The Constellation Observing System for Meteorology Ionosphere and. Climate (COSMIC) is a satellite mission for the Earth sciences that will make use of recent developments in remote sensing, communications technology, and computing to solve some of the most important geo-scientific issues today. COSMIC plans to launch eight Low Earth Orbit (LEO) satellites in 2003. Each of these spacecraft will carry three science payloads for weather and space weather research and prediction, climate monitoring, and geodesy: 1) GPS occultation receiver, 2) Tiny Ionospheric Photometer (TIP), and 3) Triband Beacon transmitters (TBB). Each of the LEOs will track the GPS satellites as they are occulted behind the Earth limb to retrieve up to 500 daily profiles of key ionospheric and atmospheric properties. Every day the constellation will provide globally about 4000 GPS soundings. TIP will measure electron densities at the peak of the F2 layer along the satellite track. TBB transmissions will be received on the ground for high-resolution tomographic reconstruction of the ionospheric electron density. COSMIC continuos precise tracking of all GPS satellites in view, also promise to benefit geodetic studies. The COSMIC system includes the LEO satellites, ground data reception and spacecraft control stations, data analysis centers and the data communications networks. This paper gives a COSMIC science overview and describes the COSMIC system.

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COSMIC System Description

[1] Temperature, pressure, and humidity profiles of the Earth's atmosphere can be derived through the radio occultation technique. This technique is based on the Doppler shift imposed, by the atmosphere, on a signal emitted from a GNSS satellite and received by a low orbiting satellite. The method is very accurate with a temperature accuracy of 1°K, when both frequencies in the GPS system are used. However, difficulties arise when the signal consists of multiple frequencies generated by multipath phenomena in the atmosphere. We demonstrate that, in general, it is possible to determine the arrival times of the different frequency components in a radio occultation signal simply as the derivatives of the phases of the conjugated Fourier spectrum of the entire occultation signal. Based on this property, a novel Full Spectrum Inversion technique for radio occultation sounding capable of disentangling multiple rays in multipath regions is presented. As the entire signal is used in the Fourier transform, a high spatial resolution in the Doppler frequency, and hence in the retrieved temperature, pressure, and humidity profiles, can be achieved. The method is conceptual and computational simple and thus easy to implement. The performance of the Full Spectrum Inversion is demonstrated by applying the technique to simulated signals generated by solving Helmholtz equation with use of the multiple phase-screen technique. Excellent agreement is found between computed bending angle profiles and corresponding solutions to the Abel integral.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002RS002763

Full Spectrum Inversion of radio occultation signals

. The launch of the proof-of-concept mission GPS/MET (Global Positioning System/Meteorology) in 1995 began a revolution in profiling Earth's atmosphere through radio occultation (RO). GPS/MET; subsequent single-satellite missions CHAMP (CHAllenging Minisatellite Payload), SAC-C (Satellite de Aplicaciones Cientificas-C), GRACE (Gravity Recovery and Climate Experiment), METOP-A, and TerraSAR-X (Beyerle et al., 2010); and the six-satellite constellation, FORMOSAT-3/COSMIC (Formosa Satellite mission {#}3/Constellation Observing System for Meteorology, Ionosphere, and Climate) have proven the theoretical capabilities of RO to provide accurate and precise profiles of electron density in the ionosphere and refractivity, containing information on temperature and water vapor, in the stratosphere and troposphere. This paper summarizes results from these RO missions and the applications of RO observations to atmospheric research and operational weather analysis and prediction.

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Exploring earth's atmosphere with radio occultation: contributions to weather, climate and space weather

