NOAA Polar Orbiter Data User's Guide

Section 3.1

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3.1 GAC Data

This section describes the data characteristics and magnetic tape format of Global Area Coverage (GAC) data. Section 3.1.1 contains the data characteristics and Section 3.1.2 contains the tape formats available. The tape formats include the full data set copy, the 16-bit unpacked format, and the selective extract subsets. NESDIS has enhanced the Level 1b format for GAC and the most recent format (for data collected after November 15, 1994) is included in this section. Previous formats are included in Appendices K and L.

3.1.1 Data Characteristics

The processor on board the satellite samples the real-time AVHRR data to produce reduced resolution GAC data. Four out of every five samples along the scan line are used to compute one average value, and the data from only every third scan line are processed. As a result, the spatial resolution of GAC data near the subpoint is actually 1.1 km by 4 km with a 3 km gap between pixels across the scan line, although generally treated as 4 km resolution. All of the GAC data computed during a complete pass are recorded on board the satellite for transmission to Earth on command. The 10-bit precision of the AVHRR data is retained.

3.1.2 Magnetic Tape Formats

The data set format for full data set copies (all channels) is different from the format for selective extract subsets (selected channels). Sections 3.1.2.1 and 3.1.2.2 contain formats for GAC full data set copies and GAC selective extract subsets, respectively. Section 3.1.2.1 also includes an explanation of the 16-bit unpacked data format.

3.1.2.1 Full Data Set Copies

Each GAC data set contains an individual satellite recorder playback (or a portion of a playback if there is an interruption in the data due to noise, etc., in which case a single playback may be fragmented into a number of data sets). Data within each GAC data set is in chronological order with one logical record per scan. Two logical records are written per 6440-byte physical record. Each logical record contains 3220 bytes written in binary format as shown in Table 3.1.2.1-1. This table contains the format of the GAC data record which was implemented on November 15, 1994.

Table 3.1.2.1-1. Format of GAC data record (implemented November 15, 1994)
Byte # # of bytes Contents
1-2 2 Scan line number from 1 to n
3-8 6 Time code (year, day, hour, minute, second)
9-12 4 Quality indicators (see Table 3.1.2.1-2)
13-52 40 Calibration coefficients
53 1 Number of meaningful Zenith angles and Earth location points appended to scan (n)
54-104 51 Solar Zenith Angles
105-308 204 Earth location
309-448 140 Telemetry (HRPT minor frame format)
449-3176 2728 GAC video data
3177-3196 20 Additional decimal portion of 51 Solar Zenith Angles
3197-3198 2 Clock drift delta in milliseconds x 2 + indicator:
0 = no time adjustment
1 = time adjustment
3199-3220 22 Spare

The time code consists of the year, Julian day, and UTC time of day in milliseconds. The year is contained in the first 7 bits of the first two bytes, the 9-bit day of year is right-justified in the first two bytes and the 27-bit millisecond UTC time of day is right-justified in the last four bytes. All other bits are zero. The time code will always have the same format for all Level 1b data sets.

The quality indicators are contained in four bytes. The first byte contains processing detected conditions and the last three bytes contain DACS quality indicators. If the bit is one (on), then the condition is true. The format of the quality bytes is contained in Table 3.1.2.1-2.

Table 3.1.2.1-2. Format of quality indicators.
Byte # Bit # Contents
9 7 FATAL FLAG - Data should not be used for product generation
6 TIME ERROR - A time sequence error was detected while processing this frame
5 DATA GAP - A gap precedes this frame
4 DATA JITTER - Resync occurred on this frame
3 CALIBRATION - Insufficient data for calibration
2 NO EARTH LOCATION - Earth location data not available
1 ASCEND/DESCEND - AVHRR Earth location indication of ascending (=0) or descending (=1) data
0 P/N STATUS - Pseudo Noise (P/N) occurred (=1) on the frame, data not used for calibration computations
10 7 BIT SYNC STATUS - Drop lock during frame
6 SYNC ERROR - Frame Sync word error greater than zero
5 FRAME SYNC LOCK - Frame Sync previously dropped lock
4 FLYWHEELING - Flywheeling detected during this frame
3 BIT SLIPPAGE - Bit slippage detected during this frame
2 CHANNEL 3 SOLAR BLACKBODY CONTAMINATION (SBBC) INDICATOR
0 = no correction
1 = solar contamination corrected
1 CHANNEL 4 SBBC INDICATOR
0 = no correction
1 = solar contamination corrected
0 CHANNEL 5 SBBC INDICATOR
0 = no correction
1 = solar contamination corrected
11 7 TIP PARITY - In first minor frame
6 TIP PARITY - In second minor frame
5 TIP PARITY - In third minor frame
4 TIP PARITY - In fourth minor frame
3 TIP PARITY - In fifth minor frame
0-2 SPARE
12 2-7 SYNC ERRORS - Number of bit errors in frame sync
0-1 SPARE

