Inter-calibration methods compare a reference instrument, with well-known calibration characteristics, with collocated observations from another instrument, in order to derive calibration corrections for the latter.
This ensures consistency between the products of the different instruments.
Satellite inter-calibration is beneficial for the following reasons:
- it can identify problems and increase the confidence in the operational calibration of individual satellites. Hence, inter-calibration can serve as a monitoring tool for the operational calibration;
- it can provide the basis for a normalised calibration, which is a prerequisite for the derivation of global products from different satellites.
EUMETSAT has performed inter-calibration using instrument measurements from both geostationary Meteosat satellites and the polar-orbiting Metop and NOAA satellites. EUMETSAT’s inter-calibration activities are now coordinated through the Global Space-based Inter-Calibration System (GSICS).
GSICS Inter-Calibration Products
The range of GSICS products includes the GSICS Correction and GSICS Bias Monitoring. GSICS Corrections are functions users can apply to correct the calibration of operationally-generated datasets from the Monitored Instrument so they are consistent with those of the Reference Instrument. GSICS Corrections are available for both Near-Real-Time and Re-Analysis applications, with different latencies.
GSICS Bias Monitoring allow users to visualise the relative biases between the monitored and reference instruments for standard radiances. These take the form of time series plots in which the latest results can be compared with recent trends.
The table (above) summarises the status of a number of different GSICS products being developed by EUMETSAT. Links are provided to the EUMETSAT GSICS Data and Product Server, where the coefficients of the GSICS Correction can be downloaded in netCDF format, and to the GSICS Bias Monitoring tool, which allows users to evaluate and plot the results of the inter-calibrations as time series of brightness temperature biases.
GSICS Data and Products Server
EUMETSAT has taken the lead in the design and development of the first operational GSICS Data and Product Server. The purpose of this server is to provide the platform for developing calibration products and to facilitate data exchange related to GSICS activities.
Its purpose, operational concepts and data exchange format (netCDF) were presented to the GSICS partners, at the joint GSICS meeting in 2008, where its development was agreed and endorsed by the GSICS partners and, then, the Executive Panel.
The EUMETSAT GSICS Data and Product Server became operational on 1 April, 2009. It currently serves the GSICS community with EUMETSAT’s instruments’ source data sets — as well as pre-operational GSICS products undergoing validation, according to the GSICS product acceptance procedure.
Our GSICS partners are in the process of developing their operational GSICS Data and Product servers. Once this is realised, a network of collaboration data and production servers will share all GSICS data and products with each other, providing a reliable data distribution service to the user community.
In the future, developments in the area of metadata conventions will be further examined. Using established conventions, or adding to them, adds value to data as all users of the convention will understand what the data is and use the same units. Another major benefit, for the user community, of using conventions is tools implemented to understand these conventions can immediately work with the data.
Heritage Inter-Calibration Products
The Meteosat-NOAA HIRS Inter-calibration page shows the calibration coefficients for infrared and water vapour channels of the Meteosat First Generation imagers, derived by comparison with collocated observations from the HIRS sounders on NOAA polar-orbiting satellites. The method is described in more detail on the MFG Calibration webpage. The calibration coefficients derived from this method are compared with those used operationally.
This method is currently under review to align it with GSICS procedures with the intention of providing an inter-calibration product for the Meteosat archive, based on a homogenised series of HIRS data as a reference.