FAQ

Show all answers | Hide all answers

1. About the VGT products

1.1 What are the different VEGETATION products?

The standard VEGETATION products are:
* VGT-P products (physical values)
* VGT-S1 products (daily MVC synthesis)
* VGT-S10 products (ten-day MVC synthesis, also in degraded resolution)
* VGT-D10 products (ten-day BDC synthesis, also in degraded resolution)

Three ten-day syntheses are made during a month:
* synthesis from the 1st to the 10th of the current month
* synthesis from the 11th to the 20th of the current month synthesis from the 21st to the end of the current month

1.2 What are VGT-P products?

VGT-P (P= physical) products are adapted for scientific applications requiring highly accurate physical measurements. The data is corrected for system errors (error registration of the different channels, calibration of all the detectors along the line-array detectors for each spectral band) and resampled to predefined geographic projections chosen by the user. The pixel brightness count is the ground area's apparent reflectance as seen at the top of atmosphere (TOA). Auxiliary data supplied with the products allow users to process the original reflectance values using their own algorithms.

Each pixel in the image represents a ground area of approximately 1km x 1 km. The image products cover all or a part of a VEGETATION segment (data strip over land).

1.3 What are VGT-S products?

VGT-S (S= synthesis) products are MVC or Maximum Value Composite Syntheses. The pixels selected for the syntheses are based on the selection of the maximum NDVI value, to ensure coverage of all landmasses worldwide with a minimum effect of cloud cover. The pixel brightness count is the ground area's reflectance (corrected for atmospheric effects); pixels in the sea area are set to 0. A map of computed normalised difference vegetation index values (NDVI image plane) is also supplied with the product.

Each pixel of a VGT-S product represents a ground area of approximately 1 km x 1 km. The global coverage of a synthesis extends from 75°N to 56°S. A VGT-S product can be ordered as a VGT-S1 or a VGT-S10 product, in a map projection specified by the user, covering the whole world or a selected part (region of interest) defined by the user.

1.4 What are VGT-S1 products?

VGT-S1 products (daily synthesis) are composed of the 'best' ground reflectance measurements of all segments received during one day for the entire surface of the Earth. This is done for each of the images covering the same geographical area. The areas distant from the equator have more overlapping parts so the choice for the best pixel will be out of more data. These products provide data from all spectral bands, the NDVI and auxiliary data on image acquisition parameters.

1.5 What are VGT-S10 products?

VGT-S10 products (ten day synthesis) are compiled by merging segments (data strips) acquired in a ten days. All the segments of this period are compared again pixel by pixel to pick out the 'best' ground reflectance values. These products provide data from all spectral bands, the NDVI and auxiliary data on image acquisition parameters.

A MVC synthesis can be delivered with several spatial resolution (1*1 km2 or 4*4 km2 or 8*8 km2). Those three products are called:
* S10 for 1*1 km2
* S10.4 for 4*4 km2
* S10.8 for 8*8 km

1.6 What are VGT-D10 products?

VGT D-10 products are BDC syntheses or BiDirectional Composite syntheses. These syntheses are based on a bidirectional reflectance distribution function.

The pixel brightness count is the ground area's reflectance; pixels in the sea area are set to 0.

A VGT-D product covers the whole world or a selected part (region of interest) and can be ordered in a map projection specified by the user. The global coverage of a synthesis extends from 75°N to 56°S. A map of computed normalised difference vegetation index values (NDVI image plane) is also supplied with the product.

A BDC synthesis can be delivered with several spatial resolution (1*1 km2 or 4*4 km2 or 8*8 km2). Those three products are called:
* D10 for 1*1 km2
* D10.4 for 4*4 km2
* D10.8 for 8*8 km2

Reference: Duchemin, B., Maisongrande, P., Dedieu, G ., Leroy, M., Roujean, J .L., Bicheron, P., Hautecoeur, O., Lacaze, R., 2000. A 10-days compositing method accounting for bidirectional effects. Proceedings of the VEGETATION 2000, Belgirate Italy (3 - 6 April 2000): 313 - 318.

Papers and a CD-rom of the Vegetation 2000 conference can be obtained by sending a mail to iuc.vegetation@jrc.it.

