CEOS / IVOS Working Group 4 (use of pseudo-invariant sites)

Methodology Intercomparisons: Objectives and approach

The intercomparisons study of CEOS/IVOS WG4 focused on four methodologies making use of pseudo-invariant sites for vicarious calibration or for radiometric intercomparisons. The sensors considered in this work were medium spatial resolution sensors with multi-spectral capabilities operating in the visible to thermal infrared. 

A report on the methodology intercomparisons is available here .

Time series of cloud screened TOA reflectance averaged over pre-defined regions of interest were generated for tree sites: 

i. Libya-4, 
ii. Niger-2
iii. Dome-C, 

The sensors for which data were extracted are: 

i. AATSR, 
ii. MERIS, 
iii. MODIS-Aqua, 

The time series of extractions covers the years: 2006 to 2009. 

These data were distributed in a predefined format and constitute what is hereafter called the reference dataset. 

Each participating team applied its vicarious calibration methodologies and/or sensor-to-sensor radiometric intercomparison methodologies to this reference dataset. MERIS data (2nd reprocessing) were chosen as a radiometric reference to which other sensor radiometry was compared. The comparison was carried out for each methodology and for each site using standardised statistical indicators associated to the time series of radiometric differences between MERIS and other sensors. 

Reference Dataset is available: You are welcome to download the reference dataset, sensor spectral responses and a ReadMe from: [DOWNLOAD DATASET HERE]

The reference dataset is fully described in the WG4 Final Report. It was generated from three separate data sources; DIMITRI (maintained by ESA and ARGANS), SADE (CNES) and RAL. It contains:   

the mean and standard deviation of TOA reflectances over pre-defined regions of interest
the associated viewing and solar geometries
the associated meteorological parameters extracted 

 Site  North Lat  South Lat  East Lon  West Lon  Surface Type
 Dome Concordia -74.9   -75.3   123.9   122.9   Snow 
 Libya 4  29.05   28.05   23.89  22.89  Desert 
 Niger 2  21.87  20.87  11.09  10.09  Desert

The reference dataset files are provided for each sensor over each site and are space delimited text files. Each row within the file represents a different satellite observation over the ROI; different sensor views are treated as separate observations (e.g. AATSR Nadir and Forward views). The different columns of data are:


 Column  Description
 Sensor  The name of the sensor and Processing Version 
 Acquisition Time  dd/mm/yyyy-hh-mm-ss 
 Processing Time  dd/mm/yyyy-hh-mm-ss (not extended, duplicate of acquisition time)  
 Site Name  The validation site name
 meanTOArho  The mean TOA reflectance for each band (*)
 stdevTOArho  The standard deviation of reflectance for each band (*)
 VZA  The mean viewing zenith angle at each band (*), 0° = nadir, 90° = horizon
 VAA   The mean viewing azimuth angle at each band (*), 0° = nadir, 90° = horizon
 NPixels  The number of pixels within the ROI for this observation
 Latitude  The site centre Latitude
 Longitude  The site centre Longitude
 SZA  The mean Solar Zenith Angle, 0° = nadir, 90° = horizon
 SAA  The mean Solar Azimuth Angle, 0° = North, 90 = East°
 WVAP  The mean Solar Zenith Angle, 0° = nadir, 90° = horizon
 OZONE  The mean ozone concentration in cm atm
 PRESS  The mean surface pressure in mbar
 WIND  The mean wind speed modulus in m s-1



Missing values are set as -999.

* Sensor Bands:

AATSR: all seven bands in ascending wavelength order, 

MERIS: all 15 bands in ascending wavelength order

MODIS-Aqua: MODIS Band Numbers 8 - 9 - 3 - 10 - 4 - 1 - 2 - 5 - 6 – 7 (wavelengths 412 - 443 - 469 – 488 - 555 - 645 - 858 - 1240 - 1640 – 2130)

PARASOL: all bands in ascending wavelength order: 443 - 490 - 565 - 670 - 763 - 765 - 865 - 910 - 1020

VEGETATION: all four bands in ascending wavelength order