22-1 : Compulsory or optional planes of P product.
P products are adapted for the first type of users for which physical quality of data is important. They correspond to data which would have been acquired by an ideal instrument:
are corrected for system errors (misregistration of the different channels,
calibration of all the detectors along the line-array detectors for each spectral
bands) and resampled to geographic projections for multitemporal analysis as
well as for comparison with high resolution data. The input system data is processed
using ancillary data necessary for radiometric and geometric corrections and
annotation of the data set. The output data will correspond to the entire lines
of the orbital segment which is necessary to overlap the geographical area requested
by the user.
The accuracies given above (Geometrical properties) apply to this data level.
Annotations giving full information on applied corrections (calibration information, geometric parameters taking into account attitude and position on the orbit), or for further non-system corrections ("standard" atmosphere parameters) are attached to the data sets.
Calibration information are used to correct detector normalization and absolute calibration. The output values are linearly related to equivalent "Top of the Atmosphere" reflectances. The linear relationship use a null offset and a spectrally constant scaling factor allowing direct comparison between spectral reflectances.
Missing measurements are corrected or indicated as follows :
- interpolation using the mean value of their two neighbors should be used on the blind detectors in the SWIR spectral band,
- identified "false detectors" in the VIS-NIR spectral bands are interpolated the same way,
- lost lines are not replaced through interpolation,
- interpolated and "missing" measurements are flagged in the status map.
All pixels are resampled onto a regular grid, taking into account :
- spectral band registration
- satellite location and attitude correction
- terrain elevation to take into account parallax distortion and provide orthoimages.
The set of projections chosen for the P products is compatible with the set of projections available for SPOT high resolution images to ensure direct registration between VEGETATION and HRVIR data.
The final resampling grid is 1 km when the projection is adapted to such regular sampling.
A status map is provided with the images, indicating for each pixel one of the following cover classes at the time of acquisition: land, water, snow, ice and cloud.
Indication of interpolated, lost pixels or lines are part of the status map.
The following information are provided for the ancillary data
- System information
- Atmosphere information
1. methods and data to associate to each pixel its location and acquisition time as well as geometry of acquisition (solar zenith and azimuth angles, viewing zenith and azimuth angles)
2. reference to actual absolute calibration parameters used to allow reverse correction assuming the correction is linear.
The standard correction method for all VEGETATION products is based on SMAC and all necessary data for application are appended. They are the aerosol optical depth at 550 nm and the vertically integrated gaseous contents for water vapor and ozone.
- Topography information
A reference to the elevation information data set that is used for the geometric corrections is indicated. With that, the users have the capability to retrieve topographic information from the known location of each pixel.