1.7 In which projections can I order VEGETATION products?

Since no cartographic projection retains its geometric qualities globally, a specific cartographic projection should only be used for the region where the major part of its qualities is best preserved. It is important to avoid using a cartographic projection in a region for which the projection is not designed. It is also advisable not to use a projection in a region with large geometric distortions (surface or angles).

The following table enumerates the available projections for VEGETATION products.

Projection Name	Class	Best quality	Do not use
Plate Carrée	Global	Equator	Poles
Goode Homolosine	Global	Equator	Poles
Sinusoidal Equal Area	Global	Equator	Poles and 180° meridian
Mercator Equatorial	Global	Equator	Poles
UTM	Regional	Central Meridian	Away from the central meridian
Lambert Europe	Regional	Europe	Outside Europe
Albers US	Regional	North America	Outside North America
Stereo polar North	Regional	North Pole	Equator
Stereo polar South	Regional	South Pole	Equator
Stereo locale	Local	Tangential point	Far from the tangential point

1.8 What are the different planes in the VEGETATION product?

Plane		Description of the plane						File format	Type of product
PHYS_VOL	Physical Volume descriptor	Information about the delivery						TXT	P/S/D
LOG	Logical Volume descriptor	Information about - the product - map projection information (general information, geodetic system parameters, projection parameters) - cartographic location - geographic location - image co-ordinates (corresponding to carto and geographic location) - geometric correction - radiometric correction - orbit parameters - date and time - algorithms references - production						TXT	P/S/D
RIG	Copyright Descriptor	Information about the copyright						TXT	P/S/D
BO	B0 spectral band	Radiometry data						HDF	P/S/D
B2	B2 spectral band	Radiometry data						HDF	P/S/D
B3	B3 spectral band	Radiometry data						HDF	P/S/D
MIR	MIR spectral band	Radiometry data						HDF	P/S/D
NDV	Vegetation Global Index	NDVI indices						HDF	S/D
SM	Status Map	Data about Bit NR 7: Radiometric quality for B0 coded as 0 if bad and 1 if good Bit NR 6: Radiometric quality for B2 coded as 0 if bad and 1 if good Bit NR 5: Radiometric quality for B3 coded as 0 if bad and 1 if good Bit NR 4: Radiometric quality for MIR coded as 0 if bad and 1 if good Bit NR 3: land code 1 or water code 0 Bit NR 2: ice/snow code 1 , code 0 if there is no ice/snow						HDF	P/S
		Bit NR 1:	0	0	1	1
		Bit NR 0:	0	1	0	1
			clear	shadow	uncertain	cloud
BSM	BDC Status Map	Data about Bit NR 7: BRDF model adjustment quality on B0: coded as 0 if bad quality and 1 if good Bit NR 6: BRDF model adjustment quality on B2: coded as 0 if bad quality and 1 if good Bit NR 5: BRDF model adjustment quality on B3: coded as 0 if bad quality and 1 if good Bit NR 4: BRDF model adjustment quality on MIR: coded as 0 if bad quality and 1 if good Bit NR 3: land/water: coded as 1 if land and 0 if sea Bit NR 2: ice/snow: coded as 1 if there is ice/snow and 0 if not Bit NR 1: cloud/shadow or clear: coded as 1 if there is cloud/shadow and 0 if not Bit NR 0: qrmir: Radiometric quality for MIR coded as 0 if bad and 1 if good						HDF	D
VZA	Viewing zenith angle grid	Data of the satellite zenith angles						HDF	P/S
VAA	Viewing azimuth angle grid	Data of the satellite azimuth angles						HDF	P/S
SZA	Solar zenith angle grid	Data of the solar zenith angles						HDF	P/S
SAA	Solar azimuth angle grid	Data of the solar azimuth angles						HDF	P/S
WVG	Water Vapour grid	Data of water vapour						HDF	P
OG	Ozone grid	Data of ozone						HDF	P
AG	Tropospheric Aerosol grid	Data of aerosols						HDF	P
1BL	1B HRVIR latitude plane	Data of the HRVIR co-location (latitude)						HDF	P
1BO	1B HRVIR longitude plane	Data of the HRVIR co-location (longitude)						HDF	P
TG	Time grid	Each pixel is expressed with a precision of one minute referring to the beginning of the first segment of the synthesis.						HDF	S
SZN	SZ NORM	This value is the normalized solar zenith angle used in the BDC correction.						HDF	D
TP	TAUP: Optical thickness	This value is the average of the optical thickness of the selected measures of the BDC period.						HDF	D
Kxx	K0, K1, K2 for the the bands B0, B2, B3 and MIR	These values are the coefficients of the BDC correction function.						HDF	D
QL	Quick look	Quick look of one spectral band (see LOG file which spectral band)						TIF	P/S/D

