Understanding our dynamically changing planet

Climate studies require data records exceeding the lifetime of a single remote sensing satellite mission. Moreover, precise altimetry measurements are needed to explore evolution of the sea level. Changes in the mean sea level allow scientists to understand the response of the ocean to continued global warming (both through thermal expansion of the ocean and the melting of major ice sheets.

As with all scientific data, it is vital that measurement tools operate within specifications. Calibration of satellite-based altimeter systems is an important element in this regard. However, calibrations at fixed sites are not without problems. The EU-funded project 'A global network of permanent sites for calibrating satellite altimetry missions' (CALT) was initiated with the aim to establish a network of calibration sites.

Specifically, the CALT project provides support to an international network of Brazilian, Chinese and European satellite altimetry sites, brought together more than 10 years ago. Significant efforts have also been devoted to including two additional in situ calibration sites located in Australia and the United States. The CALT network collectively oversees the operation and calibration of multiple satellite altimetry missions.

Each site provides observations of the sea level along with ancillary measurements collected by the satellites as they pass over the site. Observations from tide gauges and other sensors placed over the satellite ground tracks are used to ensure each mission performs as accurately as possible. Nevertheless, calibration results depend on the local conditions and duration of measurements of the calibration sites.

Among the network's goals is, therefore, to demonstrate the value of a collaborative partnership in addressing key observational gaps and sustain a continuous stream of calibration and validation data. More importantly, the CALT network offers unique opportunities to share experience and expertise in the development of calibration methodologies and to standardise procedures.

The CALT network provides the means to address the calibration needs of space-based Earth observation on a global basis ( without eroding the independence of individual agencies operating permanent sites.