The calibration coefficients consist of slope and intercept values for each of the five channels. The use of these coefficients is described in Section 3.3. Each value is stored in four bytes in the following order:

Channel 1 slope coefficient
Channel 1 intercept coefficient
Channel 2 slope coefficient
Channel 2 intercept coefficient
Channel 3 slope coefficient
Channel 3 intercept coefficient
Channel 4 slope coefficient
Channel 4 intercept coefficient
Channel 5 slope coefficient
Channel 5 intercept coefficient

A fixed number of Zenith Angles and Earth location points are appended to each scan. However, only the first n Zenith Angles and the first n Earth location points have meaningful values (n is defined in byte #53). The maximum number of points possible in a scan is 51. There are 409 points in a GAC scan line. However, the solar Zenith Angles and Earth location data (latitude and longitude) are sampled every eight points starting at the fifth point (5, 13, 21,..., 389, 397, 405). There are 51 possible Solar Zenith Angles and Earth location values for each scan line. Each Zenith Angle requires one byte and is stored as degrees x 2. The latitude and longitude values are each stored in two-byte fields in 128ths of a degree (0 to 180E positive, 0 to 180W negative). See Section 2.0.1 for explanation of negative binary integers in two's-complement notation.

The telemetry data contain information which may be used to compute calibration coefficients when these are not included in the data. The telemetry data are stored in 140 bytes. The first 103 (10 bit) words are packed three (10 bit) words in four bytes, right justified. The last four byte group contains one (10 bit) word with 20 trailing bits. All unused bits are set to zero. The contents of these 103 words are contained in Table 3.1.2.1-4, which is the entire HRPT minor frame format. For more information, refer to NOAA Technical Memorandum NESS 107 entitled, Data Extraction and Calibration of TIROS-N/NOAA Radiometers.

The GAC video data consist of five readings (one for each channel) for each of the 409 points in a scan. They are packed as three (10-bit) samples in four bytes, right-justified. The last four-byte group contains two (10-bit) samples with 10 trailing zero bits. The first two bits of each four-byte group are zero. The 2045 samples (409 points x 5 channels) are ordered scan point 1 (Channel 1, 2, 3, 4, 5), scan point 2 (Channel 1, 2, 3, 4, 5), etc., which is also known as Band Interleaved by Pixel (BIP). Note for TIROS-N, NOAA-6, NOAA-8, and NOAA-10, there is no sensor for Channel 5 so Channel 4 data is repeated in the Channel 5 position. The video data are stored in binary. See Figure 2.2.1-1 for the arrangement of the GAC channels (packed format).

Enhancements implemented on September 7, 1994, included clock drift corrections and orbit parameters format change from IBM real numbers to scaled integers. These enhancements were made to both the TOVS and AVHRR Level 1b data. However, the new system did not function properly for AVHRR and was removed the same day with the old process resumed. Problems were encountered with the data record time codes in the GAC Level 1b data and NESDIS was forced to remove the updates from the AVHRR processing. Initially, NESDIS tried turning off the clock drift corrections, but time codes were still incorrect so the old process was resumed. The AVHRR data were processed with the old on-line earth location software until updates could be reinstalled. The following list of orbits were affected for the AVHRR:

NOAA-12
clock corrections on HRPT - S1542.E1550.B1722626.GC
LHRR - S1402.E1402.B1722525.GC
(time sequence errors)
GHRR - S1359.E1539.B1722526.GC
LHRR - S1359.E1411.B1722525.GC
LHRR - S0934.E0946.B1722222.GC
HRPT - S1902.E1914.B1722828.GC
clock corrections off GHRR - S1723.E1900.B1722728.GC
GHRR - S1534.E1727.B1722627.GC
LHRR - S1729.E1738.B1722727.GC
LHRR - S1652.E1658.B1722627.GC
LHRR - S1532.E1543.B1722626.GC
NOAA-11
clock corrections on HRPT - S1722.E1734.B3068787.GC
GHRR - S1542.E1719.B3068687.GC
GHRR - S1353.E1547.B3068586.GC
LHRR - S1516.E1527.B3068586.GC
LHRR - S1342.E1353.B3068585.GC
NOAA-9
clock corrections on GHRR - S0825.E1019.B5019596.WI
GHRR - S0128.E0321.B5019092.WI

On November 15, 1994, NESDIS re-implemented the following changes to the Level 1b GAC datastream: 1) Current orbital parameters in the header were switched to four byte integers rather than 8 byte floating point numbers. 2) Calibration algorithms were updated to remove the effects of sunlight impinging on the internal calibration target (Solar Blackbody Contamination - SBBC). SBBC detection indicators were added to the AVHRR data record quality indicators. 3) Navigation software was updated to remove satellite clock errors from the data time codes. 4) Navigation quality control parameters were provided in the header record.

