NOAA Polar Orbiter Data User's Guide

Section 4.2

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4.2 SSU Data

This section describes the Stratospheric Sounding Unit (SSU), its data characteristics, and magnetic tape formats. Section 4.2.1 contains a description of the SSU instrument and its data characteristics. Section 4.2.2 contains the tape formats for full data set copies, 16-bit unpacked format, and selective extract subsets.

4.2.1 Instrument Description and Data Characteristics

The SSU is a step-scanned far-infrared spectrometer with three channels in the 15 micrometer carbon dioxide absorption band (the SSU instruments have been contributed to the TIROS-N series satellites by the British government). It makes use of the pressure modulation technique to measure radiation emitted from carbon dioxide at the top of the Earth's atmosphere. The principles of operation are based on the selective chopper radiometer flown on Nimbus 4 and 5, and the pressure modulator radiometer flown on Nimbus 6. The three SSU channels have the same frequency, but different cell pressures, as shown in Table 4.2.1-1.

Table 4.2.1-1. Cell pressures of the SSU channels.
Channel # Central Wave #
(cm-1)
Cell Pressure (mb) Pressure of Weighting Function Peak (mb) Typical NEΔN
(see Note 1)
1 668 100 15 0.30
2 668 35 5 0.40
3 668 10 1.5 1.00
Note:
1. NEΔN is noise equivalent differential radiance (mW/(m2-sr-cm-1))

The SSU consists of a single primary telescope with a 10 degree IFOV which is step-scanned perpendicular to the satellite subpoint track. Each scan line is composed of eight individual 4.0 second steps and requires a total of 32 seconds, including time for the mirror retrace. The 10 degree IFOV gives a resolution of 147 km at the satellite subpoint and the stepping produces an underlap between adjacent scan lines of approximately 62 km at nadir. A calibration sequence is initiated every 256 seconds (8 scans) during which the radiometer is in turn, stepped to a position to view unobstructed space and an internal blackbody at a known temperature. This calibration mode is synchronized with the HIRS/2 instrument.

Data is sampled at the rate of 40 samples per second, and is digitized to 12-bit precision. Therefore, the SSU data rate is 480 bits per second.

4.2.2 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 4.2.2.1 and 4.2.2.2 contain formats for SSU full data set copies and SSU selective extract subsets, respectively. Section 4.2.2.1 also includes the unpacked format for SSU data.

4.2.2.1 Full Data Set Copies

Each SSU data set normally contains an individual satellite recorder playback. Data within each SSU data set are in chronological order with one record for each SSU scan. Each record is written in binary and contains 2498 bytes (2500 bytes prior to January 1, 1995) in the format as shown in Table 4.2.2.1-1.

Table 4.2.2.1-1. Format of SSU data.
Byte # # Bytes Content
1-2 2 Identifier Field
3-4 2 Scan Line
5-10 6 Time Code
11-14 4 Scan Quality Indicator
15-16 2 Earth Location Delta
17-112 96 Calibration Coefficients
113-116 4 Height and Local Zenith Angle
117-148 32 Earth Location Data
149-2068 1920 SSU Data
2069-2100 32 Scan Position Quality
2101-2498 (2500 prior to 1/1/1995) 398 (400 prior to 1/1/1995) Spare

The identifier field contains the Spacecraft ID in the first byte (see Spacecraft ID in Section 2.0.1) and the SSU data set code in the second byte. The SSU data set code is always the number 7.

The scan line, time code, and height and Local Zenith Angle fields have the same format as the HIRS/2 data in Section 4.1.2.1.

The scan quality information is contained in four bytes. The first two bytes contain processing detected conditions, the third byte contains DACS quality indicators, and the last byte is spare. The scan quality indicators contain a summary of the quality of the 320 TIP minor frames which make up an SSU scan. The definition of the quality bytes is contained in Table 4.2.2.1-2.

