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New AREA Format Information

Copyright 1996 Space Science and Engineering Center (SSEC)
University of Wisconsin - Madison
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Storing Satellite Imagery 
in McIDAS Data Structures

February 1997

Space Science and Engineering Center
University of Wisconsin - Madison
1225 West Dayton Street
Madison, WI 53706



Additional signal transmission reference materials:
  METEOSAT High Resolution Image Dissemination,
1992, EUMETSAT.
  Operations Ground Equipment, Internal 
Specification, DRL 504-02-1 Part 1, Specification No 
E007020, released February 9, 1994, Space 
Systems/Loral, 3825 Fabian Way, Palo Alto, CA, 
94303-4604.


Introduction

Satellite imagery is collected by instruments on 
geostationary or earth orbiting satellites. These 
instruments make a series of parallel scans over a section 
of the earth. Ingestors collect and store the scanned data 
in area files. In addition to the image data, area files 
include information about the image's geography and 
physical characteristics, and modifications to the data.
This document describes the structure of McIDAS area 
files and how image data is stored and accessed. Since 
this is a only a reference document, no information is 
provided about how to manipulate data in the structures 
described. If you plan to build area structures, it is 
important that you are familiar with the data structure 
formats described here and the signal transmission 
formats described in the reference documents on page 2.
Currently, this document contains information about 
METEOSAT PDUS, GVAR and GOES-7 data structure 
formats only. Future revisions will include other 
satellites.
This document includes the following information:
  a description of McIDAS area file components 
including the Area Directory and image data
  descriptions of area file characteristics specific to 
METEOSAT, GVAR and GOES-7 satellites




Area File Description
In McIDAS, satellite imagery data and supplemental 
information are stored on disk in data structures called 
areas. Each area is a binary file containing all the 
information necessary to display and navigate the image. 
Area files are named AREAnnnn, where nnnn is a four-
digit number between 0000 and 9999. This number is 
referred to as the area file number. For example, 
AREA0013 is the name of the file that contains area 13.
Complete images are often too large to be stored 
completely in an area file. An area may be a geographic 
portion of the image or a subset produced by sampling or 
averaging the image data. Any point in the area can be 
described with image coordinates, its position in the full 
satellite image, or with area coordinates, its position in 
the area or subset of the image.

Area files consists of six sections, or blocks:
  The Area Directory block describes the area data 
and the image from which the area was created. It 
also contains pointers used for locating other blocks 
and data in the file.
  The Auxiliary (AUX) block contains information 
created by the user such as histograms or long 
comments.
  The Navigation (NAV) block contains information 
used to associate earth coordinates with the image 
pixels.
  The Calibration (CAL) block contains information 
that relates sensor data to meaningful physical units 
such as temperature, albedo or visible brightness.
  The Digital Data (DATA) block contains the actual 
data values for the image.
  The Comment Records (AUDIT) block documents 
modifications to the file data.


Area Directory Block
The first 256 bytes of every area file are the Area 
Directory block for the image. The Area Directory 
describes the kind of data in the area and contains 
pointers used for locating the remaining blocks and other 
subsets of data in the file. It also contains information 
about the image from which the area was created.
The data in the Area Directory is stored as 32-bit (4-byte) 
twos complement binary integers or as ASCII character 
data. The directory is divided in 64 words. Each word is 
described below. Some words are satellite specific. See 
the next section for specific satellites. All byte offsets are 
zero-based.

Word
Description

W1 
contains zeros if the record is valid

W2 
area format: always 4 (as of June 1985)

W3 
sensor source number; see Appendix A

W4 
nominal start date; scheduled date for image 
data collection; YYDDD

W5 
nominal start time; scheduled time for image 
data collection; HHMMSS UTC

W6
image line coordinate of area line 0, element 0 

W7 
image element coordinate of area line 0, 
element 0 

W8 
not used

W9 
number of lines in the area        

W10 
number of elements in each line           

W11 
number of bytes per element (1, 2 or 4)         

W12 
line resolution; number of image lines between 
consecutive area lines

W13 
element resolution; number of image elements 
between consecutive area elements

W14 
maximum number of bands per line of the area

W15 
length of the DATA block line prefix, in bytes; 
sum of W49, W50, W51 (+ 4 if W36 validity 
code is present)                  

W16 
McIDAS user project number under which the 
area was created                     

W17 
actual ingest date; date the area was created; 
provided by the ingesting computer; YYDDD 




W18 
actual ingest time; time the area was created; 
provided by the ingesting computer; HHMMSS

W19 
32-bit filter band map for multichannel 
images; if a bit is set, data exists for the band; 
band 1 is the least significant byte (rightmost)

W20-24 
satellite specific information                      

W25-32 
memo; 32 ASCII characters available for 
comments     

W33 
area file number; last four digits of the file 
name

W34 
byte offset to the start of the area file's DATA 
block

W35 
byte offset to the start of the area file's NAV 
block

W36 
validity code; contains zeros if this area does 
not have validity codes; if these bytes are non-
zero, they must match the first four bytes of 
each DATA block line prefix or the line's data 
is ignored; this word is usually constructed 
from the date and time of the Area Directory 
creation; DDDHHMMSS

W37-45 
satellite specific

W46
actual image start date; date the ingestor 
begins receiving image data; YYDDD

W47
actual image start time; time the ingestor 
begins receiving image data; HHMMSS

W48
actual starting scan line; the first scan line 
received by the ingestor

W49 
length of the DATA block line prefix 
documentation region, in bytes       

W50 
length of the DATA block line prefix 
calibration region, in bytes        

W51 
length of the DATA block line prefix level map 
region, in bytes         

W52 
image source type; for example, VISR, VAS, 
AAA, ERBE, AVHR

W53 
calibration type; units in which the digital data 
is stored; for example, RAW, TEMP, BRIT

W54-59
internal use only; initialized to 0            

W60  
byte offset to the beginning of the area file's 
AUX block

W61  
length of the area file's AUX block, in bytes

W62
not used

W63 
byte offset to the beginning of the area file's 
CAL block

W64 
number of comment records in the area file's 
AUDIT block 

Auxiliary (AUX) Block
This block contains information like histograms or long 
comments created by the user to describe or explain the 
area data. Word 60 of the Area Directory contains the 
number of bytes from the beginning of the area file to the 
beginning of the AUX block. Word 61 contains the total 
number of bytes in the block. If the area file does not 
contain an AUX block, Words 60 and 61 are both zero. 
Ordinarily, when an image is first ingested there is no 
AUX block in the area file.

