CIDC FTP Data
PAL NDVI IDC Data on FTP

Data Access

NDVI Data from AVHRR Land Pathfinder
rule

Readme Contents

Data Set Overview
Sponsor
Original Archive
Future Updates

The Data
Characteristics
Source

The Files
Format
Name and Directory Information
Companion Software

The Science
Theoretical Basis of Data
Processing Sequence and Algorithms
Scientific Potential of Data
Validation of Data

Contacts
Points of Contact

References

rule

Data Set Overview

This data set is produced as part of the NOAA/NASA Pathfinder AVHRR Land program. It contains global monthly composites of the Normalized Difference Vegetation Index (NDVI) at 1 degree resolution covering the period from July 1981 to September 1994. This monthly climate data set was recently updated since the original data (the 10-day composite) used for generating this monthly dataset, was recently reprocessed. The Pathfinder Program produces long-term data sets derived from the observations made by Advanced Very High Resolution Radiometers (AVHRR) on the "afternoon" NOAA operational meteorological satellites (NOAA-7, -9, 11) and processed in a consistent manner for global change research.

Sponsor
The production and distribution of this data set are funded by NASA's Earth Science enterprise. The data are not copyrighted; however, we request that, when you publish data or results using these data, please acknowledge as follows:

The authors wish to thank the Distributed Active Archive Center (Code 902.2) at the Goddard Space Flight Center, Greenbelt, MD, 20771, for producing the data in their present form and distributing them. The original data products were produced under the NOAA/NASA Pathfinder program, by a processing team headed by Ms. Mary James of the Goddard Global Change Data Center; and the science algorithms were established by the AVHRR Land Science Working Group, chaired by Dr. John Townshend of the University of Maryland. Goddard's contributions to these activities were sponsored by NASA's Earth Science enterprise.

Original Archive
This data set is part of the NOAA/NASA AVHRR Land Pathfinder data set archived at the Goddard DAAC. It is derived from the 8 km, 10-day composite data (three per month), which are composited from the Daily data. The Daily data are derived from the NOAA AVHRR Global Area Coverage (GAC) 1B data, available from NOAA's Satellite Active Archive.

Future Updates
This data set will be updated as data from other years are processed.

The Data

Characteristics

Source
These data were collected by the Advanced Very High Resolution Radiometer (AVHRR) flown on NOAA-series satellites.

Nominal orbit parameters for the NOAA-7, -9, and -11 are

Launch date: 6/23/81 (NOAA-7), 12/12/84 (NOAA-9), 9/24/88 (NOAA-11)
Orbit: Sun synchronous, near polar
Nominal altitude: 833 km
Inclination: 98.8 degrees
Orbital period: 102 minutes
Equatorial crossing times: 114.30 (NOAA-7), 14.20 (NOAA-9),
13.40 (NOAA-11) LST
Nodal Increment: 25.3 degrees
The orbital period of about 102 minutes produces 14.1 orbits per day. Because the daily number of orbits is not an integer, the suborbital tracks do not repeat daily, although the local solar time of the satellite's passage is essentially unchanged for any latitude. The 110.8 degrees cross-track scan equates to a swath of about 2700 km. This swath width is greater than the 25.3 degrees separation between successive orbital tracks and provides overlapping coverage (side-lap).

The spectral band widths and Instantaneous Field of View (IFOV) of the AVHRR instrument are given in the following table.

Channel
Wavelength (micrometer)
IFOV (milliradian)
1
0.58 - 0.68
1.39
2
0.73 - 1.10
1.41
3
3.55 - 3.93
1.51
4
10.3 - 11.3
1.41
5
11.5 - 12.5
1.30


A more detailed, comprehensive description of the NOAA series satellites, the AVHRR instrument, and the AVHRR GAC 1B data can be found in the NOAA Polar Orbiter Data User's Guide (Kidwell 1991), which can be obtained from NOAA's National Environmental Satellite Data and Information Service (NESDIS) (see DATA ACCESS AND CONTACTS).

