Zou Xiaolei, Weng Fuzhong, Tian Xiaoxu( Earth System Science Interdisciplinary Center, Department of Atmospheric & Oceanic Science, Maryland University, College Park, USA National Environmental Satellite, Data & Information Service, National Oceanic and Atmospheric Administration, Washington D. C., USA)

Serial of Applications of Satellite ObservationsAn Effective Mitigation of Radio Frequency Interference over Land by Adding a New C-Band on AMSR2

Zou Xiaolei1, Weng Fuzhong2, Tian Xiaoxu1
(1 Earth System Science Interdisciplinary Center, Department of Atmospheric & Oceanic Science, Maryland University, College Park, USA 2 National Environmental Satellite, Data & Information Service, National Oceanic and Atmospheric Administration, Washington D. C., USA)

The Global Change Observation Mission 1st- Water (GCOM-W1) satellite was successfully launched into a polar-orbit on July 4, 2012, carrying the Advanced Microwave Scanning Radiometer-2 (AMSR-2)[1]. The GCOM-W1 satellite is operated by the Japan Aerospace Exploration Agency (Japan Aerospace Exploration Agency, JAXA). Compared with its predecessor heritage Advanced Microwave Scanning Radiometer for the EOS(AMSR-E) on board EOS Aqua satellite, AMSR-2 has two additional 7.3 GHz channels for mitigating radiofrequency interference[2-6]so that the soil moisture content[7]can be reliably retrieved over most of land conditions.

Having completed an initial calibration operation①http://www.jaxa.jp/press/2013/01/20130125_shizuku_e.html, the JAXA started to provide AMSR-2 brightness temperature observations to the public on January 25, 2013. In this study, the radio frequency interference (RFI) characteristics at two AMSR-2 C-band frequencies are analyzed and their distributions over United States and central American continents are examined for an initial evaluation of the RFI mitigation by the newly added channels. In Section 1, AMSR-2 channel characteristics and spectral difference method are briefly presented. Numerical results are presented in Section 2. Section 3 provides a summary and conclusions.

1　Data Description and Methodology

1.1 AMSR-2 Instrument Characteristics

AMSR-2 is a conical-scanning microwave imager with fourteen channels at the following seven frequencies: 6.925, 7.3, 10.65, 18.7, 23.8, 36.5, and 89.0GHz[8]. It has a local incident angle of 55° from an orbit at 700km above the surface. The AMSR-2 antenna reflector size is 2.0m, which is larger than that of AMSR-E and therefore provides a better spatial resolution. Specifically, the across-track and along-track spatial resolutions of the individual ground instantaneous field-of-view (IFOV) measurements are 62km×35km at both 6.925 and 7.3GHz, 42km×24km at 10.65GHz, 22km×14km at 18.7GHz, 26km×15km at 23.8GHz, 12km×7km at 36.5GHz and 5km×3km at 89.0GHz, respectively. The sampling interval is 10km except for the 89GHz channels, whose sample interval is 5km.

1.2 The Spectral Difference Method

In general, the land surface emissivity increases with frequency, resulting higher brightness temperatures at 10.65GHz (channels 3-4) than those at 6.925GHz, i.e., TB6v＜TB10v. The natural phenomenon such as flooding and wet surface further decreases the brightness temperatures, especially at lower microwave frequencies. The measured brightness temperatures at low frequencies can thus be used for retrieving soil moisture content. The presence of RFI at 6.925GHz however increases the brightness temperature at lower frequency, resulting in a reversed spectral gradient, i.e., TB6v＞TB10v[9]. By examining the spatial distributions of the inequality about RFI-sensitive spectral difference indices TB6v－TB10vand/or TB6h－TB10h(e.g., differences between brightness temperatures at two different frequencies for a given polarization), RFI contaminated data can be identified. Since RFI signals typically originate from a wide variety of coherent point target sources and are often directional and narrow-banded, they are often isolated in space and persistent in time.