Navigation (NAV) Block
This block contains the navigation data for the image. 
This information is used to associate earth coordinates, 
usually latitude and longitude, with the pixels of the 
image. Word 35 of the Area Directory contains the 
number of bytes from the beginning of the area file to the 
beginning of the NAV block. If there is no navigation for 
the image, Word 35 is zero. The NAV block's last byte is 
the byte before the CAL block, pointed to in Area 
Directory Word 63. If Word 63 is zero, the NAV block's 
last byte is the byte before the DATA block, pointed to in 
Word 34. The navigation block format varies with each 
satellite. See the next section for specific satellites.

Calibration (CAL) Block 
This block contains the calibration data for the image. It 
is present in the area file if the data must be calibrated 
before it can be displayed. Calibration data is used to 
relate the sensor data to meaningful physical units such 
as temperature, albedo and visible brightness. Word 63 of 
the Area Directory contains the number of bytes from the 
beginning of the area file to the beginning of the CAL 
block. If the area file does not contain a CAL block, Word 
63 is zero. The CAL block's last byte is the byte before the 
DATA block, pointed to in Area Directory Word 34. The 
calibration block format varies with each satellite. See 
the next section for specific satellites.


Digital Data (DATA) Block
This block contains the actual data values for the image. 
These values are stored in the area file in a series of lines 
and elements.

Coordinate Systems
Any point in an area can be described with image 
coordinates, its position in the full satellite image, or 
with area coordinates, its position in the area or subset of 
the image.
Regardless of the source, a satellite image is a sequence 
of lines numbered from top to bottom. Each line is a 
sequence of elements numbered across the line from left 
to right. This line/element numbering scheme determines 
the coordinates for each element, called the image 
coordinates. The top line and leftmost element have 
image coordinates (1,1). This coordinate system is defined 
only by the satellite/camera combination and is 
independent of how the data is stored.
The data stored in an area file is also arranged in a 
sequence of lines and elements, like the image from 
which it was created. The line/element pair determines 
the coordinates for the elements in the area, called the 
area coordinates. The top line and leftmost element have 
area coordinates (0,0).
If entire satellite images could be stored in a single area, 
there would be no point in distinguishing between image 
and area coordinates. However, images are usually too 
large to process efficiently in their entirety. For example, 
a GOES VISSR image in the visible-light band contains 
14568 lines with 15288 elements per line. This image 
requires over 200 megabytes of storage. The data 
actually stored in an area file is a subset of the image. An 
area may be a geographic portion of an image, the USA 
taken from a global image for example. 


Resolution
An area file may be produced from an image by sampling 
or averaging the data. In the case of multiband images, 
an area may include only a portion of the measured 
spectral bands, so that each element contains fewer data 
values than are contained in the original image.
To map an area back to the original image the following 
formulas are used: 
    Image Line = UpperLeftLine + (Area Line * 
    LineResolution)
    Image Element = UpperLeftEle + (Area Element 
    * ElementResolution)
UpperLeftLine is the image line coordinate of the first 
area line. UpperLeftEle is the image element coordinate 
of the first area element.
When LineResolution and ElementResolution are both 1, 
the area is said to be at Resolution 1 or Full Visible 
Resolution. If the area is at Resolution 4, every fourth 
line and element of an image originally at Resolution 1 
are included in the area. Resolutions are relative to full 
resolution. The coverage of a pixel at full resolution will 
vary with each satellite.

Data Structure
The DATA block may be viewed as a continuous stream 
of bytes numbered from zero. The area data is arranged 
line after line, first to last. If a line of data is missing, the 
corresponding place in the data file must be either filled 
with zeros or flagged using non-matching validity codes.

 line prefix 1 line data 1 line prefix 2 line data 2 etc
|_____________|___________|_____________|___________|___...
0               byte numbers increase >>
Figure 1. Area File DATA Block Structure

Each line is divided into two parts: the line prefix and the 
actual line data (image elements). See Figure 1 below. 
The line prefix contains documentation about the image 
and the particular line. 
Although the size and content of the line prefix depend 
on the image source type defined in Area Directory Word 
52, each line in an area has the same prefix length. Area 
Directory Word 15 contains the length of the prefix, in 
bytes. If no line prefix exists, Word 15 is zero.


The line prefix may contain any of the following regions. 
The lengths of these regions, when present, are multiples 
of four bytes. 
  validity code
  documentation
  calibration
  level map
If present, the validity code is the first four bytes of the 
line prefix. When this code is present, Area Directory 
Word 36 contains a non-zero value. If the line prefix's 
validity code does not match the value in Word 36, the 
line is invalid and is ignored. 
The documentation region contains information specific 
to a particular satellite line (such as the VISSR IR 
documentation). Area Directory Word 49 contains the 
region's length, in bytes.
The calibration region contains a set of calibration 
coefficients for each band in the line. For example, AAA 
Sounder images contain 13 bands per line, each with 8 
bytes of calibration information. With an additional 12 
bytes of information, an AAA Sounder calibration region 
has 116 bytes. Calibration regions in a line prefix are 
useful when coefficients change for individual bands 
during the course of a transmission. Area Directory Word 
50 contains the region's length, in bytes.
The level map region contains a 1-byte entry for each 
band/channel in the line. These are only meaningful if 
they are greater than zero. For an image containing a 
single band, the level map region is optional; for 
multiband areas, it must be present. Area Directory 
Word 51 contains the region's length, in bytes. 

 validity code   documentation   calibration   level map    
|______________|_______________|_____________|___________|
0               byte numbers increase >>
Figure 2. Area Line Prefix Structure


Calculating Prefix, Line, and DATA Block Sizes
Area Directory Word 34 contains the number of bytes 
from the beginning of the area file to the beginning of the 
DATA block. Each line in an area has the same length. 
This length is a multiple of four bytes. The length of a 
line prefix, line data section, or the entire DATA block 
can be determined from information in the Area 
Directory. Use the information and the formulas that 
follow to calculate the sizes.