P=VGT-P product
S=VGT-S product
D=VGT-D product
TXT=text
HDF=Hierarchical Data Format
TIF=TIFF Format

1.9 What is the Physical Volume descriptor?

The Physical Volume descriptor (Phys_vol.txt) is a leader file of a support or ftp file, containing some basic infromation and the list of files available in the product. This file is also usefull when the product is split on serveral supports.

1.10 What is the Logical Volume descriptor?

The Logical Volume descriptor contains information about the product, map projection, cartographic en geographic location, image coordinates, geometric and radiometric correction, orbit parameters, date and time, algorithms references and production.

1.11 What is the relation between the digital number and the real VEGETATION reflectance value?

The physical values of VEGETATION data can be calculated by the following formula:

Real reflectance value VGT	=coefficient a * Digital Number + coefficient b
	= a * DN +b

 

Coefficient a = 0.0005

Coefficient b = 0.0

1.12 What is the relation between the digital number and the real NDVI?

Real NDVI	=coefficient a * Digital Number + coefficient b
	= a * DN +b

 

Coefficient a = 0.004

Coefficient b = -0.1

1.13 How must I read the 8 bits of the status map?

Bit NR 1:	0	0	1	1
Bit NR 0:	0	1	0	1
(LSB):
	clear	shadow	uncertain	cloud

° for P, S1 and S10 products
In the status map the following bits are used for:

from MSB to LSB

Bit NR 7 (MSB): radiometric quality for B0 coded as 0 if bad and 1 if good
Bit NR 6: radiometric quality for B2 coded as 0 if bad and 1 if good
Bit NR 5: radiometric quality for B3 coded as 0 if bad and 1 if good
Bit NR 4: radiometric quality for MIR coded as 0 if bad and 1 if good quality

Bit NR 7 - 4: coded as 0 for 'no data', missing lines, sea on VGT-S products, adjacent blind or defective MIR detectors, interpolated data, saturated data, negative data after atmospheric correction

Note : Because of the use of a bicubic interpolation to improve the geometrical accuracy, it is not straightforward to declare a product pixel as a bad pixel. The product pixel value is not a 'pure' instrument pixel value but a weighted average of the 16 nearest pixels (4x4). The radiometric quality of a product pixel is then considered as bad since 20% or more of its value is produced by bad pixels at instrument level.

Bit NR 3: land (code 1) or water (code 0), computed from the "Digital Chart of the Worlds"
Bit NR 2: ice/snow (code 1) , code 0 if there is no ice/snow, computed from thresholds from reflectances

° for D10 products
In the status map the following bits are used for:

from MSB to LSB

Bit NR 7 (MSB): radiometric quality for B0 coded as 0 if bad and 1 if good
Bit NR 6: radiometric quality for B2 coded as 0 if bad and 1 if good
Bit NR 5: radiometric quality for B3 coded as 0 if bad and 1 if good
Bit NR 4: radiometric quality for MIR coded as 0 if bad and 1 if good quality

Bit NR 7 - 4: coded as 0 when the algoritm could not generate output or when sea

Bit NR 3: land (code 1) or water (code 0), computed from the "Digital Chart of the Worlds"
Bit NR 2: ice/snow (code 1) , coded 0 if there is no ice/snow, computed from thresholds from reflectances
Bit NR 1: cloud/shadow or clear, coded as 1 if there is clouds or shadow, and 0 if not (clear sky)
Bit NR 0 (LSB): "qmir", radiometric quality for MIR band coded as 0 if bad and 1 if good

1.14 What is the maximum pixel size for the raw data?

The maximum pixel size for raw data across track is 1.7km. Attitude oscillations of the satellite, the earth's relief and the non-perfect globular shape of the earth make that this is an approximate value. Note however, that pixels in VEGETATION products are projected and interpolated, resulting in a constant pixel resolution of 1/1 km.