Table 4.2.2.1-2. Format of the SSU scan quality information.
Byte Bit Definition
1 7 FATAL FLAG - Data should not be used for product generation.
6 DATA GAP - A data gap of one or more scans precedes this scan.
5 DATA FILL - The scan contains partial data fill. Each halfword of data fill is set to hex "FFFF".
4 DWELL - Data gap was due to dwell data.
3 TIME ERROR - This scan contains data that is suspect due to time error(s).
2 DACS - This scan contains data that is suspect due to DACS QC.
1 NO EARTH LOCATION - Earth location data not available.
0 EARTH LOCATION DELTA - Predicted less actual time exceeds tolerance of 3 seconds.
2 7 CALIBRATION - Insufficient data was collected to calibrate this scan, for at least one channel. On TIROS-N, Channel 3 was not working properly and this bit is set quite often. However, calibration data for the other two channels is current.
6 SPACE VIEW - This scan contains space view data.
5 BLACKBODY VIEW - This scan contains blackbody view data.
4 MIRROR LOCKED - The mirror was in a locked position during this scan. Normal Earth scanning was disrupted.
3 SCAN SEQUENCE - A scan mirror sequence error was detected during this scan.
2 MIRROR SYNC - Mirror sync was lost during this scan. Normal Earth scanning was disrupted.
1 LINEARITY - ADC non-linearity was detected during this scan. Usually not an error unless detected continuously.
0 SPARE
3 7 BIT SYNC STATUS - Drop lock during this scan. The scan line is suspect.
6 SYNC ERROR - Number of bit errors in frame sync was non-zero during this scan. The scan line is suspect.
5 FRAME SYNC LOCK - Frame sync lock during this scan.
4 FLYWHEELING - Flywheeling detected during this scan. The scan line is suspect.
3 BIT SLIPPAGE - Bit slippage detected during this scan. The scan line is suspect.
2 TIP PARITY - DACS TIP parity detected during this scan. The scan line is suspect.
1 AUXILIARY FRAME SYNC ERRORS - The number of bit errors in auxiliary frame sync was non-zero during this scan. The scan line is suspect.
0 SPARE
4 7-4 MAJOR TIP FRAME (0-7) from which this scan was generated.
3-0 SPARE

The Earth location delta contains the time difference between the scan time code and the time code associated with the Earth location data appended to this record. The value is right-justified in the two bytes and is in milliseconds.

The calibration coefficients for SSU data contain three groups. Each group contains coefficients for all three channels. The first group of 24 bytes contains the manual calibration slope and intercept coefficients. The second group of 24 bytes contains the auto calibration slope and intercept coefficients. The third group of 48 bytes contains the normalization coefficients, stored in ascending order. Each value is stored in four bytes and contains a scaled number of fractional bits. The scaling factor for the lowest order coefficients, the constants, is 222, with increasing order coefficients scaled by 230, 244, and 256, respectively. All fields for non-computed values are set to zero. The coefficients are stored as shown in Table 4.2.2.1-3.

Table 4.2.2.1-3. Storage of SSU calibration coefficients.
Channel 1 manual slope coefficient
Channel 1 manual intercept coefficient
Channel 2 manual slope coefficient
...
Channel 1 auto slope coefficient
Channel 1 intercept coefficient
Channel 2 auto slope coefficient
...
Channel 1 normalization coefficient 1 (0th order term)
Channel 1 normalization coefficient 2 (1st order term)
Channel 1 normalization coefficient 3 (2nd order term)
Channel 1 normalization coefficient 4 (3rd order term)
Channel 2 normalization coefficient 1 (0th order term)
...

See Section 4.5 on how to use the SSU calibration coefficients. The Earth location field contains the latitude and longitude for each of the 8 Earth FOVs in the scan. The latitude and longitude values are stored in the two-byte fields in 128ths of a degree. The Earth location data is ordered as follows: FOV 1 latitude, FOV 1 longitude, FOV 2 latitude, FOV 2 longitude, etc.

Each TIP minor frame contains six bytes (3 two-byte pairs) of SSU data. The data is organized into 60-byte groups (10 TIP minor frames) and placed in the SSU Level 1b data set. Each 60-byte group contains data for one-fourth of an SSU FOV so that each SSU data set record contains 32 of the 60-byte groups. The order of the data in each 60-byte group is defined in Table 4.2.2.1-4.