An anomalous dip occurs in the AVHRR Channel 3 blackbody counts between 70 and 80S latitude shortly after the spacecraft comes out of eclipse but only for sun angles less than 25 degrees above the horizon. This dip, which produces an anomalous increase in the calibration gain coefficient, has been attributed to reflected sunlight impinging onto the internal calibration target (ICT). Evidence suggests that this dip may have occurred in all of the afternoon satellites in the current NOAA series.

When SBBC is detected, the slope and intercept are replaced by the running average of the twenty-four previous good values. If no checking is done, the value is set to zero. Currently, only NOAA-11 shows this contamination. On Dec. 1, 1993, AVHRR gain anomaly corrections were implemented for NOAA-11 beginning with dataset:
NSS.GHRR.NH.D93335.S1408.E1554.B2673132.GC. However, for 24 hours, the following datasets for NOAA-12 were affected:
NSS.GHRR.ND.D93335.S1303.E1457.B1324445.GC through
NSS.GHRR.ND.D93336.S0535.E0729.B1325455.WI.

The drift in the onboard TIP clock for each polar satellite introduces a timing error in the instrument data time codes. This error is reflected as an along-track Earth location error. SOCC measures this error and corrects it periodically to maintain a maximum of approximately ±0.5 seconds. Under Phase II of the navigation enhancements, SOCC error corrections will be used to adjust the instrument time codes and eliminate its Earth location error effect. The adjusted values will be stored in the scan data record and an indicator flag will be set for users. Consequently, users are apprised of the magnitude of the adjustment and also have the flexibility of removing the correction, if needed.

This enhancement also results in improved navigation parameters. The operational quality control (QC) of Earth location data will be improved. A nadir Earth location tolerance value will be used to indicate when the Earth location data are beyond an acceptable range. An independent calculation of the latitude and longitude values at nadir will be compared to the operational values. This comparison will be performed approximately four times per orbit. If differences are beyond the acceptable tolerance range, a "BAD EARTH LOCATION" flag is set, and a message identifying the scan is generated. The Earth location tolerance range will be provided in the header record as an integer, scaled to 0.1 km (ranges from 0.1 to 25.5). If no checking is done, the value is set to zero.

See Table 2.0.4-2 for the current format of the level 1b header record which was implemented November 15, 1994.

The 16-bit unpacked format for full copy GAC data has the same format as the "packed" data described above except for the video data. The video data values for each channel are contained in the 10 least significant bits and the 6 most significant bits are zero filled. The record length is 4540 bytes. Table 3.1.2.1-3 contains the format for the unpacked GAC video data (for all five channels).

Table 3.1.2.1-3. Format of unpacked GAC video data (five channels).
Point # Byte # Bit # Content
1 449-450 10-15
0-9
Zero filled
1st value of Channel 1
451-452 10-15
0-9
Zero filled
1st value of Channel 2
453-454 0-9 1st value of Channel 3
455-456 0-9 1st value of Channel 4
457-458 0-9 1st value of Channel 5
2 459-460 0-9 2nd value of Channel 1
... ... ... ...
409 4537-4538 0-9 409th value of Channel 5
4539-4540 0-15 Zero filled (so record will be on word boundary)

