All objects at a temperature above absolute zero, -273.15°C, emit thermal radiation. The hotter they are, the brighter the thermal radiation and the shorter the wavelength at which the radiation brightness peaks. At a high enough temperature, you can see an object's thermal radiation (e.g. an oven heating element glows red when it heats up). Objects also absorb thermal radiation, and the parameter that quantifies absorption and emission is called the emissivity. A near perfect emitter/absorber, with an emissivity very close to 1, is termed a 'blackbody'. The amount of thermal radiation emitted at long infrared / terahertz / millimetre wavelengths from such a blackbody is proportional to its absolute temperature, and it thus forms an excellent source for calibrating instruments. Such radiometer instruments are used in Earth observation, for weather forecasting and climate research, as well as in radioastronomy, to observe the distant universe.
The Millimeter Wave Technology (MMT) Group has developed an approach to create such blackbodies. An array of aluminium pyramids is conformally coated with a thin layer of an absorber, which is made from iron particles in epoxy resin. The combination of shape and absorber means that only a fraction of 1 in 20,000 if the incoming radiation is reflected. In other words, the emissivity is 0.99995, and we have created a near perfect blackbody. In collaboration with the Space Engineering & Technology division of RAL Space, the MMT group are currently using these blackbodies to produce the pre-launch calibration equipment for two instruments in the Met-Op Second Generation satellite series. This programme will deliver operational meteorology for the UK and Europe over the coming decades, providing essential information on global atmospheric humidity, temperature and pressure.
Our customers are Airbus Defence and Space (UK) for the MWS instrument, and OHB (Italy) for the Microwave Imager (MWI) instrument. In both cases our calibration targets will operate inside thermal vacuum chambers at temperatures from -190°C to 80 °C, covering frequencies from 14 GHz to above 229 GHz.
More generally, RAL Space can design, manufacture and supply pyramidal targets that are usable at cryogenic and elevated
temperatures (100°C maximum so far) and with integrated thermal stabilisation, including physical temperature readouts. We have developed the precision machining approaches needed for targets with areas in excess of 1 m2, and are able to characterise these blackbodies accurately with radiometers and vector network analysers.
Blackbody calibration target former for MetOp-SG