Area Directory Information
Word

validity code length (valcode)
4 bytes if W36 is not 0, 
0 otherwise

documentation length (doc)
W49

calibration length (cal)
W50

level map length (level)
W51

number of bands per line (nbands)
W14

number of elements per line (nele)
W10

number of bytes per element (nbytes)
W11

number of lines in the area (nlines)
W9


line prefix length = valcode (0 or 4) + doc + cal + level 
line data section length = nbands*nele*nbytes
line length = line prefix length + line data section length
DATA block length = nlines * line length

Comment Records (AUDIT) Block
Comment records may be included in an area file. These 
records are usually used to keep an audit trail of 
modifications made to data in the file. Each comment 
record is 80 ASCII characters. Area Directory Word 64 
contains the number of comment records. Ordinarily, 
when an image is first ingested there is no AUDIT block 
in the area file.


Satellite Specific Characteristics 
of McIDAS Area Files

Some aspects of McIDAS area files are satellite specific. 
This section describes characteristics specific to the 
following satellites:
  METEOSAT PDUS
  GVAR IMAGER
  GVAR BLOCK 11
  GVAR SOUNDER
  GVAR VISSR
  GOES-7
Future revisions of this document will include other 
satellites.

METEOSAT PDUS
Because METEOSAT PDUS images are remapped and 
calibrated at the ground station before the stretched 
signal is disseminated, the navigation and calibration 
data sections are simpler. This data is eight bits and each 
band is stored in a separate area file.
For more information on METEOSAT labels and headers, 
see the EUMETSAT document METEOSAT High 
Resolution Image Dissemination.

PDUS Area Directory Block

Word
Value and Description

W14
1; each band is stored separately

W19
band filter map values: 
  0 for visible image
  128 (eighth bit from right) for IR
  512 (tenth bit from right) for WV




W22
MIEC absolute calibration band value (IR or 
WV) from the calibration section of the 
METEOSAT header; stored as scaled integer 
xxxxx, value is .xxxxx

W23
space count corresponding to calibration value 
from the calibration section of the METEOSAT 
header; stored as scaled integer xxx, value is 
xx.x

W24
1 or 2; physical sensor number from the 
METEOSAT header

W37
line offset of the southeast corner of the area in 
image coordinates; 16-bit value from the 
METEOSAT header, right justified plus 1

W38
element offset of the southeast corner of the 
area in image coordinates; 16-bit value from 
the METEOSAT header, right justified plus 1

W39
satellite center longitude of rectification; 16-bit 
value from the METEOSAT header and right 
justified

W44
0

W49
24; length of the DATA block line prefix 
documentation, in bytes

W50
0; length of the DATA block line prefix 
calibration, in bytes

W51
0; length of the DATA block line prefix level 
map, in bytes

W52
MSAT; image source type

W53
RAW; calibration type

W54
0 if data was ingested as sent (full resolution); 
1 if data was sampled down (every other pixel); 
VIS is sent as Resolution 1 in some images and 
Resolution 2 in others

W55
bitmap indicating types of data included in the 
original image; bits are numbered right to left 
(least significant to most significant bit)
  bit 0: 1 if VIS was included in 
transmission, 0 if not
  bit 1: 1 if IR was included in transmission, 
0 if not
  bit 2: 1 if WV included in transmission, 0 if 
not
  all other bits = 0



PDUS DATA Block

Line Prefix
  Validity code: Optional but recommended to flag 
missing data. Missing data cannot be simply omitted; 
it must have zeros as placeholder data or a validity 
code that does not match the value in Area Directory 
Word 36.
  Documentation: 24 bytes. This is a copy of the label 
that arrives with every subframe. 
  Calibration: 0 bytes. Not used.
  Level map: 0 bytes. Not used. Each METEOSAT 
band is stored in a separate area.

Line Data 
Each value is transmitted as eight bits and is stored west 
to east and north to south in the area, the opposite of 
how it is transmitted.

PDUS NAV Block
A PDUS navigation block is divided into 256 words:

Word
Value and Description

W1
MSAT; navigation type

W2
Julian day of this navigation, YYDDD

W3
time of this navigation, HHMMSS

W4
0; reference position for the telescope

W5
0; line number corresponding to the telescope 
reference position

W6
1250; center scan line

W7
center longitude of rectification (west positive), 
DDMMSS

W8
0; not used

W9
0; not used

W10
Julian day of this navigation, YYDDD

W11-256
0; not used


PDUS CAL Block
No calibration information is needed for PDUS.


GVAR IMAGER
The tables mentioned in the line prefix, line data, 
calibration and navigation descriptions in this section are 
from Operations Ground Equipment, Internal 
Specification, DRL 504-02-1 Part 1, Specification No 
E007020. This document describes data formatted by the 
ground station and then retransmitted. It is referred to 
in this document as the OGE.

