Roland Albers

The Arctic Weather Satellite (AWS) is a single instrument mission consisting of a microwave sounder operating in the 54, 89, 183 and 325 GHz bands. The optical design of the instrument consists of a feedcluster directly illuminating a crosstrack scanning mirror, keeping the instrument compact. Due to the simple optics, the beams are not co-aligned and none are in the scanning mirror focal point, leading to beam divergence, asymmetry and scan angle dependent variations in spillover. Using Method of Moment (MoM) simulations of the optics as well as instrument structure, the complete farfield sphere of the instrument can be simulated up to 183 GHz. This paper contains detailed analysis of spillover, beam divergence and intercomparison of physical optics, MoM and nearfield antenna measurements for the mainbeam.

We present the design and characterisation of the onboard calibration target for the Arctic Weather Satellite radiometer. The Arctic Weather Satellite is a single instrument mission consisting of a cross-track scanning microwave radiometer. The radiometer optics consists of a feed cluster with four horns (54, 89, 183 and 325 GHz) directly illuminating a primary scanning mirror. The onboard calibration target is a wedge shaped cavity with an absorber consisting of an epoxy based mixture, developed and produced by the University of Bern. The coupling into the target via a secondary mirror is simulated using Ticra Tools and the effects of the divergent beams are investigated. We present the development of the target and laboratory measurements of the flight model showing a return loss of 55 dB or better for all bands in TM mode. A worst case thermal simulation is presented, highlighting possible temperature gradients in the target.

The Arctic Weather Satellite (AWS) is a prototype mission for an operational constellation of microwave sounders, complimenting existing meteorological sounders. The AWS is a microsatellite with a single cross-track scanning radiometer operating in the 54, 89, 183 and 325 GHz bands. Due to the small platform size, the core design focus of the radiometer’s quasi-optics is less complexity and a more compact setup than comparable spaceborne microwave sounders. To achieve this, the instrument utilises a splitblock feedarray which directly illuminates the off-axis parabolic scanning reflector. A secondary parabolic reflector is used to couple into a wedge-shaped load which is used for calibration in combination with cold sky measurements. The main challenge for instrument performance is that only one of the four horns can be located in the focus of the scanning reflector. Consequently, scan angle dependent spillover variations and beam asymmetries can occur. This paper details the simulation and optimisation efforts of the quasi-optics to minimise the aforementioned effects.

In this contribution, we present a comparison of three digital real-time spectrometers used in passive remote sensing of ozone and other trace gases in the middle atmosphere. During a period of six months, we connected the spectrometers to the same radiometric front-end to perform parallel observations of the ozone emission line at 110.836 GHz. This allowed us to better characterize a bias previously observed on the integrated spectra of the Acqiris AC240, a widely used digital spectrometer which has been used for more than a decade in many operational microwave radiometers. We investigated the bias under different atmospheric conditions and found that it is caused by multiple sources. Nonlinearities in the calibration are responsible for part of the bias, but a larger contribution stems from a second effect in the AC240. Although this error source is still partly unexplained, we found that a simple correction scheme simulating a spectral leakage can be applied to the integrated spectra of the AC240 and worked well on our range of observations. We also show that by applying our bias correction to the spectra, we can correct the bias in the ozone retrievals. There is still a need for further measurements to validate this approximate correction, but it could help to correct the numerous time series of ozone and other atmospheric constituents recorded by the AC240.