The Files
Format

Name and Directory Information

Naming Convention

The file naming convention for the NDVI data files is

avhrr_pf.ndvi.1nmegl.[yymm].ddd

where
avhrr_pf = AVHRR Pathfinder
ndvi = Normalized Difference Vegetation Index
1 = number of levels
n = vertical coordinate, n = not applicable
m = temporal period, m = monthly
e = horizontal grid resolution, e = 1 x 1 degree
gg = spatial coverage, gg = global (land and ocean)
yy = year
mm = month number
ddd = (bin=binary, ctl GrADS control file)

Directory Path

/data/inter_disc/biosphere/avhrr_ndvi/yyyy

where yyyy is year.

Companion Software
Several software packages have been made available on the CIDC CD-ROM set. The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is currently in use worldwide for the analysis and display of earth science data. GrADS meta-data files (.ctl) have been supplied for each of the data sets. A GrADS gui interface has been created for use with the CIDC data. See the GrADS document for information on how to use the gui interface.

Decompression software for PC and Macintosh platforms have been supplied for datasets which are compressed on the CIDC CD-ROM set. For additional information on the decompression software see the aareadme file in the directory:

software/decompression/

Sample programs in FORTRAN, C and IDL languages have also been made available to read these data. You may also acquire this software by accessing the software/read_cidc_sftwr directory on each of the CIDC CD-ROMs

The Science
Theoretical Basis of Data

Spectral Wavelengths:

On the NOAA-7, NOAA-9, and NOAA-11 satellites, the AVHRR sensor measures emitted and reflected radiation in five channels (bands) of the electromagnetic spectrum: a visible (0.58 to 0.68 micrometer) band that is used for daytime cloud and surface mapping; a near-infrared (0.725 to 1.1 micrometer) band used for surface water delineation and vegetation cover mapping; a mid-infrared (3.55 to 3.93 micrometer) band used for sea surface temperature and nighttime cloud mapping; a thermal infrared (10.5 to 11.5 micrometer) band used for surface temperature and day and night cloud mapping; and another thermal infrared (11.5 to 12.5 micrometer) band used for surface temperature mapping (Kidwell 1991).

Vegetation Index:

The first AVHRR channel is in a part of the spectrum where chlorophyll causes considerable absorption of incoming radiation, and the second channel is in a spectral region where spongy mesophyll leaf structure leads to considerable reflectance. This contrast between responses of the two bands can be shown by a ratio transform; i.e., dividing one band by the other. Several ratio transforms have been proposed for studying different land surfaces (Tucker, 1979). The Normalized Difference Vegetation Index (NDVI) is one such ratio, which has been shown to be highly correlated with vegetation parameters such as green-leaf biomass and green-leaf area and, hence, is of considerable value for vegetation discrimination (Justice et al. 1985).

NDVI Relationships With Geophysical Variables:

A ratio between bands is of considerable use in reducing variations caused by surface topography (Holben and Justice 1981). It compensates for variations in radiance as a function of Sun elevation for different parts of an image. The ratios do not eliminate additive effects caused by atmospheric attenuation, but the basis for the NDVI and vegetation relationship holds generally. The soil background contributes a reflected signal apart from the vegetation, and interacts with the overlying vegetation through multiple scattering of radiant energy. Huete (1988) found the NDVI to be as sensitive to soil darkening (moisture and soil type) as to plant density over partially vegetated areas.

Processing Sequence and Algorithms

Formulae:

Derivation Techniques and Algorithms

Recalibrated radiances for the Pathfinder data are converted to surface reflectance and brightness temperature using the following procedures and information available from NOAA (Rao, 1993a,b).

Calibration for Channels 1 and 2:

R = (Counts- Offset)(Gain)

where

R = is the radiance in [W][m-2][micrometer-1][steradian-1]
Gain = A exp((B)( d))
d = is the number of days since launch
A, B = are calibration parameters supplied by the NOAA/NASA Pathfinder Calibration Working Group (Rao 1993b).

