PHOCUS radiometer

[摘要] PHOCUS – Particles, Hydrogen and Oxygen Chemistry in the Upper Summer Mesosphereis a Swedish sounding rocket experiment, launched inJuly 2011, with the main goal of investigating the upper atmosphere in thealtitude range 50–110 km. This paper describes the SondRad instrument in thePHOCUS payload, a radiometer comprising two frequency channels (183 GHzand 557 GHz) aimed at exploring the water vapour concentration distributionin connection with the appearance of noctilucent (night shining) clouds. Thedesign of the radiometer system has been done in a collaboration betweenOmnisys Instruments AB and the Group for Advanced Receiver Development(GARD) at Chalmers University of Technology where Omnisys was responsiblefor the overall design, implementation, and verification of the radiometersand backend, whereas GARD was responsible for the radiometer optics andcalibration systems.

The SondRad instrument covers the water absorption lines at 183 GHz and 557 GHz.The 183 GHz channel is a side-looking radiometer, while the 557 GHzradiometer is placed along the rocket axis looking in the forward direction.Both channels employ sub-harmonically pumped Schottky mixers and FastFourier Transform Spectrometers (FFTS) backends with 67 kHz resolution.

The radiometers include novel calibration systems specifically adjusted foruse with each frequency channel. The 183 GHz channel employs a continuouswave CW pilot signal calibrating the entire receiving chain, while theintermediate frequency chain (the IF-chain) of the 557 GHz channel iscalibrated by injecting a signal from a reference noise source through adirectional coupler.

The instrument collected complete spectra for both the 183 GHz and the 557 GHzwith 300 Hz data rate for the 183 GHz channel and 10 Hz data rate forthe 557 GHz channel for about 60 s reaching the apogee of the flighttrajectory and 100 s after that. With lossless data compression usingvariable resolution over the spectrum, the data set was reduced to 2 × 12MByte.

The first results indicate that the instrument successfully performedmeasurements of the mesospheric water profile as planned. However, thetemperature environment for the instruments showed more extreme behaviourthan expected and accounted for. Consequently, the results of the calibrationand the final data reduction will need careful treatment. Further,simulations through finite element method (FEM), modelling and directmeasurements of the simulated thermal environment and its impact on theinstrument performance are described, as well as suggestions forimprovements in the design for future flights.

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