Date/Time(UTC) Message Issued:
06 January 2006, 1700 UTC
Satellite(s) Involved:
NOAA-15, 16, 17, and 18
Instrument(s) Involved:
AVHRR
Product(s) Involved/Affected:
All Products/no impact
Date/Time(UTC) of Initial Implementation:
25 January 2006, Time TBD UTC
Details/Specifics of Change:
On January 25, NOAA would like to update our AVHRR level 1b processing
so that the reserved CLAVR-x (Clouds from AVHRR Extended cloud mask) bytes
will be utilized and the announced 1B converter changes will go into operation.
A new flag will be added to the header record (utilizing spares) that will
indicate when the CLAVR-x processing is actually turned on or off. A brief
explanation of the CLAVR-x is provided below along with a list of the affected
level 1b bytes. Also, you will find a brief explanation of the 1B converter
change. This change was announced for testing on December 13, 2005. Test
data that includes both changes is provided on the ftp://metroweb.nesdis.noaa.gov/pub/test/
anonymous ftp site. This data is updated orbitally and will be available
until the implementation is complete. Though we believe this to be a no
impact change, we are providing all instrument level 1b data for your review.
Please take some time to try the test data and provide your comments or
concerns to us before January 23rd.
The CLAVR-x provides the cloud mask present in the AVHRR level 1b datasets. It was designed to operate under all conditions including all orbital configurations. The goal of the CLAVR-x cloud mask is to provide a useful first guess cloud mask for all AVHRR applications that require it. It is constrained to perform this task in a timely manner to avoid delays in subsequent AVHRR applications. The CLAVR-x cloud mask in level 1b data is stored as a 2-bit value for each pixel . The values of the cloud mask are 0= clear,1 =probably clear, 2= probably cloudy and 3 = cloudy. The cloud mask is a series of spectral and spatial tests based on a long heritage of AVHRR cloud detection within NESDIS. In addition to providing the 2-bit cloud mask in the level 1b data, the results of each CLAVR-x test are also available in HDF files to allow for application specific optimization of the cloud mask. A draft of the CLAVR-x cloud mask theoretical basis document can be found at http://cimss.ssec.wisc.edu/clavr/clavrx_docs/clavrx_mask_atbd.pdf .
The CLAVR specific bytes in the level 1B file are outlined below. This
information can be found in the NOAA-KLM Users' Guide (http://www2.ncdc.noaa.gov/docs/klm/html/c8/s83-1.htm)
under the NOAA-N format information.
Header Record
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CLAVR Status Bit
Field
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989
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990
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2
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1
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0
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bits
15-1: <zero fill>
bit
0: CLAVR processing (0=off; 1=on) |
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<Zero Fill>
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991
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992
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2
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1
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0
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HRPT/LAC Data Record
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<Reserved> [CLAVR
Status Bit Field]
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14977
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14980
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4
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1
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0
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bits
31-1: <undefined>
bit 0: CLAVR status (0=disable, CCM codes zero-filled; 1=enable) |
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<Reserved> [CLAVR]
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14981
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14984
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4
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1
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0
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<Reserved>
[CLAVR CCM (Clear/Cloudy/Mixed) Codes (0=clear; 1=mixed clear; 2=mixed
cloudy; 3=cloudy)]
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14985
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15496
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2
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256
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0
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Word
1
bits 15-14: CCM code, FOV 1 bits 13-12: CCM code, FOV 2 ... bits 1-0: CCM code, FOV 8 Word 2
...
Word 256
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GAC data Record
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<Reserved> [CLAVR
Status Bit Field]
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4049
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4052
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4
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1
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0
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bits
31-1: <undefined>
bit 0: CLAVR status (0=disable, CCM codes zero-filled; 1=enable) |
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<Reserved> [CLAVR]
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4053
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4056
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4
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1
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0
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<Reserved>
[CLAVR CCM (Clear/Cloudy/Mixed) Codes (0=clear; 1=mixed clear; 2=mixed
cloudy; 3=cloudy)]
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4057
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4160
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2
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52
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0
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Word
1
bits 15-14: CCM code, FOV 1 bits 13-12: CCM code, FOV 2 ... bits 1-0: CCM code, FOV 8 Word 2
...
Word 52
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The following gives a description of the changes to the converter:
A problem with certain data values in 1b data sets has been reported
by users. This problem involves 1-, 2-byte or 4-byte data fields where
a valid data value has the most significant bit set, with all other bits
0. Therefore, values affected are 1-byte fields with a valid signed integer
value of -128, 2-byte fields with a valid signed integer value of -32768,
or 4-byte fields with a valid signed integer value of -2147483648. These
values may be erroneously stored as 0 in the 1b. The 1b fields which have
been reported as occasionally erroneous are AMSU-B and MHS radiometry values
and HIRS minor frame bit flags and minor frame header elements. See December
13 notice for more details.
Contact Person(s) Name/Email/Phone Number for Questions:
John.Sapper@noaa.gov 301-457-9014 ext 148 - CLAVR questions
Emily.Harrod@noaa.gov 301-457-5247 ext. 117 - ops questions
Web site(s) that contain other relevant information (where applicable):
http://www.osdpd.noaa.gov/PSB/PPP/PPP.html - History of level 1B notices
http://www.ssd.noaa.gov/PS/SATS/messages.html - All user notices
http://www.oso.noaa.gov/poes/index.htm - SOCC polar satellite status
http://www.oso.noaa.gov/goes/index.htm - SOCC geostationary satellite
status
http://www.oso.noaa.gov/daily-news/index.asp - SOCC morning reports
--
*******************************************
*
* Emily D. Harrod
* NOAA/NESDIS/OSDPD/IPD
* Computer Operations Branch
* Pre-Product Processing Group
* E/SP11, Federal Building #4, room 0318
* 5200 Auth Road
* Suitland, Maryland 20746-4304
* USA
* voice 301-457-5247 ext 117
* fax 301-457-5199
* email Emily.Harrod@noaa.gov
*
*******************************************