Table 3.1.2.1-4. HRPT Minor Frame Format.
Function No. of Words Word Position Bit No. Plus Word Code &
1 2 3 4 5 6 7 8 9 10 Meaning
Frame Sync 6 1 1 0 1 0 0 0 0 1 0 0
2 0 1 0 1 1 0 1 1 1 1
3 1 1 0 1 0 1 1 1 0 0 (See Note 1.)
4 0 1 1 0 0 1 1 1 0 1
5 1 0 0 0 0 0 1 1 1 1
6 0 0 1 0 0 1 0 1 0 1
ID (AVHRR) 2 7 Bit 1: 0=Internal Sync; 1=AVHRR Sync
Bits 2 & 3; 00=Not used; 01=Minor Frame #1; 10=Minor Frame #2; 11=Minor Frame #3
Bits 4-7; Spacecraft Addresses; Bit 4=MSB, Bit 7=LSB
Bit 8; 0=Frame Stable; 1=Frame resync occurred
Bits 9-10; spare; bit 9=0, bit 10=1
8 Spare word; bit symbols undefined
Time Code 4 9 Bits 1-9; Binary day count; Bit 1=MSB; Bit 9=LSB
Bit 10; 0; spare
10 Bits 1-3; all zeroes; spare 1, 0, 1
Bits 4-10; Part of binary msec of day count; Bit 4=MSB of msec count
11 Bits 1-10; Part of binary msec of day count;
12 Bits 1-10; Remainder of binary msec of day count; Bit 10=LSB of msec count
Telemetry (AVHRR) 10 13 Ramp Calibration AVHRR Channel #1
14 Ramp Calibration AHVRR Channel #2
15 Ramp Calibration AHVRR Channel #3
16 Ramp Calibration AHVRR Channel #4
17 Ramp Calibration AHVRR Channel #5
18 PRT Reading 1
19 PRT Reading 2 (See Note 2.)
20 PRT Reading 3
21 AVHRR Patch Temperature
22 0 0 0 0 0 0 0 0 0 1 Spare
Internal Target Data (AVHRR) 30 23 10 words of internal target data from each AVHRR channel 3, 4 and 5. These data are time multiplexed as chan 3 (word 1), chan 4 (word 1), chan 5 (word 1), chan 3 (word 2), chan 4 (word 2), chan 5 (word 2), etc.
Space Data (AVHRR) 50 53 thru 102 10 words of space scan data from each AVHRR channel 1,2,3,4, and 5. These data are time multiplexed as chan 1 (word 1), chan 2 (word 1), chan 3 (word 1), chan 4 (word 1), chan 5 (word 1), chan 1 (word2), chan 2 (word 2), chan 3 (word2), chan 4 (word 2), chan 5 (word 2), etc.
Sync (AVHRR) 1 103 Bit 1; 0=AVHRR sync early; 1=AVHRR sync late, Bits 2-10; 9 bit binary count of 0.9984 MHz periods; Bit 2=MSB, Bit 10=LSB
TIP data 520 104 thru 623 The 520 words contain five frames of TIP data (104 TIP data words/frame). (See Note 3.)
Bits 1-8: Exact format as generated by TIP
Bit 9: Even parity check over bits 1-8
Bit 10: -bit 1.
Spare Words 127 624 1 0 1 0 0 0 1 1 1 0
625 1 1 1 0 0 0 1 0 1 1
626 0 0 0 0 1 0 1 1 1 1
627 1 0 1 1 0 0 0 1 1 1
628 1 1 0 1 0 1 0 0 1 0
... ... (See Note 4.)
748 1 0 0 1 0 1 1 0 1 0
749 1 1 0 0 1 0 0 0 1 0
750 1 0 0 0 0 0 0 0 0 0
Earth Data (AVHRR) 10,240 751 Chan 1 - Sample 1
752 Chan 2 - Sample 1
753 Chan 3 - Sample 1
754 Chan 4 - Sample 1
755 Chan 5 - Sample 1
756 Chan 1 - Sample 2
... ... (See Note 5.)
10,985 Chan 5 - Sample 2047
10,986 Chan 1 - Sample 2048
10,987 Chan 2 - Sample 2048
10,988 Chan 3 - Sample 2048
10,989 Chan 4 - Sample 2048
10,990 Chan 5 - Sample 2048
Auxiliary Sync 100 10,991 1 1 1 1 1 0 0 0 1 0
10,992 1 1 1 1 1 1 0 0 1 1
10,993 0 1 1 0 1 1 0 1 0 1
10,994 1 0 1 0 1 1 1 1 0 1
... ... (See Note 6.)
11,089 0 1 1 1 1 1 0 0 0 0
11,090 1 1 1 1 0 0 1 1 0 0
Notes:
1) First 60 bits from 63 bit PN generator started in the all 1's state. The generator polynomial is X6+X5+X2+X+1

2) AVHRR Internal Target Temperature Data. Three readings from one of the four platinum resistance thermometers (PRT). Each of these words is a 5 channel subcom, 4 words of IR data plus a subcom reference value.

3) 104 words includes 103 words of the AMSU frame plus the first word of TIP.

4) Derived by inverting the output of a 1023 bit PN sequence provided by a feedback shift register generating the polynomial: X10+X5+X2+X+1. The generator is started in all 1's state at the beginning of word 7 of each minor frame.

