Starting on 15 August 2015 (orbital rev 5064) and continuing to present
day, there has been an anomalous drop in the echo power signal in the
receiver of approximately 20 dB, which initially took place at 03:01:59 UTC.
Both noise and signal power dropped but not by equal amounts. The Signal to
Noise Ratio dropped by 10 dB. Corresponding with this drop in power was an
increase of approximately 0.20 mA in current in the Scatterometer Electronics
Subsystem (SES). ISS attitude has appeared normal during this time. Due to
this anomaly the RapidScat Science Data Systems Team was initially unable to
process the telemetry data to backscatter (Level 1B, 2A) or wind vectors
(Level 2B). The operations team is still in the process of investigating the
cause of the “reduced echo power” anomaly. Several commands
were sent to the instrument to aid in diagnosing the anomaly and to aid in
recovering data production.
With some degradation in performance at low wind speeds, the ground data
processing is now able to process science quality data and retrieve valid
wind vector data. A preliminary review of the data has shown good correlation
with pre-anomaly data. Data from orbits ranging from 5064 to 5127 and
5137 to 5152 will need special processing to account for variation in
commanded instrument settings. Data from orbits 5127 through 5136 have
been made available, as these data were processed when the instrument
was first placed into the current configuration. We have also released data
obtained from orbital rev 5152 and forward.
There were some unrelated events that also impacted the flow of RapidScat
data:
- Instrument deactivation on 24 August 2015 due to HTV-5 docking, which resulted in pitch adjustment to approximately 0 degrees, outside of RapidScat’s desirable pitch range of -1 to -4 degrees.
- Instrument deactivation on 27 August 2015 due to relocation of Soyuz 42S; instrument reactivated on 28 August 2015.
There are several changes in the data going forward as a result of this
persistent anomaly. The following changes affect all orbits forward from
5064:
- Wind direction (and to a lesser extent speed) accuracy are degraded for low winds. Winds above 6 m/s are not significantly affected but statistics for 3-6 m/s are noticeably worse especially in the middle of the swath.
- Rain flagging is now unavailable for the outer (single beam portion) of the swath. The IMUDH rain flag that the JPL wind retrieval has been using is corrupted because it uses the RapidScat Brightness temperature. With a 10 dB loss in SNR, brightness temperatures are no longer usable. So we are switching rain flags to use the Impact Quantity Rain flag which has been previously utilized to determine when to correct wind speeds for rain. The new rain flag does not use brightness temperature and is not available in the outer swath. The statistics of the rain impact quantity have not changed significantly as a result of this anomaly.
The improved rain speed correction for tropical cyclones (version 1.1) has
now been turned off because it relies on brightness temperature. We are going
back to the original rain correction used in the Version 1.0 L2B data
processing.
As of now, the 12.5-km L2B wind vector data from orbital rev 5152 to 5242 are
available. More data will continue to flow in sequentially each day with an
approximate 7-day latency.
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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