For Channel 3, the calibration is determined by the gain and offsets provided in the NOAA 1B data, using procedures described in Kidwell (1991).

For Channels 4 and 5, radiances are computed from the temperatures of the Internal Calibration Target (ICT) and the laboratory blackbody as convolutions of the Planck function over AVHRR's spectral response functions. Details are available in NOAA Technical Report (Rao 1993a).

Reflectance for Channels 1 and 2 is derived as follows:

Reflectance = (surface leaving radiance) / (incoming radiance).

The Atmospheric correction scheme follows the algorithm of Gordon et. al (1988)

Brightness temperatures for Channels 3, 4, and 5 are derived as follows:

The calibrated radiances are converted to brightness temperatures using Planck function. NOAA provides look-up tables for each satellite (Kidwell 1991, Brown et al. 1985, Weinreb et al. 1990).

NDVI is derived:

(Channel 2 reflectance - Channel 1 reflectance)
----------------------------------------------
(Channel 2 reflectance + Channel 1 reflectance)

Data Processing Sequence

Daily data are produced for a compositing period and quality controlled. The 10-day composite is created from Daily data and quality controlled. Climate data are generated from the composite. The One-Degree Monthly Composite data set is derived from the Climate data.

Processing Steps (and data sets)

The AVHRR Land Pathfinder data are created by the following steps.

  1. Ascending GAC orbit data are unpacked and staged.
  2. Ancillary data needed in processing are retrieved. These include ozone data from Nimbus-7 Total Ozone Mapping Spectrometer (TOMS), land surface elevation from the Earth Topographic Five Minute Grid (ETOPO5) data set, land or sea mask, and satellite ephemeris files.
  3. Each scan is navigated using an orbital model.
  4. Based on the precise navigation, latitudes, longitudes, solar zenith, solar azimuth, scan, and relative azimuth angles are determined for each pixel.
  5. Calibration and atmospheric corrections are applied, and counts are converted to radiances that are used to derive reflectance and brightness temperatures.
  6. Cloud flags are calculated and appended and the NDVI is calculated from the surface reflectances.
  7. The data are then resampled to 8 km x 8 km pixels in the output product (Daily data) and all ocean data are masked out.
  8. Once 10 days of daily data are processed, they are composited by choosing values for each bin based on the day that has the highest NDVI value. Only those pixels within 42 degrees of nadir are used in the composite.
  9. The Climate data are produced from the Composites.
  10. One Degree Monthly Composites are produced from the Composite data.
  11. Data products and their associated metadata are quality controlled before archiving at the Goddard DAAC.

Processing Changes

The changes included with Pathfinder data beginning in 1988 are

The change made beginning in 1986 data is

Calibration

The Pathfinder Calibration Working Group recommended time- dependent calibration coefficients that incorporate the slopes derived from several different calibration investigations and tie these to offsets corresponding to certain aircraft underflights (Staylor 1990). For channels 4 and 5, new methods for calibration were recommended based in part on reanalysis of preflight calibration data that take into consideration the nonlinear response of the instrument and provide corrections to earlier gain and offset adjustments.

In the processing stream, the satellite number and days since launch are used to calculate a revised gain. This new gain, along with offsets provided in Rao (1993b), is used to calculate radiance. For the thermal channels, the gains and offsets provided in the NOAA 1B record are corrected using the Internal Calibration Target (ICT) temperature and corrections provided in Rao (1993a). They are then applied to calculate a top of the atmosphere radiance. This is then converted into brightness temperature using a Planck function equivalent lookup table based on the response curve of each channel. Channel 3 is converted to brightness temperature following procedures described in Kidwell (1991).

Atmospheric Correction A Rayleigh correction is calculated and applied using a standard radiative transfer equation and methodology, which follows the work of Gordon et al. (1988). This includes a correction for ozone absorption and daily ozone data from the Total Ozone Mapping Spectrometer (McPeters et al. 1993) used in the correction. In addition, the pixel elevation as determined from the ETOPO5 data set (NGDC 1993) is used to correct the pressure level used in the calculation of Rayleigh coefficients. The Rayleigh correction terms are applied to the Channels 1 and 2 radiance, and the resulting reflectances are normalized for solar illumination.

