VEGETATION 传感器于1998 年3 月由SPOT-4 搭载升空，，从1998年4 月开始接收用于全地植被覆盖观测的SP0T VGT 数据。VEGETATION 数据主要由瑞典的Kiruna 地面站负责接收，由位于法国Toulouse 的图像质量监控中中负责图像质量并提供相关参数(如定标地数)，最终由位于比利时的VITO 研究所的图像处理与存档中中负责全地VEGETATION 数据存档与用户定单。

太阳同步圆形近极地轨道 轨道高度：832公里 
轨道倾角：98.721度 
轨道周期：101.469分/圈 
重复周期：369圈/26天 
降交点时间：上午10：30分 
扫描带宽度： 60 公里 
两侧侧视：+/-27度 扫描带宽：950公里

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spot vgt的几个波段及用途

• Blue,where ground reflectance of vegetation cover is minimal and atmospheric aerosol diffusion effects are maximal.

       mainly to perform atmospheric corrections 主要用于大气校正

• The influence of atmospheric water vapor, which is most important in a wide near infrared band, is severely decreased by limiting the upper portion of the near infrared band to avoid the 0.935 ?m water vapor absorption band.

•  Red and Near Infrared (NIR), sensitive to the vegetation's photosynthetic activity and cell structure 对植被的光合作用以及细胞结构敏感

   a red band centered on the absorption peak of the chlorophyll (0.665 ?m),a near infrared bandcorresponding to the maximum vegetation spectral reflectance and principally related to the structural properties of the canopies and to percentage of soil covered by vegetation,

• Short Wave Infrared (SWIR), sensitive to soil and vegetation moisture content. 对土壤和植被湿度敏感

  a short wave infrared band centered around 1.65 ?m where reflectance is related to water content of the canopy components and to its structure.

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                         their spectral responses

  http://img.bimg.126.net/photo/ch7kn0VTjEWQh2PxOY2Izw==/5764326048057673715.jpgvgt 产品基本介绍" />

  About 90% of the equatorial areas are imaged each day, the remaining 10% being imaged the next day. For latitudes higher than 35° (North and South), all regions are acquired at least once a day.

  从图上可以看出赤道地区轨道之间有缝隙，所以在赤道地区不能每天都获取影响，但是在90以上的区域都可以获取，只有10％区域不能拥有每天的图像，但是在第二天可以获取到

  All spectral bands at full spatial resolution acquired on terrestrial areas will be stored onboard in a solid state memory, allowing the use of only one receiving station to which data will be transmitted in X band. All the spectral bands will also be transmitted in L band, for possible local receiving stations.

  有两个传输波段x和l波段

•  
  Its 1 km spatial resolution is nearly constant across the whole 2,250km corridor it covers, which means that there is almost no distortion on image edges
• http://www.spot-vegetation.com/images/home/quick/sensor_tab.gifvgt 产品基本介绍" />
• http://www.spot-vegetation.com/images/home/quick/shemapro.gifvgt 产品基本介绍" />

目前spot VGT数据主要有

• 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)

  其中每旬的合成时间为Three ten-day syntheses are made during a month:

• synthesis from the 1st to the 10th of the current month   1-10
• synthesis from the 11th to the 20th of the current month  11 -20

• synthesis from the 21st to the end of the current month   21-最后一天

•  

   

  下面我们分别来介绍一下spot vgt的这几种产品

   

• 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 countis 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).Each pixel brightness count is encoded using 16 bits yielding 12 significant bits.

     其中VGT—P(prototype)数据产品主要为科研人员提供高质量的物理量原型数据以便于他们 
  研建算法和应用模型。数据经过严格的地地误差订正并重采样为经纬地投影，像元分辨率lkm， 
  像元亮度值是地物在大气顶层的反射率。除提供四个波段原始数据外，还根据用户需要提供相关 
  辅助参数，如大气状况、地地信息(太阳的天底角、方位角，视场角和接收时间)和地形数据等。

• VGT-S (S= synthesis) products are MVC or Maximum Value Composite Syntheses.，the cloud cover influences are favorably reduced.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.

• VGT—S(synthesis)最大值合成产品，提供经过大气纠正的地表反射率数据，并运用多波段合成技术来获得lkm 分辨率的归一化植被指数( w)数据集。减小云的影响，海洋的值设为0，VGI—S 产品包括每天合成的四个波段的光谱反射率及NDVI 数据集(s1)，每10 天合成的四个波段的光谱反射率及10 天最大化NDVI 数据集(S10)以减少云及BRDF 的影响，VGT—S 产品以其高时间分辨率而被广泛使用。本数据集包含的是每10 天合成的四个波段的光谱反射率及10 天最大化NDVI 数据集(S10)。SPOT 源数据的预处理包括大气校正，辐射校正，几何校正，中成了10 d 最大化合成的NDVI 数据，并将-1 到-0.1 的值设置为-0.1，再通过公式YDN=(JNDVI +0.1)/0.004 转换到0~250 的YDN 值。

  1. 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 havemore 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

• vgt s1产品是合成每天的最好的地表反射率，有的地区可以一天覆盖多次，在高纬地区存在轨道的叠加，因此可以进行合并

• 2.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.同时S10 还被重采样成4km 分辨率(S10.4)和8km 分辨率(S10.8)数据集。

  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
  • 3.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

  vgt d10 产品指的是bdc或二向反射合成，这种合成方法是基于二向反射率函数，像元的灰度值为地面反而反射率，海洋地区的反射率设为0，和s10一样有三种合成产品 1km 4km和8km

  那么mvc和bdc产品有什么区别呢，主要表现在status map 和新增以及去除一些内容

  • 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.

  不同产品的数据内容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

  • 植被产品的应用Application of Vegetation products: 
    The VEGETATION system allows operational & near real-time applications, at global, continental and regional scales, in very broad environmentally and socio-economically critical fields such as; 
    ? Agriculture; 
         Crop monitoring, crop production quantification, yields forecasting, agricultural pest prediction and monitoring, early warning systems to prevent food shortage, improve decision support tools and optimise the actions of governments, food aid agencies, international financing &/or cooperation agencies towards food security. 
    ? Water resource management; 
    Water resource mapping and monitoring for the optimisation of irrigation management and herds displacement, snow cover monitoring for flood prevention
  • Forestry; 
    Forest mapping and change detection 
    ? Land planning; 
    Land cover map to be used for zoning processes and the elaboration of resource management plans 
    ? Environment; 
    Environmental degradation phenomena, natural hazards and disaster mapping and monitoring (floods, fires, burnt scars, desertification) for damage evaluation, emergency preparedness and risk management 
    ? Terrestrial ecosystems;
  •   Characterization of terrestrial ecosystems and their seasonal dynamics, representing critical information in the framework of international conventions (biodiversity, desertification,) 
    ? Global change; 
    Carbon sinks and sources estimation through extraction of vegetation parameters (biomass, Leaf Area Index - LAI - , Net Primary Productivity - NPP -) for climate change studies, etc...
  • ndvi实际值的计算公式

  Real NDVI =coefficient a * Digital Number + coefficient b

               = a * DN +b

  Coefficient a = 0.004    Coefficient b = -0.1

  • 反射率的计算公式 （产品中包含波段数据）

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

  = a * DN +b

  Coefficient a = 0.0005   Coefficient b = 0.0