1.15 How is the coördinate localisation in a VEGETATION pixel?

The coordinate localisation in VEGETATION pixels are given for the centre of the pixel.

1.16 What is the relation between VEGETATION and HRV-IR images?

The VEGETATION instrument is carried on the SPOT4 platform, which is occupied by the High-Resolution instrument of the HRVIR series (10-20 meter resolution). The two instruments acquire the data at exactly the same time and have the same spectral channels (B2, B3 and MIR). A direct combined use of high and low-resolution views of the same target is possible by the user.

1.17 What is the strange 'shadow' effect at the land borders of the VEGETATION MVC synthesis?

For the calculation of VGT synthesis the CTIV system uses a compositing method with as main rule: selection of the pixel with the maximum NDVI. Over land, cloudy pixels generally have a lower NDVI than pixels where the land surface is visible. As a consequence, the compositing method tends to reject cloudy pixels over land in favour of non-cloudy pixels. Over sea however, cloud-pixels having a higher NDVI than sea-pixels, the situation is reversed.

The land/sea mask applied to the syntheses is slightly overdimensioned for the land masses in order to cope with localisation inaccuracies. As a result, a 3 to 4 pixel-wide border of sea pixels surrounds the land masses. The selection of cloudy pixels in this sea-border results in the above mentioned 'shadow' effect.

1.18 What are the vertical stripes in the MIR band?

The vertical stripes in the MIR band are caused by blind or aberrant detectors:

• The MIR sensor consists of 6 bricks of 300 detectors in line. At every brick junction there is a blind detector, so in-between the 6 bricks there are 5 blind 'built in' detectors.
• The MIR detectors are sensible to proton fluxes coming from the sun. When a proton hits a detector, it is disturbed or destroyed, depending on the power of the proton or the number of the previous chocks it received. On average one detector is blind or aberrant every month.

In the image processing chain at the CTIV, these stripes can be made invisible by interpolating the radiometry of the neighbor detectors.
The interpolation can only be done when the position of the blind or aberrant detector is known. The positions of the 'built in' blind detectors are well known. For the others, the Image Quality centre for Vegetation images (QIV) requires every second week a calibration campaign to update the list of the blind or aberrant detectors. This list is used to interpolate bad stripes. A algorithm used in the CTIV (installed on May 17th 2001) analyzes every incoming VEGETATION data to perform this detection without the updated list. However this automatic detection may miss some lightly deviant detectors, for these detectors the list is used.
The interpolated MIR pixels are flagged as bad in the status map. Depending on the image projection, one to three bad stripes can be found in the MIR band.
On previously computed VEGETATION products (VEGETATION products ordered before May,17th 2001), the unknown blind and aberrant detectors gave undetected bad data i.e. vertical stripes.

1.19 Which method is used for cloud and ice/snow detection (CLOUD_COVER_REF V2.0 and SNOW_COVER_REF V2.0)?

The method used for cloud detection is by comparing the reflectance values with a reference map. The reference map has 4 plains, each plain corresponding with one spectral band (TAO values). The resolution of the map is 1/10 degrees.
A pixel is defined as cloudy when the TOA reflectance, for each spectral band, is higher than the value in the corresponding reference map.
In the status map, bit number 1 is defined as cloud/ no cloud:
Code 1 = clouds
Code 0 = if there are no clouds

Threshold Values in the reference map:
B0= 674
B2 = 549
B3 = 521
MIR = 328

1.20 What is the HDF format?

The Hierarchical Data Format, or HDF, is a multi-object file format for sharing scientific data in a distributed environment. HDF was created at the National Centre for Supercomputing Applications (NCSA) to serve the needs of diverse groups of scientists working on projects in many fields. HDF was designed to address many requirements for storing scientific data, including:

• Support for the types of data and metadata commonly used by scientists.
• Efficient storage of and access to large data sets.
• Platform independence.
• Extensibility for future enhancements and compatibility with other standard formats.

