Lead Organisation: STFC RAL Space
Project Lead: Manju Henry
Partners: STAR-Dundee Ltd. and JCR Systems Ltd.
A new atmospheric remote sounding total power heterodyne radiometer is being developed within the United Kingdom (UK), and for the UK Space Agency, by a team led by the Rutherford Appleton Laboratory, and including STAR-Dundee and JCR Systems.
The radiometer employs a hyper-spectral microwave imaging technique in which hundreds of contiguous detection channels, spread across a wide instantaneous bandwidth, simultaneously sample atmospheric molecular absorption features associated with molecular oxygen (O2) centred at 60 GHz and with a pressure broadened profile extending to 68 GHz. This measurement technique substantially increases the precision of atmospheric profile retrievals and the laboratory prototype instrument, HYMS, provides a first realisation of a future space-borne Earth observation tool that will enhance weather forecasting.
Developing a hyper-spectral imager is hugely challenging as it demands a significant reduction in the detector system noise in order to meet the required radiometric precision, i.e. as defined by a noise equivalent minimum detectable temperature (NE∆T) of <0.4K, especially at a high spectral resolution of 10 MHz. This is because NE∆T is a function of the selected spectral resolution and to maintain it, a reduction in noise is required. For instance, when compared with the Microwave Sounder (MWS), the most sensitive microwave payload on MetOp-SG, the predicted sensitivities are 0.3 K and 1.8 K for a spectral resolution of 300 MHz to 12 MHz respectively. In the case of HYMS, the required sensitivity is four-fold higher than MWS and this poses a challenge to achieving the required system noise.
The solution combines a state-of-the-art low noise amplifier followed by a heterodyne down-convertor and ultra-high-speed digital backend data processor. This system architecture provides a powerful and highly integrated remote sounder capable of providing ultra-high sensitivity radiometric measurements with a high spectral resolution, i.e. of order 3 MHz. Moreover, it is suitable for future test via an airborne observation platform and targets eventual deployment in space as a next generation operational meteorological observation tool.
The HYMS lower sideband (LSB) and upper sideband (USB) system noise performance at 50.3-57.3 GHz and 63.3-67.9 GHz has been measured. Results indicate a noise temperature of 170±20K across each sideband, which equates to an NEΔT of ~0.26 K and ~0.47K in a 10 MHz (minimum) and 3 MHz (goal) spectral bandwidth respectively, both for a 0.3 second integration time. This result demonstrates state-of-the-art performance for a room temperature airborne atmospheric sounder operating within the 50 to 68 GHz frequency range.