2　Numerical Results

For a surface condition without snow, brightness temperatures at 6.925GHz (channels 3-4) are smaller than those at 10.65GHz, i.e., TB6v－TB10v＜0, since the surface emissivity over land at lower frequency is smaller than that at higher frequency. The presence of RFI at 6.925GHz increases the brightness temperature at this frequency, reversing the sign of the spectral gradient, i.e., TB6v－TB10v＞0. RFI contaminated data at 6.925 or 7.3 GHz (Fig. 1) could be identified by their excessively positive values of the spectral differences with 10.65GHz. Figure 2 presents spatial distributions of the spectral differences TB6h－TB10h(Fig. 2a), TB6v－TB10v(Fig. 2b), TB7h－TB10h(Fig. 2c) and TB7v－TB10v(Fig. 2d) for AMSR-2 data from descending nodes over North America on December 11, 2012. The typical isolated features of RFI signals characterized by large positive spectral differences of brightness temperatures at 6.925GHz (Fig. 2a and 2b) are found in many places over the United States, while RFI signals at 7.3GHz seem to occur only in Mexico, Washington D. C. and New York. Similar patterns are obtained at other days examined (picture omitted).

In order to provide a quantitative examination of the relationship of the RFI signals at 6.925 and 7.3GHz channels, we show in Fig. 3 the scatter plots of TB6h－TB10hversus TB7h－TB10h(Fig. 3a), as well as TB6v－TB10vversus TB6v－TB10v(Fig. 3b). The differences of brightness temperatures between 6.925 and 7.3GHz channels are indicated in color. Data counts at an interval of spectral difference of 0.05 are shown in Fig. 3c and3d. It is seen that the brightness temperatures at 7.3GHz increase linearly with the brightness temperatures at 6.925GHz within a fixed interval of TB6h－TB10hexcept when the RFI signals are strong (Fig. 3a and 3b). There exists a very small portion of data points with RFI occurring at both 6.925 and 7.3GHz frequencies for horizontally polarized channels (Fig. 3a). The RFI does not occur simultaneously at both 6.925 and 7.3GHz frequencies for vertically polarized channels (Fig. 3b).

The AMSR-2 measured brightness temperatures over four characteristic regions over Denver, Mexico, Washington DC and New York, indicated by boxes A, B, C and D in Fig. 2a, respectively, are shown in Fig. 4 and Fig.5. As expected, RFI signals at 6.925GHz horizontally polarized (Fig. 4a) and vertically polarized (Fig. 4b) channels over Denver are characterized as outliers with excessively large values of brightness temperatures at 6.925GHz. RFI signals at horizontally polarized (Fig. 4c) and vertically polarized (Fig. 4d) channels at 7.3GHz over Mexico are characterized as outliers with excessively large values of brightness temperatures at 7.3GHz. It is also pointed out that brightness temperatures at 7.3GHz increase linearly with brightness temperatures at 6.925GHz for RFI-free data.

RFI signals over Washington DC and New York are detected for horizontally polarized channels at both 6.925GHz (Fig. 5a) and 7.3GHz (Fig. 5c), and are characterized by higher brightness temperatures at both frequencies than those RFI-free data. For vertically polarized channels, RFI signals appear only in 6.925GHz channel over Washington D. C. and New York (Fig. 5b). The 7.3GHz vertically polarized channel is RFI-free over both Washington D. C. and New York (Fig. 5d).

3　Summary and Conclusions

RFI signal in satellite microwave imager radiances over land must be detected and removed from the contaminated data before the radiance data are used for retrieving geophysical parameters such as soil moisture content. In order to mitigate the RFI in C-band channels, two new C-band channels centered at 7.3GHz are added to AMSR-2. In this paper, we evaluated the results of a spectral difference method for detecting RFI signals in AMSR-2 data over North and Central Americas.

For the study cases of AMSR-2 data, a strong RFI is detected at the AMSR-2 C-band channels at 6.925GHz at both horizontal and vertical polarization over North America. The RFI signals are populated near the metropolitans of the United States. However, the newly added C-band channels at 7.3GHz are mostly RFI-free except in Mexico, Washington D. C. and New York. There are no RFI over Mexico at 6.925GHz for both polarization states. The only places where RFI occur at both C-bands of AMSR-2 are Washington D. C. and New York for the horizontal polarization state. It is thus concluded that a successful mitigation of RFI is achieved in AMSR-2 observations over North America.

注釋

① http://www.jaxa.jp/press/2013/01/20130125_shizuku_e.html

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