McIDAS Programmer's Manual
Version 2003

[Search Manual] [Table of Contents] [Go to Previous] [Go to Next]

AVHRR

The AVHRR (Advanced Very High Resolution Radiometer) instrument is a 5-channel scanning radiometer. It generates data in HRPT, LAC and GAC modes.

 

The band information for the AVHRR sensor is provided in Appendix B, Satellite Information.

AVHRR directory block

Word Value Description

14

5

all five bands are normally stored in one file

15

244 or 364

line prefix length in bytes. (TIRO = 244, AVHR = 364)

19

 

band map for the image, which has valid bits 1 through 6 for the new AVHRR/3 instruments on NOAA-15 (Note that if bit 3 is turned on, bit 6 must be turned on as well, and vice-versa, since both sensors share a single data band identified as either 3 or 6); determining which line belongs to which sensor can be made only from reading the line prefixes, except when an entire area contains only band 3 or only band 6

26

 

POES signal type (HRPT, LAC, GAC)

36

 

line prefix validity (VAL) code. (length=4)

47

 

time in milliseconds

49

192

length of the line prefix documentation

50

40 or 160

line prefix calibration length (40 for TIRO, 160 for AVHR)

51

8

length of the level (LEV) map section in the line prefix

52

TIRO/AVHR

image source type

53

RAW

pixel type stored in the area file

54

0, 1

sampling/averaging indicator (0=average, 1=sample)

55

1, 2, 3

signal type indicator (1=LAC, 2=GAC, 3=HRPT)

56

 

orbit position (ascending node, decending node, equatorial pass)

AVHRR navigation block

The AVHRR navigation block is divided into 128 words.

Word Value Description

1

TIRO

navigation type; 4 bytes ASCII

2

 

sensor source, year and Julian day of the navigation, ssyyddd

3

 

time of the navigation, hhmmss

4

1

orbit type

5

 

epoch date, yymmdd

6

 

epoch time, hhmmss

7

 

semi-major axis, km * 100

8

 

orbital eccentricity, *1000000

9

 

orbital inclination, degrees * 1000

10

 

mean anomaly, degrees * 1000

11

0

argument of perigee, degrees * 1000

12

 

right ascension of the ascending node, degrees * 1000

13

2048

number of samples per line

14

 

angular increment between samples, degrees * 1000

15

 

fraction of a second in epoch time

16 - 45

 

reserved

46

-1
1

satellite is in a descending pass
satellite is in an ascending pass

47

 

image coordinates of the first line to navigate

48

 

time at the start of the first line, milliseconds from start-of-day

49

 

time interval between lines, in milliseconds

50

0
1

image is displayed normally
image is inverted

51

 

number of lines in the inverted image

52

 

number of elements in the inverted image

53

 

time interval between lines, in microseconds (preferred over word 49)

54

 

time interval between individual data points * 100000000

55-120

 

reserved

121-128

 

comments; up to 32 characters

AVHRR Calibration

The calibration block is not used. Instead, calibration is done dynamically on a line-by-line basis to accommodate changing orbital conditions in the AVHRR/3 instrument. Therefore, the calibration data is contained in the data line prefixes. Refer to theTIRO and AVHR line prefix descriptions below, and Appendix D, POES AVHRR Calibration Information for additional information.

Because NOAA-12 and -14 AVHRR use the older TIRO calibration while the NOAA-15 AVHRR uses the newer AVHR calibration, changes have been made in the McIDAS-X area structure between the NOAA-14 areas and the NOAA-15 areas.

AVHRR data block

The AVHRR data block consists of data lines, each consisting of a line prefix and line data. The TIRO and AVHR line prefixes are different.

TIRO line prefix

The TIRO line prefix for the data block contains the information below.

Region Description

validity code

this is present for the realtime data

documentation

192-byte DOC section from the signal transmission

calibration

40 bytes of zeros; filled during post-processing in McIDAS

level

8 bytes; values 1 through 5 (left to right), with three pad zeros in successive bytes, indicate the order the bands will appear in each pixel in the subsequent data section

The TIRO line prefix is defined as follows:

Byte Address

Section

 Description

0

VAL

Validity code

4

DOC

Words 7-102 of HRPT minor frame (all left shifted 5 bits into a 2-byte sample)

196

CAL

Band 1 Slope/Gain 1 (all slopes and intercepts in the CAL block are scaled by 1000)

200

 

Band 1 Intercept/Offset 1

204

 

Band 1 Slope/Gain 2

208

 

Band 1 Intercept/Offset 2

212

 

Band 2 Slope/Gain 1

216

 

Band 2 Intercept/Offset 1

220

 

Band 2 Slope/Gain 2

224

 

Band 2 Intercept/Offset 2

228

 

Band 3 Slope/Gain 1

232

 

