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Case studies

Calibrating satellite instruments to aid climate observation

NPL validated and tested Earth observation instruments to enable accurate in-flight data collection. 

Case study

The need

The Earth Cloud Aerosol and Radiation Explorer (EarthCARE) satellite is a joint project between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). 

Equipped with four climate observation instruments, the EarthCARE satellite mission has been designed to make a range of measurements that, when combined, will shed new light on the role that clouds and aerosols play in regulating the Earth’s climate. 

These on-board observation instruments required high-level validation and testing to ensure reliable and accurate data recording while in orbit. 

The solution 

The satellite’s on-board radar system antenna and the associated quasi-optical feed system were developed by TK Instruments, a UK SME and regular collaborator of the National Physical Laboratory (NPL). The design of these systems required verification prior to approval by the satellite consortium.  

NPL used its large anechoic chamber antenna measurement facility to validate TK Instrument’s antenna’s reflector design and system performance, by measuring both its gain and antenna pattern. 

The radar transmitter incorporated two microwave klystron power sources. JAXA specified stringent requirements for monitoring these, to ensure that the secondary source would only be activated when the primary power source showed signs of failure. To achieve this, TK instruments incorporated four microwave power sensors into the antenna feed. 

NPL built a test assembly to characterise the microwave power sensors over wide temperature and power ranges. We were able to accurately quantify the signal loss between the klystron source and the detectors in the antenna feed so that the source power could be accurately determined once the satellite was in operation. 

NPL was also involved in the pre-flight calibration and characterisation of two of the on-board optical instruments. The on-board Broad Band Radiometer (BBR) has two channels to measure Earth emitted/reflected radiation for radiation balance. The BBR, built by RAL space, had its spectral responsivity measured pre-flight at NPL. This required the tailoring of some of our facilities to address the calibration needs of an essentially spectrally flat black detector that could only operate in vacuum.  

In addition, the thermal infrared channels of the on-board Multi-Spectral Imager (MSI), built by Surrey Satellites Ltd, was calibrated using specially developed infrared ‘black body’ reference targets built by NPL. These black body references were originally built by NPL to support the Geostationary Earth Radiation Budget (GERB) series of instruments, some of which are still operating on-board Meteosat second generation weather satellites. 

The impact

The satellite launched from the Vandenberg Space Force Base in California, US, on 29 May 2024. The data gathered from the mission, which relies on extremely accurately calibrated instruments, will deliver critical information on the role that clouds and aerosols play in Earth’s radiation balance and benefit both climate modelling and weather forecasting to help address environmental challenges. 

Since launch, the satellite has captured images of the interior structure of storm clouds from orbit, the first of their kind. From these images, scientists can see exactly where different phases of water are held. This kind of imagery will help us gain clearer insights into the physics that drives cloud formation and behaviour.

Find out more