5) Each minor frame contains the data obtained during one Earth scan of the AVHRR sensor. The data from the five sensor channels of the AVHRR are time multiplexed as indicated.

6) Derived from the non-inverted output of a 1023 bit PN sequence provided by a feedback shift register generating the polynomial: X10+X5+X2+X+1. The generator is started in the all 1's state at the beginning of word 10,991.


3.1.2.2 Selective Extract Subsets

The following sections describe the data set format for GAC data sets which have been produced using channel selection (16-bit format) or by choosing the 8-bit format offered by SAA. Section 3.1.2.2.1 describes the 16-bit format and Section 3.1.2.2.2 describes the 8-bit format. SSB is aware of a problem which affects the format of both the 8-bit and the 16-bit (unpacked) data for the Level 1b AVHRR. The problem involves the omission of 22 bytes of extra precision information containing the Solar Zenith angle and clock drift delta in each record. These bytes were added to the Level 1b data record (on October 24, 1992) after the extraction software was written, and were therefore not included. If users of the 8- or 16-bit format data want this extra precision data, they are only available if the user requests packed format output from the SAA's online archive of Level 1b data sets.

3.1.2.2.1 16-bit format

When channels are selected for GAC data, the format is identical to that described in Section 3.1.2.1 except for the GAC video data. In the GAC video data, the selected channels are packed in consecutive half words (16 bits). The data values for each channel selected are contained in the ten least significant bits and the remaining six most significant bits are zero filled. The GAC video data have the format shown in Table 3.1.2.2.1-1 when two channels are selected.

Table 3.1.2.2.1-1. Format for GAC video data (two channels).
Point # Byte # Bit # Content
1 449-450 10-15 Zero-filled.
0-9 First value, First selected channel.
2 451-452 10-15 Zero-filled.
0-9 First value, Second selected channel.
3 453-454 10-15 Zero-filled.
0-9 Second value, First selected channel.
4 455-456 10-15 Zero-filled.
0-9 Second value, Second selected channel.
... ... ... ...
409 2083-2084 10-15 Zero-filled.
0-9 409th value, Second selected channel.

The total record length for two channel selective GAC would be 4168 bytes (2 x 2084, since there are 2 scans/record) with no spare bytes trailing. Similarly, if one channel was selected the record would contain 820 bytes (409 points/scan + 1 (to make a whole word) x 2 bytes/point) for the GAC video, which would make the entire record 2536 bytes long (2 x 1268).

3.1.2.2.2 8-bit format

When the 8-bit format is chosen (from SAA), the normal 10-bit video data are truncated to 8-bits. (The 2 least significant bits of data are dropped.) The format is identical to that described in Section 3.1.2.1 except for the GAC video data. In the GAC video data, the data are packed in consecutive bytes (four data points per 32-bit word). Because there are an odd number of data points (409) in the video data, the last three bytes are zero filled in order to pad the data to a full word boundary. The GAC video data have the format shown in Table 3.1.2.2.2-1.

Table 3.1.2.2.2-1. 8-bit format for GAC video data (one channel).
Word # Point # Byte # Bit # Content
113 1 449 0-7 First video data value (First scan).
2 450 0-7 Second video data value (First scan).
3 451 0-7 Third video data value (First scan).
... ... ... ... ...
214 408 856 0-7 408th video data value (First scan).
215 409 857 0-7 409th video data value (First scan).
858 0-7 Zero filled.
859 0-7 Zero filled.
860 0-7 Zero filled.
216 . 861-862 0-15 Scan line number (Second scan).
... ... ... ... ...
327 1 1309 0-7 First video data value (Second scan).
2 1310 0-7 Second video data value (Second scan).
... ... ... ... ...
429 408 1716 0-7 408th video data value (Second scan).
430 409 1717 0-7 409th video data value (Second scan).
1718 0-7 Zero filled.
1719 0-7 Zero filled.
1720 0-7 Zero filled.

The record length for one channel selected GAC in the 8-bit format would be 1720 bytes (2 x 860, since there are 2 scans/record) with the last three trailing bytes zero filled.

Similarly, a user may select channels and request them in the 8-bit format. The output physical record length information for GAC is included in Table 3.1.2.2.2-2.

Table 3.1.2.2.2-2. Output physical record length for GAC (in bytes).
Packed: 6440 bytes
# Channels Selected Unpacked: (in bytes)
8-bit Format 16-bit Format
1 1720 2536
2 2536 4168
3 3352 5808
4 4168 7440
5 4992 9080

Amended March 22, 1999

Amended December 17, 2001



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