HDF files are ‘self-describing’. For each data object in and HDF file, there is information about the type of data, the amount of data, its dimensions, and its location in the file. Self-description means that many types of data can be included within an HDF file. For example, it is possible to have symbolic, numerical, and graphical data within one HDF file.

1.21 Where can I find the appropriate HDF software?

The SPOT-VEGETATION HDF data format can be opened using several software programmes including ENVI, ARCGIS etcetera.

Using ENVI:
1. From the ENVI main menu bar, select File/Open External File/SPOT  Vegetation.
2. When the file selection dialog appears, select the .hdf file.

Using ARCGIS:
1. Arcgis supports the direct readout from HDF4 files

Other possibilities:
* The VGTExtract tool can convert SPOT-VEGETATION S1 and S10 to TIFF or other formats and extract a region of interest. You can find the tool here (http://www.vgt4africa.org/vgtextract.do).
* CROP-VGT (http://cidoc.iuav.it/~silvio/crop_vgt.html) is a Win32 program that extracts in a totally automatic way a user-defined Region of Interest from a whole set of zipped files downloaded from the free VGT site.
* BEAM-software: http://www.brockmann-consult.de/cms/web/beam/
* An overview of other HDF reading software: http://www.hdfgroup.org/tools.html

1.22 What are the VGT2 geometric performances?

During the in-flight comissioning phase, the VGT2 geometric performances have been assessed and compared to VGT1. The table below shows the test results measured on 15 sites for VGT1 and VGT2 images (24/07 to 31/08/2002).

	Absolute location		Multi Temporal registration
Instrument	RMS	MAX(95%)	RMS	MAX(95%)
VGT1	225 m	485 m	215 m	450 m
VGT2	180 m	370 m	155 m	320 m

VGT1 processing was performed with GCPs, VGT2 with stars tracker data.

1.23 Has the SWIR-band been repaired?

The problem with the SWIR band on VGT1 is related to the technology of the sensor. Due to cosmic radiation some “pixels” on the sensor become aberrant after a certain time. As from that moment the specific “pixel” becomes an idle line in SWIR band image. Before launch it was already known that throughout time VGT2 will manifest the same aberrant lines as VGT1. Time is the only parameter here. So, reparation of the SWIR-band was not an option.

1.24 What is the relation between the digital number and angle, atmospheric and HRVIR planes (MVC synthesis)?

Angles?
Angle value = coefficient a * Digital Number + coefficient b
=a*DN +b (unit is degrees)
Viewing and solar zenith angle coefficient a = 0.5
Viewing and solar azimuth angle coefficient a = 1.5
Viewing and solar zenith angle coefficient b = 0
Viewing and solar azimuth angle coefficient b = 0

Atmospheric planes
Physical measurement = coefficient a * Digital Number + coefficient b
=a*DN +b
Water vapour coefficient a = 0.04
Water vapour coefficient b = 0
Unit water vapour = g/cm²
Ozone coefficient a = 0.004
Ozone coefficient b = 0
Unit ozon = atm.cm
Aerosol coefficient a = 0.004
Aerosol coefficient b = 0 

HRVIR planes
Localisation information = coefficient a * Digital Number + coefficient b
=a*DN + b
1BHRVIR latitude coefficient a = 0.000001
1BHRVIR latitude coefficient b = 0
1BHRVIR longitude coefficient a = 000001
1BHRVIR longitude coefficient b = 0

1.25 What is the relation between the digital number and the real K coefficient (BDC synthesis)?

Real K coefficient = coefficient a * Digital Number + coefficient b
=a*DN +b
For K0 : coefficient a = 0.004, coefficient b = 0
For K1 : coefficient a = 0.001, coefficient b = -0.12
For K2 : coefficient a = 0.006, coefficient b = -0.2

1.26 Stripes in MIR band of S10 due to aberrant detectors

As Q1.23 describes the MIR band degrades after a time. The processing software of the CTIV could interpolate up to 3 consecutive aberrant detectors. However, in September 2010, 4 consecutive aberrant detectors occurred in the MIR band leading to blind zone in the synthesis. This problem occurred from 29/9/2010 till 6/10/2010. The problem was solved by switching on a "less aberrant" detector. The blind zone is visible in the S10 of 21/09/2010 and 1/10/2010. This problem reoccurred in March 2011 (from 10 till 14/3/2011) and affected the S10 of 1/3/2011 and 11/3/2011.
Since 16/6/2011 the algorithm at the CTIV has been adopted to be able to interpolate between 4 consecutive aberrant detectors.
In December 2012, the algorithm will be adjusted again to be able to interpolate between 6 aberrant detectors.

