Stefan Heise

A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere

Ground-based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)-based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS-based), and O+-H+ ion transition level. The method employs new formulae based on Chapman, sech-squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle- and high-geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low-earth-orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real-time basis, etc.

GPS/GLONASS-based TEC measurements as a contributor for space weather forecast

Space weather monitoring and forecast require a permanent monitoring of the ionospheric state on global scale. The world-wide use of global navigation satellite systems such as GPS and GLONASS o7ers the unique chance for a permanent monitoring of the total ionization (total electron content—TEC) of the global ionosphere=plasmasphere up to about 20000 km height. In this study we turn our attention to TEC variations over the European area. Using the data of more than 15 GPS stations of the GPS tracking network of the International GPS Service (IGS), a horizontal resolution in the order of 500 km is achieved, the standard time resolution is 10 min. The total ionization of the ionosphere reacts very sensitive to solar radiation changes. As correlation studies with the solar radio =ux index F10.7 have shown, the ionospheric response over the European area is delayed by about 1–3 days depending on geophysical conditions. Consequently, the turn o7=on of the solar radiation during the solar eclipse on August 11, 1999 was seen as a signi@cant reduction of TEC following the obscuration function with a delay of up to 40 min. Ground-based GPS measurements can e7ectively be used for detecting large-scale horizontal structures and their motion (up to 30 s time resolution) during perturbation processes (see http://www.kn.nz.dlr.de/). These capabilities are demonstrated by analyzing individual storms of January 10, 1997 and of April 6, 2000. For the latter also TEC maps of the Northern polar cap down to 50 ◦ N were computed. These polar maps indicate strong ionization enhancements around the geomagnetic pole in the evening hours. Furthermore, simultaneous high rate sampled GPS and GLONASS data are presented that demonstrate the impact of perturbation-induced small-scale irregularities in the ionosphere on satellite signals in operational communication and navigation systems. c

Real‐time GPS sensing of atmospheric water vapor: Precise point positioning with orbit, clock, and phase delay corrections

The recent development of the International Global Navigation Satellite Systems Service Real-Time Pilot Project and the enormous progress in precise point positioning (PPP) techniques provide a promising opportunity for real-time determination of Integrated Water Vapor (IWV) using GPS ground networks for various geodetic and meteorological applications. In this study, we develop a new real-time GPS water vapor processing system based on the PPP ambiguity fixing technique with real-time satellite orbit, clock, and phase delay corrections. We demonstrate the performance of the new real-time water vapor estimates using the currently operationally used near-real-time GPS atmospheric data and collocated microwave radiometer measurements as an independent reference. The results show that an accuracy of 1.0 ~ 2.0 mm is achievable for the new real-time GPS based IWV value. Data of such accuracy might be highly valuable for time-critical geodetic (positioning) and meteorological applications.

The rapid and precise computation of GPS slant total delays and mapping factors utilizing a numerical weather model

In a previous study we developed an elegant technique to compute the signal travel time delay due to the neutral atmosphere, also known as slant total delay (STD), between a Global Positioning System (GPS) satellite and a ground-based receiver utilizing data from a numerical weather model (NWM). Currently, we make use of NWM data from the Global Forecast System (GFS) because short-range forecasts are easily accessible. In this study we introduce some modifications which double the speed of our algorithm without altering its precision; on an ordinary PC (using a single core) we compute about 2000 STDs per second with a precision of about 1 mm. The data throughput and precision are independent of the vacuum elevation (azimuth) angle of the receiver satellite link. Hence, the algorithm allows the computation of STDs in a mesobeta-scale NWM with an unprecedented speed and precision. A practical by-product of the algorithm is introduced as well; the Potsdam Mapping Factors (PMFs), which are generated by fast direct mapping utilizing short-range GFS forecasts. In fact, it appears that the PMFs make the application of parameterized mapping in GPS processing obsolete.

TEC monitoring by GPS - a possible contribution to space weather monitoring

Abstract The world-wide use of GPS satellites for navigation and positioning offers a unique chance for permanent monitoring the total electron content (TEC) of the ionosphere on regional/global scale. Using permanent operating GPS ground station networks (e.g. that from IGS) the derived TEC maps may contribute to monitor the actual development of large scale structures in electron content and their dynamics during ionospheric perturbations (e.g.www.kn.nz.dlr.de/ → Ionosphere Impact → Storm). Based on GPS measurements at European IGS ground stations and subsequent TEC estimations and mapping, both individual as well as common features of ionospheric storms shall be discussed for the European ionosphere. The paper demonstrates the power of this new GPS tool to detect and to study the dynamics of large-scale spatial structures (e.g. ionospheric response of solar eclipse on 11 August 1999). Detected TEC-perturbations are closely related to space weather characterizing quantities such as solar radio emission, solar wind and geomagnetic activity indices. Due to close relationships between TEC and space weather parameters a permanent space weather monitoring can help to control the ionospheric impact on operational navigation satellite systems.