Imager Documentation (Block 0)
The GVAR Imager documentation, Block 0, is 
supplemental control information about an image, not 
image data. Some of the information is also contained in 
Imager sensor data. For each line of GVAR Imager 
sensor data transmitted, one line of Block 0 
documentation is transmitted and stored in a separate 
area.
Area Directory Block

Word
Value and Description

W12
8; line resolution

W13
1; element resolution

W14
1; number of bands 

W19
1; band number

W25-32
ordinarily contains the ASCII string RT IMGR DOC

W49
44; length of the DATA block line prefix 
documentation region, in bytes

W50
0; length of the DATA block line prefix 
calibration region, in bytes

W51
0; length of the DATA block line prefix level 
map region, in bytes

W52
GVAR; image source type

W53
RAW; calibration type



Line Prefix 
  Validity Code: 4 bytes
  Documentation: 44 bytes. The documentation region 
consists of the following:
- Block Header CRC: 2 bytes (OGE Table 3-5)
- Scan Status: 4 bytes (OGE Table 3-6)
- Year, Day, Time from Block 0: 8 bytes (OGE 
Table 3-6)
- Block Header: 30 bytes (OGE Table 3-5)
  Bytes 17-24 now contain the time the block was disseminated
  from the ground station.
The rest of the line is made up of 8040 bytes of 8-bit data. 
(OGE Table 3-6)

Imager Area DIRECTORY Block   (For blocks 1-10, bands 1-5)

Word
Value and Description

W5
1; size of the z-dimension

W14
1; number of bands requested

W19
band filter map; 1 for VIS; 2, 4, 8 or 16 for IR 
bands; only one bit should be set

W25-32
RT IMGR IR; (ASCII string)
RT IMGR VIS if the band is visible


W49
76; length of the DATA block line prefix 
documentation region, in bytes

W50
0; length of the DATA block line prefix 
calibration region, in bytes

W51
0; length of the DATA block line prefix level 
map region, in bytes

W52
GVAR; image source type

W53
RAW; calibration type

W55
1



Imager area DATA Block
GOES Variable Imager data is transmitted in five 
spectral bands: VISIBLE (VIS) and four INFRARED (IR) 
bands. An area contains only one of these five bands. 
Area Directory Word 19 contains a band filter map 
indicating the area file's band. 
The highest resolution (lowest values of line and element 
resolution in the Area Directory) possible for a visible 
area is 1. The highest resolution for an IR area is 4 
because longer wavelengths inherently have less 
resolution. For a GVAR satellite, Resolution 1 means 
approximately 1 km resolution at the satellite subpoint.
Every element in a GOES-8 area contains one 10-bit 
pixel representing raw data from the instrument. Each 
pixel is stored as two bytes in the McIDAS area file. The 
hardware shifts the data so that the 10 bits are formatted 
as follows:
| 0 | x | x | x | x | x | x | x | x | x | x | 0 | 0 | 0 | 0 | 0 |
The x represents a data bit and the rest is 0-filled after 
shifting.

Line Prefix
  Validity Code: 4 bytes
  Documentation: 76 bytes. The documentation region 
consists of the following:
- Block Header CRC: 2 bytes. Last three bits only; 
bit is set if Block Header copy is good. This data 
is usually 00,07.
- Scan Status: 4 bytes (OGE Table 3-6)
- Year, Day, Time from Block 0: 8 bytes
- Block Header: 30 bytes (OGE Table 3-5)
  Bytes 17-24 now contain the time the block was disseminated from
  the ground station.
- Additional Line Documentation: 32 bytes. 16 10-
bit fields, right-justified. (OGE Table 3-7)
The 6 bit field on the left hand side is not zero filled.
To get the 10 bit field a logical AND against 03FF (hex) must be used.

Data
The rest of the line consists of up to 41920 bytes of data. 
Because it is 2-byte data, half that many pixels are 
represented.


Imager NAV Block
Navigation blocks are divided into 640 words. Unless 
otherwise noted, words are twos complement binary 
integers. 
This navigation information comes from Block 0 records. 
Bytes designated R*4 in OGE Tables are in Gould 
format. They must be scaled and then converted to 
integers; or converted to Real on the machine doing the 
decoding, scaled as designated below, and then converted 
to integer.
Word
Value and Description

W1
GVAR; navigation type

W2
ASCII string; usually a letter followed by three 
integers, for example, U001

W3
imager scan status; bits 0-15 are right justified, bit 
15 is the least significant bit; IMC active flag is bit 
8, counting from the least significant bit; 1=active; 
see OGE Table 3-6, bytes 3-6

W4-5
0; not used

W6-62
see OGE Table 3-6, bytes 295-522

W6
reference longitude, rad * 10000000

W7
reference distance from nominal, km * 10000000

W8
reference latitude, rad * 10000000

W9
reference yaw, rad * 10000000

W10
reference attitude roll, rad * 10000000

W11
reference attitude pitch, rad * 10000000

W12
reference attitude yaw, rad * 10000000

W13-14
epoch date/time, BCD format

W15
delta from epoch time, minutes * 100

W16
image motion compensation roll, rad * 10000000

W17
image motion compensation pitch, rad * 10000000

W18
image motion compensation yaw, rad * 10000000

W19-31
longitude delta from reference values, rad * 
10000000

W32-42
radial distance delta from reference values, rad * 
10000000

W43-51
geocentric latitude delta values, no units * 
10000000

W52-60
orbit yaw delta values, no units * 10000000

W61
daily solar rate, rad/min * 10000000

W62
exponential start time from epoch, minutes * 100

W63-117
roll attitude angle information; 
see OGE Table 3-6, bytes 523-742

W63
exponential magnitude, rad * 10000000

W64
exponential time constant, minutes * 100

W65
mean attitude angle, rad * 10000000

W66
number of sinusoids/angles, no units

W67
magnitude of first order sinusoid, rad * 10000000

W68
phase angle of first order sinusoid, rad * 10000000

   :

W95
magnitude of fifteenth sinusoid, rad * 10000000

W96
phase angle of fifteenth sinusoid, rad * 10000000

W97
number of monomial sinusoid, no units

W98
order of applicable sinusoid, no units

W99
order of first monomial sinusoid, no units

W100
magnitude of monomial sinusoid, rad * 10000000

W101
phase angle of monomial sinusoid, rad * 10000000

W102
angle from epoch at daily solar rate, rad * 10000000

W103-107
repeat of Words 98-102 for second monomial

W108-112
repeat of Words 98-102 for third monomial

W113-117
repeat of Words 98-102 for fourth monomial

W118-127
reserved

W128
MORE; 4-byte ASCII

W129
GVAR; 4-byte ASCII

W130-239
attitude angles

W130-184
repeat of Words 63-117 for pitch attitude angle;
see OGE Table 3-6, bytes 743-962