2. About VGT syntheses

2.1 What is the difference between S10 and D10 synthesis?

BDC syntheses differ from MVC syntheses in the following aspects:
* The status map of an BDC synthesis includes the following indicators: sea/land, ice-snow/not ice-snow, clouds or shadow/clear, MIR quality, BRDF model adjustment quality flag for each band.
* The following new planes are added: SzNorm, TauP and for each spectral band: K0, K1, K2.
* The following planes are dropped: viewing angles, solar angles, time grid.

2.2 What is the Goode Homolosine projection?

Figure : The 12 zones of the "Interrupted Goode Homolosine" projection: 2 zones northwards of 45°N, 6 in the equatorial belt, and 4 southwards of 45°S.

The Goode Homolosine projection is often used for global data sets. In fact, this projection is made of two different projections: in the equatorial belt the Goode Homolosine projection uses the sinusoidal projection, while in the polar regions the Mollweide projection is adopted. One of the main advantages of this projection is its equivalency. To avoid the large distortions, typical for the family of equivalent projections, the Goode Homolosine projection divides the globe in 12 longitudinal zones. As one can see in the figure, the margins of these zones are often situated in the oceans. Hence, all the large continents are constituted of a few neighbouring zones. Antarctica and Greenland are the only exceptions to this rule. Each zone is projected separately around a central meridian what causes only small distortions on the continental scale. A result of this method is that between some zones there are large gaps that do not correspond to physical parts of the globe.

2.3 What What are the free VEGETATION products?

Free VEGETATION Products are:

• extracted from ten-day global syntheses.

• available 3 months after insertion in the VEGETATION archive.

• in full resolution (1km).

• in plate carrée projection.

• available on 10 predefined regions of interest.

• in the standard VEGETATION product format.

• only delivered from the http://free.vgt.VITO.be site via http.
• CROP_VGT is a Win32 program that extracts in a totally automatic way a user-defined Region of Interest from a whole set of zipped files downloaded from the free VGT site, provided by Silvio Griguolo.

2.4 Where can I download the free VEGETATION S10 products?

Free VEGETATION S10 products can be downloaded at the following site : http://free.vgt.VITO.be

3. About the VGT instrument

3.1 What are the characteristics of the instrument?

* Spatial resolution: in both directions 1.15 km at nadir;
* A continuous and global monitoring of the continental areas either through the centralised archiving and processing facility, or, for local or regional studies;
* Long term data sets with accurate calibration and positioning, continuity and consistency through the renewal of the system with further satellites;
* High quality related to the radiometric and geometric characteristics;
* Operate with a frequency of acquisition adequate for following changes in the vegetation cover (growth cycle, season,...) over all land surfaces of the earth;
* It uses the push-broom technology so the images will present much less geometric distortions (it will be easier to superimpose images taken at different dates and a larger proportion of the orbital strip will be useful);
* Multi-scale approaches of surface parameters and processes using simultaneous measurements acquired through the VEGETATION instrument and the High Resolution instrument on the same SPOT4 platform;
* Local receiving stations will make VEGETATION data directly available to the local users.

3.2 What are the specific characteristics of the VEGETATION images?

The next table gives an overview of the VEGETATION spectral bands, developed specific for monitoring vegetation conditions:

Spectral band	Wavelength
B0(Blue)	0.43 - 0.47 µm
B2(Red)	0.61 - 0.68 µm
B3(NIR)	0.78 - 0.89µm
MIR(SWIR)	1.58 - 1.75 µm

3.3 What took VEGETATION so long to switch to VGT2 (SPOT5 was launched in May 2002)?

- An extensive calibration campaign was launched for VGT2.
- As it was know that VGT2 will deliver the same products as VGT1 while SPOT4 with VGT1 was operating fully within specifications, there was no reason to push an early switch from VGT1 to VGT2.
- It seems better to switch from VGT1 to VGT2 outside the growing season.

