Ensuring the accuracy of ESA’s FORUM climate mission

Our planet continuously receives energy from the Sun, reflects some of it back to space and emits thermal energy of its own. The delicate balance between the incoming and outgoing energy is known as Earth’s radiation budget, and it is a factor directly controlling our planet’s surface temperature.

The European Space Agency (ESA) is working on a mission to address a major gap in climate monitoring. The agency’s ninth Earth Explorer mission FORUM, short for Far-infrared Outgoing Radiation Understanding and Monitoring, will deliver the first-ever measurements from space of Earth’s outgoing radiation over an extremely large range extending into the far-infrared spectrum with unprecedented resolution and accuracy.

To achieve something that has never been done before, engineers and scientists often need to adapt existing methods, processes, or even invent entirely new technologies. The National Metrology Institute of Germany PTB is one of the organisations making FORUM’s ambitious goal possible by doing just that.

PTB’s contribution to the mission was funded by ESA’s General Support Technology Programme (GSTP), the agency’s initiative enabling the development of new technologies.

“The spectrometer onboard the FORUM mission will measure Earth’s outgoing radiation with very low uncertainty, meaning its measurements will be very accurate,” explains Christian Monte, head of the Detector Radiometry and Radiation Thermometry department at PTB. “A level of uncertainty this low was at the limit of what was achieved in the best labs, on ground, five years ago. Never in space.”

To operate, the FORUM spectrometer needs a reference source, a device that will be used to calibrate the instrument to make sure its measurements remain as accurate as possible while in space. For a spectrometer of this kind, this reference source is a ‘blackbody’ – a physical object that, in theory, absorbs all electromagnetic radiation and reflects none.

“We have developed a new on-ground reference blackbody, with half the uncertainties so far achieved on ground. This allowed us to calibrate the blackbody which will actually fly on FORUM with the desired level of uncertainty,” adds Christian. “In the process of making this device, we created an entirely new system to measure its temperature, applied a new kind of coating, and developed a dedicated manufacturing process involving a very special type of welding to avoid degrading the temperature-sensitive coating material.”

“A blackbody can only serve as a reference source with the lowest uncertainty if its surrounding temperature is stable, known and uniform,” adds Daniela Narezo Guzmán, leading the Infrared Radiometry working group at PTB. “For this we developed a coldscreen, which is essentially a highly emissive black plate very uniform in its temperature, mounted in front of the blackbody’s aperture.”

Christian notes: “At PTB, we have the only facility in the world capable of calibrating in the far-infrared range with FORUM’s challenging accuracy requirements. Our expertise and extremely committed team allowed us to successfully calibrate the onboard blackbody that will fly on the FORUM satellite.”

“To make sure that the low uncertainty is maintained during its operational lifetime in space, we proposed the inclusion of extra temperature sensors that would help us identify any variations in the measurements,” notes Daniela. “Micos, who developed the flying blackbody, liked our idea and implemented it.”

“We also proposed developing a twin blackbody, identical to the one in space, which would be operated on ground, in conditions resembling space as closely as possible – vacuum, radiation, as well as vibrations simulating launch,” she adds.

“This would help us understand which set of sensors degrades, if any, and correct the blackbody flying in space. If this proves to be an effective way of detecting degradation, it could become a valuable method applicable to other missions.”

Felice Vanin, ESA’s FORUM payload manager, comments: “The success of this GSTP activity was reflected in the synergy and collaboration across different nations and entities. It was remarkable to witness how we all work together towards mission success.”