William S. Schreiner

THE COSMIC/FORMOSAT-3 MISSION : Early Results

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO dat...

Analysis and validation of GPS/MET data in the neutral atmosphere

The Global Positioning System/Meteorology ( GPS/MET) Program was established in 1993 by the University Corporation for Atmospheric Research ( UCAR) to demonstrate active limb sounding of the Earths atmosphere using the radio occultation technique. The demonstration system observes occulted GPS satellite signals received by a low Earth orbiting ( LEO) satellite, MicroLab-1, launched April 3,1995. The system can profile ionospheric electron density and neutral atmospheric properties. Neutral atmospheric refractivity, density, pressure, and temperature are derived at altitudes where the amount of water vapor is low. At lower altitudes, vertical profiles of density, pressure, and water vapor pressure can be derived from the GPS/MET refractivity profiles if temperature data from an independent source are available. This paper describes the GPS/MET data analysis procedures and validates GPS/MET data with statistics and illustrative case studies. We compare more than 1200 GPS/MET neutral atmosphere soundings to correlative data from operational global weather analyses, radiosondes, and the GOES, TOVS, UARS/MLS and HALOE orbiting atmospheric sensors. Even though many GPS/MET soundings currently fail to penetrate the lowest 5 km of the troposphere in the presence of significant water vapor, our results demonstrate 1°C mean temperature agreement with the best correlative data sets between 1 and 40 km. This and the fact that GPS/MET observations are all-weather and self-calibrating suggests that radio occultation technology has the potential to make a strong contribution to a global observing system supporting weather prediction and weather and climate research.

GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results

Abstract This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides informat...

Analysis and validation of GPS/MET radio occultation data in the ionosphere

Global Positioning System (GPS) radio occultation signals received by a low Earth orbit (LEO) satellite provide information about the global distribution of electron den- sity in the ionosphere. We examine two radio occultation inversion algorithms. The first algo- rithm utilizes the Abel integral transform, which assumes spherical symmetry of the electron density field. We test this algorithm with two approaches: through the computation of bend- ing angles and through the computation of total electron content (TEC) assuming straight line propagation. We demonstrate that for GPS frequencies and for observations in LEO, the as- sumption of straight-line propagation (neglecting bending) introduces small errors when monitoring the F2 layer. The second algorithm, which also assumes straight-line propagation, is a three-dimensional (3-D) inversion constrained with the horizontal structure of a priori electron density fields. As a priori fields we use tomographic solutions and the parameterized real-time ionospheric specification model (PRISM) when adjusted with ionosonde data or ground-based GPS vertical TEC maps. For both algorithms we calibrate the occultation data by utilizing observations from the part of the LEO that is closer to the GPS satellite. For in- versions we use dual-frequency observational data (the difference of L1 and L2 phase ob- servables) which cancel orbit errors (without applying precise orbit determination) and clock errors (without requiring synchronous ground data) and thus may allow inversions to be computed close to real time in the future. The Abel and 3-D constrained algorithms are vali- dated by statistically comparing 4 days of inversions with critical frequency (foF2) data from a network of 45 ionosonde stations and with vertical TEC data from the global network of GPS ground receivers. Globally, the Abel inversion approach agrees with the foF2 correlative data at the 13% rms level, with a negligible mean difference. All tested 3-D constrained in- version approaches possess a statistically significant mean difference when compared with the ionosonde data. The vertical TEC correlative comparisons for both the Abel and 3-D con- strained inversions are significantly biased (-30%) by the electrons above the 735-km LEO altitude.

Artificial plasma cave in the low‐latitude ionosphere results from the radio occultation inversion of the FORMOSAT‐3/COSMIC

Previous studies report unexpected electron density reductions, termed “plasmacaves,” located underneath the equatorial ionization anomaly (EIA) crests. A radiooccultation (RO) observation simulation experiment has been built to evaluate possiblebiases introduced by the spherical symmetry assumption in the standard (Abel) ROinversion processes. The experiment simulates the electron density profiles andreconstructs the plasma structure of the EIA at low latitudes, where the horizontal gradientis most significant. The reconstruction shows that artificial plasma caves are createdunderneath the EIA crests along with three density enhancements in adjacent latitudes. Theartifact appears mainly below 250 km altitudes and becomes pronounced when theEIAs are well developed. Above that altitude, the two EIA features in the original (truth)model, the International Reference Ionosphere (IRI‐2007), and in the inversion are similar,but the inversion reconstructs less distinct EIA crests with underestimation of the electrondensity. A simple correction has been introduced by multiplying the ratio between thetruth and inversion with actual FORMOSAT‐3/COSMIC observations. This initialcorrection shows that the artificial plasma caves are mitigated. Results also reveal that theRO technique is not suitable to detect or rule out possible existence of the plasma caves.

