SYN Level-2 VGT processing aims to combine information from the OLCI and SLSTR instruments to provide comparable products to SPOT-VGT to allow continuity of data delivery to the existing user community of SPOT-VGT.
The products selected for continuity are the P, S1 and S10 products named SY_2_VGP, SY_2_VGS1 and SY_2_VGS10 , respectively.
The P projection is defined at top of atmosphere (TOA), while the S1 and S10 are composite products defined at top of canopy (TOC), i.e. after an atmospheric correction. S1 is defined over a daily time period and S10 over a 10-day (decade) period.
The starting point of the processing is the co-located TOA L2 product from the SYN branch. It is divided into four steps:
- Spectral band mapping to simulate the SPOT-VGT spectral bands (four bands) at top of atmosphere from the OLCI and SLSTR bands.
- Pixel flagging, indicating land, water, cloud cover, ice/snow are transferred from SYN L2 datasets.
- Projection from the ortho-geolocation grid to the SPOT VGT Plate-Carrée grid at 1 km resolution. At the end of this sub-step, the SY_2_VGP product can be written.
- Atmospheric correction of the VGT-P product to obtain TOC reflectance and computation of the NDVI. This last step aims to produce the VGS1&10 products.
VGT S1 products give a single ‘best’ value for TOC reflectance at the four VGT channels (B0, B2, B3, MIR), based on those giving the maximum of Normalised Difference Vegetation Index (NDVI). The NDVI is a simple numerical indicator that is widely used to give an indicator of vegetation amount, derived from the NIR and RED VGT channels:
The method and principals for generation of the VGT S10 product are identical to the VGT S1, except that S10 is formed using the maximum value composite over a 10‐day period.
S1 and S10 can either be generated in parallel, as is the case in CTIV, (Centre de Traitement des Images VEGETATION) or alternatively S10 can be generated from S1. The advantage of the former approach is that the same compositing program can be used to generate S1 and S10.