NOAA Polar Orbiter Data User's Guide

Section 5.2.2

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5.2.2 SST Observation File

There has been a steady evolution of the SST Observation file since it was first generated in December 1978 using TIROS-N data. In 1978, the SST Observation file contained seven days of SST data at a resolution of 50 km. On Nov. 17, 1981, the operational technique for calculating SSTs was changed to a multichannel technique with separate algorithms for day and night observations. This new multichannel technique yielded a much improved resolution of 8 km. On November 1, 1986, the SST Observation File was changed to eight days to satisfy requirements for the Tropical Ocean and Global Atmosphere (TOGA) project. The basic structure of the two types of SST Observation Files is very similar. However, there are differences in the format of the Block Directory, Subblock Directory, and the Observation Unit. Section 5.2.2.1 describes the Seven Day Observation File and Section 5.2.2.2 describes the Eight Day Observation File.

The SST Observation File contains either seven or eight days of SST observations (depending on time period of data) which are organized in 5 x 5 degree blocks. These 5 x 5 degree blocks are further subdivided into 1 x 1 degree subblocks. Global coverage requires 2,592 blocks. The block number IBLOCK for an observation located at ILAT latitude (+N,-S) and ILONG longitude (+E,-W) can be calculated using the following equation:

IBLOCK=((ILAT-LA) over LAO) times INBC + ((ILONG-LO) over LOO)+1

where LA is the latitude origin of the file (-90 degrees), LO is the longitude origin of the file (-180 degrees), LAO is the size of the block in the latitudinal direction (5 degrees), LOO is the size of the block in the longitudinal direction (5 degrees), and INBC is the number of column blocks (360 degrees/LOO). Each block includes the minimum whole latitude and longitude, and excludes the maximum whole latitude and longitude which borders the block. For example, the limits of Block 1 are: -90.0S to -85.01S and -180.0W to -175.01W. Since all subblocks are 1 degree boxes, the Subblock number SBN for a given latitude and longitude can be defined as:

SBN=(ILAT-LLA) times LOO + ILONG - LLL +1

where LLA and LLL are respectively the lower left latitude and longitude of the 5 degree block.

On July 26, 1995, NESDIS began archiving an 8-day observation file created from SST retrievals processed at the Naval Oceanographic Center at Stennis Space Center, MS. This was initiated under a Department of Commerce/Department of Defense agreement for shared processing. This file is identical in format to the NESDIS 8-day observation file.

5.2.2.1 Seven Day SST Observation File

The Seven Day SST Observation File is organized with the first record containing documentation and directory information. This Block Directory Record points to the location of the beginning record for each block in the file. The first record in each block contains a Subblock directory. The Subblock Directories point to the first Observation Unit (6 words) in each of 25 subblocks for each block. The structure of the SST Observation File is shown in Figure 5.2.2.1-1.

Figure showing structure of the seven day SST Observation file

The first record in the SST Observation File is the Block Directory and it contains 13,024 bytes. The Block Directory contains location information, dates of data, and pointers to the location of the beginning record for each block. The format of the Block Directory for the Seven Day SST Observation File is contained in Table 5.2.2.1-1.

Table 5.2.2.1-1. Format of the Block Directory for the Seven Day SST Observation File.
Halfword # Contents
1 LA, the latitude origin of the file
2 LO, the longitude origin of the file
3 LAO, size of block in latitudinal direction
4 LOO, size of block in longitudinal direction
5 First free record pointer
6 Number of records in file
7 Start of Block Directory Information (in halfwords)
8 Day of year of most recent information
9 Year of most recent information
10 NESDIS archive flag (1 if archived; 0 if not archived) of most recent information
11-13 Day of year, year, NESDIS flag of 2nd most recent data
14-16 Day of year, year, NESDIS flag of 3rd most recent data
17-19 Day of year, year, NESDIS flag of 4th most recent data
20-22 Day of year, year, NESDIS flag of 5th most recent data
23-25 Day of year, year, NESDIS flag of 6th most recent data
26-28 Day of year, year, NESDIS flag of 7th most recent data
29-31 Day of year, year, NESDIS flag of 8th most recent data
32-40 Unused
41 Record position for block 1 (zero if no data in block)
42 Record position for block 2 (zero if no data in block)
... ...

