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THE NERC MST RADAR FACILITY AT ABERYSTWYTH
ERROR IN LASER CEILOMETER CLOUD BASE FILES, 2nd NOVEMBER 2010 - 10th
The Vaisala LD40 laser ceilometer cloud base altitude files contain an error in the observation times for the period 2nd November 2010 - 10th September 2011, inclusive. The recorded times were offset relative to the actual times by:
The NASA-Ames cloud base data files, and their corresponding quick-look plots, were removed from the BADC on 12th September 2011. Data files for 11th September 2011 onwards are not affected by this problem.
Replacing the affected files
Replacement files, with corrected observation times, can easily be generated for individual days on request. However, this will rely on a manual inspection of the raw data files on a day-by-day basis. Owing to the underlying and ongoing problem of slow instrument clock drift, an automatic software solution is required in the longer term. All of the removed files will be recreated once the updated software is operational. A confirmation will be added to this page when the task has been completed.
Cause of the problem
Data messages produced by the instrument contain a date-time stamp which is generated by its internal, free-running clock. The latter
When the current time error came to light, it was thought that an incorrect offset value had been applied when it was last updated on 2nd August 2010. On that occasion, the instrument's clock had been passively reset as a result of the electricity supply to the site being down for most of the day. However, further investigation revealed that the internal clock had again been passively reset at 09:14 UT on 2nd November 2010. This appears to have been caused by the first of three brief mains disruptions which were detected by the Uninterruptible Power Supply (UPS) units. The LD40 is not powered by a UPS. The clock reset went unnoticed at the time.
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Last updated 15th September 2011