Comparison of Water Vapor and Temperature Results From GPS Radio Occultation Aboard CHAMP With MOZAIC Aircraft Measurements

Global positioning system (GPS) radio occultation (RO) observations aboard low earth orbiting (LEO) satellites provide a powerful tool for global atmospheric sounding. Almost continuously activated since mid-2001, the challenging minisatellite payload (CHAMP) GPS RO experiment provides up to 200 vertical atmospheric profiles per day. In this paper, we intercompare CHAMP RO humidity results and analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF) with coinciding measurement of ozone and water vapor by airbus in-service aircraft (MOZAIC) data collected during aircraft ascents and descents. About 320 coinciding profiles with CHAMP were found from 2001 to 2006 (coincidence radius: 3 h, 300 km). Between about 650 and 300 hPa, the CHAMP-MOZAIC humidity bias is smaller than the ECMWF-MOZAIC bias. On the other hand, the standard deviation between MOZAIC and CHAMP humidity is slightly higher than that between MOZAIC and ECMWF through the entire altitude range. Apart from the water vapor validation (ascent and descent data), we also compare MOZAIC cruise data at an altitude of typically 10-11 km with CHAMP refractivity and temperature results (dry retrieval), and corresponding ECMWF analysis data. Whereas refractivity data from MOZAIC, CHAMP, and ECMWF show excellent agreement, the CHAMP temperature exhibits a cold bias of about 0.9 K in comparison to MOZAIC and ECMWF.

A forward operator and its adjoint for GPS slant total delays

In a recent study we developed a fast and accurate algorithm to compute Global Positioning System (GPS) Slant Total Delay (STDs) utilizing numerical weather model data. Having developed a forward operator we construct in this study the tangent linear (adjoint) operator by application of the chain rule of differential calculus in forward (reverse) mode. Armed with these operators we show in a simulation study the potential benefit of GPS STDs in inverse modeling. We conclude that the developed operators are tailored for three (four)-dimensional variational data assimilation and/or travel time tomography.

Differential Code Bias of GPS Receivers in Low Earth Orbit: An Assessment for CHAMP and SAC-C

The knowledge of the transmitter and receiver differential code biases (DCB) plays a key role for the calibration of GPS based measurements of total electron content (TEC). To estimate the DCB of the CHAMP receiver concerning the zenith looking antenna a model assisted technique has been developed which takes advantage of the known GPS biases and comparatively low TEC above Low Earth Orbiter (LEO) altitudes in polar regions and during nighttime intervals. For assistance the Parameterized Ionospheric Model (PIM) is used. Applying this method we derived a reliable bias solution for the CHAMP receiver varying within a spread RMS below 1 TECU for the years 2001 and 2002. The SAC-C receiver DCB (zenith antenna) could be derived comparatively stable. In this paper we describe the model assisted calibration technique and present differential code bias estimations for CHAMP and SAC-C. The model assisted approach has been used to estimate the CHAMP occultation DCB as well. This bias could also be assessed by simultaneous GPS observations of the zenith and occultation antenna for selected intervals in 2001. DCB estimations from both approaches are shown.

Satellite technology glimpses ionospheric response to solar eclipse

The millenniums last solar eclipse was observed on August 11, 1999, over most of Europe, along the northeast coast of North America, and in the Near East and Middle East. The eclipse was also observed by the global navigation satellite systems (GNSS), the Global Positioning System (GPS), and the Russian GNSS (GLONASS), because the transmitted signals can be used to infer the total electron content of the ionosphere. Disruption of photoionization and thermospheric heating leads to numerous complex phenomena in the ionosphere. Because of the supersonic speed of the Moons cool shadow in the atmosphere, atmospheric gravity waves may be generated; these propagate upward and trace as traveling ionosphere disturbances in the ionosphere [Chimonas and Hines, 1970]. Analogous to the atmospheric pressure on the Earths surface, total electron content (TEC) can be understood as the “pressure” of the electron gas of the ionosphere, which will decrease with reduced energy input.

GPS Radio Occultation with CHAMP and GRACE: Recent Results

The German CHAMP (CHAllenging Minisatellite Payload) satellite provides continuously GPS radio occultation data since February 2001. The measurements are analyzed by an operational orbit and occultation processing system at GFZ. In total ∼170 000 high quality globally distributed vertical profiles of refractivity, temperature and water vapor are provided as of October 2004. The ground infrastructure from GFZ allows for the demonstration of a rapid data analysis since February 2003. The average delay between each measurement and provision of atmospheric excess phase data was reduced to ∼4 hours by mid April 2004 and is continuously reached. The complete set of the available refractivity profiles is compared with corresponding analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) between 0 km and 30 km altitude. The comparison shows nearly bias-free refractivity between ∼7 km and 30 km, the standard deviation is ∼1%. The known negative refractivity bias of the CHAMP data in relation to ECMWF is significantly reduced in comparison to earlier product versions by applying the Full Spectrum Inversion (FSI) method for the data analysis in the lower troposphere. First radio occultation measurements from the GRACE-B (Gravity Recovery And Climate Experiment) satellite are available for a 25 h period on July 28/29, 2004. The stability of the satellite clock from GRACE-B is significantly improved in relation to CHAMP. This allows for precise occultation analysis using 30 s clock solutions applying a zero difference technique. Thus the disadvantageous use of a reference GPS satellite link to eliminate the clock error from GRACE-B can be avoided.