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Calibrating satellites in-flight

The need

Satellites can determine the health and state of the planet by measuring the spectral reflectance of sunlight off the Earth’s surface. However, a satellite's radiometric performance changes over time. The stress of launch and the harshness of space can affect the calibration of sensors used in satellites for Earth observation. Without post-launch calibration there is added uncertainty into the data collected, which can impact our ability to monitor and mitigate climate change. To counter this, regular checks against known values are essential to enable trust in the data they deliver.

The impact

The Committee on Earth Observation Satellites (CEOS) has established the Radiometric Calibration Network (RadCalNet), a network of radiometric test sites against which satellite-borne sensors in the optical domain can be calibrated during flight. These mainly desert-like sites are equipped with autonomous instrumentation to provide continuous measurements of both surface reflectance and local environmental/atmospheric conditions.

The sites allow Top of Atmosphere (TOA) reflectance values to be determined on the ground every 30 minutes, from 9 am to 3 pm local time. The values are then provided on a free and open access web portal for any satellite operator to compare to their satellite's measurements and correct the data post-launch. It is important that the methodologies used to take these measurements are consistent between sites and between the different teams taking these measurements.

Our expertise as the UK’s National Measurement Institute makes us uniquely placed to provide traceability of the whole RadCalNet network, including sites in France, China and the USA. Our work with the network will result in more confidence in the data used for climate change, weather systems and monitoring disaster areas. In addition, we established and continue to operate one of the member sites, Gobabeb in Namibia, on behalf of the European Space Agency.  

The ongoing work of RadCalNet will help to reduce overall uncertainty in data collected from Earth observation satellites. It will ensure a consistent and harmonised absolute radiometric calibration to the same SI-traceable scale.