Starting on 25 April 2015 (orbital rev 3333) and continuing to present
day, the ISS spacecraft experienced changes in attitude in excess of the
nominal spacecraft attitude for RapidScat wind retrieval in comparison to the
previous portion of the mission. For this reason, RapidScat has moved into a
regime where it was not always possible to obtain accurate data from the
outer antenna beam for every azimuth angle. For a small portion of each
antenna rotation near the southernmost part of each orbit, the large
spacecraft pitch led to loss of signal on the outer beam. This problem was
made worse in that the processor did not correctly detect the condition and
thus backscatter values were marked as usable that contained only noise.
These anomalously low backscatter measurements were detected by the JPL rain
flag and the KNMI quality control flag and led to large portions of the swath
being marked as "rainy" or "poor quality." For the JPL
product, the amount of data flagged as rainy went up from 2% to 6% of the
overall data, with much larger amounts (~30%) flagged in the center of the
swath for latitudes south of 30 S. The affected wind vector cells also had
poorer wind speed and wind direction accuracy especially for the JPL rain
corrected wind speed product.
The RapidScat Project is in the process of delivering a set of fixes to the
data to correct many of the issues as noted in the summary below.
Here is a brief summary of the known data quality issues as observed from
April 25 until today.
- Condition was initially undetected as the backscatter data was incorrectly flagged as usable even though it contained only noise.
- Unable to obtain accurate data from the outer beam for a small portion of each antenna rotation near the southernmost part of each orbit.
-
Anomalously low sigma-0 measurements were detected by the JPL rain flag and
the KNMI quality control flag, which led to large portions of the swath being
flagged as “rainy
or “poor quality”. - The amount of data flagged as rainy increased from 2% to 6% of the overall data; much larger percentages of rain flagged data (approximately 30%) were observed in the center of the swath for latitudes south of 30° S.
- The affected wind vector cells also contained poor accuracy in the wind speed (including corrected and uncorrected) and direction.
To minimize the impact of potentially poor quality data, we have quarantined all data starting from orbit 3333 (25 April 2015) and extending to present day. All additional data from orbits beyond the present will remain in quarantine until a fix has been implemented by the RapidScat project. Once the corrected data has been reprocessed and made available through PO.DAAC, an immediate follow-up announcement will be sent out. We expect a fix to be implemented by June 25th 2015 and the data reprocessed.
Concerned data users who are not currently registered with the PO.DAAC email list are encouraged to register to receive timely data announcement updates via email by contacting podaac@podaac.jpl.nasa.gov.
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