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[CIDC FTP Data]
[PAL NDVI IDC Data on FTP]
Data Access
NDVI Data from AVHRR Land Pathfinder
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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
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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
* Parameters: Normalized Difference Vegetation Index (NDVI),
derived from the visible and near-infrared channel
reflectances (0.58 to 0.68 um and 0.73 to 1.10 um,
respectively). The NDVI is highly correlated to surface
vegetation.
* Units: Dimensionless
* Typical Range: -0.200 to 0.730
* Temporal Coverage: July 1981 to September 1994
* Temporal Resolution: All gridded values are monthly
composites.
* Spatial Coverage: Global
* Spatial Resolution: 1 degree x 1 degree
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 IFOV (milliradian)
(micrometer)
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
* File Size: 259200 bytes, 64800 data values
* Data Format: IEEE floating point notation
* Headers, trailers, and delimiters: none
* Fill value: -99.999
* Mask: Sea mask applied, value = -9.999
* Image orientation: North to South
Start position: (179.5W, 89.5N)
End position: (179.5E, 89.5S)
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
* Relative azimuth representation
* Bug fixes to CLAVR
* Bug fixes to thermal calibration
* Modification of ozone input to atmospheric correction
The change made beginning in 1986 data is
* Composite routine changes
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
* Global land cover classification
* Regional agricultural crop monitoring
* Desertification studies and drought monitoring
* Terrestrial environmental monitoring
* Global water and energy balance studies.
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.
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