Global 3-D ionospheric electron density reanalysis based on multisource data assimilation

[1] We report preliminary results of a global 3-D ionospheric electron density reanalysis demonstration study during 2002–2011 based on multisource data assimilation. The monthly global ionospheric electron density reanalysis has been done by assimilating the quiet days ionospheric data into a data assimilation model constructed using the International Reference Ionosphere (IRI) 2007 model and a Kalman filter technique. These data include global navigation satellite system (GNSS) observations of ionospheric total electron content (TEC) from ground-based stations, ionospheric radio occultations by CHAMP, GRACE, COSMIC, SAC-C, Metop-A, and the TerraSAR-X satellites, and Jason-1 and 2 altimeter TEC measurements. The output of the reanalysis are 3-D gridded ionospheric electron densities with temporal and spatial resolutions of 1 h in universal time, 5 in latitude, 10 in longitude, and 30 km in altitude. The climatological features of the reanalysis results, such as solar activity dependence, seasonal variations, and the global morphology of the ionosphere, agree well with those in the empirical models and observations. The global electron content derived from the international GNSS service global ionospheric maps, the observed electron density profiles from the Poker Flat Incoherent Scatter Radar during 2007–2010, and foF2 observed by the global ionosonde network during 2002–2011 are used to validate the reanalysis method. All comparisons show that the reanalysis have smaller deviations and biases than the IRI-2007 predictions. Especially after April 2006 when the six COSMIC satellites were launched, the reanalysis shows significant improvement over the IRI predictions. The obvious overestimation of the low-latitude ionospheric F region densities by the IRI model during the 23/24 solar minimum is corrected well by the reanalysis. The potential application and improvements of the reanalysis are also discussed.

Quantitative evaluation of the low Earth orbit satellite based slant total electron content determination

[1] With the increased number of low Earth orbit (LEO) satellites equipped with GPS receivers, LEO based GPS observations play a more important role in space weather research because of better global coverage and higher vertical resolution. GPS slant total electron content (TEC) is one of the most important space weather products. In this paper, the LEO based slant TEC derivation method and the main error sources, including the multipath calibration, the leveling of phase to the pseudorange TEC, and the differential code bias (DCB) estimation, are described systematically. It is found that the DCB estimation method based on the spherical symmetry ionosphere assumption can obtain reasonable results by analyzing data from multiple LEO missions. The accuracy of the slant TEC might be enhanced if the temperature dependency of DCB estimation is considered. The calculated slant TEC is validated through comparison with empirical models and analyzing the TEC difference of COSMIC colocated clustered observations during the initial stage. Quantitatively, the accuracy of the LEO slant TEC can be estimated at 1–3 tecu, depending on the mission. Possible use of the LEO GPS data in ionosphere and plasmasphere is discussed.

Preparing for COSMIC: Inversion and Analysis of Ionospheric Data Products

The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) is scheduled for launch in 2006. COSMIC will consist of six low earth orbiting satellites in planes separated by 24° to provide global atmospheric and ionospheric observations. One of the goals is to demonstrate near real-time processing of data products for numerical weather prediction and space weather applications. Each COSMIC satellite will carry three payloads: (1) a Global Positioning System (GPS) occultation receiver with two high-gain limb viewing antennas and two antennas for precision orbit determination, (2) a Tiny Ionospheric Photometer (TIP) for monitoring the electron density via nadir radiance measurements along the sub-satellite track, and (3) a Tri-Band Beacon (TBB) transmitter for ionospheric tomography and scintillation studies. The data from all these payloads will be processed at the COSMIC Data Analysis and Archival Center (CDAAC). Here we give an overview of the ionospheric data products from COSMIC and focus on the plans and preliminary simulation studies for analyzing the ionospheric occultation data and combining them with ground-based GPS, TIP, and TBB observations.

Data assimilation retrieval of electron density profiles from radio occultation measurements

[1] In this paper, the Kalman filter is used to retrieve the electron density profile along the tangent points by assimilating the slant total electron content data observed during a radio occultation (RO) event into an empirical background model. The RO data observed by COSMIC satellites on day of year 266 in 2009 are selected to do both the simulation work and the real data retrieval test. The results show that the data assimilation technique can improve the electron density retrieval in comparison with the Abel inversion. It is less influenced by the ionospheric inhomogeneity than the Abel method. Some pseudo‐large‐scale features made by the Abel retrieval, such as the plasma cave underneath the equatorial ionization anomaly region and the three peaks along the latitude direction in the E layer, disappear in the data assimilation retrieval results. Independent validation by ground‐based ionosonde observations confirms the improvement of data assimilation retrieval below the F2 peak. In addition, some potential research on RO data assimilation is also discussed.

Space Weather Observations by GNSS Radio Occultation: From FORMOSAT-3/COSMIC to FORMOSAT-7/COSMIC-2

The joint Taiwan-United States FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission, hereafter called COSMIC, is the first satellite constellation dedicated to remotely sense Earths atmosphere and ionosphere using a technique called Global Positioning System (GPS) radio occultation (RO). The occultations yield abundant information about neutral atmospheric temperature and moisture as well as space weather estimates of slant total electron content, electron density profiles, and an amplitude scintillation index, S4. With the success of COSMIC, the United States and Taiwan are moving forward with a follow-on RO mission named FORMOSAT-7/COSMIC-2 (COSMIC-2), which will ultimately place 12 satellites in orbit with two launches in 2016 and 2019. COSMIC-2 satellites will carry an advanced Global Navigation Satellite System (GNSS) RO receiver that will track both GPS and Russian Global Navigation Satellite System signals, with capability for eventually tracking other GNSS signals from the Chinese BeiDou and European Galileo system, as well as secondary space weather payloads to measure low-latitude plasma drifts and scintillation at multiple frequencies. COSMIC-2 will provide 4–6 times (10–15X in the low latitudes) the number of atmospheric and ionospheric observations that were tracked with COSMIC and will also improve the quality of the observations. In this article we focus on COSMIC/COSMIC-2 measurements of key ionospheric parameters.