE Transactions on Geoscience and Remote Sensing, Vol. 46, No. 7, , Brehovskih, L. M., Waves in Layered Media, Nauka, Moscow 1973 (in Russian). 6. Kleshchenko, V. N., S. A. Komarov, and V. L. Mironov, Dielectric properties of needle litter, Journal of Communications Technology and Electronics, Vol. 47, No. 11, , 2002.