3.4 What is the origin of the VGT2 calibration problem (statement from the VEGETATION programme scientist?

More information here

3.5 How to recognize products corrected for the VGT2 calibration problem?

Every VEGETATION product is delivered with an associated “_LOG.TXT” file. This file contains a.o. the field “RADIOM_ABS_CAL_REF ” with as value something like “V2_A_034_2003_12_22_02.PCI”. The numerical part at the end of this value (02 in this example) indicates the version of the calibration coefficients that was used to calculate the product. All products that are corrected for the current calibration problem will be recognizable by a value of 05 or higher in this part of the value.

3.6 What is recalculated by the VGT2 recalibration campaign?

The VGT-2 recalibration campaign regenerates the following data sets :
- the level-1B and the S10 archives at the CTIV
- the free-S10 products on the http://free.vgt.vito.be website
Already delivered end products will not be regenerated due to the overhead and consequent delays this would cause in the recalibration campaign. Users who need the updated S10 products are requested to download these from the http://free.vgt.vito.be website. Users of VGT-P products should use the correction parameters that are published this paper.
The reprocessing of the VGT2 archive (February 2003 until May 2006) will be according to the following priorities:
- June 2005 (simultaneously with the real time of June 2006)
- synchronously April and May 2005 and 2006
- growing season of 2005 (October included)
- rest of 2005
- whole year 2004
- February 2003
- December 2003

3.7 What is the current status of the VGT2 recalibration campaign?

Reclibration has been succesfully fullfilled since April 2007
Documentation about the recalibration campaign can be found here.

3.8 Short announcement "VGT1 Recalibration"

In order to improve the radiometry consistency of the entire SPOT/VEGETATION Archive, a careful vicarious re-calibration of the VGT1 data has recently been carried on with the same methodology as the one applied to VGT2 acquisitions. Performed at CTIV under CNES supervision, the new processed VGT products have currently been distributed since the 1st of June 2010. A brief checking of the RADIOM_ABS_CAL_REF field in the "0001_LOG.txt" file of every product informs users on the calibration version which is applied. The "up-to-date" calibration reference should be : "RADIOM_ABS_CAL_REF V1_A_000_YYYY_MM_01_05.PCI (with YYYY = year of the current synthesis and MM = month of the current synthesis).

4. About the VGT entity

4.1 Who are the VEGETATION product distributors?

VEGETATION products are distributed from 1st January 2007 onwards by VITO (worldwide distribution) and by the distribution partner SPOT Image (L-Band Station).

Contact Spot Image
Patrice Galey
Tel : +33 562 19 43 36
Fax : +33 562 19 40 51

Contact VITO
Tel : +32 14 33 68 77
Fax : +32 14 32 27 95

4.2 How can I order VEGETATION products?

VEGETATION products can be ordered on the Internet through the VEGETATION Catalogue. The catalogue is freely accessible for browsing, but for ordering you should be registered. To register, you should contact VITO.

The products can be ordered:
* by catalogue: for images in the catalogue
* by subscription: for images over a future period (a few weeks to several years) covering a particular region of interest

The following order information is needed for both orders:
* Region of interest (ROI)
* Period
* Product type (P, S1, S10, S10.4, S10.8, D10, D10.4, D10.8)
* Projection
* Delivery media

4.3 What are the delivery supports?

There are 3 kind of supports for delivery of VEGETATION products:
* (default) FTP (products are compressed by default in ZIP format)
* CD-ROM

The supports (CD-ROM) are dispatched to the user.

The products by FTP are obtained from the FTP server at the Vegetation Image Processing Centre (CTIV). The user receives an e-mail with the information when a product is ready. However, if the user does not retrieve the product after 10 days the product will automatically be redelivered on FTP unless specified otherwise.