The GPSIMET (Global Positioning System/Meteorology, Ware et a1. 19996) project demonstrated atmospheric limb sounding from low-earth-orbit (LEO) with high vertical resolution, high accuracy, and global coverage in all weather. Based on the success and scientific results of GPS/MET, Taiwan’s National Space Program Office (NSPO), the University Corporation for Atmospheric Research (UCAR), the Jet Propulsion Laboratory (JPL), the Naval Research Laboratory (NRL), the University of Texas at Austin, the University of Arizona, Florida State University and other partners in the university community are developing COSMIC (Constellation observing System for Meteorology, Ionosphere and Climate), a follow-on project for weather and climate research, climate monitoring, space weather, and geodetic science. COSMIC plans to launch eight LEO satellites in 2004. Each COSMIC satellite will retrieve about 500 daily profiles of key ionospheric and atmospheric properties from the tracked GPS radio-signals as they are occulted behind the Earth limb. The constellation will provide frequent global snapshots of the atmosphere and ionosphere with about 40000 daily soundings.
This paper discusses some of the applications of COSMIC data for meteorology, including polar meteorology, numerical weather prediction (NWP), and climate. Applications to ionospheric research including space weather and geodesy are described elsewhere in this issue of TAO.
In meteorology COSMIC will provide high vertical resolution temperature, pressure and water vapor information for a variety of atmospheric process studies and improve the forecast accuracy of numerical weather prediction models. The COSMIC data set will allow investigation of the global water vapor distribution and map the atmospheric flow of water vapor that is so crucial for understanding and predicting weather and climate. The data set will provide accurate geopotential heights, enable the detection of gravity waves from the upper troposphere to the stratosphere, reveal the height and shape of the tropopause globally with unprecedented accuracy, support the investigation of fronts and other baroclinic structures, and improve our understanding of tropopause-stratosphere exchange processes. 
COSMIC data will complement other observing systems and improve global weather analyses, particularly over the oceans and polar regions, and NWP forecasts made from these analyses. Through assimilation in numerical models, COSMIC data will improve the resolution and accuracy of the global temperature, pressure and water vapor fields, and through the model’s dynamical and physical adjustment mechanisms, the wind fields as well. These improved analyses and forecasts will provide significant benefits to aviation and other industries. 
For climate research and monitoring COSMIC will provide an accurate global thermometer that will monitor Earth’s atmosphere in all weather with unprecedented long-term stability, resolution, coverage, and accuracy. COSMIC will provide a data set for the detection of climate variability and change, the separation of natural and anthropogenic causes, the calibration of other satellite observing systems and the verification and improve events especially in remote oceanic regions, and it will enable scientists to monitor the response of the global atmosphere to regional events such as Volcanic eruptions, the Kuwait oil fires, or the recent Indonesian and Mexican forest fires. Upper-tropospheric refractivity data from COSMIC may shed new light on the controversy over the role that tropical convection of COSMIC data will provide new insights into the global hydrologic cycle.

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Applications of COSMIC to Meteorology and Climate

Propagation of radio occultation signals through the tropical lower troposphere with severe refractivity gradients results in significant spreading of the signal spectrum. Under such conditions a signal acquisition technique which tracks large random troposphere-induced phase accelerations more reliably than a generic phase-locked loop has to be applied. This paper discusses the results of simulations of open loop tracking of radio occultation signals that were generated with data from high-resolution tropical radiosondes. The signal has to be down-converted in real time in the receiver on orbit to a low mean residual frequency by use of a phase (Doppler) model based on predicted orbits and refractivity climatology. The down-converted complex signal is then low-pass filtered and sampled. The phase in excess of the phase model must be reconstructed from the sampled and down-linked signal in postprocessing. This may require an additional down-conversion to eliminate (minimize) aliasing of harmonics in the spectrum. Then the accumulated phase can be reconstructed by resampling the signal at a higher rate to resolve the cycle ambiguities. A fast algorithm for prediction of the Doppler based on the refractivity climatology and an algorithm for the detection of Doppler mismodeling based on sliding window spectral analysis of the down-converted signal are developed and tested. The accuracy of the Doppler modeling, ±(15-20) Hz, the required filter bandwidth, 100 Hz, and the sampling rate, 50-100 Hz, are estimated.

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Tracking tropospheric radio occultation signals from low Earth orbit

Interpretation of the Doppler-shifted radio frequencies from the proof-of-concept GPS/MET radio occultation experiment is impeded by the effects of multipath propagation caused by strong variations of atmospheric refractivity. In this paper, we describe and validate a method of deciphering the ray structure of the measured electromagnetic field in regions of multipath propagation. The method is based on diffraction theory and the principles of the synthesized aperture, which allows for the refinement of the vertical resolution beyond the Fresnel scale as well as for the improvement of the signal/noise ratio. The validation of the method is based on artificial occultation data simulated using global fields from a NCEP analysis. An example of the application of the diffraction correction algorithm for the processing of real GPS/MET data is given. The comparison of the inversion results with and without diffraction correction indicates a very significant role of multipath propagation in the lower troposphere, which suggests that the diffraction correction can significantly improve the quality of the data processing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD00806

Alexander S. Gurvich

Papers

Microlab-1 experiment: Multipath effects in the lower troposphere