W185-239
repeat of Words 63-117 for yaw attitude angle;
see OGE Table 3-6, bytes 963-1182

W240-255
reserved

W256
MORE; 4-byte ASCII

W257
GVAR; 4-byte ASCII

W258-367
misalignment angles

W258-312
repeat of Words 63-117 for roll misalignment angle;
see OGE Table 3-6, bytes 1183-1402

W313-367
repeat of Words 63-117 for pitch misalignment 
angle; see OGE Table 3-6, bytes 1403-1622

W368
year and Julian day, YYDDD

W369
nominal start time of image, HHMMSS

W370
1; Imager/Sounder instrument flag; 1 = Imager, 
2 = Sounder

W371-379
reserved

W380
instrument nadir, north/south cycles; 
see OGE Table 3-6, byte 6305

W381
instrument nadir, east/west cycles; 
see OGE Table 3-6, byte 6306

W382
instrument nadir, north/south increments; 
see OGE Table 3-6, byte 6307-6308

W383
instrument nadir, east/west increments; 
see OGE Table 3-6, byte 6309-6310

W384
MORE; 4-byte ASCII

W385
GVAR; 4-byte ASCII

W386-511
reserved

W512
MORE; 4-byte ASCII

W513
GVAR; 4-byte ASCII

W514-640
reserved


Imager CAL Block
There are two instruments on the GOES-8 through 
GOES-12 series of satellites: the Sounder and the 
Imager. Even sensor source numbers represent Imager 
data and odd sensor source numbers represent Sounder 
data. Area Directory Word 52 contains the image source 
type:
  GVAR for GVAR data as it is ingested (2 byte)
  VISR for 1-byte visible or infrared imagery
VISR data cannot be ingested; GVAR data can be 
transformed to VISR with a McIDAS utility such as AA 
or SENAA.

Area Directory Word 53 contains the units in which the 
data is stored:
  RAW for 2-byte raw GVAR data
  BRIT for 1-byte visible or infrared values
Area Directory Word 14 contains the number of spectral 
bands present in an Imager area. The filter band map in 
Area Directory Word 19 describes the bands in an area. A 
bit is set for each band appearing in the area. The 
number of bands must match the value in Word 14.


The Imager calibration block is made up of 128 words 
(512 bytes). The data is in the Gould format.

Word
Value and Description

W1-W8
visible bias coefficients; one per detector (OGE Table 
3-6, bytes 6399-6430)

W9-W16
visible first order gain coefficients; one per detector 
(OGE Table 3-6, bytes 6431-6462)

W17-W24
visible second order gain coefficients; one per detector 
(OGE Table 3-6, bytes 6463-6494)

W25
visible radiance to albedo conversion factor (OGE 
Table 3-6, bytes 6495-6498)

W26-W29
det side 1 IR bias scaling factors; one per IR channel 
(OGE Table 3-6: bytes 6667-6670 Ch 4, Side 1; bytes 
6675-6679 Ch 5, Side 1; bytes 6683-6686 Ch 2, Side 1; 
bytes 6691-6694 Ch 3, Side 1)

W30-W33
det side 2 IR bias scaling factors; one per IR channel 
(OGE Table 3-6: bytes 6695-6698 Ch 4, Side 2; bytes 
6703-6706 Ch 5, Side 2; bytes 6711-6714 Ch 2, Side 2; 
bytes 6719-6722 Ch 3, Side 2)

W34-W37
det side 1 IR gain scaling factors; one per IR channel 
(OGE Table 3-6: bytes 6723-6726 Ch 4, Side 1; bytes 
6731-6734 Ch 5, Side 1; bytes 6739-6742 Ch 2, Side 1; 
bytes 6747-6750 Ch 3, Side 1)

W38-W41
det side 2 IR gain scaling factors; one per IR channel 
(OGE Table 3-6: bytes 6751-6753 Ch 4, Side 2; bytes 
6759-6762 Ch 5, Side 2;bytes 6767-6770 Ch 2, Side 2; 
bytes 6775-6778 Ch 3, Side 2)

W42-W128
0



BLOCK 11
Block 11 Holding areas contain data for Sounder images. 
This data cannot be easily accessed. A decoder must 
reformat the raw Block 11 data and place it in Sounder 
image areas, where it is available for analysis and 
display.

Block 11 Holding Area DIRECTORY Block

Word
Value and Description

W11
number of bytes per element; 1 or 2 depending 
on the element size of the desired band; a 
holding area cannot contain both 1- and 2-byte 
data

W14
1; number of bands in the image

W19
block type filter; positions of set bits 
correspond to the block types requested; the 
least significant bit is the right hand bit; a 
value of 787968 translates to 0c0600 hex with 
bits set in positions 20, 19, 11 and 10

W25-32
ordinarily contains the ASCII string RT BK11 BYT1

W49
40; length of the DATA block line prefix 
documentation region, in bytes

W50
0; length of the DATA block line prefix 
calibration region, in bytes

W51
0; length of the DATA block line prefix level 
map region, in bytes

W52
BK11; image source type

W53
RAW; calibration type



Block 11 Holding Area DATA Block
GVAR transmits 22 types of Block 11 data. This can be 
6-, 8-, or 10-bit data. The user can specify any type to be 
stored in a single holding area. Control fields in the line 
prefix or the first portion of the data (called the SAD ID) 
are used by post-processes, such as the Sounder decoder, 
to determine the block type. Each DATA block line 
consists of a single Block 11 sector (or block). All blocks 
are 8040 bytes. Refer to the OGE, sections 3.3.7 - 3.3.7.14 
for a detailed breakdown of the contents of each block 
type.
The 10-bit data is formatted as follows:
| 0 | x | x | x | x | x | x | x | x | x | x | 0 | 0 | 0 | 0 | 0 |
The x represents a data bit and the rest is zero-filled 
after shifting.
The 8-bit data is formatted as follows:
| x | x | x | x | x | x | x | x |
The 6-bit data is formatted as follows:
| 0 | 0 | x | x | x | x | x | x |

Line Prefix
  Validity Code: 4 bytes
  Documentation: 40 bytes. The documentation region 
consists of the following:
- Block Header CRC: 2 bytes (OGE Table 3-5)
- Current Year, Day and Time from Block 0: 8 
bytes (OGE Table 3-6)
- Block Header: 30 bytes (OGE Table 3-5)
  Bytes 17-24 now contain the time the block was disseminated from
  the ground station.
The rest of the line consists of up to 8040 bytes of data, 
depending on block type. 
WARNING: The Block Header CRC field is overwritten 
in the mainframe by a 2-byte counter. This is used to 
check sequencing of the data flow.