Table 4.2.2.1-4. Format of each 60-byte group of SSU data.
SSU Data Bytes Tip Minor Frame
Digital Word 1 1,2
1
Digital Word 2 3,4
Digital Word 3 5,6
Space Port Temperature 7,8 2
Earth Port Temperature 9,10
PMC Bulkhead Temperature 11,12
Detector Temperature 13,14 3
Blackbody Temperature (Space Side) 15,16
Blackbody Temperature (Sun Side) 17,18
Cell Temperature Channel 1 19,20 4
Cell Temperature Channel 2 21,22
Cell Temperature Channel 3 23,24
Base Plate Temperature 25,26 5
Middle Bulkhead Temperature 27,28
Optics Baseplate Temperature 29,30
Signal Output Channel 1 31,32 6
Signal Output Channel 2 33,34
Signal Output Channel 3 35,36
Thermistor Reference 37,38 7
Mirror Fine Position 39,40
Blackbody Temperature (Point) 41,42
PMC Amplitude Channel 1 43,44 8
PMC Amplitude Channel 2 45,46
PMC Amplitude Channel 3 47,48
ADC Calibration 5% of Full Scale 49,50 9
ADC Calibration 50% of Full Scale 51,52
ADC Calibration 90% of Full Scale 53,54
Signal Output Channel 1 55,56 10
Signal Output Channel 2 57,58
Signal Output Channel 3 59,60

The scan position quality is contained in 32 bytes, one byte for each 60-byte group of SSU data. The definition of the quality byte is contained in Table 4.2.2.1-5.

Table 4.2.2.1-5. Definition of the scan position quality for SSU data.
Byte Bit Definition
1 1 TIME ERROR - This 30-word group contains data that is suspect due to a time error.
2 MISSING DATA - This 30-word group contains data fill due to a data gap.
3 DWELL - This 30-word group contains data fill due to a dwell condition.
4 DACS - This 30-word group contains data that is suspect due to DACS QC error(s) (DROP LOCK, SYNC ERROR, FLYWHEEL, BIT SLIP, TIP PARITY).
5 SCAN SEQUENCE ERROR - A scan mirror position sequence error was detected for this 30-word block.
6 MIRROR SYNC ERROR - The mirror sync was lost during this 30-word block.
7-8 SPARE

The unpacked format for a full copy SSU data set is very similar to the "packed" format described above except for the field containing the SSU data. The signal output for each selected channel (contained in TIP minor frames 6 and 10) is substituted for the complete SSU data field. The unpacked full copy SSU data field has the format shown in Table 4.2.2.1-6. Note: the 400 spare bytes have been eliminated, so the unpacked full copy SSU data record contains only 564 bytes.

Table 4.2.2.1-6. Format of unpacked full copy SSU data.
Byte # # Bytes Content
149-150 2 Signal Output Channel 1, Group 1, TIP Minor Frame 6
151-152 2 Signal Output Channel 2, Group 1, TIP Minor Frame 6
153-154 2 Signal Output Channel 3, Group 1, TIP Minor Frame 6
155-156 2 Signal Output Channel 1, Group 1, TIP Minor Frame 10
157-158 2 Signal Output Channel 2, Group 1, TIP Minor Frame 10
159-160 2 Signal Output Channel 3, Group 1, TIP Minor Frame 10
161-162 2 Signal Output Channel 1, Group 2, TIP Minor Frame 6
... ... ...
527-528 2 Signal Output Channel 1, Group 32, TIP Minor Frame 10
529-530 2 Signal Output Channel 2, Group 32, TIP Minor Frame 10
531-532 2 Signal Output Channel 3, Group 32, TIP Minor Frame 10
533-564 32 Scan position quality

4.2.2.2 Selective Extract Subsets

When channels are selected for SSU data sets, the format of the SSU data is the only field that changes from the format described in Section 4.2.2.1. The signal output for each selected channel (contained in TIP minor frames 6 and 10) is substituted for the complete SSU data field. One or two channels can be selected for the SSU data sets. If Channels 2 and 3 were selected, the SSU data field would have the format shown in Table 4.2.2.2-1. Immediately following the SSU data values, the scan position quality is written starting on the next halfword boundary.

Table 4.2.2.2-1. Format of SSU data field when Channels 2 and 3 are selected.
Byte # # Bytes Content
149-150 2 Signal Output Channel 2, Group 1, TIP Minor Frame 6
151-152 2 Signal Output Channel 3, Group 1, TIP Minor Frame 6
153-154 2 Signal Output Channel 2, Group 1, TIP Minor Frame 10
155-156 2 Signal Output Channel 3, Group 1, TIP Minor Frame 10
157-158 2 Signal Output Channel 2, Group 2, TIP Minor Frame 6
... ... ...
397-398 2 Signal Output Channel 2, Group 32, TIP Minor Frame 6
399-400 2 Signal Output Channel 3, Group 32, TIP Minor Frame 6
401-402 2 Signal Output Channel 2, Group 32, TIP Minor Frame 10
403-404 2 Signal Output Channel 3, Group 32, TIP Minor Frame 10

Amended April 6, 2000



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