Scan Format

SDI Operator's Manual
Revised July 2000

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Scan Format

The GVAR transmission sequence is depicted in Table A-1. It consists of twelve blocks numbered 0 through 11. Blocks 0 through 10 are transmitted when an Imager scan line is completed. Block 10 is followed by a variable number of block 11s (0-N) according to the data available for transmission. Block 11 (Sounder/Auxiliary Data-SAD) transmits sounder data and all non-imager data. Blocks 1 through 10 vary in length directly in accordance with the length of the imaging instrument scan line. Table A-2 provides further details about the structure of the GVAR format. In turn, Tables A-3 through A-7 further explain the GVAR block formats.

Table A-2. GVAR Scan Characteristics

Scan Characteristics
Period		Variable
Blocks/Imager Scan		11
Bit Rate		2,111,360 bits/second

Block Characteristics
Period		15.25 to 104.6 msec
Sync Length		10032 bits
Header Word Length		8 bits/word
Header Length (triple redundant)		90 words (720 bits)

Information Field:
	Block 0 - Documentation Block (Table A-3)
	Word Size	8 bits
	Field Length	8040 words (64,320 bits)

	Block 1 - Infrared Block 1 (Table A-4)
	Word Size	10 bits
	Field Length	68 to 21008 words*
	Number of Records	4 per block
	Line Documentation	16 words
	IR Detector Data	1 to 5236 words

	Block 2 - Infrared Block 2 (Table A-5)
	Word Size	10 bits
	Field Length	51 to 15756 words*
	Number of Records	3 per block
	Line Documentation	16 words
	IR Detector Data	1 to 5236 words

	Blocks 3 to 10 - Visible Blocks (Table A-6)
	Word Size	10 bits
	Field Length	20 to 20960 words*
	Number of Records	1 per block
	Line Documentation	16 words
	Visible Detector Data	4 to 20944 words

	Block 11 - Sounder Auxiliary Data (Table A-7)
	Word Size	6, 8 or 10 bits
	Field Length	10720, 8040 or 6432 words
	Record Types	1 to 8

	Cyclic Redundancy Check (CRC)	16 bits

* Variable length information fields are subjected to zero packing to meet the minimum block length of 32208 bits. The maximum values above assume a 19.2 degree scan width.

Table A-3. GVAR Block 0 (Documentation Block) Format

Sync Code 1254 Bytes	Header 90 Bytes Triple Redundant	Information Field - 8,040 Bytes						CRCC 2 Bytes
Sync Code 1254 Bytes	Header 90 Bytes Triple Redundant	Instrument and Scan Status	Orbit and Attitude Parameters 1412 Bytes	Scan Reference Data 616 Bytes	Grid Data 3080 Bytes	Spares 918 Bytes	Factory Parameters 1736 Bytes	CRCC 2 Bytes
10,032 Bits	720 Bits	2,224 Bits	11,296 Bits	4,928 Bits	24,640 Bits	7,344 Bits	13,888 Bits	16 Bits

Note: Block 0 provides documentation for blocks 1-10 (imager data blocks) only. It does not apply to Block 11.

Table A-4. GVAR Block 1 (IR1 Block) Format

Sync Code 1254 Bytes	Header 90 Bytes Triple Redundant	Data Field - Maximum 15,756 Words (10-bit)						CRCC 2 Bytes
		Channel 4, Detector 1		Channel 4, Detector 2		Channel 5, Detector 2
		Line Doc. 16 Words	Data * 1-5,236 Words	Line Doc. 16 Words	Data * 1-5,236 Words	Line Doc. 16 Words	Data * 1-5,236 Words
10032 Bits	720 Bits	160 Bits	10-52,360 Bits	160 Bits	10-52,360 Bits	160 Bits	10-52,360 Bits	16 Bits

* The minimum Data Field is specified at 21,440 bits (2,1440 words). Therefore, the minimum detector field length, including the 160-bit Line Doc, is 715 words. Detector data fields less than 715 words are padded out to 715 words with zeros.

Table A-5. GVAR Block 2 (IR2 Block) Format

Sync Code 1254 Bytes	Header 90 Bytes Triple Redundant	Data Field - Maximum 15,756 Words (10-bit)						CRCC 2 Bytes
		Channel 2, Detector 1		Channel 2, Detector 2		Channel 3
		Line Doc. 16 Words	Data * 1-5,236 Words	Line Doc. 16 Words	Data * 1-5,236 Words	Line Doc. 16 Words	Data * 1-5,236 Words
10032 Bits	720 Bits	160 Bits	10-52,360 Bits	160 Bits	10-52,360 Bits	160 Bits	10-52,360 Bits	16 Bits

Table A-6. GVAR Blocks 3-10 (Visible Image Data) Format

Sync Code
1254 Bytes

Header
90 Bytes
Triple Redundant

Data Field - Variable Length - 2,144-20,960 Words (10-bit)

CRCC
2 Bytes

Line Doc.

16 Words

Data

4-20,944 Words

Zeros *

0-2,124 Words

10032 Bits

720 Bits

160 Bits

40-209,440 Bits

0-21,240 Zero Bits

16 Bits

* The minimum Data Field is specified at 21,440 bits (2,1440 words). Therefore, blocks having less than 2,128 data words (21,280 bits) are padded with zeros to achieve the minimum Data Field length.

Table A-7. GVAR Block 11 Format for Sounder Scan Data

Sync Code 1254 Bytes	Header 90 Bytes Triple Redundant	Data Field - Variable Length - 64,320 Bits (6-, 8- or 10-bit Words)				CRCC 2 Bytes
		SAD Block ID 40 6-bit Words	Data - 64,080 Bits
			Twelve blocks of raw data samples consisting of one sample from each of 76 sensors.	Line and pixel locators	Seventy-six (76) calibrated channels of 12 pixels each
10032 Bits	720 Bits	240 Bits	4,096 Bits/Block 49,152 Bits Total	336 Bits	16 Bits/Pixel 14,592 Bits Total	16 Bits

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