Scientific Potential of Data
This data set product (NDVI) is particularly useful for studies of temporal and interannual behavior of surface vegetation and for developing surface background characteristics for use in climate modeling. Some uses of NDVI include

Validation of Data
A few validation checks have been built into the Pathfinder data processing (Quality Control Flags). Automated quality checks are made for consistency in fields such as date and satellite or scan times. Geophysical values are checked to see that they are within a reasonable range. Certain anomalies may exist in the data set because of conditions inherent in the input data, for example, missing scan lines or orbits, incorrect or incomplete calibration coefficients, and many of these data are flagged with the Quality control indicator.

Contacts

Points of Contact
For information about or assistance in using any DAAC data, contact

EOS Distributed Active Archive Center(DAAC)
Code 902.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771

Internet: daacuso@daac.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)

References
Brown, O.W., J.W. Brown, and R.H. Evans. 1985. Calibration of Advanced Very High Resolution Radiometer observations. Journal of Geophysical Research, 90:11667- 11677.

Gordon, H.R., J.W. Brown, and R.H. Evans. 1988. Exact Rayleigh scattering calculations for use with the Nimbus- 7 coastal zone color scanner. Applied Optics, 27:2111-2122.

Holben, B.N., and C.O. Justice. 1981. An examination of spectral band ratioing to reduce the topographic effect on remotely sensed data, International Journal of Remote Sensing, 2:115-133.

Huete, A.R. 1988. A soil adjusted vegetation index (SAVI), Remote Sensing of the Environment, 25:295-309.

Justice, C.O., J.R.G. Townshend, B.N. Holben, and C.J. Tucker. 1985. Analysis of the phenology of global vegetation using meteorological satellite data, International Journal of Remote Sensing, 6:1271-1318.

Kidwell, K. 1991. NOAA Polar Orbiter Data User's Guide. NCDC/SDSD. National Climatic Data Center, Washington, DC.

McPeters, R.D., et al. 1993. Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) Data Products User's Guide. NASA Reference Publication 1323.

NGDC. 1993. 5 Minute Gridded World Elevation. NGDC Data, Announcement DA 93-MGG-01. Boulder.

Rao, C.R.N. 1993a. Nonlinearity corrections for the thermal infrared channels of the Advanced Very High Resolution Radiometer: assessment and recommendations. NOAA Technical Report NESDIS-69. NOAA/NESDIS. Washington, DC.

Rao, C.R.N. 1993b. Degradation of the visible and near-infrared channels of the Advanced Very High Resolution Radiometer on the NOAAP9 spacecraft: assessment and recommendations for corrections. NOAA Technical Report NESDIS- 70. NOAA/NESDIS. Washington, DC.

Staylor, W.F. 1990. Degradation rates of the AVHRR visible channel from the NOAA-6, -7, and -9 spacecraft. Journal of Atmospheric and Oceanic Technology, 7:411-423.

Tucker, C.J. 1979. Red and photographic infrared linear combinations for monitoring vegetation. Remote Sensing of the Environment, 8:127-150.

Weinreb, M.P., G. Hamilton, S. Brown, and R.J. Koczor. 1990. Nonlinearity corrections in calibration of Advanced Very High Resolution Radiometer infrared channels. Journal of Geophysical Research, 95:381-7388.

NASA GSFC Goddard DAAC cidc site
NASAGoddardGDAACCIDC

Last update:Fri Dec 12 12:40:21 EST 1997
Page Author: William Teng -- teng@daac.gsfc.nasa.gov
Web Curator: Daniel Ziskin -- ziskin@daac.gsfc.nasa.gov
NASA official: Paul Chan, DAAC Manager -- chan@daac.gsfc.nasa.gov