The GRAS instrument on the Metop-A satellite provides more than 600 radio occultation measurement profiles per day. The instrument is characterized by its wide antenna coverage, high signal-to-noise ratio and an ultra-stable clock reference. The conventional dual-frequency tracking of GPS signals is under dynamic atmosphere conditions complemented by open loop tracking with sampling of the signal at a 1 kHz rate, providing an unprecedented view of the signal spectral environment. This paper presents the instrument performance as derived from analysis of in-orbit measurement data. We show that the noise figure is low enough to enable mapping of external radio noise variations over the earth's surface. An error propagation model is presented to relate instrument characteristics to bending angle performance. This model is also used to illustrate the relation between filter bandwidth, resolution and measurement noise. The Doppler model, guiding open loop measurements, is found to be accurate to better than 20 Hz with a possibility for improvement to 10 Hz. The high performance at low altitudes enables the presence of surface reflections at the -20-dB level to be identified in more than 50% of the occultations. The potential performance improvements for next generation receivers are discussed.
Millimeter and Sub-mm-wave imagers/sounders are considered for future meteorological geostationary satellite missions. A novel interferometric Geostationary Atmospheric Sounder (GAS) has been developed and a concept demonstrator is under construction. The concept is a response to the requirements of observations for nowcasting and short range forecasting in 2015-2025, as determined by EUMETSAT for post-MSG operational satellites observations. Prioritized parameters include vertical profiles of temperature and humidity with high temporal and horizontal resolution (15 min and 30 km) under all weather conditions. Frequency bands around 53GHz, 118GHz, 183GHz, 380GHz have the highest user priority and are all supported by GAS. The instrument relies on an innovative configuration of interferometer elements which enables the use of a sparse array and simplifies calibration.
The GEO atmospheric sounder (GAS) is a potential European future instrument, which utilizes interferometric synthetic aperture radiometry to obtain the desired spatial (30 km) and temporal (30 minutes) resolution for measurement of atmospheric temperature and humidity profiles under all weather conditions. This paper presents a simulation model for the entire imaging process. It enables detailed simulation of the impact of the instrument characteristics on the imaging performance. The model has been used as a tool to predict the performance of an instrument demonstrator under development.
This paper presents the concept and initial breadboarding results of Geostationary Atmospheric Sounder (GAS), which is being developed by Saab Space AB and Omnisys AB, Sweden, and funded by European Space Agency (ESA). GAS utilizes interferometric synthetic aperture radiometry to obtain desired spatial (30 km) and temporal (nowcasting) resolution for measurement of atmospheric temperature and humidity profiles under all weather conditions. These parameters are decisively important to meteorological and climate models at all time scales.
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