Under normal circumstances, there can be a maximum of 3,250 blocks. The remaining records in the SST Observation File contain the Subblock Directories and Observation Units. The format for the Subblock Directory is contained in Table 5.2.2.1-2.

Table 5.2.2.1-2. Format of the Subblock Directory.
  Halfword # Byte # Contents
1 1 Record number
2 3 Block number
3 5 Pointer (in halfwords) to start position of Subblock directory information
4 7 Length of observation unit in words (normally 6 words)
5 9 LLA, lower left latitude of block
6 11 LLL, lower left longitude of block
7 13 Pointer (in halfwords) for start of observation data
8 15 Unused
Repeated for rest of Subblocks 9 17 Position (in halfwords) for start of Subblock 1 Observation Units
10 19 Position for end of Subblock 1 Observation Units
11 21 Record number containing Subblock 1
... ... ... ...

There is only one Subblock Directory for each block. It is not repeated at the start of each new record. If no data is contained in a Subblock, then all corresponding location parameters are set to zero. All unused bytes in each record are set to zero.

The Observation Unit contains the actual SST data for each Subblock. Each unit contains 6 words and has the format shown in Table 5.2.2.1-3. The first bit in the Observation Unit must be on as this signifies the beginning of the unit. Thus, the type of observation is coded in one byte as a number between 129 and 255. Also, there cannot be a high order bit on in any odd full word of the unit because it will be interpreted as the beginning of a unit. Note: in this format a longitude value of +180 degrees is not allowed and a value of -3000 for temperature signifies no information.

Table 5.2.2.1-3. Format of the Observation Unit.
Halfword # Byte # Content Range
1 1 Type of observation (see Table 5.2.2.1-5) 129-255
1 2 Source of observation (see Table 5.2.2.1-6) 0-255
2 3 Year of century (last 2 digits) 0-99
2 4 Month of year 1-12
3 5-6 Latitude (+N, -S) x 100 -9000 to 9000
4 7-8 Longitude (+E, -W) x 100 -18000 to 17999
5 9 Day of month 1-31
5 10 Hour 0-23
6 11 Minute 0-59
6 12 Second 0-59
7 13-14 SST Measurement (degrees C x 10) -3000 to 1000
8 15-16 Reliability Variable (normal=100)
9-12 17-24 Variable information (depending on observation type) n/a

The record structure of the Seven Day SST Observation File is contained in Table 5.2.2.1-4 (assuming that data for 25 subblocks can be stored in one record). If there are no data in a block, there will be no record for that block. If there are numerous data for a block, there will be more than one consecutive record allocated to that block. Note: Currently, the SST Observation File has a maximum of 4,002 records.

Table 5.2.2.1-4. Record structure of the Seven Day SST Observation File.
Record # Contents
1 Block Directory
2 1st block, Subblock directory and data for 25 subblocks
3 2nd block, Subblock directory and data for 25 subblocks
... ...
2593 2592nd block, Subblock directory and data for 25 subblocks

Tables 5.2.2.1-5 and 5.2.2.1-6 contain the SST Observation types and source codes, respectively.

Table 5.2.2.1-5. SST Observation Types.
Code Type
0 No type
1-128 Illegal type code
129 Nominal SST
130 AVHRR only SST
131 HIRS/2 only SST
132 Coastal type
133 Reserved
134 Reserved
135 Reserved
136 Reserved
137 Reserved
138 Test type
139-149 Reserved
150 Heat Budget observation
151 AVHRR-only day operational
152 AVHRR-only night operational
153 HIRS-only day operational
154 HIRS-only night operational
155 AVHRR + HIRS day operational
156 AVHRR + HIRS night operational
157 Reserved
158 Aerosol contaminated night operational
159 Reserved
160 Reserved
161 AVHRR-only day test
162 AVHRR-only night test
163 HIRS-only day test
164 HIRS-only night test
165 AVHRR + HIRS day test
166 AVHRR + HIRS night test
167-168 Reserved
179 ITOS SST
180-199 Reserved
200 Independent SST (Ship or Buoy)
201-254 Reserved
255 Erroneous Data - Do not use this Observation
Note: Codes having values between 0 and 149 (inclusive) indicate the operational technique in use between December 1, 1978 and November 16, 1981. Codes having values between 150 and 166 (inclusive) indicate a multichannel technique in use between November 17, 1981 and the present.

