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This document briefly describes some caveats of the AMSR and AMSR-E Level 1B brightness temperature product (Version 1.0). Please refer to [1] for information regarding instrument characteristics.
Due to the non-uniform physical temperature over the High-Temperature Noise Source (HTS, hot load), additional correction procedures were applied to derive an HTS effective temperature [2]. In determining some coefficients of the procedure, oceanic Earth brightness temperatures (Tbs) were used. The oceanic Earth Tbs were prepared based on the Reynolds OI-SST data [3] with a radiative transfer computation for the 6.9 and 10.65GHz channels and the SSM/I Tbs [4] with corrections for the differences of incidence angles and center frequencies for the 18.7GHz channels and above. Therefore, the present calibration method is statistically tied to these data sets, used radiative transfer models, and comparison method.. When comparing these data sets, we believe that the current brightness temperatures are consistent within a difference of 1 to 2K (over the oceans) depending on the channel.
The current procedure may be overestimating higher Tbs (e.g., over land) particularly in the 6.9GHz channels up to 5 to 10K. These findings are reported by several land investigators and seem to be consistent with the fact that the 6.9-GHz effective temperature of HTS (derived by the present method) indicates unexpectedly high values compared to other frequencies. Further investigations including other frequencies are ongoing. Based on the results, quadratic calibration curves will be applied in the Version 2.0 release.
Systematic degradation of Tbs (or scan-bias error) at the edge of the scan is evident in the 6.9GHz channels. However, this is consistent with the instrument design. Due to the possible interference in the main beam's field of view, the valid scan angle of 6.9GHz observation was restricted between -61 and +58 degrees [1]. At the beginning of the scans (on the right-hand side of the scans when looking in the satellite traveling direction), the Tbs degrade approximately 1.5K and require 20 footprints to recover. This is not corrected in Version 1.0, so depending on their purposes, we recommend data users exclude (or correct) these footprints.
Some contamination sources were found in the cold sky mirror (CSM) calibration counts. One is lunar emission that enters the CSM field of view. The affected counts were excluded and interpolated based on the outer valid counts. The other contamination source is correlated with the Earth Tbs. One possible cause is a spillover between the feed horn and CSM. The energy of this spillover portion may see the Earth surface via reflection by the main reflector. Based on this assumption, we statistically determined the spillover factor and corrected for it in the L1B processing. The current calibration procedure is separately applied to ascending and descending passes (not for an entire revolution). Due to the limitation of the method, some discontinuities of Tbs may exist at the corresponding data edges between ascending and descending passes.
For more details of the present radiometric calibration, please refer to [5].
The absolute geolocation error was assessed by comparing the 89-GHz (A-scan) Tb maps with land-ocean flag information over predetermined areas. The land-ocean flag is included in the L1B product. For AMSR-E, the tendency of the error differs for ascending and descending passes. The errors were 4km (2km) in the along-track direction and 5km (-5km) in the scan direction for ascending (descending) passes. The scan-direction error is correlated with the argument of latitude (or position in orbit). Also, these errors seem to have seasonal variations up to 5km. Co-registration errors between 89GHz A-scan and other lower frequency channels were a maximum of 5km in the scan direction. For AMSR, there were no apparent correlations between the geolocation errors and the position in orbit. The errors were 1 to 3km in the along-track direction and 3 to 5km in the scan direction. The tendency and size of co-registration errors were similar to those of AMSR-E.
Geolocation errors were not corrected in Version 1.0. Based on the above analysis, an empirical correction will be applied to the Version 2.0 product.
Possible Radio Frequency Interference (RFI) is observed in the 6.9GHz channels. RFI was also present at 10.65GHz, but was less prominent than at 6.9GHz. The RFI spots are observed worldwide, including in Japan, Europe, the Middle East, and southern Africa, but are most prominent over North America. Thus far, RFI effects seem to be minuscule over the oceans, except around some islands. Note that the 6.9GHz band is not primarily allocated for Earth observation. We did not exclude or correct these RFI effects in the Version 1.0 product. For AMSR-E, a paper was published by American investigators [6].
The sun-glitter areas, which are not due to instrument error, are significant in the 6.9 and 10.65 GHz channels over the ocean in descending node (AMSR) and ascending node (AMSR-E) passes. For general information about AMSR and AMSR-E L1B products, please visit the following websites.
[1] T. Kawanishi, T. Sezai, Y. Ito, K. Imaoka, T. Takeshima, Y. Ishido, A. Shibata, M. Miura, H. Inahata, and R. W. Spencer, "The Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), JAXA's contribution to the EOS for global energy and water cycle studies," IEEE Trans. Geosci. Remote Sensing, vol. 41, pp. 184-194, 2003. [3] R. W. Reynolds, N. A. Rayner, T. M. Smith, D. C. Stokes, and W. Wang, "An improved in situ and satellite SST analysis for climate," J. Climate, vol. 15, pp. 1609-1625, 2002. [4] MSFC SSM/I Brightness Temperatures from DMSP Satellites (Grid and Swath Products), http://ghrc.msfc.nasa.gov/uso/readme/ssmitb.html [5] K. Shiomi, K. Imaoka, Y. Fujimoto, A. Shibata, and M. Yoshikawa, "Status of mutual calibration and evaluation between ADEOS-II/AMSR and Aqua/AMSR-E," 8th Specialist Meeting on Microwave Radiometry and Remote Sensing Applications, Rome, Italy, 2004 [6] L. Li, E.G. Njoku, E. Im, P.S. Chang, and K. St.Germain, "A preliminary survey of radio-frequency interference over the U.S. in Aqua AMSR-E Data," IEEE Trans. Geosci. Remote Sensing, vol. 42, issue 2, pp. 380-390, 2004. SSM/I data were provided by the Global Hydrology Resource Center (GHRC) at the Global Hydrology and Climate Center, Huntsville, Alabama, USA. The Reynolds OI-SST datasets were made available by NOAA. |