GVAR SOUNDER
Sounder areas are decoded from Block 11 holding area 
data.

Block 11 Decoded Sounder Area DIRECTORY Block

Word
Value and Description

W12
line resolution; 10 is the base resolution; if 
lines are sampled or averaged, the resolution 
is in multiples of 10.

W13
element resolution; 10 is the base resolution; 
if pixels are sampled or averaged, the 
resolution is in multiples of 10.

W14
19; number of bands

W19
54287; band filter map; translates to 0007ffff 
hex; bits set for bands 1-19

W25-32
ordinarily contains the ASCII string PRIORITY
COMPLETED

W49
36; length of the DATA block line prefix 
documentation region, in bytes

W50
0; length of the DATA block line prefix 
calibration region, in bytes

W51
24; length of the DATA block line prefix level 
map region, in bytes

W52
GVAR; image source type

W53
RAW; calibration type



Block 11 Decoded Sounder Area DATA Block
The GVAR Sounder Decoder reads Block 11 Holding 
areas, which contain blocks of type 32 (20hex) and type 
35 (23hex) as well as others. These blocks are 
documented in OGE, section 3.3.7.2 and section 3.3.7.3. 
Navigation and calibration data is read from type 32 
blocks (Sounder documentation blocks). Sensor data is 
read from type 35 blocks (Sounder scan data blocks) and 
then reformatted and placed in the Sounder image area.
All Sounder data fields are 13 bits and are placed in 2-
byte (16-bit) fields. There are 23 bands of data for each 
pixel in a Sounder scan data block. The pixels correspond 
to a geographic area 11 pixels west-east and 4 pixels 
north-south. For each image line, the decoder produces 
11 sets of 23 interleaved fields of data.
Bands 20-23 of this data are not displayable; they hold 
the latitude and longitude of the first 19 bands. The 
latitude and longitudes are 32-bit values. Since the 
actual Sounder data is 16 bits, the latitude and longitude 
values must be split in half to store them in the area 
structure. Band 20 holds the two most significant bytes 
and band 21 holds the two least significant bytes of the 
latitude. Band 22 holds the two most significant bytes 
and band 23 holds the two least significant bytes of the 
longitude.
These latitude and longitude values are in the Gould 
floating point format. See OGE, section 3.5.4. For 
example, if the latitude of a pixel is 100.1640625, the hex 
representation is 42642A00; band 20 holds 4264, and 
band 21 holds 2A00.
No code is provided in this package to use these latitudes 
and longitudes; they are included only for reference 
purposes.
There are four Sounder sensors: A through D. Each 
DATA block line contains information from only one 
sensor. The first line contains sensor A information, the 
second line, sensor B, etc. This pattern is repeated for the 
entire DATA block.


Line Prefix
  Validity Code: 4 bytes
  Documentation: 36 bytes. The documentation region 
consists of the following:
- First 9 words of SAD Block ID: 9 bytes 
(OGE Table 3-8)
- Year, Day, Time of scan line start: 8 bytes 
(OGE Table 3-11)
- Sounder scan status: 2 bytes (OGE Table 3-11)
- Number of Block 11 blocks in the scan: 2 bytes 
(OGE Table 3-11)
- O&A Location: 2 bytes. No longer used.
- Detector status: 10 bytes (OGE Table 3-11)
- Detector used in this area line: 1 byte
- padding: 2 bytes
  Level map; indicates band order for this line: 24 bytes
The rest of the line is made up of the interleaved Sounder 
data.

Block 11 Decoded Sounder Area NAV Block
Navigation blocks are divided into 640 words. Unless 
otherwise noted, words are twos complement binary 
integers. 
This navigation information comes from Block 11 records, 
type 32. Bytes designated R*4 in the OGE Tables are in 
Gould format in the holding areas. They must be scaled 
and then converted to integers; or converted to Real on 
the machine doing the decoding, scaled as designated 
below, and then converted to integer.

Word
Value and Description

W1
GVAR; navigation type

W2
ASCII string; usually a letter followed by three 
integers, for example, U001

W3
imager scan status; bits 0-15 are right justified, bit 
15 is the least significant bit; IMC active flag is bit 
8, counting from the least significant bit; 1=active; 
see OGE Table 3-11, bytes 3-6

W4-5
0; not used

W6-62
see OGE Table 3-11, bytes 323-550

W6
reference longitude, rad * 10000000

W7
reference distance from nominal, km * 10000000

W8
reference latitude, rad * 10000000

W9
reference yaw, rad * 10000000

W10
reference attitude roll, rad * 10000000

W11
reference attitude pitch, rad * 10000000

W12
reference attitude yaw, rad * 10000000

W13-14
epoch date/time, BCD format

W15
delta from epoch time, minutes * 100

W16
image motion compensation roll, rad * 10000000

W17
image motion compensation pitch, rad * 10000000

W18
image motion compensation yaw, rad * 10000000

W19-31
longitude delta from reference values, rad * 
10000000

W32-42
radial distance delta from reference values, rad * 
10000000

W43-51
geocentric latitude delta values, no units * 
10000000

W52-60
orbit yaw delta values, no units * 10000000

W61
daily solar rate, rad/min * 10000000

W62
exponential start time from epoch, minutes * 100

W63-117
roll attitude angle information; 
see OGE Table 3-11, bytes 551-770

W63
exponential magnitude, rad * 10000000

W64
exponential time constant, minutes * 100

W65
mean attitude angle, rad * 10000000

W66
number of sinusoids/angles, no units

W67
magnitude of first order sinusoid, rad * 10000000

W68
phase angle of first order sinusoid, rad * 10000000

   :

