Anders Carlström

GAS: the Geostationary Atmospheric Sounder

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.

A Geostationary Atmospheric Sounder for now-casting and short-range weather forecasting

Millimeter and sub-millimeter-wave imagers and sounders are considered for future meteorological and climate observation satellites [1]. The Geostationary Atmospheric Sounder (GAS) [2], is a development project for an imaging sounder at frequency bands around 53 GHz, 118 GHz, 183 GHz, and 380 GHz. These frequency bands are included to satisfy the user requirements for vertical profiles of temperature and humidity under all weather conditions. The primary advantage of a Geostationary Earth Orbit (GEO) for remote sensing, compared with a Low-Earth Orbit (LEO), is that continuous monitoring is possible over a large area of the Earths surface and atmosphere. This is desirable for nowcasting, and short range forecasting of rapidly evolving meteorological phenomena.

Image Retrieval Simulations for the GEO Atmospheric Sounder (GAS)

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.

The Geostationary Atmospheric Sounder (GAS)

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 GPS Occultation Sensor for NPOESS

The Global Positioning System Occultation Sensor (GPSOS) is a precision GPS instrument carried by the NPOESS spacecraft. It measures primarily the electron density profile and scintillation parameters of the ionosphere. The measurements are obtained by tracking signals of GPS satellites that are observed to rise or set through the atmosphere while recording the signal amplitude and phase.

Spaceborne submm-wave interferometry

The primary advantage of a geostationary Earth orbit (GEO) for remote sensing, compared with a low-Earth orbit (LEO), is that continuous monitoring is possible over a large area of the Earths surface and atmosphere. The main technical challenges are the very large antenna aperture required for achieving the required spatial resolution (40 folds increase in the distance to the Earth as compared to the LEO) and the necessity for imaging of two-dimensional scanning due to the absence of a relative spacecraft-Earth movement. The requirements derived from this application includes an effective aperture diameter of more than 8 m, which can not be fulfilled by a classical reflector-based instrument. Hence, an interferometric concept has been studied. In this paper a theoretical derivation of the minimum number of receivers that is required is presented along with a discussion on the implementation of such a system in space within a 10-year time frame

Improved GNSS radio occultation with the next generation GRAS instrument

The next generation GNSS radio occultation instrument under development at RUAG Space will take advantage of the new signals of Galileo, GPS, GLONASS, and Compass. The measurement performance is improved as compared to the present instrument generation that operates on the MetOp satellites. A tracking algorithm has been developed which ensures that all signal information throughout the atmosphere is made available for ground processing with a minimum of losses. Simulation results are presented for a demanding case of a rising occultation.

Information content in reflected global navigation satellite system signals

The direct signals from satellites in global satellite navigation satellites systems (GNSS) as, GPS, GLONASS and GALILEO, constitute the primary source for positioning, navigation and timing from space. But also the reflected GNSS signals contain an important information content of signal travel times and the characteristics of the reflecting surfaces and structure. Ocean reflected signals from GNSS satellite systems reveal the mean height, the significant wave height and the roughness of the ocean. The estimated accuracy of the average surface height can be as low as 10 cm. For low elevations, the signals reveal the incoherent scatter process at the reflection zone. By using open-loop high-precision GNSS receivers, it is possible to provide the in-phase and quadrature components of the signal at high sample rates, which enables investigation of the spectral signatures of the observations. The retrieval method consists of a radio occultation technique for the phase differences between the direct and reflected signal combined with a statistical method. Results are derived through a sequential Bayesian estimation method, where the retrieval algorithms are based on a particle filtering technique. The horizontal size of the probability density function, which uniquely describes the ocean reflection zone using the recursive particle filter method, totals from 200 to 500 meters for all data sets.

MetOp GRAS: Signal tracking performance results

The GRAS radio occultation instrument on board MetOp has continuously provided measurement data during almost four years. The measurement data are here used as a means to analyze the signal tracking performance of the GRAS instrument. The results demonstrate that the atmospheric Doppler shift is measured to an accuracy of 1.1 mm/s and that the on-board Doppler model is accurate within 4 m/s (or 20 Hz). It is also found that rapid signal amplitude fluctuation, exceeding 10 dB, frequently occur at higher altitudes than expected and that it is possible to improve the signal tracking during such events.