Band 3 Intercept/Offset 1

236

 

Band 3 Space scan five sample average (rounded and left shifted 5 bits)

240

 

Band 3 Internal target temperature (scaled by 100)

244

 

Band 4 Slope/Gain 1

248

 

Band 4 Intercept/Offset 1

252

 

Band 4 Space scan five sample average (rounded and left shifted 5 bits)

256

 

Band 4 Internal target temperature (scaled by 100)

260

 

Band 5 Slope/Gain 1

264

 

Band 5 Intercept/Offset 1

268

 

Band 5 Space scan five sample average (rounded and left shifted 5 bits)

272

 

Band 5 Internal target temperature (scaled by 100)

276

LEV

Band number: 01

277

 

Band number: 02

278

 

Band number: 03 or 06

279

 

Band number: 04

280

 

Band number: 05

281

 

Unused Band numbers: 00 00 00 (three bytes)

AVHR line prefix

The AVHR line prefix for the data block contains the information below.

Region Description

validity code

this is present for the realtime data

documentation

192-byte DOC section from the signal transmission

calibration

160 bytes from the signal transmission

level

8 bytes; values 1 through 5 (left to right), with three pad zeros in successive bytes, indicate the order the bands will appear in each pixel in the subsequent data section

The AVHR line prefix for the data block is defined as folllows.

Section Byte Address Size (bytes) Contents

VAL

0

4

Validity code

DOC

4

192

Words 7-102 of HRPT minor frame (all left shifted 5 bits into a 2-byte sample)

CAL

196

4

Band 1 Slope/Gain 1 (all slopes and intercepts in the CAL section are scaled by 100,000 except for the band 3 slope)

 

200

4

Band 1 Intercept/Offset 1

 

204

4

Band 1 Slope/Gain 2

 

208

4

Band 1 Intercept/Offset 2

 

212

16

Unused

 

228

4

Band 2 Slope/Gain 1

 

232

4

Band 2 Intercept/Offset 1

 

236

4

Band 2 Slope/Gain 2

 

240

4

Band 2 Intercept/Offset 2

 

244

16

Unused

 

260

4

Band 6 Slope/Gain 1

 

264

4

Band 6 Intercept/Offset 1

 

268

4

Band 6 Slope/Gain 2

 

272

4

Band 6 Intercept/Offset 2

 

276

16

Unused

...or...

 

260

4

Band 3 Slope/Gain (scaled by 10,000,000)

 

264

4

Band 3 Intercept/Offset (scaled by 100,000)

 

268

8

Unused

 

276

4

Band 3 Space scan five sample average (rounded and left shifted 5 bits)

 

280

4

Band 3 Target scan five sample average (rounded and left shifted 5 bits)

 

284

4

Band 3 Internal target temperature (scaled by 100)

 

288

4

Unused

 

292

4

Band 4 Slope/Gain (scaled by 100,000)

 

296

4

Band 4 Intercept/Offset

 

300

8

Unused

 

308

4

Band 4 Space scan five sample average (rounded and left shifted 5 bits)

 

312

4

Band 4 Target scan five sample average (rounded and left shifted 5 bits)

 

316

4

Band 4 Internal target temperature (scaled by 100)

 

320

4

Unused

 

324

4

Band 5 Slope/Gain (scaled by 100,000)

 

328

4

Band 5 Intercept/Offset

 

332

8

Unused

 

340

4

Band 5 Space scan five sample average (rounded and left shifted 5 bits)

 

344

4

Band 5 Target scan five sample average (rounded and left shifted 5 bits)

 

348

4

Band 5 Internal target temperature (scaled by 100)

 

352

4

Unused

LEV

356

1

Band number: 01

 

357

1

Band number: 02

 

358

1

Band number: 03 or 06

 

359

1

Band number: 04

 

360

1

Band number: 05

 

361

3

Unused Band numbers: 00

 

 

For a complete description of the AVHRR instruments on the satellites and their calibration, see the NOAA-KLM User's Guide at http://www2.ncdc.noaa.gov/docs/klm or http://www2.ncdc.noaa.gov/docs/intro.htm. For instruments prior to 1988, see NOAA Technical Memorandum NESS 107, 1988 (now obsolete) or http://noaasis.noaa.gov/NOAASIS/ml/calibration.html.

AVHRR line data

AVHRR data is transmitted as 10 bits and stored as 16 bits. The 10-bit data is formatted as follows, with x representing a data bit and the rest being zero-filled after shifting.

| 0 | x | x | x | x | x | x | x | x | x | x | 0 | 0 | 0 | 0 | 0 |

The 16-bit values from each of the five channels covering the same geographic area are stored interleaved in one image. The lines of data are stored in a time-ordered sequence and as the satellite scans right to left.

[Search Manual] [Table of Contents] [Go to Previous] [Go to Next]