Table 5.2.2.1-6. SST Observation Source Codes.
Source code
(archived data)
Source

128

No source
129 AVHRR #1 (TIROS-N)
130 AVHRR #2 (NOAA-6)
131 AVHRR #3 Not used
132 AVHRR #4 (NOAA-7)
133 AVHRR #5 Not used
134 AVHRR #6 (NOAA-8) (see Note 1)
135, 7 (see Note 2) AVHRR #7 (NOAA-9)
8 AVHRR #8 (NOAA-10) (see Note 1)
1 AVHRR #9 (NOAA-11)
5 AVHRR #10 (NOAA-12)
2 AVHRR #11 (NOAA-13) (see Note 3)
3 AVHRR #12 (NOAA-14)
TBD AVHRR #15-20 (TBD)
21-50 ITOS NOAA-1 Sensor #1
51 ITOS NOAA-1 Sensor #2
53 ITOS NOAA-2 Sensor #1
54 ITOS NOAA-2 Sensor #2
55-58 ITOS NOAA-3 and -4
59-62 ITOS NOAA-5
63-127 Reserved
Notes:

1. No SSTs were archived for these satellites.

2. NOAA-9 source code was 135 prior to August 4, 1986. NOAA-9 source code was 7 beginning on August 4, 1986.

3. No SSTs were ever produced for this satellite.

5.2.2.2 Eight Day SST Observation File

The Eight Day SST Observation File is arranged similarly to the Seven Day Observation File (see Fig. 5.2.2.1-1). Currently, it has 5 x 5 degree blocks, which are further subdivided into 1 x 1 degree subblocks. There are 25 subblocks in each block. The first record in the Eight Day SST Observation File is the Block Directory which contains 13,028 bytes using the record format VS option. The format of the Block Directory is contained in Table 5.2.2.2-1.

Table 5.2.2.2-1. Format of the Block Directory record.
Halfword # Contents
1 LA, the latitude origin of the file (range: -90 to 90)
2 LO, the longitude origin of the file (range: -180 to 180)
3 LAO, size of block in latitudinal direction (range: 1 to +5)
4 LOO, size of block in longitudinal direction (range: 1 to +5)
5 First free record pointer (points to record number of 1st record available as an overflow track; 0 if no more tracks available).
6 Number of records in file (3100 initially)
7 Start of Block Directory Information in halfwords (11 initially)
8 Day of year of most recent information (range: 1 to 366)
9 File availability: 0=available; 1=unavailable, update in progress
10 Year of century of last data (range: 0 to 99)
11 Record number for Block 1 (range: 2 to 3100; 0 for no data in block)
12 Record number for Block 2 (range: 2 to 3100; 0 for no data in block)
... ...

Using the block number, the record number can be calculated and found in the portion of the Block Directory which serves as a lookup table. If the record number entry is zero, there is no data for that corresponding block in the file. The record number points to the Observation Data record, of which the first portion is a subdirectory. The file contains 8,446 records of 13,028 bytes each. The Observation Data record has the format shown in Table 5.2.2.2-2.