5. About VGT1 <-> VGT2

5.1 What are the radiometric differences between VGT1 and VGT2?

Test results on radiometry are given in the table below. There are 2 reasons for VGT1/VGT2 reflectance discrepancy on the products:
. spectral bands are not exactly the same (filters manufacturing)
. improved calibration methods used for VGT2 result in slightly improved radiometry.

	B0	B2	B3	MIR	NDVI
Difference of Reflectance over miscellaneous targets
RMS Difference	0.9 %	2.0 %	1.6 %	2.2 %	/
Difference of reflectance over vegetation (spectral effect)
RMS Difference	0.4 %	1.7 %	0.7 %	1.4 %	3.3 %
Max. Difference	1.6 %	5.4 %	2.5 %	4.6 %	12.1 %
Difference of reflectance due to calibration
reflectance bias	1.3 %	2.1 %	6.3 %	0.7 %	3.4 % ndvi>0.3

5.2 What are the differences between VGT1 and VGT2 products from the users point of view (on level P, S1, S10)?

VGT2 will generate the same type of products as VGT1 with no significant difference, 2 slight differences can be noticed between VGT1 and VGT2 products:
- the use of the on-board SPOT5 stars tracker for VGT2 geometric modelling allows to achieve high geometric performances without any need for GCPs
- the VGT2 spectral bands are very similar to VGT1 one’s but small discrepancies can induce small reflectance variations.

5.3 What are the new changes in the VEGETATION product processing chain?

Since 11 May 2001 the CTIV made 4 new changes in the VEGETATION product processing chain:

1. The cloud flag in the status map of the products is still compatible with the old version. However, some additional information is available:
The new Status map is:

bit7(MSB)	0 (poor)		1(good) Quality B0
bit6	0 (poor)		1(good) Quality B2
bit5	0 (poor)		1(good) Quality B3
bit4	0 (poor)		1(good) Quality MIR
bit3	0 (sea)		1(land)
bit2	0 (no)		1(ice/snow)
bit1	0	0	1	1
bit0(LSB)	0	1	0	1
	clear	shadow	uncertain	cloud

The impact of the change for VGT products (VGT-P, VGT-S1, VGT-S10) is minor.

2. Improved parameters in the bicubic interpolation resulting in a minor improvement of radiometric accuracy in projected end-user products. Impact (VGT-P, VGT-S1, VGT-S10) is minor (almost unnoticeable).

3. Improved atmospheric correction for the tropospheric aerosols based on the radiometry of the blue band. Impact on end-user products : improved quality for end-products (VGT-S1, VGT-S10).

4. Improved algorithm to correct the data from aberrant MIR detectors. This algorithm analyzes the image data from the MIR band, detects the abnormal rows, and interpolates or corrects the radiometric count of the impacted pixels. The impact of the change for VGT products (VGT-P, VGT-S1, VGT-S10) should be a drastically reduction of aberrant MIR lines in images.

5.4 Where can I find information about the new distribution policy?

New Spot VEGETATION data policy in support of GMES (2002-01-26)

5.5 What will happen february 1st 2003 in the VEGETATION programme?

February 1st 2003, the VEGETATION 1 instrument on board of SPOT4 will stop capturing data from the entire landmass. At that moment the VEGETATION 2 instrument will become the nominal VEGETATION instrument and will start capturing the entire globe. VGT1 will then be used for L-band and special programming.

5.6 What will happen with VGT1 and with SPOT4?

The SPOT4 platform will continue operations. VGT1 will become the redundant instrument in the VEGETATION programme. It will be charged with special programming for e.g. Antarctica, Oceans measurements, L band programming or other users requests.

5.7 Is there continuity for subscription orders?

If you have a subscription order, you must be aware that an order is instrument related. When VGT2 takes over the nominal daily task, users with subscription orders will have to re-submit their order as VGT2 orders (done by spotimage for the current subscriptions).

6. VEGETATION catalogue

6.1 What is the band combination of the browse images in the catalogue?

The band combination used for the browse products in the Catalogue is:

B0(0.43 - 0.47 µm)	BLUE
B2(0.61 - 0.68 µm)	GREEN
B3(0.78 - 0.89 µm)	RED

7. other

7.1 My question is not answered, what should I do?

If your question is not answered here, then please contact us by email for assistance.