| Paper: | TU-P1.5 |
| Session: | Advanced Radiometry |
| Time: | Tuesday, March 27, 14:40 - 15:00 |
| Presentation: |
Oral
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| Topic: |
Advanced radiometer techniques: |
| Title: |
Data Processing and Experimental Performance of GIMS-II (Geostationary Interferometric Microwave Sounder-Second Generation) Demonstrator |
| Authors: |
Xi Guo; National Space Science Center, Chinese Academy of Sciences | | |
| | Hao Liu; National Space Science Center, Chinese Academy of Sciences | | |
| | Lijie Niu; National Space Science Center, Chinese Academy of Sciences | | |
| | Cheng Zhang; National Space Science Center, Chinese Academy of Sciences | | |
| | Hao Lu; National Space Science Center, Chinese Academy of Sciences | | |
| | Changxing Huo; National Space Science Center, Chinese Academy of Sciences | | |
| | Te Wang; National Space Science Center, Chinese Academy of Sciences | | |
| | Ji Wu; National Space Science Center, Chinese Academy of Sciences | | |
| Abstract: |
The Geostationary Interferometric Microwave Sounder (GIMS) is a millimeter wave imaging sounder concept proposed for China’s next generation geostationary meteorological satellite (FY-4M). It aims to observe the full-earth disk in the geostationary orbit. Compared with LEO satellites, it has the advantage of high time resolution. Thus, it has great potential to monitoring rapidly changing weather, such as tropic cyclones, which cause severe damages to the eastern coastal area in China every year.
The concept of GIMS is based on a microwave interferometric radiometer technology with a rotating circular thinned array. 70 antennas units is optimized to form an effectively thinned circular array with 3.7-meter diameter, including 3 small antennas to form the minimum baseline. With the maximum baseline of 3.7 meters and the minimum baseline of 0.0187 meters, GIMS realizes the spatial resolution of 50km in the geostationary orbit and ±0.15 FOV (in direction cosine) to observe the full-earth disk. As working frequency varies from 50-56GHz in 7 channels, GIMS has the ability to sound the height profiles of atmospheric temperature.
The digital correlation unit of GIMS-Ⅱ consists of 14 customized 10-channel 3-level digitizers, which works at 500Msps. After digital quantization by ADCs, correlators produce the auto-correlation of 70 pairs of I/Q noise signals and cross-correlation of every different pairs of I/Q noise signals (which means II/IQ/QI/QQ cross-correlation in every different combination of receiving unit pairs). In the ideal case, cross-correlation of a pair of receiving units follows the relationship II=QQ, IQ=-QI. So we have redundant measurement of cross-correlation to reduce noise. Also, we obtain cross-correlation between I channel and Q channel of the same receiving unit, which could be used to correct quadrature errors.
According to the three-level quantization theory, we convert digital covariance to correlation coefficient, which is so-called normalized correlation. In order to obtain the Tsys of each receiving unit, we conduct end-to-end two-point calibration. Blackbody filled with liquid nitrogen is used to be the cold target. And a temperature-control blackbody is used to be the hot target, which temperature could vary from 55℃ to 65℃ with the temperature resolution of 0.1℃. Once we have the amplitude-calibrated visibility functions, the phase error is still needed to be calibrated. Strong target such as the sun, active point source is chosen to be the inter-element phase calibration target. All different baselines visibility functions and reductant baselines visibility functions are used to resolve every receiver’s inherent phase. Then closure phase and equal length baseline are used to unwrap the phase ambiguity. Finally, brightness temperature image is reconstructed by an imaging algorithm based on IPPFFT.
Several field imaging experiments have been conducted outdoors. The impulse response measurement result using active point source shows that the angular resolution of GIMS-Ⅱ is better than 0.07°. Due to the calibrated phase results, equal length baseline’s phases have a very good consistency. After a long-time continuous observation of the sun, the solar moving track could be clearly seen in dynamic process. As the sun is a dynamic target, maximum array rotation speed of 1RPM is chosen, in order to achieve minimum imaging period of one minute. It reveals that GIMS-Ⅱ has the ability to observe dynamic target, but it still needs validation in further experiment. Due to the high angular resolution, buildings imaging results turn out to be dramatic, Besides the building contour line, the frame of windows as well as the air conditioner on the roof could be clearly seen.
We are still conducting field experiments of GIMS-Ⅱ demonstrator, and it needs further test to carry out better performance. We are looking forward to sharing detailed testing results on the MicroRad 2018.
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