Until July 2019, the L1C and L2A products were generated at the Venµs Imapge Processing center (VIP) with a specific format. From July 2019, the L2A are processed at MUSCATE with a slghtly different format which is described here below. The L1C are still produced at the VIP, but they are now integrated with MUSCATE, and have a slightly different format. The reason of this change is the homogeneity of formats within the Theia platform. As a result, the Venµs L1C format is close to that of Sentinel-2 L2A products generated by Theia.
The main changes are:
- the metadata format is different, the keywords have changed.
- the images are still provided with TIFF format, but there is one file per band. The scaling factor for L1C is still 1000.
- the grid used corresponds to that of Sentinel-2, you only have to account for the upper left origin of each site to obtain the registration of the images. No reprojection and no resampling is necessary.
When you download a Venµs L2A product from Theia website, you obtain a zip file.
unzip -o VENUS-XS_20190414-173938-000_L1C_ARM_C_V2-13.zip
After unzipping, you get a folder with the same name as the zipfile, without ".zip".
The main product format contains a metadata file in xml format, the surface reflectances in two flavours, the atmospheric variables computed by MAJA, and a MASK folder.
Useful information in metadata
- Instrument, date and time
- projection and geographic coverage
- Solar and viewing angles
- For the solar angles, you can use the values at the scene center.
- For the viewing angles, you have to account for the fact that the angles differ depending on the spectral band.Venµs channels are regrouped per triplets and the viewing angles are provided for each triplet. For instance, for the third one :
Important quality indexes
The registration quality of the images of Venµs micro satellite is not that of a big platform such as Sentinel-2. The platform is less stable and its sensors are less accurate. As a result, most of the registration accuracy of Venµs is base on the L1C geometrical processing, which makes a large use of image matching measurements. If usually the registration results are correct, when a site changes quickly (snow melt for instance), or is covered by clouds, or is covered by a large part of water or uniform landscape, the results may be degraded.
We recommend users to watch two quality indicators, which are provided in the Quality Index fields of the header of Level 1C products.
- The first index concerns multi-temporal registration. It provides a statistical error on the registration of the current image B05 on the reference image, expressed in meter (5.155 m in the example below). To meet the Venµs Image quality specification we had issued before flight (which apply to 10m resolution images), You should check that IMAGE_RESIDUES_REFIMG < 2.85m. If not, use the image with precaution as its multi-temporal registration with the other images in the time series is probably degraded.
- The second index (field N°18) is related to multi-spectral registration. It provides a statistical error between various spectral bands and the reference spectral band (B05), expressed in meter (here 0.637 m). You should check that IMAGE_RESIDUES_INTERDETECTORS < 1m. If not, use the image with precaution, as the registration between spectral bands is maybe degraded.
- the filenames end with "REF_BXX.TIF" (SRE for Surface REflectance) were corrected for atmospheric effects, including adjacency effets
The files contain :
- - TOA reflectances for spectral band XX
- coded in 16 bits signed integers: you have to divide per 1000 to obtain reflectances
- No_Data value (outside the image), is -10000
The MASK directory:
This directory provides information on cloud cover (which is not as accurate than that provided at L2A), saturated and interpolated pixels. The files in that folder are compressed, wich is not too convenient but saves a lot of space, except if you need to use the information and unzip the files.
- pixel is saturated if >0
- pixel is interpolated if >0
- pixel is cloudy if >0