Product version 5 data was release in November 1999. One of the major change of this product is to utilize calibration table in 1B21 instead of giving linear equation, in order to utilize both weaker and stronger signal (than linear part) information, especially for stronger signal utilized for surface backscattering cross section (ƒÐ0)[sigma zero] calculation (see Fig. 1). The calibration curve in of version 5 consist of three parts: lower non-linear part, linear part, and upper non-linear part. Non-linear parts are estimated by fitting parabolic equation to the calibration data (triangles in Fig. 1), where the constant values are allocated in version 4.
Fig.1 Examples of 1B21 of 1B21 calibration table for product version 4 (solid line), version 5 (bold line), and calibration data (triangles).
After a few month operation of version 5 algorithm, we found that there are small gap between linear part and non-linear part in calibration curve shown in Fig. 2. The gaps shown in lower panel in Fig. 2 may affect statistics of ƒÐ0[sigma zero], if the sampling intervals are narrow. Since the non-linear part is expressed as parabolic equation, the interval of input power versus output count value increases with count value shown in Fig. 2. This also causes statistical gaps.
On the other hand, reprocessing result from the beginning of the data (December 1997), shows that unexpected echo appeared on December 17 through 20, 1997, when the initial checkout of PR was performed by switching internal attenuation. This echo apparently does not come from rainfall. Lower non-linear curve is determined to cause this unexpected echo.
Fig.2 Calibration curve for product version 5 (upper panel) and gradient of calibration curve (lower panel).
Users should be careful dealing with the strong echo (especially for ƒÐ0[sigma zero]) and weak echoes on December 21 and 22, 1997.
For more information, contact to:
Dr. Nobuhiro Takahashi
NASDA/EOSD
World Trade Center Bldg.
2-4-1 Hamamatsu-cho, Minato-ku Tokyo, JAPANphone: +81-3-3438-6352
email: takahashi.nobuhiro3@nasda.go.jp