W95
magnitude of fifteenth sinusoid, rad * 10000000

W96
phase angle of fifteenth sinusoid, rad * 10000000

W97
number of monomial sinusoid, no units

W98
order of applicable sinusoid, no units

W99
order of first monomial sinusoid, no units

W100
magnitude of monomial sinusoid, rad * 10000000

W101
phase angle of monomial sinusoid, rad * 10000000

W102
angle from epoch at daily solar rate, rad * 10000000

W103-107
repeat of Words 98-102 for second monomial

W108-112
repeat of Words 98-102 for third monomial

W113-117
repeat of Words 98-102 for fourth monomial

W118-127
reserved

W128
MORE; 4-byte ASCII

W129
GVAR; 4-byte ASCII

W130-239
attitude angles see OGE Table 3-11, bytes 771-1210

W130-184
repeat of Words 63-117 for pitch attitude angle;
see OGE Table 3-11, bytes 771-990

W185-239
repeat of Words 63-117 for yaw attitude angle;
see OGE Table 3-11, bytes 991-1210

W240-255
reserved

W256
MORE; 4-byte ASCII

W257
GVAR; 4-byte ASCII

W258-367
misalignment angles

W258-312
repeat of Words 63-117 for roll misalignment angle;
see OGE Table 3-11; bytes 1211-1430

W313-367
repeat of Words 63-117 for pitch misalignment 
angle; see OGE Table 3-11; bytes 1431-1650

W368
year and Julian day, YYDDD

W369
nominal start time of the image, HHMMSS

W370
2; Imager/Sounder instrument flag; 1 = Imager, 
2 = Sounder

W371-379
reserved

W380
instrument nadir, north/south cycles; 
see OGE Table 3-6, byte 3005

W381
instrument nadir, east/west cycles; 
see OGE Table 3-6, byte 3006

W382
instrument nadir, north/south increments; 
see OGE Table 3-6, byte 3007-3008

W383
instrument nadir, east/west increments; 
see OGE Table 3-6, byte 3009-3010

W384
MORE; 4-byte ASCII

W385
GVAR; 4-byte ASCII

W386-511
reserved

W512
MORE; 4-byte ASCII

W513
GVAR; 4-byte ASCII

W514-640
reserved


Block 11 Decoded Sounder Area CAL Block
Sounder calibration blocks are made up of 128 words 
(512 bytes):

Word
Value and Description

W1-W4
visible bias coefficients; one per detector 
(OGE Table 3-11, bytes 3075-3090)

W5-W8
visible first order gain coefficients; one per detector 
(OGE Table 3-11, bytes 3091-3106)

W9-W12
visible second order gain coefficients; one per 
detector (OGE Table 3-11, bytes 3107-3122)

W13
visible radiance to albedo conversion factor (OGE 
Table 3-11, bytes 3123-3126)

W14-W31
IR bias scaling factors; one per IR channel (OGE 
Table 3-11, bytes 3127-3414)

W32-W49
IR gain scaling factors; one per IR channel (OGE 
Table 3-11, bytes 3415-3702)

W50-W128
0


GVAR VISSR

Brightness to Temperature Conversion Algorithm
The following description is for image source type VISR 
from Area Directory Word 52. GVAR VISSR areas can be 
made with the McIDAS commands AA and ARAGET.
Every element in a VISSR area contains one 8-bit pixel, 
representing raw data from the instrument. If the area 
contains IR data, the observed temperature may be 
calculated from the pixel value using the following 
formulas:
    T = 418 - B  (B>176 OR B=176)
    T = 330 - (B / 2)  (B<176 OR B=176)
T is the brightness temperature (degrees K). B is the 
pixel value (0 to 255). For IR data, the highest pixel 
values correspond to the coldest temperatures (space is 
white).
The line prefix in a VISSR area may be absent or it may 
contain only the 4-byte validity code. The highest 
resolution, lowest values of line and element resolution in 
the Area Directory, possible for a visible area is 1. The 
highest resolution for an IR area is 4 because longer 
wavelengths inherently have less resolution. For a GOES 
satellite, resolution 1 means approximately 1 km 
resolution at the satellite subpoint.


GOES-7

GOES-7 Area NAV Block
Unless otherwise noted, the words in the navigation 
block are twos complement binary integers.

Word
Value and Description

W1
GOES; navigation type

W2
satellite ID, year, and Julian day, 
SSYYDDD

W3
nominal start time of image, HHMMSS

Orbit parameters

W4
1; orbit type

W5
epoch date (ETIMY), YYMMDD

W6
epoch time (ETIMH), HHMMSS

W7
semimajor axis (SEMIMA), Km * 100

W8
orbital eccentricity (ECCEN) * 100000

W9
orbital inclination (ORBINC), Deg * 1000

W10
mean anomaly (MEANA), Deg * 1000

W11
argument of perigee (PERIGEE), Deg * 
1000

W12
right ascension of ascending node 
(ASNODE), Deg * 1000

Attitude parameters

W13
declination of satellite axis (DECLIN), 
DDDMMSS (+ = NORTH)

W14
right ascension of satellite axis (RASCEN), 
DDDMMSS

W15
picture center line number (PICLIN)

Spin
 

W16
spin period (SPINP); the satellite period, in 
microseconds, or the spin rate in 
revolutions/minute.