Table 5.2.2.2-2. Format of Observation Data Record.
Halfword # Contents Range

1

Record number 2 to 4002
2 Block number 1 to 2592
3 Extent number (number of records removed from primary) 0 (if primary)
4 Pointer to succeeding overflow record. Last overflow record points to primary record. 0 if no overflow
5 Pointer to halfword position of start of Observation Unit 61
6 Pointer to start of Subblock Directory 11
7 Lower left latitude of block LLA (+N,-S) in degrees -90 to +90
8 Lower left longitude of block LLL (+E,-W) in degrees -180 to +180
9 Pointer to last halfword containing data n/a
10 Unused n/a
11 Halfword of start of data for Subblock #1 0 if no data
12 Halfword of end of data for Subblock #1 (other extents may or may not contain data for this Subblock). 0 if no data
13-60 Similar to halfwords # 11 and #12 for remaining subblocks n/a
61-6512 Observation Unit n/a

If the block size is changed in the future, a block may contain a different number of subblocks, thus changing the number of Subblock pointers and the starting halfword of the Observation Unit. If the observations for a block do not fit on one record, then as many records (extents) are allocated as needed. Each additional record will include the subdirectory and Observation Unit. If the Subblock contains no information, then the start and end position contain a zero. Subblocks may cross record boundaries. If an entire Subblock cannot fit into one record, it will be split and a new record will be allocated for the remainder of the Subblock. Unused portions of records and records containing no data will be zero filled.

The Observation Unit for the Eight Day SST Observation File is of variable length, ranging from a minimum of 4 words to a maximum of 24 words. The length must be an even number of full words with no odd full word (except the first word which is always negative). The first three words of an Observation Unit contain identification information including the type of algorithm used, the satellite, date, time, and location. The fourth word contains the actual SST data and the reliability assigned to the observation. The remainder of the Observation Unit is unique to the type of algorithm used. The format of the Eight Day SST Observation Unit is contained in Table 5.2.2.2-3.

Table 5.2.2.2-3. Format of the Eight Day SST Observation Unit.
Halfword # Byte # Contents Range
1 1 Type of Observation (see Table 5.2.2.1-5) 129 to 255
1 2 Source of Observation (see Table 5.2.2.1-6) 0 to 255
2 3 Year 0 to 99
2 4 Month 1 to 12
3 5-6 Latitude (+N,-S) x 100 -9000 to 9000
4 7-8 Longitude (+E, -W) x 100 -18000 to 17999
5 9 Day 1 to 31
5 10 Hour 0 to 23
6 11 Minute 0 to 59
6 12 Second 0 to 59
7 13-14 SST (degrees C x 10) -20 to 350
8 15-16 Reliability 0 to 32767
9 17-18 Solar Zenith Angle (degrees x 10) 0 to 1800
10 19-20 Satellite Zenith Angle (degrees x 100) -600 to 600
11 21-22 Analyzed Field SST (degrees C x 10) -20 to 350
12 23-24 Internal Error (RMS x 100) 0 to 1000
13 25-26 Solar azimuth angle (degrees x 10) 0 to 1800
14 27-28 Climatological SST (degrees C x 10) -20 to 350
15 29 Beginning Row if unit array 1 to 11
15 30 Beginning Column of unit array 1 to 11
16 31-32 AVHRR Channel 1 average (% x 100) 0 to 10000
17 33-34 AVHRR Channel 2 average (% x 100) 0 to 10000
18 35-36 AVHRR Channel 3 average (K x 100) 0 to 32767
19 37-38 AVHRR Channel 4 average (K x 100) 0 to 32767
20 39-40 AVHRR Channel 5 average (K x 100) 0 to 32767
21 41-42 Space view σ Channel 1 (% x 100) 0 to 10000
22 43-44 Space view σ Channel 2 (% x 100) 0 to 10000
23 45-46 Space view σ Channel 3 (K x 100) 0 to 32767
24 47-48 Channel 4 Blackbody Temperature (K x 100) 0 to 32767
25 49-50 Channel 5 Blackbody Temperature (K x 100) 0 to 32767
26 51-52 Year of observation (see Note 1) 1998 to 32767
27 53-56 Spares n/a
Note:

1. This change makes the software Year 2000 compliant with a 4-digit year. Change went into effect on 4/29/98 with the 12Z NOAA-14 GAC orbit.


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