Frame geometry

W17
total sweep angle, line direction (DEGLIN), 
DDDMMSS

W18
number of scan lines (LINTOT), NNLLLLL 
where NN is number of sensors, LLLLL is 
number of scans; total number of actual 
lines is NN * LLLLL

W19
total sweep angle, element direction 
(DEGELE), DDDMMSS

W20
number of elements in a scan line 
(ELETOT)

Camera geometry

W21
forward-leaning (PITCH), DDDMMSS

W22
sideways-leaning (YAW), DDDMMSS

W23
rotation (ROLL), DDDMMSS

W24
reserved

W25
east-west adjustment value (IAJUST), in 
visible elements (+ or -)

W26
time computed by IAJUST from the first 
valid landmark of the day (IAJTIM), 
HHMMSS

W27
reserved

W28
angle between the VISSR and sun sensor 
(ISEANG), DDDMMSS

W29
0; reserved for later implementation of 
*SKEW*

W30
reserved

Betas for this area

W31
scan line of first the beta

W32
time of first beta's scan line (beginning), 
HHMMSS

W33
time of first beta's scan line (continued), 
milliseconds * 10

W34
beta count 1

W35
scan line of second beta

W36
time of second beta's scan line (beginning), 
HHMMSS

W37
time of second beta's scan line (continued), 
milliseconds * 10

W38
beta count 2

Gammas for this area

W39
gamma, element offset * 100; this is the 
nominal offset at time zero of this day

W40
gamma-dot, element drift per hour * 100

W41-120
reserved

W121-128
memo; up to 32 ASCII characters of 
comments

Appendix A
 Sensor Source Numbers
Use this table to find the number used by McIDAS 
software to identify sensors. 

Number  Sensor Source 
0   Non-Image Derived Data        
2   Graphics               
3   MDR Radar
4   PDUS METEOSAT Visible
5   PDUS METEOSAT Infrared
6   PDUS METEOSAT Water Vapor
7   Radar                
8   Miscellaneous Aircraft Data (MAMS)  
9   Raw METEOSAT
12  GMS Visible prior to GMS-5
13  GMS Infrared prior to GMS-5
14  ATS 6 Visible            
15  ATS 6 Infrared            
16  SMS-1 Visible            
17  SMS-1 Infrared            
18  SMS-2 Visible            
19  SMS-2 Infrared            
20  GOES-1 Visible            
21  GOES-1 Infrared           
22  GOES-2 Visible            
23  GOES-2 Infrared           
24  GOES-3 Visible            
25  GOES-3 Infrared           
26  GOES-4 Visible (VAS)         
27  GOES-4 Infrared and Water Vapor (VAS) 
28  GOES-5 Visible            
29  GOES-5 Infrared and Water Vapor (VAS) 
30  GOES-6 Visible            
31  GOES-6 Infrared           
32  GOES-7 Visible            
33  GOES-7 Infrared           
41  TIROS-N (POES)               
42  NOAA-6                
43  NOAA-7                
44  NOAA-8                
45  NOAA-9          
46-49   MARINER X Spacecraft         
50  Hubble Space Telescope      
54  METEOSAT-3           
55  METEOSAT-4
56  METEOSAT-5
57  METEOSAT-6
60  NOAA-10
61  NOAA-11
62  NOAA-12
63  NOAA-13
64  NOAA-14
70  GOES-8 (Imager)
71  GOES-8 (Sounder)
72  GOES-9 (Imager)
73  GOES-9 (Sounder)
74  GOES-10 (Imager)
75  GOES-10 (Sounder)
76  GOES-11 (Imager)
77  GOES-11 (Sounder)
78  GOES-12 (Imager)
79  GOES-12 (Sounder)
80  ERBE
82  GMS-4
83  GMS-5
84  GMS-6
85  GMS-7
87  DMSP F-8
88  DMSP F-9
89  DMSP F-10
90  DMSP F-11
91  DMSP F-12
95  FY-1b                
96  FY-1c                
97  FY-1d                


Appendix B
GVAR Calibration Tables

Imager Sensor Characteristics
The IMAGER produces observation data for a given 
spatial location in four different IR spectral bands and a 
visible band. The characteristics of the Imager bands are 
given below:


Band No.

Wavelength Range (um)

Range of Measurement

Meteorological Objective 
And Maximum Temp. Range

1 
00.55   00.75 
1.6 to 100 % albedo 
Cloud Cover

2 
03.80   04.00 
4 to 320 K    
Night clouds (space - 340 K)

3 
06.50   07.00 
4 to 320 K    
Water vapor (space - 290 K)

4 
10.20   11.20 
4 to 320 K    
Surf Temp (space - 340 K)

5 
11.50   12.50 
4 to 320 K    
Sea surface temp and water 
vapor (space - 335 K)




Sounder Detector Characteristics
The Sounder produces data for a given spatial location in 
18 IR spectral bands and a visible band. The 
characteristics of the Sounder bands are listed below:


Band No.

Central Wavelength (um)

Wave No. (cm -1)

Meteorological Objective And 
Maximum Temp. Range

Longwave

1 
14.71  
680 
Temperature (space - 280 K)

2 
14.37  
696 
Sounding (space - 280 K)

3 
14.06  
711 
Sounding (space - 290 K)

4 
13.64  
733 
Sounding (space - 310 K)

5 
13.37  
748 
Sounding (space - 320 K)

6 
12.66  
790 
Sounding (space - 330 K)

7 
12.02  
832 
Surface temp. (space - 340 K)

Midwave

8 
11.03  
907 
Surface temp (space - 345 K)

9 
9.71  
1030 
Total ozone (space - 330 K)

10 
7.43  
1345 
Water vapor (space - 310 K)

11 
7.02  
1425 
Sounding (space - 295 K)

12 
6.51  
1535 
Sounding (space - 290 K)

Shortwave

13 
4.57  
2188 
Temperature (space - 320 K)

14 
4.52 
2210 
Sounding   (space - 310 K)

15 
4.45  
2248 
Sounding   (space - 295 K)

16 
4.13  
2420 
Sounding   (space - 240 K)

17 
3.98  
2513 
Surface temp (space - 345 K)

18 
3.74  
2671 
Temperature (space - 345 K)

Visible

19 
0.70  
14367 
Cloud cover


The number of spectral bands present in Imager and 
Sounder areas is contained in Area Directory Word 14. 
The filter map in Word 19 describes the bands in an area. 
A bit is set for each band in the area. The number of 
bands must match the value in Word 14.




Storing Satellite Imagery in McIDAS Data Structures
Preliminary Issue 12/95

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